Bird

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Birds
Temporal range:
Late Cretaceouspresent, 72–0 Ma[1][2] Possible Early Cretaceous or early Late Cretaceous origin based on molecular clock[3][4][5]
Red-crested turacoSteller's sea eagleRock doveSouthern cassowaryGentoo penguinBar-throated minlaShoebillGrey crowned craneAnna's hummingbirdRainbow lorikeetGrey heronEurasian eagle-owlWhite-tailed tropicbirdIndian peafowlAtlantic puffinAmerican flamingoBlue-footed boobyKeel-billed toucan
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Sauropsida
Clade: Archosauria
Clade: Avemetatarsalia
Clade: Dinosauria
Clade: Theropoda
Clade: Ornithurae
Class: Aves
Linnaeus, 1758[6]
Extant clades
Synonyms

Birds are a group of

skeleton. Birds live worldwide and range in size from the 5.5 cm (2.2 in) bee hummingbird to the 2.8 m (9 ft 2 in) common ostrich. There are over 11,000 living species, more than half of which are passerine, or "perching" birds. Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Wings, which are modified forelimbs, gave birds the ability to fly, although further evolution has led to the loss of flight in some birds, including ratites, penguins, and diverse endemic island species. The digestive and respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. The study of birds is called ornithology
.

Birds are

theropod dinosaurs and constitute the only known living dinosaurs. Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Birds are descendants of the primitive avialans (whose members include Archaeopteryx) which first appeared during the Late Jurassic. According to recent estimates, modern birds (Neornithes) evolved in the Late Cretaceous and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 million years ago, which killed off the pterosaurs and all non-avian dinosaurs.[7]

Many

incubated
by the parents. Most birds have an extended period of parental care after hatching.

Many species of birds are economically important as food for human consumption and raw material in manufacturing, with domesticated and undomesticated birds being important sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano (bird excrement) is harvested for use as a fertiliser. Birds figure throughout human culture. About 120 to 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Recreational birdwatching is an important part of the ecotourism industry.

Evolution and classification

Slab of stone with fossil bones and feather impressions
Archaeopteryx is often considered the oldest known true bird.

The first

classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[8]

Definition

Aves and a sister group, the order

fossils, and assigning them, instead, to the broader group Avialae,[12] on the principle that a clade based on extant species should be limited to those extant species and their closest extinct relatives.[12]

Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem.[13] 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,[13] which corresponds to meaning number 4 below. They assigned other names to the other groups.[13]

Reptiles
  
Squamates
  

  Lizards and snakes

  Turtles

  Archosaurs  

  Crocodiles

  Birds

The birds' phylogenetic relationships to major living reptile groups
  1. Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)
  2. Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)
  3. Aves can mean those feathered dinosaurs that fly (alternately Avialae)
  4. 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.[14]

Maniraptoromorpha

Coelurus

Ornitholestes

Maniraptoriformes

Ornithomimosauria

Maniraptora

Alvarezsauridae

Pennaraptora

Oviraptorosauria

Paraves

Cladogram showing the results of a phylogenetic study by Cau, 2018.[15]

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",[16][17] with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus.[18] 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.[13][19]

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.[20]

Dinosaurs and the origin of birds

Paraves

Scansoriopterygidae

Eosinopteryx

Eumaniraptora

Jinfengopteryx

Aurornis

Dromaeosauridae

Troodontidae

Avialae

Cladogram following the results of a phylogenetic study by Cau et al., 2015[21]
Simplified phylogenetic tree showing the relationship between modern birds and other dinosaurs[22]

Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of

theropod dinosaurs[23] and, more specifically, members of Maniraptora, a group of theropods which includes dromaeosaurids and oviraptorosaurs, among others.[24] As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. By the 2000s, discoveries in the Liaoning Province of northeast China, which demonstrated many small theropod feathered dinosaurs, contributed to this ambiguity.[25][26][27]

Anchiornis huxleyi is an important source of information on the early evolution of birds in the Late Jurassic period.[28]

The consensus view in contemporary

arboreal, have been able to glide, or both.[30][31] Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, studies suggest that the first avialans were omnivores.[32]

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.[33]

Early evolution

Confuciusornis sanctus, a Cretaceous bird from China that lived 125 million years ago, is the oldest known bird to have a beak.[34]

Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest

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.[15][35] 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.[15] The integument evolved into complex, pennaceous feathers.[35]

The oldest known paravian (and probably the earliest avialan) fossils come from the

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 maneuvering.[36]

Avialans diversified into a wide variety of forms during the

primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves).[37] 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.[15] 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.[38] Around 95 million years ago, they evolved a better sense of smell.[39]

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. [15]

Avialae

Anchiornis

Archaeopteryx

Xiaotingia

Rahonavis

Jeholornis

Jixiangornis

Euavialae

Balaur

Avebrevicauda

Zhongjianornis

Sapeornis

Pygostylia

Confuciusornithiformes

Protopteryx

Pengornis

Ornithothoraces

Cladogram following the results of a phylogenetic study by Cau et al., 2015[21]

Early diversity of bird ancestors

Ornithothoraces

Enantiornithes

Euornithes

Archaeorhynchus

Ornithuromorpha

Patagopteryx

Vorona

Schizooura

Hongshanornithidae

Jianchangornis

Songlingornithidae

Gansus

Apsaravis

Ornithurae

Hesperornithes

Ichthyornis

Vegavis

Aves

Mesozoic bird phylogeny simplified after Wang et al., 2015's phylogenetic analysis[40]
Ichthyornis, which lived 93 million years ago, was the first known prehistoric bird relative preserved with teeth.

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.[37][41]

Many species of the second major avialan lineage to diversify, the

perching adaptations and likely included shorebird-like species, waders, and swimming and diving species.[42]

The latter included the superficially

Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic.[37] 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).[44] Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers,[45] which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight.[36]

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.[46]

Diversification of modern birds

Aves
Palaeognathae

(

tinamous
)

Neognathae
 
Galloanserae
 

(

waterfowl
)

Neoaves

(all other birds including perching birds)

Major groups of modern birds based on
Sibley-Ahlquist taxonomy

Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the Early Cretaceous[3][47] to the latest Cretaceous.[48][4] Similarly, there is no agreement on whether most of the early diversification of modern birds occurred in the Cretaceous and associated with breakup of the supercontinent Gondwana or occurred later and potentially as a consequence of the Cretaceous–Palaeogene extinction event.[49] 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).

The discovery of

Galloanserae, the earliest diverging lineage within Neognathae.[1]

Attempts to reconcile molecular and fossil evidence using genomic-scale DNA data and comprehensive fossil information have not resolved the controversy.

Paleotropics.[7] On the other hand, the occurrence of Asteriornis in the Northern Hemisphere suggest that Neornithes dispersed out of East Gondwana before the Paleocene.[1]

Classification of bird orders

All modern birds lie within the

taxonomic viewpoint, the number of known living bird species is around 10,906[56][57]
although other sources may differ in their precise number.

Cladogram of modern bird relationships based on Braun & Kimball (2021)[58]

Aves
Palaeognathae

Struthioniformes (ostriches)

Rheiformes (rheas)

Apterygiformes (kiwis)

Tinamiformes (tinamous)

cassowaries)

Neognathae
Galloanserae

Galliformes (chickens and relatives)

Anseriformes (ducks and relatives)

Neoaves
Mirandornithes

flamingos)

Podicipediformes (grebes)

Columbimorphae

Columbiformes (pigeons and doves)

Mesitornithiformes (mesites)

Pterocliformes (sandgrouse)

Passerea

Otidiformes (bustards)

Cuculiformes (cuckoos)

Musophagiformes (turacos)

Gruiformes (rails and cranes)

Charadriiformes (waders and relatives)

Opisthocomiformes (hoatzin)

Strisores

Caprimulgiformes (nightjars)

Vanescaves

Nyctibiiformes (potoos)

Steatornithiformes (oilbird)

Podargiformes (frogmouths)

Daedalornithes

Aegotheliformes (owlet-nightjars)

Apodiformes (swifts, treeswifts and hummingbirds)

Phaethoquornithes
Eurypygimorphae

Phaethontiformes (tropicbirds)

Eurypygiformes (sunbittern and kagu)

Aequornithes

Gaviiformes[59] (loons)

Austrodyptornithes

Procellariiformes (albatrosses and petrels)

Sphenisciformes (penguins)

Ciconiiformes (storks)

Suliformes (boobies, cormorants, etc.)

Pelecaniformes (pelicans, herons and ibises)

(Ardeae)
Telluraves
Accipitrimorphae

Cathartiformes (New World vultures)

Accipitriformes (hawks and relatives)

Strigiformes (owls)

Coraciimorphae

Coliiformes (mousebirds)

Cavitaves

Leptosomiformes (cuckoo roller)

Trogoniformes (trogons and quetzals)

Picocoraciae

Bucerotiformes (hornbills and relatives)

Picodynastornithes

Coraciiformes (kingfishers and relatives)

Piciformes (woodpeckers and relatives)

Australaves

Cariamiformes (seriemas)

Eufalconimorphae

Falconiformes (falcons)

Psittacopasserae

Psittaciformes (parrots)

Passeriformes (passerines)

The classification of birds is a contentious issue.

Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the subject.[60] Most evidence seems to suggest the assignment of orders is accurate,[61] but scientists disagree about the relationships among 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. Fossil and molecular evidence from the 2010s is providing an increasingly clear picture of the evolution of modern bird orders.[48][52]

Genomics

As of 2010, the genome had been sequenced for only two birds, the chicken and the zebra finch. As of 2022 the genomes of 542 species of birds had been completed. At least one genome has been sequenced from every order.[62][63] These include at least one species in about 90% of extant avian families (218 out of 236 families recognised by the Howard and Moore Checklist).[64]

Being able to sequence and compare whole genomes gives researchers many types of information, about genes, the DNA that regulates the genes, and their evolutionary history. This has led to reconsideration of some of the classifications that were based solely on the identification of protein-coding genes. Waterbirds such as

flamingos, for example, may have in common specific adaptations suited to their environment that were developed independently.[62][63]

Distribution

small bird withpale belly and breast and patterned wing and head stands on concrete
The range of the house sparrow has expanded dramatically due to human activities.[65]

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.[66] 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 studies from the 2000s found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics.[67] 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,[68] while some penguins have been recorded diving up to 300 metres (980 ft) deep.[69]

Many bird species have established breeding populations in areas to which they have been

game bird.[70] Others have been accidental, such as the establishment of wild monk parakeets in several North American cities after their escape from captivity.[71] Some species, including cattle egret,[72] yellow-headed caracara[73] and galah,[74] have spread naturally far beyond their original ranges as agricultural expansion created alternative habitats although modern practices of intensive agriculture have negatively impacted farmland bird populations.[75]

Anatomy and physiology

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, 23 Eyestripe

Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight.

Skeletal system

The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the

extinct moa and elephant birds.[80]

Excretory system

Like the

urinary bladder or external urethral opening and (with exception of the ostrich) uric acid is excreted along with faeces as a semisolid waste.[81][82][83] However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.[84] They also excrete creatine, rather than creatinine like mammals.[77] This material, as well as the output of the intestines, emerges from the bird's cloaca.[85][86] 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.[87]

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.[88]

Reproductive system

Males within

penis, which is never present in Neoaves.[89][90] The length is thought to be related to sperm competition.[91] For male birds to get an erection, they depend on lymphatic fluid instead of blood.[92] When not copulating, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. Female birds have sperm storage tubules[93] that allow sperm to remain viable long after copulation, a hundred days in some species.[94] 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,[95] 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.[95] 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.[96][97] 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.[95] While rare, mostly abortive, parthenogenesis is not unknown in birds and eggs can be diploid, automictic and results in male offspring.[98]

Birds are solely gonochoric.[99] 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).[77] A complex system of disassortative mating with two morphs is involved in the white-throated sparrow Zonotrichia albicollis, where white- and tan-browed morphs of opposite sex pair, making it appear as if four sexes were involved since any individual is compatible with only a fourth of the population.[100]

In nearly all species of birds, an individual's sex is determined at fertilisation. However, one 2007 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.[101] This, however, was later proven to not be the case. These birds do not exhibit temperature-dependent sex determination, but temperature-dependent sex mortality.[102]

Respiratory and circulatory systems

Birds have one of the most complex

syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea;[104] the trachea being elongated in some species, increasing the volume of vocalisations and the perception of the bird's size.[105]

In birds, the main arteries taking blood away from the heart originate from the right

red blood cells in birds retain their nucleus.[106]

Heart type and features

Didactic model of an avian heart

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

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.[108]

The

epicardial layers.[107] 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.[109]

Organisation

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.[109] 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.[110] 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.[citation needed]

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.[110] 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.[citation needed]

Nervous system

The

accommodation for vision in air and water.[77] 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.[115] They also have double cones, likely to mediate achromatic vision.[116]

The nictitating membrane as it covers the eye of a masked lapwing

Many birds show plumage patterns in

blue tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.[117] 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.[118] With the exception of pigeons and a few other species,[119] the eyelids of birds are not used in blinking. Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally.[120] The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.[77] The bird retina has a fan shaped blood supply system called the pecten.[77]

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.[123]

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,[124] and some species of pitohuis from New Guinea have a powerful neurotoxin in their skin and feathers.[125]

A lack of field observations limit our knowledge, but intraspecific conflicts are known to sometimes result in injury or death.

Swans, for instance, may strike with the bony spurs and bite when defending eggs or young.[126]

Feathers, plumage, and scales

disruptively patterned 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

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,[127] and sex.[128]

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.[129] 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.[130]

A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.[131] As a general rule, the tail feathers are moulted and replaced starting with the innermost pair.[130] Centripetal moults of tail feathers are however seen in the Phasianidae.[132] 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.[130][133] The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from centre outward.[134] 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.[135]

Red parrot with yellow bill and wing feathers in bill
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.

anting, to remove feather parasites.[138]

The

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.[139]

Flight

Black bird with white chest in flight with wings facing down and tail fanned and down pointing
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 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.[77]

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.[140] Flightlessness often arises in birds on isolated islands, most likely due to limited resources and the absence of mammalian land predators.[141] Flightlessness is almost exclusively correlated with gigantism due to an island's inherent condition of isolation.[142] 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.[143]

Behaviour

Most birds are

crepuscular (active during twilight hours), and many coastal waders feed when the tides are appropriate, by day or night.[144]

Diet and feeding

Illustration of the heads of 16 types of birds with different shapes and sizes of beak
Feeding adaptations in beaks

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.[145] Some species such as pigeons and some psittacine species do not have a gallbladder.[146] Most birds are highly adapted for rapid digestion to aid with flight.[147] 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.[148]

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.[77] 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.[149] Combined, insectivorous birds eat 400–500 million metric tons of arthropods annually.[150]

Nectar feeders such as

dabbling ducks are primarily grazers.[155][156]

Some species, including

corvids, or other birds of prey, are opportunists.[160]

Water and drinking

Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands.[161] Some desert birds can obtain their water needs entirely from moisture in their food. Some have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting.[162] Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils.[163]

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

bustard families are capable of sucking up water without the need to tilt back their heads.[164] Some desert birds depend on water sources and sandgrouse are particularly well known for congregating daily at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers.[165] 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.[166]

Feather care

Feathers, being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi,

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.[168][169]

Migration

Canada geese in V formation

Many bird species migrate to take advantage of global differences of

tropical regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.[170][171]

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),[77] although the bar-tailed godwit is capable of non-stop flights of up to 10,200 km (6,300 mi).[172] 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).[173] 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.[174]

A map of the Pacific Ocean with several coloured lines representing bird routes running from New Zealand to Korea
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.[175] 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.[176] 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.[177]

Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations.[179]

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

geomagnetism through specialised photoreceptors.[182]

Communication

Birds communicate primarily using visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).

Birds sometimes use plumage to assess and assert social dominance,[183] 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.[184]

Large brown patterned ground bird with outstretched wings each with a large spot in the centre
The startling display of the sunbittern mimics a large predator.

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.[77] The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements;[185] males' breeding success may depend on the quality of such displays.[186]

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.[104]
Calls are used for a variety of purposes, including mate attraction,
Coenocorypha snipes of New Zealand drive air through their feathers,[191] woodpeckers drum for long-distance communication,[192] and palm cockatoos use tools to drum.[193]

Flocking and other associations

massive flock of tiny birds seen from distance so that birds appear as specks
Red-billed queleas, the most numerous species of wild bird,[194] form enormous flocks – sometimes tens of thousands strong.

While some birds are essentially territorial or live in small family groups, other birds may form large

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.[195] Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.[196] Some species have a mixed system with breeding pairs maintaining territories, while unmated or young birds live in flocks where they secure mates prior to finding territories.[197]

Birds sometimes also form associations with non-avian species. Plunge-diving

birds of prey and other predators.[199]

Resting and roosting

Pink flamingo with grey legs and long neck pressed against body and head tucked under wings
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.[200] 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.[201] It has been suggested that there may be certain kinds of sleep which are possible even when in flight.[202]

Some birds have also demonstrated the capacity to fall into

predators by viewing the outer margins of the flock. This adaptation is also known from marine mammals.[203] Communal roosting is common because it lowers the loss of body heat and decreases the risks associated with predators.[204] Roosting sites are often chosen with regard to thermoregulation and safety.[205] Unusual mobile roost sites include large herbivores on the African savanna that are used by oxpeckers.[206]

Many sleeping birds bend their heads over their backs and tuck their

Loriculus roost hanging upside down.[207] Some hummingbirds go into a nightly state of torpor accompanied with a reduction of their metabolic rates.[208] 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.[209] Birds do not have sweat glands, but can lose water directly through the skin, and they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or using special behaviours like urohidrosis to cool themselves.[210][211]

Breeding

Social systems

Bird faces up with green face, black breast and pink lower body. Elaborate long feathers on the wings and tail.
Like others of its family, the male Raggiana bird-of-paradise has elaborate breeding plumage used to impress females.[212]

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.

forced copulation in ducks and other anatids.[216]

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.[217] Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.[218]

Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur.[77] Polygamous breeding systems arise when females are able to raise broods without the help of males.[77] Mating systems vary across bird families[219] but variations within species are thought to be driven by environmental conditions.[220] A unique system is the formation of trios where a third individual is allowed by a breeding pair temporarily into the territory to assist with brood raising thereby leading to higher fitness.[221][188]

Breeding usually involves some form of courtship display, typically performed by the male.

billing and allopreening are commonly performed between partners, generally after the birds have paired and mated.[225]

Homosexual behaviour has been observed in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks.[226] 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."[227]

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.[228]

All birds lay

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.[229]

Yellow weaver (bird) with black head hangs an upside-down nest woven out of grass fronds.
Male golden-backed weavers construct elaborate suspended nests out of grass.

Bird eggs are usually laid in a

nidifugous) by their parents soon after hatching.[232]

Nest made of straw with five white eggs and one grey speckled egg
Nest of an eastern phoebe that has been parasitised by a brown-headed cowbird

Incubation, which regulates temperature for chick development, usually begins after the last egg has been laid.[77] 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.[233] The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or volcanic sources.[234] Incubation periods range from 10 days (in woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis).[77]

The diversity of characteristics of birds is great, sometimes even in closely related species. Several avian characteristics are compared in the table below.[235][236]

Species Adult weight
(grams)
Incubation
(days)
Clutches
(per year)
Clutch size
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 (Phoebastria 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

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".[237] Chicks at neither extreme but favouring one or the other may be termed semi-precocial[238] or semi-altricial.[239]

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.[240] 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.[241] 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.[242]

Hummingbird perched on edge of tiny nest places food into mouth of one of two chicks
A female calliope hummingbird feeding fully grown chicks

In some species, both parents care for nestlings and fledglings; in others, such care is the responsibility of only one sex. In some species,

fairy-wrens,[244] 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.[77] 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.[245]

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.[246] 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.[247] Chicks may also follow their parents during their first migration.[248]

Brood parasites

brood parasite

conspecifics to increase their reproductive output even though they could have raised their own young.[250] One hundred bird species, including honeyguides, icterids, and ducks, are obligate parasites, though the most famous are the cuckoos.[249] 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.[251]

Sexual selection

The peacock tail in flight, the classic example of a Fisherian runaway

Birds have

courtship behaviour are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours.[253]

Inbreeding depression

Inbreeding causes early death (inbreeding depression) in the zebra finch Taeniopygia guttata.[254] Embryo survival (that is, hatching success of fertile eggs) was significantly lower for sib-sib mating pairs than for unrelated pairs.[255]

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.[256]

Inbreeding avoidance

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).[257] 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.[257]

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.[258]

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.[259]

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.[260] 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.[261] Cross-fertilisation between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.[262][263]

Ecology

Gran Canaria blue chaffinch, an example of a bird highly specialised in its habitat, in this case in the Canarian pine forests

Birds occupy a wide range of ecological positions.

forest canopy, others beneath the canopy, and still others on the forest floor. Forest birds may be insectivores, frugivores, or nectarivores. Aquatic birds generally feed by fishing, plant eating, and piracy or kleptoparasitism. Many grassland birds are granivores. Birds of prey specialise in hunting mammals or other birds, while vultures are specialised scavengers. Birds are also preyed upon by a range of mammals including a few avivorous bats.[264] A wide range of endo- and ectoparasites depend on birds and some parasites that are transmitted from parent to young have co-evolved and show host-specificity.[265]

Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal.[266] Plants and pollinating birds often coevolve,[267] and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.[268]

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

kokako today.[266] Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.[269]

Many birds act as ecosystem engineers through the construction of nests, which provide important microhabitats and food for hundreds of species of invertebrates.[270][271] Nesting seabirds may affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of guano, which may enrich the local soil[272] and the surrounding seas.[273]

A wide variety of avian ecology field methods, including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.[274]

Relationship with humans

Industrial farming of chickens

Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man.

avian lead poisoning, pesticides, roadkill, wind turbine kills[282] and predation by pet cats and dogs are common causes of death for birds).[283]

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.[284]

Economic importance

Illustration of fisherman on raft with pole for punting and numerous black birds on raft
The use of cormorants by Asian fishermen is in steep decline but survives in some areas as a tourist attraction.

Domesticated birds raised for meat and eggs, called

doves, partridge, grouse, snipe, and woodcock.[citation needed] Muttonbirding is also popular in Australia and New Zealand.[287] Although some hunting, such as that of muttonbirds, may be sustainable, hunting has led to the extinction or endangerment of dozens of species.[288]

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.[289]

Birds have been domesticated by humans both as pets and for practical purposes. Colourful birds, such as

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,[291]

Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly,

birders) number in the millions.[292] 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.[293]

In religion and mythology

Woodcut of three long-legged and long-necked birds
The 3 of Birds by the Master of the Playing Cards, 15th-century Germany

Birds play prominent and diverse roles in religion and mythology. In religion, birds may serve as either messengers or priests and leaders for a

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.[295]

They may also serve as

Canaanite mother goddess Asherah,[298][299][300] and the Greek goddess Aphrodite.[298][299][301][302][303] In ancient Greece, Athena, the goddess of wisdom and patron deity of the city of Athens, had a little owl as her symbol.[304][305][306] 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.[307] Indigenous peoples of the central Andes maintain legends of birds passing to and from metaphysical worlds.[307]

In culture and folklore

tiles with design of birds from Qajar dynasty

Birds have featured in culture and art since prehistoric times, when they were represented in early

Mughal and Persian emperors.[311] With the advent of scientific interest in birds, many paintings of birds were commissioned for books.[citation needed
]

Among the most famous of these bird artists was

Sisserou Parrot
, its national bird.

Perceptions of bird species vary across cultures.

coats of arms[320] In vexillology, birds are a popular choice on flags. Birds feature in the flag designs of 17 countries and numerous subnational entities and territories.[321] Birds are used by nations to symbolize a country's identity and heritage, with 91 countries officially recognizing a national bird. Birds of prey are highly represented, though some nations have chosen other species of birds with parrots being popular among smaller, tropical nations.[322]

In music

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.[323][324][325][326]

A 2023 archaeological excavation of a 10000-year-old site in Israel yielded hollow wing bones of coots and ducks with perforations made on the side that are thought to have allowed them to be used as flutes or whistles possibly used by Natufian people to lure birds of prey.[327]

Threats and conservation

Large black bird with featherless head and hooked bill
The California condor once numbered only 22 birds, but conservation measures have raised that to over 500 today.

Human activities have caused population decreases or

IUCN in 2009.[330][331]

The most commonly cited human threat to birds is

long-line fishing bycatch,[333] pollution (including oil spills and pesticide use),[334] competition and predation from nonnative invasive species,[335] and climate change
.

Governments and

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.[336]

Human activities have allowed the expansion of a few temperate area species, such as the

European starling. In the tropics and sub-tropics, relatively more species are expanding due to human activities, particularly due to the spread of crops such as rice whose expansion in south Asia has benefitted at least 64 bird species, though may have harmed many more species.[337] Some of the species benefitting from traditional rice farming includes Bar-headed Goose, Lesser Whistling Duck, Ruff, Red-wattled Lapwing, Black-shouldered Kite, several heron species,[338] Asian Openbill, Woolly-necked Stork, Black-headed Ibis
, Streak-throated Swallow, Ashy-crowned Sparrow-lark, Bengal Bushlark, Paddyfield Pipit, and Baya Weaver.

See also

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