Rook
On Dartmoor, Devon, England
Conservation status
Least Concern (IUCN 3.1) [1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Passeriformes
Family:	Corvidae
Genus:	Corvus
Species:	C. frugilegus
Binomial name
Corvus frugilegus Linnaeus, 1758
Rook range

The Rook (Corvus frugilegus) is a member of the Corvidae family in the passerine order of birds. Named by Carl Linnaeus in 1758,^[2] the species name frugilegus is Latin for "food-gathering".

This species is similar in size (45–47 cm in length) to or slightly smaller than the Carrion Crow with black feathers often showing a blue or bluish-purple sheen in bright sunlight. The feathers on the head, neck and shoulders are particularly dense and silky. The legs and feet are generally black and the bill grey-black.

Rooks are distinguished from similar members of the crow family by the bare grey-white skin around the base of the adult's bill in front of the eyes. The feathering around the legs also looks shaggier and laxer than the congeneric Carrion Crow. The juvenile is superficially more similar to the Crow because it lacks the bare patch at the base of the bill, but it loses the facial feathers after about six months. Collective nouns for rooks include building, parliament, clamour and storytelling.^[3][4] Their nesting patterns gave rise to the term rookery.

Distribution and habitat[link]

Though resident in Great Britain, Ireland and much of north and central Europe, vagrant to Iceland and northern Scandinavia, it also occurs as an eastern race in Asia where it differs in being very slightly smaller and more efficient on average, and having a somewhat more fully feathered face. In the north of its range the species has a tendency to move south during autumn though more southern populations are apt to range sporadically also. The species has been introduced to New Zealand, with several hundred birds being released there from 1862 to 1874, though today their range is very localised.^[5] There the species is an agricultural pest and it is being eradicated.

Behaviour[link]

Diet[link]

Food is predominantly earthworms and insect larvae, which the bird finds by probing the ground with its strong bill. It also eats cultivated cereal grain, smaller amounts of fruit, small mammals, acorns, small birds, their eggs and young and carrion. In urban sites, human food scraps are taken from rubbish dumps and streets, usually in the early hours when it is relatively quiet. It has also been seen along the seashore, feeding on insects, crustaceans and suitable food flotsam.

Nesting[link]

The distribution of rook colony sizes in Normandy.^[6] Most colonies are small, a few are large (smoothed).

Nesting in a rookery is always colonial, usually in the very tops of the trees. Branches and twigs are broken off trees (very rarely picked up off the ground), though as many are likely to be stolen from nearby nests as are collected from trees. Eggs are usually 3–5 in number, can appear by the end of February or early March and are incubated for 16–18 days. Both adults feed the young, which are fledged by the 32nd or 33rd day.

In autumn, the young birds of the summer collect into large flocks together with unpaired birds of previous seasons, often in company with Jackdaws. It is during the autumn that spectacular aerial displays can be seen by adult birds that seem to delight in the autumn gales.

Voice[link]

The call is usually described as "kaah" – it is similar to that of the Carrion Crow, but usually rather flatter in tone. It is given both in flight and while perched, when the bird fans its tail and bows on each caw. Calls in flight are usually given singly, in contrast to the Carrion Crow's which are in groups of three or four. Solitary birds often "sing" apparently to themselves, uttering strange clicks, wheezes and almost human-like notes.

In The Rooks Have Returned (1871) by Alexei Savrasov, the arrival of the rooks is an early portent of the coming spring.

Intelligence[link]

In captivity, when confronted with problems, rooks have recently been documented as one of multiple species of bird capable of tool use to obtain a goal. They learned that if they push a stone off a ledge into a tube, they will get food. The rooks even quickly discovered they could go get a stone and carry it to the tube if no stone was there already. They also used sticks, wire and even figured out how to bend a wire into a hook to reach an item.^[7] Rooks are as clever at making and using simple tools with their beaks as chimpanzees are with their hands.^[8]

In an experiment, a rook was placed near a tube of water, with a worm floating on top of the water, and some rocks next to the tube. The water level was too low for the rook to reach the worm. The rook placed rocks into the tube until the water level was high enough for the rook to reach the worm.^[9]

References[link]

• Handbook of the Birds of Europe, the Middle East, and North Africa: The Birds of the Western Palearctic Volume VIII: Crows to Finches Chief Editor: the late Stanley Cramp, Edited by C. M. Perrins ISBN 0-19-854679-3
• Crows and Jays by Madge and Burn, ISBN 0-7136-3999-7

External links[link]

Wikimedia Commons has media related to: Corvus frugilegus

• Data related to Corvus frugilegus at Wikispecies
•  Chisholm, Hugh, ed. (1911). "rook". Encyclopædia Britannica (11th ed.). Cambridge University Press. 
• Rook videos, photos & sounds on the Internet Bird Collection
• "Rooks calling" (WAV). http://www.scricciolo.com/eurosongs/Corvus.frugilegus.wav. 

v t e Heraldic creatures Beasts Bear Boar Bull/Ox Dog Camelopard (giraffe) Hind/Stag (deer) Kangaroo Leopard Lion Tiger Wolf Birds Cock Dove Eagle Martlet Pelican Rook Legendary creatures Basilisk Biscione Chollima Cockatrice Dragon Enfield Garuda Griffin/Keythong Harpy Lindworm Manticore Mermaid Phoenix Salamander Sea-horse Sea-lion Turul Unicorn Wyvern Fish Dolphin Ged Hippocampus Lucy (esox) Scallop Others Bat Bee Crapaudy (toad) Emmet (ant) Serpent

Look up rook or rooks in Wiktionary, the free dictionary.

Rook or rooks may refer to:

Bird[link]

• Rook (bird) (Corvus frugilegus), a member of the crow family

Games[link]

• Rook (chess) or castle, a piece in the board game of chess
• Rook (card game), a trick-taking game, usually played with a specialized deck of cards

Literature[link]

• The Rook, A 2011 supernatural thriller by Daniel O'Malley

People[link]

• Alan Rook, editor of the 1936 issue of New Oxford Poetry, one of the Cairo poets
• Jean Rook (1931–1991), British newspaper columnist
• Jerry Rook (born 1943), American former professional basketball player
• Susan Rook, journalist and photographer
• Albert Harold Rooks (1891–1942), Captain in U.S. Navy, World War II Medal of Honor recipient

Places[link]

• Montes Rook, a mountain range on the Moon
• Rooks County, Kansas
• Rooks Creek, Illinois

Culture[link]

• Rook (band), alternative rock band (Lyman Smith, Mark Tillman, Eric Koller)
• Rook (album), 2008 album by Shearwater
• The Dirty Rooks, American rock band
• The Rook, the title character in a 1940s series of stories by Barry Reese
• Rook Bartley of the anime series Genesis Climber Mospeada and Robotech
• Rook (Transformers), a fictional Transformers character
• Rook (G.I. Joe), a fictional character in the G.I. Joe universe
• Rook (comics), comic book character from Eerie magazine
• Rooks (band), American extreme metal band from Northbrook, Illinois

Technology[link]

• Rook (rocket), a British rocket launched between 1959 and 1972
• "Rook", a Russian nickname for the Sukhoi Su-25 jet aircraft
• USS Rooks (DD-804) (1944–1962), destroyer, World War II and Korea
• Rook (1874–1892), GWR no. 2176, one of seven steam South Devon Railway 0-4-0 locomotives, Raven class

Other[link]

• Rook (piercing), a piercing on the anti-helix of the ear

See also[link]

• Rookery
• Rookery (slum)
• Rookie

This disambiguation page lists articles associated with the same title. If an internal link led you here, you may wish to change the link to point directly to the intended article.

Birds Temporal range: Late Jurassic–Recent, 150–0 Ma PreЄ Є O S D C P T J K Pg N
White-throated Rock Thrush (Monticola gularis)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
clade:	Avialae
Class:	Aves Linnaeus, 1758[1]
Subclasses
†Archaeornithes * †Enantiornithes †Hesperornithes †Ichthyornithes Neornithes And see text

Birds (class Aves) are feathered, winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. With around 10,000 living species, they are the most speciose class of tetrapod vertebrates. All present species belong to the subclass Neornithes, and inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds emerged within theropod dinosaurs during the Jurassic period, around 160 million years (Ma) ago. Paleontologists regard birds as the only clade of dinosaurs to have survived the Cretaceous–Paleogene extinction event 65.5 Ma ago.

Modern birds are characterised by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings—the now extinct flightless moa of New Zealand were the only exception. Wings are evolved forelimbs, and most bird species can fly. Flightless birds include ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.

Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviours, including cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority of bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have polygynous ("many females") or, rarely, polyandrous ("many males") breeding systems. Eggs are usually laid in a nest and incubated by the parents. Most birds have an extended period of parental care after hatching.

Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly songbirds and parrots, are popular as pets. Other uses include the harvesting of guano (droppings) for use as a fertiliser. Birds figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to protect them.

Evolution and taxonomy[link]

Archaeopteryx lithographica is often considered the oldest known bird

The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.^[2] Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.^[3] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.^[4] Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. During the late 20th century, Aves was commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.^[5] However, an alternate definition proposed by scientists including Jacques Gauthier and adherents of the Phylocode system defined Aves to include only the modern bird groups, the crown group. This was done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae,^[6] in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

All modern birds lie within the crown group 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.^[4] These two subdivisions are often given the rank of superorder,^[7] although Livezey and Zusi assigned them "cohort" rank.^[4] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800^[8] to 10,050.^[9]

Dinosaurs and the origin of birds[link]

Confuciusornis, a Cretaceous bird from China

Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of theropod dinosaurs.^[10] More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.^[11] As scientists have discovered more nonavian theropods closely related to birds, the previously clear distinction between nonbirds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod dinosaurs had feathers, contribute to this ambiguity.^[12]

The consensus view in contemporary paleontology is that the birds, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids, troodontids and possibly archaeopterygids.^[13] Together, these three form a group called Paraves. Some basal members of this group, such as Microraptor and Archaeopteryx, have features which may have enabled them to glide or fly. The most basal deinonychosaurs are very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, may have been able to glide, or both.^[14][15] Unlike Archaeopteryx and the feathered dinosaurs, who primarily ate meat, recent studies suggest that the first birds were herbivores.^[16]

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 reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, though it is possibly closely related to the real ancestor.^[17]

Alternative theories and controversies[link]

Early disagreements on the origins of birds included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp, there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors.^[18] Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure independently.^[19] In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae.

A small minority of researchers, such as paleornithologist Alan Feduccia of the University of North Carolina, challenge the majority view, contending that birds are not dinosaurs, but evolved from early archosaurs like Longisquama.^[20][21]

Early evolution of birds[link]

Birds diversified into a wide variety of forms during the Cretaceous Period.^[22] Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors,^[22] the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved better olfactory senses.^[23]

The first large, diverse lineage of short-tailed birds 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.^[22]

Many species of the second major bird lineage to diversify, the Ornithurae (including the ancestors of modern birds), specialised in eating fish, like the superficially gull-like subclass Ichthyornithes (fish birds).^[24] One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments, they lost the ability to fly and became primarily aquatic. Despite their extreme specializations, the Hesperornithiformes represent some of the closest relatives of modern birds.^[22]

Diversification of modern birds[link]

Containing all modern birds, the subclass Neornithes is, due to the discovery of Vegavis, now known to have evolved into some basic lineages by the end of the Cretaceous^[25] and is split into two superorders, the Palaeognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The dates for the splits are much debated by scientists. The Neornithes are agreed to have evolved in the Cretaceous, and the split between the Galloanseri from other Neognathes occurred before the Cretaceous–Paleogene extinction event, but there are different opinions about whether the radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.^[26] This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while fossil evidence supports a Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.^[26][27]

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,^[28] although it is frequently debated and constantly revised. Most evidence seems to suggest the assignment of orders is accurate,^[29] 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.

Classification of modern bird orders[link]

Cladogram showing the most recent classification of Neoaves, based on several phylogenetic studies.^[30]

This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. This list uses the traditional classification (the so-called Clements order), revised by the Sibley-Monroe classification. The list of birds gives a more detailed summary of the orders, including families.

Subclass Neornithes
The subclass Neornithes has two extant superorders –

Superorder Palaeognathae:

The name of the superorder is derived from paleognath, the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds. The Palaeognathae consists of two orders which comprise 49 existing species.

• Struthioniformes—ostriches, emus, kiwis, and allies
• Tinamiformes—tinamous

Superorder Neognathae:

The superorder Neognathae comprises 27 orders which have a total of nearly ten thousand species. The Neognathae have undergone adaptive radiation to produce the staggering diversity of form (especially of the bill and feet), function, and behaviour that are seen today.

The orders comprising the Neognathae are:

• Anseriformes—waterfowl
• Galliformes—fowl
• Charadriiformes—gulls, button-quails, plovers and allies
• Gaviiformes—loons
• Podicipediformes—grebes
• Procellariiformes—albatrosses, petrels, and allies
• Sphenisciformes—penguins
• Pelecaniformes—pelicans and allies
• Phaethontiformes—tropicbirds
• Ciconiiformes—storks and allies
• Cathartiformes—New World vultures
• Phoenicopteriformes—flamingos
• Falconiformes—falcons, eagles, hawks and allies
• Gruiformes—cranes and allies
• Pteroclidiformes—sandgrouse
• Columbiformes—doves and pigeons
• Psittaciformes—parrots and allies
• Cuculiformes—cuckoos and turacos
• Opisthocomiformes—hoatzin
• Strigiformes—owls
• Caprimulgiformes—nightjars and allies
• Apodiformes—swifts and hummingbirds
• Coraciiformes—kingfishers and allies
• Piciformes—woodpeckers and allies
• Trogoniformes—trogons
• Coliiformes—mousebirds
• Passeriformes—passerines

The radically different Sibley-Monroe classification (Sibley-Ahlquist taxonomy), based on molecular data, found widespread adoption in a few aspects, as recent molecular, fossil, and anatomical evidence supported the Galloanserae.^[26]

Distribution[link]

The range of the House Sparrow has expanded dramatically due to human activities.^[31]

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.^[32] 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.^[33] Several families of birds have adapted to life both on the world's oceans and in them, with some seabird species coming ashore only to breed^[34] and some penguins have been recorded diving up to 300 metres (980 ft).^[35]

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.^[36] Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity.^[37] Some species, including Cattle Egret,^[38] Yellow-headed Caracara^[39] and Galah,^[40] have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.

Anatomy and physiology[link]

External anatomy of a bird (example: Yellow-wattled Lapwing): 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser coverts, 7 Scapulars, 8 Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 Vent, 13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus, 16 Foot, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast, 21 Throat, 22 Wattle

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.^[41] The skull bones in adults are fused and do not show cranial sutures.^[42] The orbits are large and separated by a bony septum. 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.^[43] The last few are fused with the pelvis to form the synsacrum.^[42] 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.^[44]

Like the reptiles, birds are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodstream and excrete it as uric acid instead of urea or ammonia via 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 feces as a semisolid waste.^[45][46][47] However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.^[48] They also excrete creatine, rather than creatinine like mammals.^[42] This material, as well as the output of the intestines, emerges from the bird's cloaca.^[49][50] The cloaca is a multi-purpose opening: waste is expelled through it, birds mate by joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate pellets.^[51] 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.^[52] Most birds are highly adapted for rapid digestion to aid with flight.^[53] Some migratory birds have adapted to use protein from many parts of their bodies, including protein from the intestines, as additional energy during migration.^[54]

Birds have one of the most complex respiratory systems of all animal groups.^[42] 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 lung 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.^[55] Sound production is achieved using the syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea;^[56] the trachea being elongated in some species, increasing the volume of vocalizations and the perception of the bird's size.^[57] The bird's heart has four chambers like a mammalian heart. 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.^[42] 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.^[58]

The nictitating membrane as it covers the eye of a Masked Lapwing

The nervous system is large relative to the bird's size.^[42] 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,^[59] New World vultures^[60] and tubenoses.^[61] The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water.^[42] 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.^[62] This allows them to perceive ultraviolet light, which is involved in courtship. 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.^[63] 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.^[64] The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally.^[65] The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.^[42] The bird retina has a fan shaped blood supply system called the pecten.^[42] Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant.^[66] 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.^[67] 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.^[68]

A few species are able to use chemical defenses against predators; some Procellariiformes can eject an unpleasant oil against an aggressor,^[69] and some species of pitohuis from New Guinea have a powerful neurotoxin in their skin and feathers.^[70]

Chromosomes[link]

Birds have two sexes: male and female. 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).^[42]

In nearly all species of birds, an individual's sex is determined at fertilization. However, one recent study demonstrated temperature-dependent sex determination among Australian Brush-turkeys, for which higher temperatures during incubation resulted in a higher female-to-male sex ratio.^[71]

Feathers, plumage, and scales[link]

The plumage of the African Scops Owl allows it to blend in with its surroundings.

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 signaling.^[42] 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,^[72] and sex.^[73]

Plumage is regularly moulted; the standard plumage of a bird that has moulted after breeding is known as the "non-breeding" 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 "alternate" plumages.^[74] 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 primary being the first. When the fifth of sixth primary is replaced, the outermost tertiaries begin to drop. After the innermost tertiaries are moulted, the secondaries starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary coverts are moulted in synchrony with the primary that they overlap.^[75] A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.^[76] As a general rule, the tail feathers are moulted and replaced starting with the innermost pair.^[75] Centripetal moults of tail feathers are however seen in the Phasianidae.^[77] 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.^[75][78] The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward.^[79] 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.^[80]

Red Lory preening

Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this.^[81] 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.^[82] 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.^[83]

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.^[84]

Flight[link]

Restless Flycatcher in the downstroke of flapping flight

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 breeding, feeding, and predator avoidance and escape. 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.^[42] Wing shape and size generally determine a bird species' type of flight; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are flightless, as were many extinct birds.^[85] Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators.^[86] Though flightless, penguins use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.^[87]

Behaviour[link]

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.^[88]

Diet and feeding[link]

Feeding adaptations in beaks

Birds' diets are varied and often include nectar, fruit, plants, seeds, carrion, and various small animals, including other birds.^[42] Because birds have no teeth, their digestive system is adapted to process unmasticated food items that are swallowed whole.

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.^[42] Birds' feeding strategies vary 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 programs.^[89] 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.^[90] Kiwis and shorebirds with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of ecological niches.^[42][91] Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion,^[34] 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.^[92][93] Geese and dabbling ducks are primarily grazers.

Some species, including frigatebirds, gulls,^[94] and skuas,^[95] 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%.^[96] 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.^[97]

Water and drinking[link]

Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands.^[98] 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.^[99] Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils.^[100]

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.^[101] 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.^[102] 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.^[103]

Migration[link]

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 arctic/antarctic 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.^[54][104] 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),^[105] although the Bar-tailed Godwit is capable of non-stop flights of up to 10,200 km (6,300 mi).^[106] 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).^[107] 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.^[108]

The routes of satellite-tagged Bar-tailed Godwits migrating north from New Zealand. This species has the longest known non-stop migration of any species, up to 10,200 km (6,300 mi).

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.^[109] 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.^[110] 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.^[111] Altitudinal migration is a form of short distance migration in which birds spend the breeding season at higher altitudes elevations 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.^[112] 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.^[113]

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 returned to its colony in Skomer, Wales, within 13 days, a distance of 5,150 km (3,200 mi).^[114] 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.^[42] Orientation with the stellar compass depends on the position of the constellations surrounding Polaris.^[115] These are backed up in some species by their ability to sense the Earth's geomagnetism through specialised photoreceptors.^[116]

Communication[link]

The startling display of the Sunbittern mimics a large predator.

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,^[117] 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.^[118] 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.^[42] The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements;^[119] males' breeding success may depend on the quality of such displays.^[120]

Call of the House Wren, a common North American songbird

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.^[56] Calls are used for a variety of purposes, including mate attraction,^[42] evaluation of potential mates,^[121] bond formation, the claiming and maintenance of territories,^[42] the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),^[122] and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat.^[123] Some birds also use mechanical sounds for auditory communication. The Coenocorypha snipes of New Zealand drive air through their feathers,^[124] woodpeckers drum territorially,^[53] and Palm Cockatoos use tools to drum.^[125]

Red-billed Queleas, the most numerous species of bird,^[126] form enormous flocks—sometimes tens of thousands strong.

Flocking and other associations[link]

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.^[42] 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.^[127] Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.^[128]

Birds sometimes also form associations with non-avian species. Plunge-diving seabirds associate with dolphins and tuna, which push shoaling fish towards the surface.^[129] 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.^[130]

Resting and roosting[link]

Many birds, like this American Flamingo, tuck their head into their back when sleeping

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.^[131] 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.^[132] It has been suggested that there may be certain kinds of sleep which are possible even when in flight.^[133] 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.^[134] Communal roosting is common because it lowers the loss of body heat and decreases the risks associated with predators.^[135] Roosting sites are often chosen with regard to thermoregulation and safety.^[136]

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.^[137] Some hummingbirds go into a nightly state of torpor accompanied with a reduction of their metabolic rates.^[138] 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.^[139] 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.

Breeding[link]

Social systems[link]

Like others of its family the male Raggiana Bird of Paradise has elaborate breeding plumage used to impress females.^[140]

Ninety-five percent 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.^[141] Monogamy allows for biparental care, which is especially important for species in which females require males' assistance for successful brood-rearing.^[142] Among many socially monogamous species, extra-pair copulation (infidelity) is common.^[143] 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.^[144] 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.^[145] Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.^[146]

Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur.^[42] Polygamous breeding systems arise when females are able to raise broods without the help of males.^[42] 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.^[147] 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,^[148] although in the polyandrous phalaropes, this is reversed: plainer males choose brightly coloured females.^[149] Courtship feeding, billing and allopreening are commonly performed between partners, generally after the birds have paired and mated.^[53]

Homosexual behaviour has been observed in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks.^[150]

Territories, nesting and incubation[link]

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.^[151]

All birds lay amniotic eggs with hard shells made mostly of calcium carbonate.^[42] 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.^[152]

Male Golden-backed Weavers construct elaborate suspended nests out of grass

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.^[153] 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,^[154] and feathers are often used for nest insulation.^[153] 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.