Location: Urban area near the Western Ghats, Kerala, South India.
Date: 20th April, 2017
Current Climate: Hot and humid. Occasional rains.
Not large, about the size of a house sparrow. Didn't make any characteristic sound/chirping during the period of observation. Pointed,slightly curved, thin beak. Probably feeds on insects, grain, nuts. Has a black crest atop its head, similar to that of a woodpecker. Appears to have an off-white or pale yellow "mouth" (couldn't get a picture of that, sorry). Chin and throat have white feathers. Foot and legs appear dark gray, if not black. Possesses conspicuous red infraorbital patches (of feathers?) below each eye.
The locality is often frequented by feral pigeons. However, this particular bird made its appearance when there were no pigeons in sight.
That is a red whiskered bulbul.
(image from the Wikipedia article)
The crest isn't as prominent in your pictures - maybe it has just been to the barbers.
Bird flight is the primary mode of locomotion used by most bird species in which birds take off and fly. Flight assists birds with feeding, breeding, avoiding predators, and migrating.
Bird flight is one of the most complex forms of locomotion in the animal kingdom. Each facet of this type of motion, including hovering, taking off, and landing, involves many complex movements. As different bird species adapted over millions of years through evolution for specific environments, prey, predators, and other needs, they developed specializations in their wings, and acquired different forms of flight.
Various theories exist about how bird flight evolved, including flight from falling or gliding (the trees down hypothesis), from running or leaping (the ground up hypothesis), from wing-assisted incline running or from proavis (pouncing) behavior.
The eye of a bird most closely resembles that of the reptiles. Unlike the mammalian eye, it is not spherical, and the flatter shape enables more of its visual field to be in focus. A circle of bony plates, the sclerotic ring, surrounds the eye and holds it rigid, but an improvement over the reptilian eye, also found in mammals, is that the lens is pushed further forward, increasing the size of the image on the retina. 
Eyes of most birds are large, not very round and capable of only limited movement in the orbits,  typically 10-20° (but in some passerines, >80°) horizontally. That's why head movements in birds play bigger role than eye movements.  Two eyes usually move independently,   and in some species they can move coordinatedly in opposite directions. 
Birds with eyes on the sides of their heads have a wide visual field, useful for detecting predators, while those with eyes on the front of their heads, such as owls, have binocular vision and can estimate distances when hunting.   The American woodcock probably has the largest visual field of any bird, 360° in the horizontal plane, and 180° in the vertical plane. 
The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third concealed eyelid that sweeps horizontally across the eye like a windscreen wiper.  The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds when they are under water.  When sleeping, the lower eyelid rises to cover the eye in most birds, with the exception of the horned owls where the upper eyelid is mobile. 
The eye is also cleaned by tear secretions from the lachrymal gland and protected by an oily substance from the Harderian glands which coats the cornea and prevents dryness. The eye of a bird is larger compared to the size of the animal than for any other group of animals, although much of it is concealed in its skull. The ostrich has the largest eye of any land vertebrate, with an axial length of 50 mm (2 in), twice that of the human eye. 
Bird eye size is broadly related to body mass. A study of five orders (parrots, pigeons, petrels, raptors and owls) showed that eye mass is proportional to body mass, but as expected from their habits and visual ecology, raptors and owls have relatively large eyes for their body mass. 
Behavioural studies show that many avian species focus on distant objects preferentially with their lateral and monocular field of vision, and birds will orientate themselves sideways to maximise visual resolution. For a pigeon, resolution is twice as good with sideways monocular vision than forward binocular vision, whereas for humans the converse is true. 
The performance of the eye in low light levels depends on the distance between the lens and the retina, and small birds are effectively forced to be diurnal because their eyes are not large enough to give adequate night vision. Although many species migrate at night, they often collide with even brightly lit objects like lighthouses or oil platforms. Birds of prey are diurnal because, although their eyes are large, they are optimised to give maximum spatial resolution rather than light gathering, so they also do not function well in poor light.  Many birds have an asymmetry in the eye's structure which enables them to keep the horizon and a significant part of the ground in focus simultaneously. The cost of this adaptation is that they have myopia in the lower part of their visual field. 
Birds with relatively large eyes compared to their body mass, such as common redstarts and European robins sing earlier at dawn than birds of the same size and smaller body mass. However, if birds have the same eye size but different body masses, the larger species sings later than the smaller. This may be because the smaller bird has to start the day earlier because of weight loss overnight.  Overnight weight loss for small birds is typically 5-10% and may be over 15% on cold winter nights.  In one study, robins put on more mass in their dusk feeding when nights were cold. 
Nocturnal birds have eyes optimised for visual sensitivity, with large corneas relative to the eye's length, whereas diurnal birds have longer eyes relative to the corneal diameter to give greater visual acuity. Information about the activities of extinct species can be deduced from measurements of the sclerotic ring and orbit depth. For the latter measurement to be made, the fossil must have retained its three-dimensional shape, so activity pattern cannot be determined with confidence from flattened specimens like Archaeopteryx, which has a complete sclerotic ring but no orbit depth measurement. 
The main structures of the bird eye are similar to those of other vertebrates. The outer layer of the eye consists of the transparent cornea at the front, and two layers of sclera — a tough white collagen fibre layer which surrounds the rest of the eye and supports and protects the eye as a whole. The eye is divided internally by the lens into two main segments: the anterior segment and the posterior segment. The anterior segment is filled with a watery fluid called the aqueous humour, and the posterior segment contains the vitreous humour, a clear jelly-like substance.
The lens is a transparent convex or 'lens' shaped body with a harder outer layer and a softer inner layer. It focuses the light on the retina. The shape of the lens can be altered by ciliary muscles which are directly attached to the lens capsule by means of the zonular fibres. In addition to these muscles, some birds also have a second set, Crampton's muscles, that can change the shape of the cornea, thus giving birds a greater range of accommodation than is possible for mammals. This accommodation can be rapid in some diving water birds such as in the mergansers. The iris is a coloured muscularly operated diaphragm in front of the lens which controls the amount of light entering the eye. At the centre of the iris is the pupil, the variable circular area through which the light passes into the eye.  
The retina is a relatively smooth curved multi-layered structure containing the photosensitive rod and cone cells with the associated neurons and blood vessels. The density of the photoreceptors is critical in determining the maximum attainable visual acuity. Humans have about 200,000 receptors per mm 2 , but the house sparrow has 400,000 and the common buzzard 1,000,000. The photoreceptors are not all individually connected to the optic nerve, and the ratio of nerve ganglia to receptors is important in determining resolution. This is very high for birds the white wagtail has 100,000 ganglion cells to 120,000 photoreceptors. 
Rods are more sensitive to light, but give no colour information, whereas the less sensitive cones enable colour vision. In diurnal birds, 80% of the receptors may be cones (90% in some swifts) whereas nocturnal owls have almost all rods. As with other vertebrates except placental mammals, some of the cones may be double cones. These can amount to 50% of all cones in some species. 
Towards the centre of the retina is the fovea (or the less specialised, area centralis) which has a greater density of receptors and is the area of greatest forward visual acuity, i.e. sharpest, clearest detection of objects. In 54% of birds, including birds of prey, kingfishers, hummingbirds and swallows, there is second fovea for enhanced sideways viewing. The optic nerve is a bundle of nerve fibres which carry messages from the eye to the relevant parts of the brain. Like mammals, birds have a small blind spot without photoreceptors at the optic disc, under which the optic nerve and blood vessels join the eye. 
The pecten is a poorly understood body consisting of folded tissue which projects from the retina. It is well supplied with blood vessels and appears to keep the retina supplied with nutrients,  and may also shade the retina from dazzling light or aid in detecting moving objects.  Pecten oculi is abundantly filled with melanin granules which have been proposed to absorb stray light entering the bird eye to reduce background glare. Slight warming of pecten oculi due to absorption of light by melanin granules has been proposed to enhance metabolic rate of pecten. This is suggested to help increase secretion of nutrients into the vitreous body, eventually to be absorbed by the avascular retina of birds for improved nutrition.  Extra-high enzymic activity of alkaline phosphatase in pecten oculi has been proposed to support high secretory activity of pecten to supplement nutrition of the retina. 
The choroid is a layer situated behind the retina which contains many small arteries and veins. These provide arterial blood to the retina and drain venous blood. The choroid contains melanin, a pigment which gives the inner eye its dark colour, helping to prevent disruptive reflections.
There are two sorts of light receptors in a bird's eye, rods and cones. Rods, which contain the visual pigment rhodopsin are better for night vision because they are sensitive to small quantities of light. Cones detect specific colours (or wavelengths) of light, so they are more important to colour-orientated animals such as birds.  Most birds are tetrachromatic, possessing four types of cone cells each with a distinctive maximal absorption peak. In some birds, the maximal absorption peak of the cone cell responsible for the shortest wavelength extends to the ultraviolet (UV) range, making them UV-sensitive.  In addition to that, the cones at the bird's retina are arranged in a characteristic form of spatial distribution, known as hyperuniform distribution, which maximizes its light and color absorption. This form of spatial distributions are only observed as a result of some optimization process, which in this case can be described in terms of birds’ evolutionary history. 
The four spectrally distinct cone pigments are derived from the protein opsin, linked to a small molecule called retinal, which is closely related to vitamin A. When the pigment absorbs light the retinal changes shape and alters the membrane potential of the cone cell affecting neurons in the ganglia layer of the retina. Each neuron in the ganglion layer may process information from a number of photoreceptor cells, and may in turn trigger a nerve impulse to relay information along the optic nerve for further processing in specialised visual centres in the brain. The more intense a light, the more photons are absorbed by the visual pigments the greater the excitation of each cone, and the brighter the light appears. 
By far the most abundant cone pigment in every bird species examined is the long-wavelength form of iodopsin, which absorbs at wavelengths near 570 nm. This is roughly the spectral region occupied by the red- and green-sensitive pigments in the primate retina, and this visual pigment dominates the colour sensitivity of birds.  In penguins, this pigment appears to have shifted its absorption peak to 543 nm, presumably an adaptation to a blue aquatic environment. 
The information conveyed by a single cone is limited: by itself, the cell cannot tell the brain which wavelength of light caused its excitation. A visual pigment may absorb two wavelengths equally, but even though their photons are of different energies, the cone cannot tell them apart, because they both cause the retinal to change shape and thus trigger the same impulse. For the brain to see colour, it must compare the responses of two or more classes of cones containing different visual pigments, so the four pigments in birds give increased discrimination. 
Each cone of a bird or reptile contains a coloured oil droplet these no longer exist in mammals. The droplets, which contain high concentrations of carotenoids, are placed so that light passes through them before reaching the visual pigment. They act as filters, removing some wavelengths and narrowing the absorption spectra of the pigments. This reduces the response overlap between pigments and increases the number of colours that a bird can discern.  Six types of cone oil droplets have been identified five of these have carotenoid mixtures that absorb at different wavelengths and intensities, and the sixth type has no pigments.  The cone pigments with the lowest maximal absorption peak, including those that are UV-sensitive, possess the 'clear' or 'transparent' type of oil droplets with little spectral tuning effect. 
The colours and distributions of retinal oil droplets vary considerably among species, and is more dependent on the ecological niche utilised (hunter, fisher, herbivore) than genetic relationships. As examples, diurnal hunters like the barn swallow and birds of prey have few coloured droplets, whereas the surface fishing common tern has a large number of red and yellow droplets in the dorsal retina. The evidence suggests that oil droplets respond to natural selection faster than the cone's visual pigments.  Even within the range of wavelengths that are visible to humans, passerine birds can detect colour differences that humans do not register. This finer discrimination, together with the ability to see ultraviolet light, means that many species show sexual dichromatism that is visible to birds but not humans. 
Migratory songbirds use the Earth's magnetic field, stars, the Sun, and other unknown cues to determine their migratory direction. An American study suggested that migratory Savannah sparrows used polarised light from an area of sky near the horizon to recalibrate their magnetic navigation system at both sunrise and sunset. This suggested that skylight polarisation patterns are the primary calibration reference for all migratory songbirds.  However, it appears that birds may be responding to secondary indicators of the angle of polarisation, and may not be actually capable of directly detecting polarisation direction in the absence of these cues. 
Ultraviolet sensitivity Edit
Many species of birds are tetrachromatic, with dedicated cone cells for perceiving wavelengths in the ultraviolet and violet regions of the light spectrum. These cells contain a combination of short wave sensitive (SWS1) opsins, SWS1-like opsins (SWS2), and long-wave filtering carotenoid pigments  for selectively filtering and receiving light between 300 and 400 nm. There are two types of short wave color vision in birds: violet sensitive (VS) and ultraviolet sensitive (UVS).  Single nucleotide substitutions in the SWS1 opsin sequence are responsible blue-shifting the spectral sensitivity of the opsin from violet sensitive (λmax = 400) to ultraviolet sensitive (λmax = 310–360).  This is the proposed evolutionary mechanism by which ultraviolet vision originally arose. The major clades of birds that have UVS vision are Palaeognathae (ratites and tinamous), Charadriiformes (shorebirds, gulls, and alcids), Trogoniformes (trogons), Psittaciformes (parrots), and Passeriformes (perching birds, representing more than half of all avian species). 
UVS vision can be useful for courtship. Birds that do not exhibit sexual dichromatism in visible wavelengths are sometimes distinguished by the presence of ultraviolet reflective patches on their feathers.   Male blue tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.  Male blue grosbeaks with the brightest and most UV-shifted blue in their plumage are larger, hold the most extensive territories with abundant prey, and feed their offspring more frequently than other males.  Mediterranean storm petrels do not show sexual dimorphism in UV-patterns, but the correlation between UV-reflectance and male body condition suggests a possible role in sexual selection. 
The bill's appearance is important in the interactions of the blackbird. Although the UV component seems unimportant in interactions between territory-holding males, where the degree of orange is the main factor, the female responds more strongly to males with bills with good UV-reflectiveness. 
UVS is also demonstrated to serve functions in foraging,  prey identification,  and frugivory. Similar advantages afforded to trichromatic primates over dichromatic primates in frugivory  are generally considered to exist in birds. The waxy surfaces of many fruits and berries reflect UV light that advertise their presence to UVS birds.  Common kestrels are able to locate the trails of voles with vision these small rodents lay scent trails of urine and feces that reflect UV light, making them visible to the kestrels.  However, this view has been challenged by the finding of low UV sensitivity in raptors and weak UV reflection of mammal urine. 
While tetrachromatic vision is not exclusive to birds (insects, reptiles, and crustaceans are also sensitive to short wavelengths), some predators of UVS birds cannot see ultraviolet light. This raises the possibility that ultraviolet vision gives birds a channel in which they can privately signal, thereby remaining inconspicuous to predators.  However, recent evidence does not appear to support this hypothesis. 
Contrast sensitivity Edit
Contrast (or more precisely Michelson-contrast) is defined as the difference in luminance between two stimulus areas, divided by the sum of luminance of the two. Contrast sensitivity is the inverse of the smallest contrast that can be detected a contrast sensitivity of 100 means that the smallest contrast that can be detected is 1%. Birds have comparably lower contrast sensitivity than mammals. Humans have been shown to detect contrasts as low as 0.5-1%  whereas most birds tested require ca. 10% contrast to show a behavioural response.    A contrast sensitivity function describes an animal's ability to detect the contrast of grating patterns of different spatial frequency (i.e. different detail). For stationary viewing experiments the contrast sensitivity is highest at a medium spatial frequency and lower for higher and lower spatial frequencies. 
Birds can resolve rapid movements better than humans, for whom flickering at a rate greater than 50 light pulse cycles per second appears as continuous movement. Humans cannot therefore distinguish individual flashes of a fluorescent light bulb oscillating at 60 light pulse cycles per second, but budgerigars and chickens have flicker or light pulse cycles per second thresholds of more than 100 light pulse cycles per second. [ citation needed ] A Cooper's hawk can pursue agile prey through woodland and avoid branches and other objects at high speed to humans such a chase would appear as a blur. 
Birds can also detect slow moving objects. The movement of the sun and the constellations across the sky is imperceptible to humans, but detected by birds. The ability to detect these movements allows migrating birds to properly orient themselves. 
To obtain steady images while flying or when perched on a swaying branch, birds hold the head as steady as possible with compensating reflexes. Maintaining a steady image is especially relevant for birds of prey.  Because the image can be centered on the deep fovea of only one eye at a time, most falcons when diving use a spiral path to approach their prey after they have locked on to a target individual. The alternative of turning the head for a better view slows down the dive by increasing drag while spiralling does not reduce speeds significantly.  
Edges and shapes Edit
When an object is partially blocked by another, humans unconsciously tend to make up for it and complete the shapes (See Amodal perception). It has however been demonstrated that pigeons do not complete occluded shapes.  A study based on altering the grey level of a perch that was coloured differently from the background showed that budgerigars do not detect edges based on colours. 
Magnetic fields Edit
The perception of magnetic fields by migratory birds has been suggested to be light dependent.  Birds move their head to detect the orientation of the magnetic field,  and studies on the neural pathways have suggested that birds may be able to "see" the magnetic fields.  The right eye of a migratory bird contains photoreceptive proteins called cryptochromes. Light excites these molecules to produce unpaired electrons that interact with the Earth's magnetic field, thus providing directional information.  
Diurnal birds of prey Edit
The visual ability of birds of prey is legendary, and the keenness of their eyesight is due to a variety of factors. Raptors have large eyes for their size, 1.4 times greater than the average for birds of the same weight,  and the eye is tube-shaped to produce a larger retinal image. The resolving power of an eye depends both on the optics, large eyes with large apertures suffers less from diffraction and can have larger retinal images due to a long focal length, and on the density of receptor spacing. The retina has a large number of receptors per square millimeter, which determines the degree of visual acuity. The more receptors an animal has, the higher its ability to distinguish individual objects at a distance, especially when, as in raptors, each receptor is typically attached to a single ganglion.  Many raptors have foveas with far more rods and cones than the human fovea (65,000/mm 2 in American kestrel, 38,000 in humans) and this provides these birds with spectacular long distance vision. [ citation needed ] It is proposed that the shape of the deep central fovea of raptors can create a telephoto optical system,  increasing the size of the retinal image in the fovea and thereby increasing the spatial resolution. Behavioural studies show that some large eyed raptors (Wedge-tailed eagle, Old world vultures) have a 2 times higher spatial resolution than humans, but many medium and small sized raptors have comparable or lower spatial resolution.      
The forward-facing eyes of a bird of prey give binocular vision, which is assisted by a double fovea.  The raptor's adaptations for optimum visual resolution (an American kestrel can see a 2–mm insect from the top of an 18–m tree) has a disadvantage in that its vision is poor in low light level, and it must roost at night.  Raptors may have to pursue mobile prey in the lower part of their visual field, and therefore do not have the lower field myopia adaptation demonstrated by many other birds.  Scavenging birds like vultures do not need such sharp vision, so a condor has only a single fovea with about 35,000 receptors mm 2 . Vultures, however have high physiological activity of many important enzymes to suit their distant clarity of vision.  [ citation needed ] Crested caracara also only have a single fovea as this species forages on the ground for carrion and insects. However, they do have a higher degree of binocular overlap than other falcons, potentially to enable the caracara to manipulate objects, such as rocks, whilst foraging. 
Like other birds investigated, raptors do also have coloured oil droplets in their cones.    The generally brown, grey and white plumage of this group, and the absence of colour displays in courtship, suggests that colour is relatively unimportant to these birds. 
In most raptors, a prominent eye ridge and its feathers extend above and in front of the eye. This "eyebrow" gives birds of prey their distinctive stare. The ridge physically protects the eye from wind, dust, and debris and shields it from excessive glare. The osprey lacks this ridge, although the arrangement of the feathers above its eyes serves a similar function it also possesses dark feathers in front of the eye which probably serve to reduce the glare from the water surface when the bird is hunting for its staple diet of fish. 
Nocturnal birds Edit
Owls have very large eyes for their size, 2.2 times greater than the average for birds of the same weight,  and positioned at the front of the head. The eyes have a field overlap of 50–70%, giving better binocular vision than for diurnal birds of prey (overlap 30–50%).  The tawny owl's retina has about 56,000 light-sensitive rods per square millimetre (36 million per square inch) although earlier claims that it could see in the infrared part of the spectrum have been dismissed. 
Adaptations to night vision include the large size of the eye, its tubular shape, large numbers of closely packed retinal rods, and an absence of cones, since cone cells are not sensitive enough for a low-photon nighttime environment. There are few coloured oil droplets, which would reduce the light intensity, but the retina contains a reflective layer, the tapetum lucidum. This increases the amount of light each photosensitive cell receives, allowing the bird to see better in low light conditions.  Owls normally have only one fovea, and that is poorly developed except in diurnal hunters like the short-eared owl. 
Besides owls, bat hawks, frogmouths and nightjars also display good night vision. Some bird species nest deep in cave systems which are too dark for vision, and find their way to the nest with a simple form of echolocation. The oilbird is the only nocturnal bird to echolocate,  but several Aerodramus swiftlets also utilise this technique, with one species, Atiu swiftlet, also using echolocation outside its caves.  
Water birds Edit
Seabirds such as terns and gulls that feed at the surface or plunge for food have red oil droplets in the cones of their retinas. This improves contrast and sharpens distance vision, especially in hazy conditions.  Birds that have to look through an air/water interface have more deeply coloured carotenoid pigments in the oil droplets than other species. 
This helps them to locate shoals of fish, although it is uncertain whether they are sighting the phytoplankton on which the fish feed, or other feeding birds. 
Birds that fish by stealth from above the water have to correct for refraction particularly when the fish are observed at an angle. Reef herons and little egrets appear to be able to make the corrections needed when capturing fish and are more successful in catching fish when strikes are made at an acute angle and this higher success may be due to the inability of the fish to detect their predators.  Other studies indicate that egrets work within a preferred angle of strike and that the probability of misses increase when the angle becomes too far from the vertical leading to an increased difference between the apparent and real depth of prey. 
Birds that pursue fish under water like auks and divers have far fewer red oil droplets,  but they have special flexible lenses and use the nictitating membrane as an additional lens. This allows greater optical accommodation for good vision in air and water.  Cormorants have a greater range of visual accommodation, at 50 dioptres, than any other bird, but the kingfishers are considered to have the best all-round (air and water) vision. 
Tubenosed seabirds, which come ashore only to breed and spend most of their life wandering close to the surface of the oceans, have a long narrow area of visual sensitivity on the retina  This region, the area giganto cellularis, has been found in the Manx shearwater, Kerguelen petrel, great shearwater, broad-billed prion and common diving-petrel. It is characterised by the presence of ganglion cells which are regularly arrayed and larger than those found in the rest of the retina, and morphologically appear similar to the cells of the retina in cats. The location and cellular morphology of this novel area suggests a function in the detection of items in a small binocular field projecting below and around the bill. It is not concerned primarily with high spatial resolution, but may assist in the detection of prey near the sea surface as a bird flies low over it. 
The Manx shearwater, like many other seabirds, visits its breeding colonies at night to reduce the chances of attack by aerial predators. Two aspects of its optical structure suggest that the eye of this species is adapted to vision at night. In the shearwater's eyes the lens does most of the bending of light necessary to produce a focused image on the retina. The cornea, the outer covering of the eye, is relative flat and so of low refractive power. In a diurnal bird like the pigeon, the reverse is true the cornea is highly curved and is the principal refractive component. The ratio of refraction by the lens to that by the cornea is 1.6 for the shearwater and 0.4 for the pigeon the figure for the shearwater is consistent with that for a range of nocturnal birds and mammals. 
The shorter focal length of shearwater eyes give them a smaller, but brighter, image than is the case for pigeons, so the latter has sharper daytime vision. Although the Manx shearwater has adaptations for night vision, the effect is small, and it is likely that these birds also use smell and hearing to locate their nests. 
It used to be thought that penguins were far-sighted on land. Although the cornea is flat and adapted to swimming underwater, the lens is very strong and can compensate for the reduced corneal focusing when out of water.  Almost the opposite solution is used by the hooded merganser which can bulge part of the lens through the iris when submerged. 
Getting the air to move over and under the wing also requires the wing to be moving. This is called thrust. Thrust is created when birds flap their wings using their strong breast muscles. Planes use another method for thrust. They use engines. These can be either propeller or jet engines. With both birds and planes, thrust is the other part of creating lift and the ability to fly. So the shape of the wing and the ability to move it through the air are the two things needed for bird and plane flight.
Birds use their strong breast muscles to flap their wings and give them the thrust to move through the air and fly. In a way, birds use a swimming motion to get the lift needed to fly. Plane wings have a similar shape as bird wings, but instead of flapping their wings, we use engines to thrust them into the air and create the lift needed to fly.
Practice, Perfect: How Birds Learn Songs
Songbirds listen, learn, and practice a lot like we do. As nestlings they tune in to neighborhood songs by listening closely and committing them to memory. It is only later, after they’ve fledged, that young birds begin to practice. The early practice songs are messy and unstructured, a lot like the babbling in humans, to practice talking by stringing together sounds, as babies do before they can produce clear words or sentences, analogous to plastic song in songbirds of a young child. After many months of practice, songbirds refine their songs and settle on a repertoire, which often stays fixed for the rest of their lives.
Songbirds need tutors too
Plastic song of a young White-throated Sparrow:
Crystallized song of an adult White-throated Sparrow:
Practice makes perfect
Many songbirds are prolific singers. Carolina Wrens (Thryothorus ludovicianus) repeat the same song hundreds of times before moving to the next song in their large repertoires the full range of sounds that an animal makes, each used in context to communicate specific messages . Becoming an accomplished performer requires a focused period of song learning (sensory period in songbirds, the early months of life before they practice singing that they most easily learn the songs of nearby tutors ) and an intensive period of practice (sensorimotor period in songbirds, the period during which the bird practices its song, usually following fledging ). For most songbirds these are distinct learning phases: (1) as nestlings, birds memorize the songs of their neighborhoods in birds, the set of adjacent territories or display sites surrounding a focal territory or site > then (2) as juveniles, they move to a new territory and practice those songs until they can masterfully defend a territory.
This practicing Carolina Wren is starting to get into the rhythm, but still needs some practice before it’s perfect
Plastic song of a young Carolina Wren
Here’s an adult Carolina Wren song, with its precise rhythm perfected.
Crystallized song of an adult Carolina Wren
Eavesdropping is best
You can learn an awful lot from what you overhear. Birds can too. In fact, research on Song Sparrows (Melospiza melodia) has revealed that young males learn more from listening in on interactions among neighborhood in birds, the set of adjacent territories or display sites surrounding a focal territory or site males than from solo tutor in songbirds which do not develop their songs instinctively, one of the adults that a young bird listens to as a nestling and models its adult song on performances. This may seem surprising at first, but there is a lot more information packed into a heated exchange than a solo, including cues about which song patterns and singing strategies are most winning. This is crucial know-how: the birds most successful at defending territories are those whose song types (adults learn up to 13 separate types) most accurately match the various songs of their neighbors. 4
All the elements are here for this young Song Sparrow he just needs to get them in the right order.
Plastic song of a young Song Sparrow
Recorded by Chris Templeton
Here, the same Song Sparrow has grown up a bit and finally mastered the crystallized adult form of this song.
Crystallized song of an adult Song Sparrow
Recorded by Chris Templeton
Not a true songbird, but still learns its song
Along with parrots and bellbirds, hummingbirds are one of the rare bird groups that learn their songs even though they are not true songbirds a species from the oscine (ah-SEEN) group of passerine (PASS-er-een) birds, songbirds (including sparrows, thrushes, and warblers) have a specialized voice box called a syrinx that can produce complex sounds, songbirds must learn their songs rather than developing them instinctively —a good example of how learning can evolve multiple times from different ancestors. 5 The Little Hermit (Phaethornis longuemareus) is a tiny hummingbird that uses its song to attract females at display sites in tropical South America. Each Little Hermit sings in a local dialect a unique set of sounds made by a subpopulation of animals of the same species very similar to his direct neighbors but quite different from more distant birds.
This Little Hermit’s early song is simpler than what he’ll sing as an adult.
Plastic song of a young Little Hermit:
Recorded by Julian Kapoor
This is the same Little Hermit months later: he sings a more complex song the same way each time.
Crystallized song of an adult Little Hermit
Recorded by Julian Kapoor
Learning like the birds
Young singer from Shutterstock
Songbirds and humans have a lot in common on the learning front. Baby birds babble in humans, to practice talking by stringing together sounds, as babies do before they can produce clear words or sentences, analogous to plastic song in songbirds just as humans do. Like birds, humans need to hear themselves and others in order to produce normal adult sounds. And just as it is much harder for us to learn languages after childhood, most birds experience a critical period in songbirds, the time as a nestling during which the bird is most sensitive to learning the sounds of nearby birds as nestlings when they are best able to learn song. There is even recent evidence that some songbirds can learn syntax a particular ordering of vocalizations that produces a specific meaning in a similar way to how we learn to string together sentences. 6
Listen as this two-year-old learns the ABCs.
Plastic song of a young child
Recorded by a proud parent
Now for a jazzy grown up version by musician Nate Marshall.
Crystallized (and jazzy) song of a adult musician:
Compare learning juveniles and seasoned adult songbirds side-by-side
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.  Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.  Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. 
Aves and a sister group, the order Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.  However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system. Gauthier defined Aves to include only the crown group of the set of modern birds. This was done by excluding most groups known only from fossils, and assigning them, instead, to the broader group Avialae,  in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
Gauthier and de Queiroz  identified four different definitions for the same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. He assigned other names to the other groups.
- Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
- Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
- Aves can mean those feathered dinosaurs that fly (alternately Avialae)
- Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "crown group", in this sense synonymous with Neornithes)
Under the fourth definition Archaeopteryx, traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and bird evolution, though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers. 
Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all theropods closer to birds than to Deinonychus",   with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus.  Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Jacques Gauthier, who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered wings used in flapping flight, and the birds that descended from them.  
Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition is synonymous to Avifilopluma. 
Dinosaurs and the origin of birds
Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs,  and more specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurids and oviraptorosaurs, among others.  As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod feathered dinosaurs, contribute to this ambiguity.   
The consensus view in contemporary palaeontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids.  Together, these form a group called Paraves. Some basal members of Deinonychosauria, such as Microraptor, have features which may have enabled them to glide or fly. The most basal deinonychosaurs were very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both.   Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, recent studies suggest that the first avialans were omnivores. 
The Late Jurassic Archaeopteryx is well known as one of the first transitional fossils to be found, and it provided support for the theory of evolution in the late 19th century. Archaeopteryx was the first fossil to display both clearly traditional reptilian characteristics—teeth, clawed fingers, and a long, lizard-like tail—as well as wings with flight feathers similar to those of modern birds. It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor. 
Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs, i.e. the first dinosaurs closer to living birds than to Tyrannosaurus rex. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase.   After the appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of neotenic (juvenile-like) characteristics. Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer.  The integument evolved into complex, pennaceous feathers. 
The oldest known paravian (and probably the earliest avialan) fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui. 
The well-known probable early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial manoeuvreing. 
Avialans diversified into a wide variety of forms during the Cretaceous Period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves).  Increasingly stiff tails (especially the outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the pygostyle, an ossification of fused tail vertebrae.  In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size.  Around 95 million years ago, they evolved a better sense of smell. 
A third stage of bird evolution starting with Ornithothoraces (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, keeled sternum and the alula, and the loss of grasping hands. 
Early diversity of bird ancestors
The first large, diverse lineage of short-tailed avialans to evolve were the Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. While they were the dominant group of avialans during the Cretaceous period, enantiornithes became extinct along with many other dinosaur groups at the end of the Mesozoic era. 
Many species of the second major avialan lineage to diversify, the Euornithes (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked perching adaptations and seem to have included shorebird-like species, waders, and swimming and diving species.
The latter included the superficially gull-like Ichthyornis  and the Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.  The early euornithes also saw the development of many traits associated with modern birds, like strongly keeled breastbones, toothless, beaked portions of their jaws (though most non-avian euornithes retained teeth in other parts of the jaws).  Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers,  which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight. 
A study on mosaic evolution in the avian skull found that the last common ancestor of all Neornithes might have had a beak similar to that of the modern hook-billed vanga and a skull similar to that of the Eurasian golden oriole. As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor. 
Diversification of modern birds
All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds.  These two subdivisions are often given the rank of superorder,  although Livezey and Zusi assigned them "cohort" rank.  Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800  to 10,758. 
The discovery of Vegavis from the Maastrichtian, the last stage of the Late Cretaceous proved that the diversification of modern birds started before the Cenozoic era.  The affinities of an earlier fossil, the possible galliform Austinornis lentus, dated to about 85 million years ago,  are still too controversial to provide a fossil evidence of modern bird diversification. In 2020, Asteriornis from the Maastrichtian was described, it appears to be a close relative of Galloanserae, the earliest diverging lineage within Neognathae. 
Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the Middle Cretaceous  to the latest Late Cretaceous.   Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after the Cretaceous–Palaeogene extinction event.  This disagreement is in part caused by a divergence in the evidence most molecular dating studies suggests a Cretaceous evolutionary radiation, while fossil evidence points to a Cenozoic radiation (the so-called 'rocks' versus 'clocks' controversy). Previous attempts to reconcile molecular and fossil evidence have proved controversial,   but more recent estimates, using a more comprehensive sample of fossils and a new way of calibrating molecular clocks, showed that while modern birds originated early in the Late Cretaceous in Western Gondwana, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Modern birds expanded from West Gondwana to the Laurasia through two routes. One route was an Antarctic interchange in the Paleogene. This can be confirmed with the presence of multiple avian groups in Australia and New Zealand. The other route was probably through North American, via land bridges during the Paleocene. This allowed the expansion and diversification of Neornithes into the Holarctic and Paleotropics. 
Classification of bird orders
Cladogram of modern bird relationships based on Kuhl, H. et al. (2020) 
The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds,  although it is frequently debated and constantly revised. Most evidence seems to suggest the assignment of orders is accurate,  but scientists disagree about the relationships between the orders themselves evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.  
As of 2020 [update] , the genome has been sequenced for at least one species in about 90% of extant avian families (218 out of 236 families recognised by the Howard and Moore Checklist). 
Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the snow petrel's breeding colonies up to 440 kilometres (270 mi) inland in Antarctica.  The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics however recent studies found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics.  Many species migrate annually over great distances and across oceans several families of birds have adapted to life both on the world's oceans and in them, and some seabird species come ashore only to breed,  while some penguins have been recorded diving up to 300 metres (980 ft) deep. 
Many bird species have established breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate the ring-necked pheasant, for example, has been introduced around the world as a game bird.  Others have been accidental, such as the establishment of wild monk parakeets in several North American cities after their escape from captivity.  Some species, including cattle egret,  yellow-headed caracara  and galah,  have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.
Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight.
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the respiratory system.  The skull bones in adults are fused and do not show cranial sutures.  The orbital cavities that house the eyeballs are large and separated from each other by a bony septum (partition). The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae.  The last few are fused with the pelvis to form the synsacrum.  The ribs are flattened and the sternum is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings.  The wings are more or less developed depending on the species the only known groups that lost their wings are the extinct moa and elephant birds. 
Like the reptiles, birds are primarily uricotelic, that is, their kidneys extract nitrogenous waste from their bloodstream and excrete it as uric acid, instead of urea or ammonia, through the ureters into the intestine. Birds do not have a urinary bladder or external urethral opening and (with exception of the ostrich) uric acid is excreted along with faeces as a semisolid waste.    However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.  They also excrete creatine, rather than creatinine like mammals.  This material, as well as the output of the intestines, emerges from the bird's cloaca.   The cloaca is a multi-purpose opening: waste is expelled through it, most birds mate by joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate pellets. 
It is a common but not universal feature of altricial passerine nestlings (born helpless, under constant parental care) that instead of excreting directly into the nest, they produce a fecal sac. This is a mucus-covered pouch that allows parents to either dispose of the waste outside the nest or to recycle the waste through their own digestive system. 
Males within Palaeognathae (with the exception of the kiwis), the Anseriformes (with the exception of screamers), and in rudimentary forms in Galliformes (but fully developed in Cracidae) possess a penis, which is never present in Neoaves.   The length is thought to be related to sperm competition.  When not copulating, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. Female birds have sperm storage tubules  that allow sperm to remain viable long after copulation, a hundred days in some species.  Sperm from multiple males may compete through this mechanism. Most female birds have a single ovary and a single oviduct, both on the left side,  but there are exceptions: species in at least 16 different orders of birds have two ovaries. Even these species, however, tend to have a single oviduct.  It has been speculated that this might be an adaptation to flight, but males have two testes, and it is also observed that the gonads in both sexes decrease dramatically in size outside the breeding season.   Also terrestrial birds generally have a single ovary, as does the platypus, an egg-laying mammal. A more likely explanation is that the egg develops a shell while passing through the oviduct over a period of about a day, so that if two eggs were to develop at the same time, there would be a risk to survival. 
Birds are solely gonochoric.  Meaning they have two sexes: either female or male. The sex of birds is determined by the Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ). 
In nearly all species of birds, an individual's sex is determined at fertilisation. However, one recent study claimed to demonstrate temperature-dependent sex determination among the Australian brushturkey, for which higher temperatures during incubation resulted in a higher female-to-male sex ratio.  This, however, was later proven to not be the case. These birds do not exhibit temperature-dependent sex determination, but temperature-dependent sex mortality. 
Respiratory and circulatory systems
Birds have one of the most complex respiratory systems of all animal groups.  Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lungs and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation.  Sound production is achieved using the syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea  the trachea being elongated in some species, increasing the volume of vocalisations and the perception of the bird's size. 
In birds, the main arteries taking blood away from the heart originate from the right aortic arch (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the aorta.  The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating red blood cells in birds retain their nucleus. 
Heart type and features
The avian circulatory system is driven by a four-chambered, myogenic heart contained in a fibrous pericardial sac. This pericardial sac is filled with a serous fluid for lubrication.  The heart itself is divided into a right and left half, each with an atrium and ventricle. The atrium and ventricles of each side are separated by atrioventricular valves which prevent back flow from one chamber to the next during contraction. Being myogenic, the heart's pace is maintained by pacemaker cells found in the sinoatrial node, located on the right atrium.
The sinoatrial node uses calcium to cause a depolarising signal transduction pathway from the atrium through right and left atrioventricular bundle which communicates contraction to the ventricles. The avian heart also consists of muscular arches that are made up of thick bundles of muscular layers. Much like a mammalian heart, the avian heart is composed of endocardial, myocardial and epicardial layers.  The atrium walls tend to be thinner than the ventricle walls, due to the intense ventricular contraction used to pump oxygenated blood throughout the body. Avian hearts are generally larger than mammalian hearts when compared to body mass. This adaptation allows more blood to be pumped to meet the high metabolic need associated with flight. 
Birds have a very efficient system for diffusing oxygen into the blood birds have a ten times greater surface area to gas exchange volume than mammals. As a result, birds have more blood in their capillaries per unit of volume of lung than a mammal.  The arteries are composed of thick elastic muscles to withstand the pressure of the ventricular contractions, and become more rigid as they move away from the heart. Blood moves through the arteries, which undergo vasoconstriction, and into arterioles which act as a transportation system to distribute primarily oxygen as well as nutrients to all tissues of the body.  As the arterioles move away from the heart and into individual organs and tissues they are further divided to increase surface area and slow blood flow. Blood travels through the arterioles and moves into the capillaries where gas exchange can occur.
Capillaries are organised into capillary beds in tissues it is here that blood exchanges oxygen for carbon dioxide waste. In the capillary beds, blood flow is slowed to allow maximum diffusion of oxygen into the tissues. Once the blood has become deoxygenated, it travels through venules then veins and back to the heart. Veins, unlike arteries, are thin and rigid as they do not need to withstand extreme pressure. As blood travels through the venules to the veins a funneling occurs called vasodilation bringing blood back to the heart.  Once the blood reaches the heart, it moves first into the right atrium, then the right ventricle to be pumped through the lungs for further gas exchange of carbon dioxide waste for oxygen. Oxygenated blood then flows from the lungs through the left atrium to the left ventricle where it is pumped out to the body.
The nervous system is large relative to the bird's size.  The most developed part of the brain is the one that controls the flight-related functions, while the cerebellum coordinates movement and the cerebrum controls behaviour patterns, navigation, mating and nest building. Most birds have a poor sense of smell  with notable exceptions including kiwis,  New World vultures  and tubenoses.  The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water.  Some species also have dual fovea. Birds are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones.  They also have double cones, likely to mediate achromatic vision. 
Many birds show plumage patterns in ultraviolet that are invisible to the human eye some birds whose sexes appear similar to the naked eye are distinguished by the presence of ultraviolet reflective patches on their feathers. Male blue tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.  Ultraviolet light is also used in foraging—kestrels have been shown to search for prey by detecting the UV reflective urine trail marks left on the ground by rodents.  With the exception of pigeons and a few other species,  the eyelids of birds are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally.  The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.  The bird retina has a fan shaped blood supply system called the pecten. 
Eyes of most birds are large, not very round and capable of only limited movement in the orbits,  typically 10-20°.  Birds with eyes on the sides of their heads have a wide visual field, while birds with eyes on the front of their heads, such as owls, have binocular vision and can estimate the depth of field.   The avian ear lacks external pinnae but is covered by feathers, although in some birds, such as the Asio, Bubo and Otus owls, these feathers form tufts which resemble ears. The inner ear has a cochlea, but it is not spiral as in mammals. 
Defence and intraspecific combat
A few species are able to use chemical defences against predators some Procellariiformes can eject an unpleasant stomach oil against an aggressor,  and some species of pitohuis from New Guinea have a powerful neurotoxin in their skin and feathers. 
A lack of field observations limit our knowledge, but intraspecific conflicts are known to sometimes result in injury or death.  The screamers (Anhimidae), some jacanas (Jacana, Hydrophasianus), the spur-winged goose (Plectropterus), the torrent duck (Merganetta) and nine species of lapwing (Vanellus) use a sharp spur on the wing as a weapon. The steamer ducks (Tachyeres), geese and swans (Anserinae), the solitaire (Pezophaps), sheathbills (Chionis), some guans (Crax) and stone curlews (Burhinus) use a bony knob on the alular metacarpal to punch and hammer opponents.  The jacanas Actophilornis and Irediparra have an expanded, blade-like radius. The extinct Xenicibis was unique in having an elongate forelimb and massive hand which likely functioned in combat or defence as a jointed club or flail. Swans, for instance, may strike with the bony spurs and bite when defending eggs or young. 
Feathers, plumage, and scales
Feathers are a feature characteristic of birds (though also present in some dinosaurs not currently considered to be true birds). They facilitate flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signalling.  There are several types of feathers, each serving its own set of purposes. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status,  and sex. 
Plumage is regularly moulted the standard plumage of a bird that has moulted after breeding is known as the "" plumage, or—in the Humphrey–Parkes terminology—"basic" plumage breeding plumages or variations of the basic plumage are known under the Humphrey–Parkes system as "" plumages.  Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines, flight feathers are replaced one at a time with the innermost being the first. When the fifth of sixth primary is replaced, the outermost begin to drop. After the innermost tertiaries are moulted, the starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary are moulted in synchrony with the primary that they overlap. 
A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.  As a general rule, the tail feathers are moulted and replaced starting with the innermost pair.  Centripetal moults of tail feathers are however seen in the Phasianidae.  The centrifugal moult is modified in the tail feathers of woodpeckers and treecreepers, in that it begins with the second innermost pair of feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail.   The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from centre outward.  Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. The skin there is well supplied with blood vessels and helps the bird in incubation. 
Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this.  The bill is used to brush away foreign particles and to apply waxy secretions from the uropygial gland these secretions protect the feathers' flexibility and act as an antimicrobial agent, inhibiting the growth of feather-degrading bacteria.  This may be supplemented with the secretions of formic acid from ants, which birds receive through a behaviour known as anting, to remove feather parasites. 
The scales of birds are composed of the same keratin as beaks, claws, and spurs. They are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers. The scales of birds are thought to be homologous to those of reptiles and mammals. 
Most birds can fly, which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for searching for food and for escaping from predators. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb (wing) that serves as an aerofoil. 
Wing shape and size generally determine a bird's flight style and performance many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are flightless, as were many extinct birds.  Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators.  Although flightless, penguins use similar musculature and movements to "fly" through the water, as do some flight-capable birds such as auks, shearwaters and dippers. 
Most birds are diurnal, but some birds, such as many species of owls and nightjars, are nocturnal or crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night. 
Diet and feeding
are varied and often include nectar, fruit, plants, seeds, carrion, and various small animals, including other birds.  The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth.  Some species such as pigeons and some psittacine species do not have a gallbladder.  Most birds are highly adapted for rapid digestion to aid with flight.  Some migratory birds have adapted to use protein stored in many parts of their bodies, including protein from the intestines, as additional energy during migration. 
Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists, while others that concentrate time and effort on specific food items or have a single strategy to obtain food are considered specialists.  Avian foraging strategies can vary widely by species. Many birds glean for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek pest insects are considered beneficial 'biological control agents' and their presence encouraged in biological pest control programmes.  Combined, insectivorous birds eat 400–500 million metric tons of arthropods annually. 
Nectar feeders such as hummingbirds, sunbirds, lories, and lorikeets amongst others have specially adapted brushy tongues and in many cases bills designed to fit co-adapted flowers.  Kiwis and shorebirds with long bills probe for invertebrates shorebirds' varied bill lengths and feeding methods result in the separation of ecological niches.   Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion,  while aerial predators such as sulids, kingfishers and terns plunge dive after their prey. Flamingos, three species of prion, and some ducks are filter feeders.   Geese and dabbling ducks are primarily grazers.
Some species, including frigatebirds, gulls,  and skuas,  engage in kleptoparasitism, stealing food items from other birds. Kleptoparasitism is thought to be a supplement to food obtained by hunting, rather than a significant part of any species' diet a study of great frigatebirds stealing from masked boobies estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%.  Other birds are scavengers some of these, like vultures, are specialised carrion eaters, while others, like gulls, corvids, or other birds of prey, are opportunists. 
Water and drinking
Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands.  Some desert birds can obtain their water needs entirely from moisture in their food. They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting.  Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils. 
Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the pigeon, finch, mousebird, button-quail and bustard families are capable of sucking up water without the need to tilt back their heads.  Some desert birds depend on water sources and sandgrouse are particularly well known for their daily congregations at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers.  Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called crop milk that they provide to their chicks. 
Feathers, being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, ectoparasitic feather mites and bird lice.  The physical condition of feathers are maintained by often with the application of secretions from the . Birds also bathe in water or dust themselves. While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Birds of arid regions make use of loose soil to dust-bathe. A behaviour termed as anting in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Many species will spread out their wings and expose them to direct sunlight and this too is thought to help in reducing fungal and ectoparasitic activity that may lead to feather damage.  
Many bird species migrate to take advantage of global differences of seasonal temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, shorebirds, and waterbirds undertake annual long-distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the temperate or polar regions and a non-breeding season in the tropical regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.  
Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi),  although the bar-tailed godwit is capable of non-stop flights of up to 10,200 km (6,300 mi).  Seabirds also undertake long migrations, the longest annual migration being those of sooty shearwaters, which nest in New Zealand and Chile and spend the northern summer feeding in the North Pacific off Japan, Alaska and California, an annual round trip of 64,000 km (39,800 mi).  Other seabirds disperse after breeding, travelling widely but having no set migration route. Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons. 
Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. Irruptive species such as the boreal finches are one such group and can commonly be found at a location in one year and absent the next. This type of migration is normally associated with food availability.  Species may also travel shorter distances over part of their range, with individuals from higher latitudes travelling into the existing range of conspecifics others undertake partial migrations, where only a fraction of the population, usually females and subdominant males, migrates.  Partial migration can form a large percentage of the migration behaviour of birds in some regions in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory. 
Altitudinal migration is a form of short-distance migration in which birds spend the breeding season at higher altitudes and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when the normal territories also become inhospitable due to lack of food.  Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations. 
The ability of birds to return to precise locations across vast distances has been known for some time in an experiment conducted in the 1950s, a Manx shearwater released in Boston in the United States returned to its colony in Skomer, in Wales within 13 days, a distance of 5,150 km (3,200 mi).  Birds navigate during migration using a variety of methods. For diurnal migrants, the sun is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an internal clock.  Orientation with the stellar compass depends on the position of the constellations surrounding Polaris.  These are backed up in some species by their ability to sense the Earth's geomagnetism through specialised photoreceptors. 
Birds communicate using primarily visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).
Birds sometimes use plumage to assess and assert social dominance,  to display breeding condition in sexually selected species, or to make threatening displays, as in the sunbittern's mimicry of a large predator to ward off hawks and protect young chicks.  Variation in plumage also allows for the identification of birds, particularly between species.
Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds.  The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements  males' breeding success may depend on the quality of such displays. 
Bird calls and songs, which are produced in the syrinx, are the major means by which birds communicate with sound. This communication can be very complex some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs.  Calls are used for a variety of purposes, including mate attraction,  evaluation of potential mates,  bond formation, the claiming and maintenance of territories,  the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),  and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat.  Some birds also use mechanical sounds for auditory communication. The Coenocorypha snipes of New Zealand drive air through their feathers,  woodpeckers drum for long-distance communication,  and palm cockatoos use tools to drum. 
Flocking and other associations
While some birds are essentially territorial or live in small family groups, other birds may form large flocks. The principal benefits of flocking are safety in numbers and increased foraging efficiency.  Defence against predators is particularly important in closed habitats like forests, where ambush predation is common and multiple eyes can provide a valuable early warning system. This has led to the development of many mixed-species feeding flocks, which are usually composed of small numbers of many species these flocks provide safety in numbers but increase potential competition for resources.  Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases. 
Birds sometimes also form associations with non-avian species. Plunge-diving seabirds associate with dolphins and tuna, which push shoaling fish towards the surface.  Hornbills have a mutualistic relationship with dwarf mongooses, in which they forage together and warn each other of nearby birds of prey and other predators. 
Resting and roosting
The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Sleeping birds often use a type of sleep known as vigilant sleep, where periods of rest are interspersed with quick eye-opening "peeks", allowing them to be sensitive to disturbances and enable rapid escape from threats.  Swifts are believed to be able to sleep in flight and radar observations suggest that they orient themselves to face the wind in their roosting flight.  It has been suggested that there may be certain kinds of sleep which are possible even when in flight. 
Some birds have also demonstrated the capacity to fall into slow-wave sleep one hemisphere of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for predators by viewing the outer margins of the flock. This adaptation is also known from marine mammals.  Communal roosting is common because it lowers the loss of body heat and decreases the risks associated with predators.  Roosting sites are often chosen with regard to thermoregulation and safety. 
Many sleeping birds bend their heads over their backs and tuck their bills in their back feathers, although others place their beaks among their breast feathers. Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. Perching birds have a tendon-locking mechanism that helps them hold on to the perch when they are asleep. Many ground birds, such as quails and pheasants, roost in trees. A few parrots of the genus Loriculus roost hanging upside down.  Some hummingbirds go into a nightly state of torpor accompanied with a reduction of their metabolic rates.  This physiological adaptation shows in nearly a hundred other species, including owlet-nightjars, nightjars, and woodswallows. One species, the common poorwill, even enters a state of hibernation.  Birds do not have sweat glands, but they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or by using special behaviours like urohidrosis to cool themselves.
Ninety-five per cent of bird species are socially monogamous. These species pair for at least the length of the breeding season or—in some cases—for several years or until the death of one mate.  Monogamy allows for both paternal care and biparental care, which is especially important for species in which females require males' assistance for successful brood-rearing.  Among many socially monogamous species, extra-pair copulation (infidelity) is common.  Such behaviour typically occurs between dominant males and females paired with subordinate males, but may also be the result of forced copulation in ducks and other anatids. 
For females, possible benefits of extra-pair copulation include getting better genes for her offspring and insuring against the possibility of infertility in her mate.  Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise. 
Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur.  Polygamous breeding systems arise when females are able to raise broods without the help of males.  Some species may use more than one system depending on the circumstances.
Breeding usually involves some form of courtship display, typically performed by the male.  Most displays are rather simple and involve some type of song. Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal lekking. Females are generally the ones that drive partner selection,  although in the polyandrous phalaropes, this is reversed: plainer males choose brightly coloured females.  Courtship feeding, billing and are commonly performed between partners, generally after the birds have paired and mated. 
Homosexual behaviour has been observed in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks.  Over 130 avian species around the world engage in sexual interactions between the same sex or homosexual behaviours. "Same-sex courtship activities may involve elaborate displays, synchronized dances, gift-giving ceremonies, or behaviors at specific display areas including bowers, arenas, or leks." 
Territories, nesting and incubation
Many birds actively defend a territory from others of the same species during the breeding season maintenance of territories protects the food source for their chicks. Species that are unable to defend feeding territories, such as seabirds and swifts, often breed in colonies instead this is thought to offer protection from predators. Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense. 
All birds lay amniotic eggs with hard shells made mostly of calcium carbonate.  Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay camouflaged eggs. There are many exceptions to this pattern, however the ground-nesting nightjars have pale eggs, and camouflage is instead provided by their plumage. Species that are victims of brood parasites have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts. 
Bird eggs are usually laid in a nest. Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows.  Some bird nests, however, are extremely primitive albatross nests are no more than a scrape on the ground. Most birds build nests in sheltered, hidden areas to avoid predation, but large or colonial birds—which are more capable of defence—may build more open nests. During nest construction, some species seek out plant matter from plants with parasite-reducing toxins to improve chick survival,  and feathers are often used for nest insulation.  Some bird species have no nests the cliff-nesting common guillemot lays its eggs on bare rock, and male emperor penguins keep eggs between their body and feet. The absence of nests is especially prevalent in ground-nesting species where the newly hatched young are precocial.
Incubation, which optimises temperature for chick development, usually begins after the last egg has been laid.  In monogamous species incubation duties are often shared, whereas in polygamous species one parent is wholly responsible for incubation. Warmth from parents passes to the eggs through brood patches, areas of bare skin on the abdomen or breast of the incubating birds. Incubation can be an energetically demanding process adult albatrosses, for instance, lose as much as 83 grams (2.9 oz) of body weight per day of incubation.  The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or volcanic sources.  Incubation periods range from 10 days (in woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis). 
The diversity of characteristics of birds is great, sometimes even in closely related species. Several avian characteristics are compared in the table below.  
|Species||Adult weight |
|Ruby-throated hummingbird (Archilochus colubris)||3||13||2.0||2|
|House sparrow (Passer domesticus)||25||11||4.5||5|
|Greater roadrunner (Geococcyx californianus)||376||20||1.5||4|
|Turkey vulture (Cathartes aura)||2,200||39||1.0||2|
|Laysan albatross (Diomedea immutabilis)||3,150||64||1.0||1|
|Magellanic penguin (Spheniscus magellanicus)||4,000||40||1.0||1|
|Golden eagle (Aquila chrysaetos)||4,800||40||1.0||2|
|Wild turkey (Meleagris gallopavo)||6,050||28||1.0||11|
Parental care and fledging
At the time of their hatching, chicks range in development from helpless to independent, depending on their species. Helpless chicks are termed altricial, and tend to be born small, blind, immobile and naked chicks that are mobile and feathered upon hatching are termed precocial. Altricial chicks need help thermoregulating and must be brooded for longer than precocial chicks. The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on an "altricial-precocial spectrum".  Chicks at neither extreme but favouring one or the other may be termed  or . 
The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in megapodes ends at hatching the newly hatched chick digs itself out of the nest mound without parental assistance and can fend for itself immediately.  At the other extreme, many seabirds have extended periods of parental care, the longest being that of the great frigatebird, whose chicks take up to six months to fledge and are fed by the parents for up to an additional 14 months.  The chick guard stage describes the period of breeding during which one of the adult birds is permanently present at the nest after chicks have hatched. The main purpose of the guard stage is to aid offspring to thermoregulate and protect them from predation. 
In some species, both parents care for nestlings and fledglings in others, such care is the responsibility of only one sex. In some species, other members of the same species—usually close relatives of the breeding pair, such as offspring from previous broods—will help with the raising of the young.  Such alloparenting is particularly common among the Corvida, which includes such birds as the true crows, Australian magpie and fairy-wrens,  but has been observed in species as different as the rifleman and red kite. Among most groups of animals, male parental care is rare. In birds, however, it is quite common—more so than in any other vertebrate class.  Although territory and nest site defence, incubation, and chick feeding are often shared tasks, there is sometimes a division of labour in which one mate undertakes all or most of a particular duty. 
The point at which chicks fledge varies dramatically. The chicks of the Synthliboramphus murrelets, like the ancient murrelet, leave the nest the night after they hatch, following their parents out to sea, where they are raised away from terrestrial predators.  Some other species, such as ducks, move their chicks away from the nest at an early age. In most species, chicks leave the nest just before, or soon after, they are able to fly. The amount of parental care after fledging varies albatross chicks leave the nest on their own and receive no further help, while other species continue some supplementary feeding after fledging.  Chicks may also follow their parents during their first migration. 
Brood parasitism, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism.  After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Brood parasites may be either obligate brood parasites, which must lay their eggs in the nests of other species because they are incapable of raising their own young, or non-obligate brood parasites, which sometimes lay eggs in the nests of conspecifics to increase their reproductive output even though they could have raised their own young.  One hundred bird species, including honeyguides, icterids, and ducks, are obligate parasites, though the most famous are the cuckoos.  Some brood parasites are adapted to hatch before their host's young, which allows them to destroy the host's eggs by pushing them out of the nest or to kill the host's chicks this ensures that all food brought to the nest will be fed to the parasitic chicks. 
Birds have evolved a variety of mating behaviours, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Commonly occurring sexual dimorphisms such as size and colour differences are energetically costly attributes that signal competitive breeding situations.  Many types of avian sexual selection have been identified intersexual selection, also known as female choice and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Conflicts between an individual fitness and signalling adaptations ensure that sexually selected ornaments such as plumage colouration and courtship behaviour are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours. 
Inbreeding causes early death (inbreeding depression) in the zebra finch Taeniopygia guttata.  Embryo survival (that is, hatching success of fertile eggs) was significantly lower for sib-sib mating pairs than for unrelated pairs.
Darwin's finch Geospiza scandens experiences inbreeding depression (reduced survival of offspring) and the magnitude of this effect is influenced by environmental conditions such as low food availability. 
Incestuous matings by the purple-crowned fairy wren Malurus coronatus result in severe fitness costs due to inbreeding depression (greater than 30% reduction in hatchability of eggs).  Females paired with related males may undertake extra pair matings (see Promiscuity#Other animals for 90% frequency in avian species) that can reduce the negative effects of inbreeding. However, there are ecological and demographic constraints on extra pair matings. Nevertheless, 43% of broods produced by incestuously paired females contained extra pair young. 
Inbreeding depression occurs in the great tit (Parus major) when the offspring produced as a result of a mating between close relatives show reduced fitness. In natural populations of Parus major, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative. 
Southern pied babblers Turdoides bicolor appear to avoid inbreeding in two ways. The first is through dispersal, and the second is by avoiding familiar group members as mates.  Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated.
Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin.  Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups. In general, inbreeding is avoided because it leads to a reduction in progeny fitness (inbreeding depression) due largely to the homozygous expression of deleterious recessive alleles.  Cross-fertilisation between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.  
Birds occupy a wide range of ecological positions.  While some birds are generalists, others are highly specialised in their habitat or food requirements. Even within a single habitat, such as a forest, the niches occupied by different species of birds vary, with some species feeding in the forest canopy, others beneath the canopy, and still others on the forest floor. Forest birds may be insectivores, frugivores, and nectarivores. Aquatic birds generally feed by fishing, plant eating, and piracy or kleptoparasitism. Birds of prey specialise in hunting mammals or other birds, while vultures are specialised scavengers. Avivores are animals that are specialised at preying on birds.
Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal.  Plants and pollinating birds often coevolve,  and in some cases a flower's primary pollinator is the only species capable of reaching its nectar. 
Birds are often important to island ecology. Birds have frequently reached islands that mammals have not on those islands, birds may fulfil ecological roles typically played by larger animals. For example, in New Zealand nine species of moa were important browsers, as are the kererū and kokako today.  Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.  Nesting seabirds may also affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of guano, which may enrich the local soil  and the surrounding seas. 
A wide variety of avian ecology field methods, including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.
Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man.  Sometimes, these relationships are mutualistic, like the cooperative honey-gathering among honeyguides and African peoples such as the Borana.  Other times, they may be commensal, as when species such as the house sparrow  have benefited from human activities. Several bird species have become commercially significant agricultural pests,  and some pose an aviation hazard.  Human activities can also be detrimental, and have threatened numerous bird species with extinction (hunting, avian lead poisoning, pesticides, roadkill, wind turbine kills  and predation by pet cats and dogs are common causes of death for birds). 
Birds can act as vectors for spreading diseases such as psittacosis, salmonellosis, campylobacteriosis, mycobacteriosis (avian tuberculosis), avian influenza (bird flu), giardiasis, and cryptosporidiosis over long distances. Some of these are zoonotic diseases that can also be transmitted to humans. 
Domesticated birds raised for meat and eggs, called poultry, are the largest source of animal protein eaten by humans in 2003, 76 million tons of poultry and 61 million tons of eggs were produced worldwide.  Chickens account for much of human poultry consumption, though domesticated turkeys, ducks, and geese are also relatively common. Many species of birds are also hunted for meat. Bird hunting is primarily a recreational activity except in extremely undeveloped areas. The most important birds hunted in North and South America are waterfowl other widely hunted birds include pheasants, wild turkeys, quail, doves, partridge, grouse, snipe, and woodcock.  Muttonbirding is also popular in Australia and New Zealand.  Although some hunting, such as that of muttonbirds, may be sustainable, hunting has led to the extinction or endangerment of dozens of species. 
Other commercially valuable products from birds include feathers (especially the down of geese and ducks), which are used as insulation in clothing and bedding, and seabird faeces (guano), which is a valuable source of phosphorus and nitrogen. The War of the Pacific, sometimes called the Guano War, was fought in part over the control of guano deposits. 
Birds have been domesticated by humans both as pets and for practical purposes. Colourful birds, such as parrots and mynas, are bred in captivity or kept as pets, a practice that has led to the illegal trafficking of some endangered species.  Falcons and cormorants have long been used for hunting and fishing, respectively. Messenger pigeons, used since at least 1 AD, remained important as recently as World War II. Today, such activities are more common either as hobbies, for entertainment and tourism,  or for sports such as pigeon racing.
Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly, birders) number in the millions.  Many homeowners erect bird feeders near their homes to attract various species. Bird feeding has grown into a multimillion-dollar industry for example, an estimated 75% of households in Britain provide food for birds at some point during the winter. 
In religion and mythology
Birds play prominent and diverse roles in religion and mythology. In religion, birds may serve as either messengers or priests and leaders for a deity, such as in the Cult of Makemake, in which the Tangata manu of Easter Island served as chiefs  or as attendants, as in the case of Hugin and Munin, the two common ravens who whispered news into the ears of the Norse god Odin. In several civilisations of ancient Italy, particularly Etruscan and Roman religion, priests were involved in augury, or interpreting the words of birds while the "auspex" (from which the word "auspicious" is derived) watched their activities to foretell events. 
They may also serve as religious symbols, as when Jonah (Hebrew: יוֹנָה , dove) embodied the fright, passivity, mourning, and beauty traditionally associated with doves.  Birds have themselves been deified, as in the case of the common peacock, which is perceived as Mother Earth by the people of southern India.  In the ancient world, doves were used as symbols of the Mesopotamian goddess Inanna (later known as Ishtar),   the Canaanite mother goddess Asherah,    and the Greek goddess Aphrodite.      In ancient Greece, Athena, the goddess of wisdom and patron deity of the city of Athens, had a little owl as her symbol.    In religious images preserved from the Inca and Tiwanaku empires, birds are depicted in the process of transgressing boundaries between earthly and underground spiritual realms.  Indigenous peoples of the central Andes maintain legends of birds passing to and from metaphysical worlds. 
In culture and folklore
Birds have featured in culture and art since prehistoric times, when they were represented in early cave paintings.  Some birds have been perceived as monsters, including the mythological Roc and the Māori's legendary Pouākai, a giant bird capable of snatching humans.  Birds were later used as symbols of power, as in the magnificent Peacock Throne of the Mughal and Persian emperors.  With the advent of scientific interest in birds, many paintings of birds were commissioned for books.
Among the most famous of these bird artists was John James Audubon, whose paintings of North American birds were a great commercial success in Europe and who later lent his name to the National Audubon Society.  Birds are also important figures in poetry for example, Homer incorporated nightingales into his Odyssey, and Catullus used a sparrow as an erotic symbol in his Catullus 2.  The relationship between an albatross and a sailor is the central theme of Samuel Taylor Coleridge's The Rime of the Ancient Mariner, which led to the use of the term as a metaphor for a 'burden'.  Other English metaphors derive from birds vulture funds and vulture investors, for instance, take their name from the scavenging vulture. 
Perceptions of bird species vary across cultures. Owls are associated with bad luck, witchcraft, and death in parts of Africa,  but are regarded as wise across much of Europe.  Hoopoes were considered sacred in Ancient Egypt and symbols of virtue in Persia, but were thought of as thieves across much of Europe and harbingers of war in Scandinavia.  In heraldry, birds, especially eagles, often appear in coats of arms. 
In music, birdsong has influenced composers and musicians in several ways: they can be inspired by birdsong they can intentionally imitate bird song in a composition, as Vivaldi, Messiaen, and Beethoven did, along with many later composers they can incorporate recordings of birds into their works, as Ottorino Respighi first did or like Beatrice Harrison and David Rothenberg, they can duet with birds.    
Although human activities have allowed the expansion of a few species, such as the barn swallow and European starling, they have caused population decreases or extinction in many other species. Over a hundred bird species have gone extinct in historical times,  although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonisation of Melanesian, Polynesian, and Micronesian islands.  Many bird populations are declining worldwide, with 1,227 species listed as threatened by BirdLife International and the IUCN in 2009.  
The most commonly cited human threat to birds is habitat loss.  Other threats include overhunting, accidental mortality due to collisions with buildings or vehicles, long-line fishing bycatch,  pollution (including oil spills and pesticide use),  competition and predation from nonnative invasive species,  and climate change.
Governments and conservation groups work to protect birds, either by passing laws that preserve and restore bird habitat or by establishing captive populations for reintroductions. Such projects have produced some successes one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the California condor and Norfolk parakeet. 
Birds maintain their body temperatures by producing heat through their metabolism rather than relying on heat from the environment. Their feathers also play an important role in regulating their temperatures by controlling how much heat leaves the body.
A bird’s body temperature is usually maintained within the high range of 40-42°C and this allows birds to be very active but means they are required to eat a lot of food. A bird will consume around 20 times as much food as a reptile of a similar size. Birds also regulate their body temperatures by shivering when they are cold, panting or fluttering when they are hot, and increasing or reducing blood flow to their feet. Temperatures over 46°C are fatal.
Characteristics of Birds
Birds are endothermic and, because they fly, they require large amounts of energy, necessitating a high metabolic rate. As with mammals, which are also endothermic, birds have an insulating covering that keeps heat in the body: feathers. Specialized feathers called down feathers are especially insulating, trapping air in spaces between each feather to decrease the rate of heat loss. Certain parts of a bird&rsquos body are covered in down feathers the base of other feathers have a downy portion, while newly-hatched birds are covered in down.
Feathers not only act as insulation, but also allow for flight, enabling the lift and thrust necessary to become airborne. The feathers on a wing are flexible, so the collective feathers move and separate as air moves through them, reducing the drag on the wing. Flight feathers are asymmetrical, which affects airflow over them and provides some of the lifting and thrusting force required for flight. Two types of flight feathers are found on the wings: primary feathers and secondary feathers. Primary feathers are located at the tip of the wing and provide thrust. Secondary feathers are located closer to the body, attach to the forearm portion of the wing, and provide lift. Contour feathers are those found on the body. They help reduce drag produced by wind resistance during flight, creating a smooth, aerodynamic surface allowing air to flow smoothly over the bird&rsquos body for efficient flight.
Figure (PageIndex<1>): Bird feathers: Primary feathers are located at the wing tip and provide thrust secondary feathers are located close to the body and provide lift.
Flapping of the entire wing occurs primarily through the actions of the chest muscles: the pectoralis and the supracoracoideus. These muscles, highly developed in birds and accounting for a higher percentage of body mass than in most mammals, attach to a blade-shaped keel, similar to that of a boat, located on the sternum. The sternum of birds is larger than that of other vertebrates, which accommodates the large muscles required to generate enough upward force to generate lift with the flapping of the wings. Another skeletal modification found in most birds is the fusion of the two clavicles (collarbones), forming the furcula or wishbone. The furcula is flexible enough to bend and provide support to the shoulder girdle during flapping.
An important requirement of flight is a low body weight. As body weight increases, the muscle output required for flying increases. The largest living bird is the ostrich. While it is much smaller than the largest mammals, it is flightless. For birds that do fly, reduction in body weight makes flight easier. Several modifications are found in birds to reduce body weight, including pneumatization of bones. Pneumatic bones are hollow rather than filled with tissue. They contain air spaces that are sometimes connected to air sacs and they have struts of bone to provide structural reinforcement. Pneumatic bones are not found in all birds they are more extensive in large birds than in small birds. Not all bones of the skeleton are pneumatic, although the skulls of almost all birds are.
Figure (PageIndex<1>): Pneumatic bones of birds: Many birds have hollow, pneumatic bones, which make flight easier.
Other modifications that reduce weight include the lack of a urinary bladder. Birds possess a cloaca: a structure that allows water to be reabsorbed from waste back into the bloodstream. Uric acid is not expelled as a liquid, but is concentrated into urate salts, which are expelled along with fecal matter. In this way, water is not held in the urinary bladder, which would increase body weight. Most bird species possess only one ovary rather than two, further reducing body mass.
The air sacs that extend into bones, making them pneumatic, also join with the lungs and function in respiration. In contrast to mammalian lungs in which air flows in two directions, as it is breathed in and out, airflow through bird lungs travels in one direction. Air sacs allow for this unidirectional airflow, which also creates a cross-current exchange system with the blood. In a cross-current or counter-current system, the air flows in one direction and the blood flows in the opposite direction, creating a very efficient means of gas exchange.
Figure (PageIndex<1>): Avian respiration: Avian respiration is an efficient system of gas exchange with air flowing unidirectionally. During inhalation, air passes from the trachea into posterior air sacs, then through the lungs to anterior air sacs. The air sacs are connected to the hollow interior of bones. During exhalation, air from air sacs passes into the lungs and out the trachea.
Main Characteristics of Birds
The Class Aves or Birds is just one of the classes of the bigger Kingdom Animalia. The birds share some characteristics with another closely related class – Reptiles, but they also have some features that are unique to these fantastic creatures. Let us go through their main traits.
- Birds are vertebrates – they have an inner skeleton that contains a spine, limbs, and a skull
- Birds are endothermic – they maintain constant body temperature by themselves, without depending on outside factors
- Birds are bipedal (i.e., they move on two limbs when they are on the ground)
- The upper limbs of birds have evolved into specialized structures – wings, that allow flight
- The body of the most bird is spindle-shaped (to make the flight more comfortable and more effective)
- The birds’ bones are hollow inside, to lower the overall weight of the body
- The birds have a beak instead of a mouth with teeth
- The body is covered with unique structures – feathers. Only legs are covered with scales
- The birds have a heart with 4 chambers – this way, the oxygen-rich and oxygen-poor blood is divided correctly, which helps with maintaining constant temperatures
- The birds have a complex nervous system and a well-developed brain. Many birds are known to be highly teachable and intelligent
- There are two sexes in birds: male and female
- Fertilization in birds is internal. As a result of fertilization, a closed-off, waterproof egg is formed.
- The eggs develop outside the body, usually under the care of both parents or sometimes only one.
If we want to define birds strictly, we would say they are warm-blooded, bipedal vertebrate animals, with forelimbs evolved into wings, covered in feathers and with a beak instead of a mouth.
Studying Feathers: How do scientists use Tinbergen’s four questions?
We’ve just used Tinbergen’s approach to look at feathers from several different perspectives—but it’s not just a learning exercise. Scientists like those in the evo-devo crowd, are making discoveries in just the same way, by linking findings from across the biological disciplines.
One such scientist is Kim Bostwick, who used this integrated approach to untangle the mysteries of a bird whose feathers work like a musical instrument. This may sound like an outrageous idea, but male Club-winged Manakins of Central and South America use a highly modified feather structure to play a powerful one-note tune. Strong evolutionary pressure on these males to attract females has made them unique in the bird world, but it took years of scientific investigation by Bostwick and colleagues to work out the full story of how and why these birds sing with their wings.
So how do they do it? Club-winged Manakins sing with their wings by rubbing specialized feathers together. One of these feathers is club-shaped with ridges along its edge. The adjacent feather is slender, and bent at a 45-degree angle. This bent feather acts as a pick, while its ridged counterpart acts as a comb to produce a one-note song. This method of producing sound is called stridulation stridulation <span act of rubbing together body parts to make a sound and also occurs in insects, such as crickets.
Kim Bostwick began her study of Club-winged Manakins by asking questions about how they sing with their wings. She spent years piecing together how the birds accomplish this feat mechanically, but she did not stop there. Because Kim had always been interested in evolution, she also asked questions about how their specialized feathers and associated behaviors evolved. This led her to study other birds closely related to Club-winged Manakins to see what behavioral innovations occurred in their evolutionary history that contributed to the display we see today. It turns out that the behavior evolved through a series of small steps, including short wing clicks and backwards hopping, into one of the most unusual displays in the animal world. Like Niko Tinbergen, Kim is one of the many scientists who prefer to ask scientific questions from many angles, going beyond the mechanics to make discoveries about function, development, and evolution.
To learn more about Kim’s story at the Singing Wings website.
Further LearningWatch a five-part video on the Club winged Manakin.
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Suggested citation: Cornell Lab of Ornithology. 2013. All About Feathers. All About Bird Biology <birdbiology.org>. Cornell Lab of Ornithology, Ithaca, New York. < add date accessed here: e.g. 02 Oct. 2013 >.
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