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For other uses, see Bird (disambiguation) and Birds (disambiguation). Bird_sentence_0

"Aves" and "Avifauna" redirect here. Bird_sentence_1

For other uses, see Aves (disambiguation) and Avifauna (disambiguation). Bird_sentence_2



Temporal range: Early Cretaceous (Aptian) – present, 121–0 Ma PreꞒ O S D C P T J K Pg NBird_header_cell_0_0_0

Scientific classification AvesBird_header_cell_0_1_0
Kingdom:Bird_cell_0_2_0 AnimaliaBird_cell_0_2_1
Phylum:Bird_cell_0_3_0 ChordataBird_cell_0_3_1
Clade:Bird_cell_0_4_0 SauropsidaBird_cell_0_4_1
Clade:Bird_cell_0_5_0 AvemetatarsaliaBird_cell_0_5_1
Clade:Bird_cell_0_6_0 OrnithuraeBird_cell_0_6_1
Class:Bird_cell_0_7_0 Aves

Linnaeus, 1758Bird_cell_0_7_1

Extant orders and temporal rangeBird_header_cell_0_8_0

Birds are a group of warm-blooded vertebrates constituting the class Aves /ˈeɪviːz/, characterized by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton. Bird_sentence_3

Birds live worldwide and range in size from the 5 cm (2 in) bee hummingbird to the 2.75 m (9 ft) ostrich. Bird_sentence_4

There are about ten thousand living species, more than half of which are passerine, or "perching" birds. Bird_sentence_5

Birds have wings whose development varies according to species; the only known groups without wings are the extinct moa and elephant birds. Bird_sentence_6

Wings, which evolved from 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. Bird_sentence_7

The digestive and respiratory systems of birds are also uniquely adapted for flight. Bird_sentence_8

Some bird species of aquatic environments, particularly seabirds and some waterbirds, have further evolved for swimming. Bird_sentence_9

Birds are a group of feathered theropod dinosaurs, and constitute the only living dinosaurs. Bird_sentence_10

Likewise, birds are considered reptiles in the modern cladistic sense of the term, and their closest living relatives are the crocodilians. Bird_sentence_11

Birds are descendants of the primitive avialans (whose members include Archaeopteryx) which first appeared about 160 million years ago (mya) in China. Bird_sentence_12

According to DNA evidence, modern birds (Neornithes) evolved in the Middle to Late Cretaceous, and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66 mya, which killed off the pterosaurs and all non-avian dinosaurs. Bird_sentence_13

Many social species pass on knowledge across generations, which is considered a form of culture. Bird_sentence_14

Birds are social, communicating with visual signals, calls, and songs, and participating in such behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. Bird_sentence_15

The vast majority of bird species are socially (but not necessarily sexually) monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Bird_sentence_16

Other species have breeding systems that are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Bird_sentence_17

Birds produce offspring by laying eggs which are fertilised through sexual reproduction. Bird_sentence_18

They are usually laid in a nest and incubated by the parents. Bird_sentence_19

Most birds have an extended period of parental care after hatching. Bird_sentence_20

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. Bird_sentence_21

Songbirds, parrots, and other species are popular as pets. Bird_sentence_22

Guano (bird excrement) is harvested for use as a fertiliser. Bird_sentence_23

Birds figure throughout human culture. Bird_sentence_24

About 120 to 130 species have become extinct due to human activity since the 17th century, and hundreds more before then. Bird_sentence_25

Human activity threatens about 1,200 bird species with extinction, though efforts are underway to protect them. Bird_sentence_26

Recreational birdwatching is an important part of the ecotourism industry. Bird_sentence_27

Evolution and classification Bird_section_0

Main article: Evolution of birds Bird_sentence_28

The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Bird_sentence_29

Carl Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Bird_sentence_30

Birds are categorised as the biological class Aves in Linnaean taxonomy. Bird_sentence_31

Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Bird_sentence_32

Definition Bird_section_1

Aves and a sister group, the order Crocodilia, contain the only living representatives of the reptile clade Archosauria. Bird_sentence_33

During the late 1990s, Aves was most commonly defined phylogenetically as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica. Bird_sentence_34

However, an earlier definition proposed by Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the Phylocode system. Bird_sentence_35

Gauthier defined Aves to include only the crown group of the set of modern birds. Bird_sentence_36

This was done by excluding most groups known only from fossils, and assigning them, instead, to the broader group Avialae, in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs. Bird_sentence_37

Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem. Bird_sentence_38

The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. Bird_sentence_39

He assigned other names to the other groups. Bird_sentence_40


  1. Aves can mean all archosaurs closer to birds than to crocodiles (alternately Avemetatarsalia)Bird_item_0_0
  2. Aves can mean those advanced archosaurs with feathers (alternately Avifilopluma)Bird_item_0_1
  3. Aves can mean those feathered dinosaurs that fly (alternately Avialae)Bird_item_0_2
  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)Bird_item_0_3

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. Bird_sentence_41

These proposals have been adopted by many researchers in the field of palaeontology and bird evolution, though the exact definitions applied have been inconsistent. Bird_sentence_42

Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers. Bird_sentence_43

Most researchers define Avialae as branch-based clade, though definitions vary. Bird_sentence_44

Many authors have used a definition similar to "all theropods closer to birds than to Deinonychus", with Troodon being sometimes added as a second external specifier in case it is closer to birds than to Deinonychus. Bird_sentence_45

Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics). Bird_sentence_46

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. Bird_sentence_47

Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Bird_sentence_48

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. Bird_sentence_49

Their alternative definition is synonymous to Avifilopluma. Bird_sentence_50

Dinosaurs and the origin of birds Bird_section_2

Main article: Origin of birds Bird_sentence_51

Based on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of theropod dinosaurs, and more specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurids and oviraptorosaurs, among others. Bird_sentence_52

As scientists have discovered more theropods closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Bird_sentence_53

Recent discoveries in the Liaoning Province of northeast China, which demonstrate many small theropod feathered dinosaurs, contribute to this ambiguity. Bird_sentence_54

The consensus view in contemporary palaeontology is that the flying theropods, or avialans, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Bird_sentence_55

Together, these form a group called Paraves. Bird_sentence_56

Some basal members of Deinonychosauria, such as Microraptor, have features which may have enabled them to glide or fly. Bird_sentence_57

The most basal deinonychosaurs were very small. Bird_sentence_58

This evidence raises the possibility that the ancestor of all paravians may have been arboreal, have been able to glide, or both. Bird_sentence_59

Unlike Archaeopteryx and the non-avialan feathered dinosaurs, who primarily ate meat, recent studies suggest that the first avialans were omnivores. Bird_sentence_60

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. Bird_sentence_61

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. Bird_sentence_62

It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor. Bird_sentence_63

Early evolution Bird_section_3

See also: List of fossil bird genera Bird_sentence_64

Over 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest bird-line archosaurs to the first maniraptoromorphs, i.e. the first dinosaurs closer to living birds than to Tyrannosaurus rex. Bird_sentence_65

The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase. Bird_sentence_66

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. Bird_sentence_67

Hypercarnivory became increasingly less common while braincases enlarged and forelimbs became longer. Bird_sentence_68

The integument evolved into complex, pennaceous feathers. Bird_sentence_69

The oldest known paravian (and probably the earliest avialan) fossils come from the Tiaojishan Formation of China, which has been dated to the late Jurassic period (Oxfordian stage), about 160 million years ago. Bird_sentence_70

The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui. Bird_sentence_71

The well-known probable early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany. Bird_sentence_72

Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. Bird_sentence_73

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. Bird_sentence_74

Avialans diversified into a wide variety of forms during the Cretaceous Period. Bird_sentence_75

Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). Bird_sentence_76

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. Bird_sentence_77

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. Bird_sentence_78

Around 95 million years ago, they evolved a better sense of smell. Bird_sentence_79

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. Bird_sentence_80

Particularly significant are the development of an enlarged, keeled sternum and the alula, and the loss of grasping hands. Bird_sentence_81

Early diversity of bird ancestors Bird_section_4

See also: Protobirds and Avialae Bird_sentence_82

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. Bird_sentence_83

Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters. Bird_sentence_84

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. Bird_sentence_85

Many species of the second major avialan lineage to diversify, the Euornithes (meaning "true birds", because they include the ancestors of modern birds), were semi-aquatic and specialised in eating fish and other small aquatic organisms. Bird_sentence_86

Unlike the Enantiornithes, which dominated land-based and arboreal habitats, most early euornithes lacked perching adaptations and seem to have included shorebird-like species, waders, and swimming and diving species. Bird_sentence_87

The latter included the superficially gull-like Ichthyornis and the Hesperornithiformes, which became so well adapted to hunting fish in marine environments that they lost the ability to fly and became primarily aquatic. Bird_sentence_88

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). Bird_sentence_89

Euornithes also included the first avialans to develop true pygostyle and a fully mobile fan of tail feathers, which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight. Bird_sentence_90

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. Bird_sentence_91

As both species are small aerial and canopy foraging omnivores, a similar ecological niche was inferred for this hypothetical ancestor. Bird_sentence_92

Diversification of modern birds Bird_section_5

See also: Sibley–Ahlquist taxonomy of birds and dinosaur classification Bird_sentence_93

All modern birds lie within the crown group Aves (alternately Neornithes), which has two subdivisions: the Palaeognathae, which includes the flightless ratites (such as the ostriches) and the weak-flying tinamous, and the extremely diverse Neognathae, containing all other birds. Bird_sentence_94

These two subdivisions are often given the rank of superorder, although Livezey and Zusi assigned them "cohort" rank. Bird_sentence_95

Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,758. Bird_sentence_96

The discovery of Vegavis, a late Cretaceous member of the Anatidae, proved that the diversification of modern birds started before the Cenozoic era. Bird_sentence_97

The affinities of an earlier fossil, the possible galliform Austinornis lentus, dated to about 85 million years ago, are still too controversial to provide a fossil evidence of modern bird diversification. Bird_sentence_98

Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the Middle Cretaceous to the latest Late Cretaceous. Bird_sentence_99

Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after the Cretaceous–Palaeogene extinction event. Bird_sentence_100

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). Bird_sentence_101

Previous attempts to reconcile molecular and fossil evidence have proved controversial, but more recent estimates, using a more comprehensive sample of fossils and a new way of calibrating molecular clocks, showed that while modern birds originated early in the Late Cretaceous in Western Gondwana, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Bird_sentence_102

Modern birds expanded from West Gondwana to the Laurasia through two routes. Bird_sentence_103

One route was an Antarctic interchange in the Paleogene. Bird_sentence_104

This can be confirmed with the presence of multiple avian groups in Australia and New Zealand. Bird_sentence_105

The other route was probably through North American, via land bridges during the Paleocene. Bird_sentence_106

This allowed the expansion and diversification of Neornithes into the Holarctic and Paleotropics. Bird_sentence_107

Classification of bird orders Bird_section_6

See also: List of birds Bird_sentence_108

Cladogram of modern bird relationships based on Kuhl, H. et al. Bird_sentence_109

(2020) Bird_sentence_110

The classification of birds is a contentious issue. Bird_sentence_111

Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds, although it is frequently debated and constantly revised. Bird_sentence_112

Most evidence seems to suggest the assignment of orders is accurate, but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. Bird_sentence_113

More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders. Bird_sentence_114

Genomics Bird_section_7

As of 2020, the genome has been sequenced for at least one species in about 90% of extant avian families (218 out of 236 families recognised by the Howard and Moore Checklist). Bird_sentence_115

See list of sequenced animal genomes. Bird_sentence_116

Distribution Bird_section_8

See also: Lists of birds by region and List of birds by population Bird_sentence_117

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. Bird_sentence_118

The highest bird diversity occurs in tropical regions. Bird_sentence_119

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. Bird_sentence_120

Many species migrate annually over great distances and across oceans; several families of birds have adapted to life both on the world's oceans and in them, and some seabird species come ashore only to breed, while some penguins have been recorded diving up to 300 metres (980 ft) deep. Bird_sentence_121

Many bird species have established breeding populations in areas to which they have been introduced by humans. Bird_sentence_122

Some of these introductions have been deliberate; the ring-necked pheasant, for example, has been introduced around the world as a game bird. Bird_sentence_123

Others have been accidental, such as the establishment of wild monk parakeets in several North American cities after their escape from captivity. Bird_sentence_124

Some species, including cattle egret, yellow-headed caracara and galah, have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat. Bird_sentence_125

Anatomy and physiology Bird_section_9

Main articles: Bird anatomy and Bird vision Bird_sentence_126

See also: Egg tooth Bird_sentence_127

Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight. Bird_sentence_128

Skeletal system Bird_section_10

The skeleton consists of very lightweight bones. Bird_sentence_129

They have large air-filled cavities (called pneumatic cavities) which connect with the respiratory system. Bird_sentence_130

The skull bones in adults are fused and do not show cranial sutures. Bird_sentence_131

The orbits are large and separated by a bony septum. Bird_sentence_132

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. Bird_sentence_133

The last few are fused with the pelvis to form the synsacrum. Bird_sentence_134

The ribs are flattened and the sternum is keeled for the attachment of flight muscles except in the flightless bird orders. Bird_sentence_135

The forelimbs are modified into wings. Bird_sentence_136

The wings are more or less developed depending on the species; the only known groups that lost their wings are the extinct moa and elephant birds. Bird_sentence_137

Excretory system Bird_section_11

Like the reptiles, birds are primarily uricotelic, that is, their kidneys extract nitrogenous waste from their bloodstream and excrete it as uric acid, instead of urea or ammonia, through the ureters into the intestine. Bird_sentence_138

Birds do not have a urinary bladder or external urethral opening and (with exception of the ostrich) uric acid is excreted along with faeces as a semisolid waste. Bird_sentence_139

However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia. Bird_sentence_140

They also excrete creatine, rather than creatinine like mammals. Bird_sentence_141

This material, as well as the output of the intestines, emerges from the bird's cloaca. Bird_sentence_142

The cloaca is a multi-purpose opening: waste is expelled through it, most birds mate by joining cloaca, and females lay eggs from it. Bird_sentence_143

In addition, many species of birds regurgitate pellets. Bird_sentence_144

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. Bird_sentence_145

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. Bird_sentence_146

Reproductive system Bird_section_12

Males within Palaeognathae (with the exception of the kiwis), the Anseriformes (with the exception of screamers), and in rudimentary forms in Galliformes (but fully developed in Cracidae) possess a penis, which is never present in Neoaves. Bird_sentence_147

The length is thought to be related to sperm competition. Bird_sentence_148

When not copulating, it is hidden within the proctodeum compartment within the cloaca, just inside the vent. Bird_sentence_149

Female birds have sperm storage tubules that allow sperm to remain viable long after copulation, a hundred days in some species. Bird_sentence_150

Sperm from multiple males may compete through this mechanism. Bird_sentence_151

Most female birds have a single ovary and a single oviduct, both on the left side, but there are exceptions: species in at least 16 different orders of birds have two ovaries. Bird_sentence_152

Even these species, however, tend to have a single oviduct. Bird_sentence_153

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. Bird_sentence_154

Also terrestrial birds generally have a single ovary, as does the platypus, an egg-laying mammal. Bird_sentence_155

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. Bird_sentence_156

Birds have two sexes: either female or male. Bird_sentence_157

The sex of birds is determined by the Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals. Bird_sentence_158

Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ). Bird_sentence_159

In nearly all species of birds, an individual's sex is determined at fertilisation. Bird_sentence_160

However, one recent study claimed to demonstrate temperature-dependent sex determination among the Australian brushturkey, for which higher temperatures during incubation resulted in a higher female-to-male sex ratio. Bird_sentence_161

This, however, was later proven to not be the case. Bird_sentence_162

These birds do not exhibit temperature-dependent sex determination, but temperature-dependent sex mortality. Bird_sentence_163

Respiratory and circulatory systems Bird_section_13

Birds have one of the most complex respiratory systems of all animal groups. Bird_sentence_164

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. Bird_sentence_165

The other 25% of the air goes directly into the lungs. Bird_sentence_166

When the bird exhales, the used air flows out of the lungs and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Bird_sentence_167

Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation. Bird_sentence_168

Sound production is achieved using the syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea; the trachea being elongated in some species, increasing the volume of vocalisations and the perception of the bird's size. Bird_sentence_169

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. Bird_sentence_170

The postcava receives blood from the limbs via the renal portal system. Bird_sentence_171

Unlike in mammals, the circulating red blood cells in birds retain their nucleus. Bird_sentence_172

Heart type and features Bird_section_14

The avian circulatory system is driven by a four-chambered, myogenic heart contained in a fibrous pericardial sac. Bird_sentence_173

This pericardial sac is filled with a serous fluid for lubrication. Bird_sentence_174

The heart itself is divided into a right and left half, each with an atrium and ventricle. Bird_sentence_175

The atrium and ventricles of each side are separated by atrioventricular valves which prevent back flow from one chamber to the next during contraction. Bird_sentence_176

Being myogenic, the heart's pace is maintained by pacemaker cells found in the sinoatrial node, located on the right atrium. Bird_sentence_177

The sinoatrial node uses calcium to cause a depolarising signal transduction pathway from the atrium through right and left atrioventricular bundle which communicates contraction to the ventricles. Bird_sentence_178

The avian heart also consists of muscular arches that are made up of thick bundles of muscular layers. Bird_sentence_179

Much like a mammalian heart, the avian heart is composed of endocardial, myocardial and epicardial layers. Bird_sentence_180

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. Bird_sentence_181

Avian hearts are generally larger than mammalian hearts when compared to body mass. Bird_sentence_182

This adaptation allows more blood to be pumped to meet the high metabolic need associated with flight. Bird_sentence_183

Organisation Bird_section_15

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. Bird_sentence_184

As a result, birds have more blood in their capillaries per unit of volume of lung than a mammal. Bird_sentence_185

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. Bird_sentence_186

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. Bird_sentence_187

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. Bird_sentence_188

Blood travels through the arterioles and moves into the capillaries where gas exchange can occur. Bird_sentence_189

Capillaries are organized into capillary beds in tissues; it is here that blood exchanges oxygen for carbon dioxide waste. Bird_sentence_190

In the capillary beds, blood flow is slowed to allow maximum diffusion of oxygen into the tissues. Bird_sentence_191

Once the blood has become deoxygenated, it travels through venules then veins and back to the heart. Bird_sentence_192

Veins, unlike arteries, are thin and rigid as they do not need to withstand extreme pressure. Bird_sentence_193

As blood travels through the venules to the veins a funneling occurs called vasodilation bringing blood back to the heart. Bird_sentence_194

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. Bird_sentence_195

Oxygenated blood then flows from the lungs through the left atrium to the left ventricle where it is pumped out to the body. Bird_sentence_196

Nervous system Bird_section_16

The nervous system is large relative to the bird's size. Bird_sentence_197

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. Bird_sentence_198

Most birds have a poor sense of smell with notable exceptions including kiwis, New World vultures and tubenoses. Bird_sentence_199

The avian visual system is usually highly developed. Bird_sentence_200

Water birds have special flexible lenses, allowing accommodation for vision in air and water. Bird_sentence_201

Some species also have dual fovea. Bird_sentence_202

Birds are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells in the eye as well as green, red and blue ones. Bird_sentence_203

They also have double cones, likely to mediate achromatic vision. Bird_sentence_204

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. Bird_sentence_205

Male blue tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers. Bird_sentence_206

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. Bird_sentence_207

With the exception of pigeons and a few other species, the eyelids of birds are not used in blinking. Bird_sentence_208

Instead the eye is lubricated by the nictitating membrane, a third eyelid that moves horizontally. Bird_sentence_209

The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds. Bird_sentence_210

The bird retina has a fan shaped blood supply system called the pecten. Bird_sentence_211

Eyes of most birds are large, not very round and capable of only limited movement in the orbits, typically 10-20°. Bird_sentence_212

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. Bird_sentence_213

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. Bird_sentence_214

The inner ear has a cochlea, but it is not spiral as in mammals. Bird_sentence_215

Defence and intraspecific combat Bird_section_17

A few species are able to use chemical defences against predators; some Procellariiformes can eject an unpleasant stomach oil against an aggressor, and some species of pitohuis from New Guinea have a powerful neurotoxin in their skin and feathers. Bird_sentence_216

A lack of field observations limit our knowledge, but intraspecific conflicts are known to sometimes result in injury or death. Bird_sentence_217

The screamers (Anhimidae), some jacanas (Jacana, Hydrophasianus), the spur-winged goose (Plectropterus), the torrent duck (Merganetta) and nine species of lapwing (Vanellus) use a sharp spur on the wing as a weapon. Bird_sentence_218

The steamer ducks (Tachyeres), geese and swans (Anserinae), the solitaire (Pezophaps), sheathbills (Chionis), some guans (Crax) and stone curlews (Burhinus) use a bony knob on the alular metacarpal to punch and hammer opponents. Bird_sentence_219

The jacanas Actophilornis and Irediparra have an expanded, blade-like radius. Bird_sentence_220

The extinct Xenicibis was unique in having an elongate forelimb and massive hand which likely functioned in combat or defence as a jointed club or flail. Bird_sentence_221

Swans, for instance, may strike with the bony spurs and bite when defending eggs or young. Bird_sentence_222

Feathers, plumage, and scales Bird_section_18

Main articles: Feather and Flight feather Bird_sentence_223

Feathers are a feature characteristic of birds (though also present in some dinosaurs not currently considered to be true birds). Bird_sentence_224

They facilitate flight, provide insulation that aids in thermoregulation, and are used in display, camouflage, and signalling. Bird_sentence_225

There are several types of feathers, each serving its own set of purposes. Bird_sentence_226

Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae. Bird_sentence_227

The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. Bird_sentence_228

The arrangement and appearance of feathers on the body, called plumage, may vary within species by age, social status, and sex. Bird_sentence_229

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. Bird_sentence_230

Moulting is annual in most species, although may have two moults a year, and large birds of prey may moult only once every few years. Bird_sentence_231

Moulting patterns vary across species. Bird_sentence_232

In passerines, flight feathers are replaced one at a time with the innermost primary being the first. Bird_sentence_233

When the fifth of sixth primary is replaced, the outermost tertiaries begin to drop. Bird_sentence_234

After the innermost tertiaries are moulted, the secondaries starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). Bird_sentence_235

The greater primary coverts are moulted in synchrony with the primary that they overlap. Bird_sentence_236

A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless. Bird_sentence_237

As a general rule, the tail feathers are moulted and replaced starting with the innermost pair. Bird_sentence_238

Centripetal moults of tail feathers are however seen in the Phasianidae. Bird_sentence_239

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. Bird_sentence_240

The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from centre outward. Bird_sentence_241

Before nesting, the females of most bird species gain a bare brood patch by losing feathers close to the belly. Bird_sentence_242

The skin there is well supplied with blood vessels and helps the bird in incubation. Bird_sentence_243

Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this. Bird_sentence_244

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. Bird_sentence_245

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. Bird_sentence_246

The scales of birds are composed of the same keratin as beaks, claws, and spurs. Bird_sentence_247

They are found mainly on the toes and metatarsus, but may be found further up on the ankle in some birds. Bird_sentence_248

Most bird scales do not overlap significantly, except in the cases of kingfishers and woodpeckers. Bird_sentence_249

The scales of birds are thought to be homologous to those of reptiles and mammals. Bird_sentence_250

Flight Bird_section_19

Main article: Bird flight Bird_sentence_251

Most birds can fly, which distinguishes them from almost all other vertebrate classes. Bird_sentence_252

Flight is the primary means of locomotion for most bird species and is used for searching for food and for escaping from predators. Bird_sentence_253

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. Bird_sentence_254

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. Bird_sentence_255

About 60 extant bird species are flightless, as were many extinct birds. Bird_sentence_256

Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators. Bird_sentence_257

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. Bird_sentence_258

Behaviour Bird_section_20

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. Bird_sentence_259

Diet and feeding Bird_section_21

Birds' diets are varied and often include nectar, fruit, plants, seeds, carrion, and various small animals, including other birds. Bird_sentence_260

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. Bird_sentence_261

Most birds are highly adapted for rapid digestion to aid with flight. Bird_sentence_262

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. Bird_sentence_263

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. Bird_sentence_264

Avian foraging strategies can vary widely by species. Bird_sentence_265

Many birds glean for insects, invertebrates, fruit, or seeds. Bird_sentence_266

Some hunt insects by suddenly attacking from a branch. Bird_sentence_267

Those species that seek pest insects are considered beneficial 'biological control agents' and their presence encouraged in biological pest control programmes. Bird_sentence_268

Combined, insectivorous birds eat 400–500 million metric tons of arthropods annually. Bird_sentence_269

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. Bird_sentence_270

Kiwis and shorebirds with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of ecological niches. Bird_sentence_271

Loons, diving ducks, penguins and auks pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as sulids, kingfishers and terns plunge dive after their prey. Bird_sentence_272

Flamingos, three species of prion, and some ducks are filter feeders. Bird_sentence_273

Geese and dabbling ducks are primarily grazers. Bird_sentence_274

Some species, including frigatebirds, gulls, and skuas, engage in kleptoparasitism, stealing food items from other birds. Bird_sentence_275

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%. Bird_sentence_276

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. Bird_sentence_277

Water and drinking Bird_section_22

Water is needed by many birds although their mode of excretion and lack of sweat glands reduces the physiological demands. Bird_sentence_278

Some desert birds can obtain their water needs entirely from moisture in their food. Bird_sentence_279

They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting. Bird_sentence_280

Seabirds can drink seawater and have salt glands inside the head that eliminate excess salt out of the nostrils. Bird_sentence_281

Most birds scoop water in their beaks and raise their head to let water run down the throat. Bird_sentence_282

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. Bird_sentence_283

Some desert birds depend on water sources and sandgrouse are particularly well known for their daily congregations at waterholes. Bird_sentence_284

Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers. Bird_sentence_285

Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. Bird_sentence_286

The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called crop milk that they provide to their chicks. Bird_sentence_287

Feather care Bird_section_23

Feathers, being critical to the survival of a bird, require maintenance. Bird_sentence_288

Apart from physical wear and tear, feathers face the onslaught of fungi, ectoparasitic feather mites and bird lice. Bird_sentence_289

The physical condition of feathers are maintained by preening often with the application of secretions from the preen gland. Bird_sentence_290

Birds also bathe in water or dust themselves. Bird_sentence_291

While some birds dip into shallow water, more aerial species may make aerial dips into water and arboreal species often make use of dew or rain that collect on leaves. Bird_sentence_292

Birds of arid regions make use of loose soil to dust-bathe. Bird_sentence_293

A behaviour termed as anting in which the bird encourages ants to run through their plumage is also thought to help them reduce the ectoparasite load in feathers. Bird_sentence_294

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. Bird_sentence_295

Migration Bird_section_24

Main article: Bird migration Bird_sentence_296

Many bird species migrate to take advantage of global differences of seasonal temperatures, therefore optimising availability of food sources and breeding habitat. Bird_sentence_297

These migrations vary among the different groups. Bird_sentence_298

Many landbirds, shorebirds, and waterbirds undertake annual long-distance migrations, usually triggered by the length of daylight as well as weather conditions. Bird_sentence_299

These birds are characterised by a breeding season spent in the temperate or polar regions and a non-breeding season in the tropical regions or opposite hemisphere. Bird_sentence_300

Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs. Bird_sentence_301

Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Bird_sentence_302

Landbirds have a flight range of around 2,500 km (1,600 mi) and shorebirds can fly up to 4,000 km (2,500 mi), although the bar-tailed godwit is capable of non-stop flights of up to 10,200 km (6,300 mi). Bird_sentence_303

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). Bird_sentence_304

Other seabirds disperse after breeding, travelling widely but having no set migration route. Bird_sentence_305

Albatrosses nesting in the Southern Ocean often undertake circumpolar trips between breeding seasons. Bird_sentence_306

Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. Bird_sentence_307

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. Bird_sentence_308

This type of migration is normally associated with food availability. Bird_sentence_309

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. Bird_sentence_310

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. Bird_sentence_311

Altitudinal migration is a form of short-distance migration in which birds spend the breeding season at higher altitudes and move to lower ones during suboptimal conditions. Bird_sentence_312

It is most often triggered by temperature changes and usually occurs when the normal territories also become inhospitable due to lack of food. Bird_sentence_313

Some species may also be nomadic, holding no fixed territory and moving according to weather and food availability. Bird_sentence_314

Parrots as a family are overwhelmingly neither migratory nor sedentary but considered to either be dispersive, irruptive, nomadic or undertake small and irregular migrations. Bird_sentence_315

The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s, a Manx shearwater released in Boston in the United States returned to its colony in Skomer, in Wales within 13 days, a distance of 5,150 km (3,200 mi). Bird_sentence_316

Birds navigate during migration using a variety of methods. Bird_sentence_317

For diurnal migrants, the sun is used to navigate by day, and a stellar compass is used at night. Bird_sentence_318

Birds that use the sun compensate for the changing position of the sun during the day by the use of an internal clock. Bird_sentence_319

Orientation with the stellar compass depends on the position of the constellations surrounding Polaris. Bird_sentence_320

These are backed up in some species by their ability to sense the Earth's geomagnetism through specialised photoreceptors. Bird_sentence_321

Communication Bird_section_25

See also: Bird vocalisation Bird_sentence_322

Birds communicate using primarily visual and auditory signals. Bird_sentence_323

Signals can be interspecific (between species) and intraspecific (within species). Bird_sentence_324

Birds sometimes use plumage to assess and assert social dominance, to display breeding condition in sexually selected species, or to make threatening displays, as in the sunbittern's mimicry of a large predator to ward off hawks and protect young chicks. Bird_sentence_325

Variation in plumage also allows for the identification of birds, particularly between species. Bird_sentence_326

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. Bird_sentence_327

These displays may signal aggression or submission or may contribute to the formation of pair-bonds. Bird_sentence_328

The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements; males' breeding success may depend on the quality of such displays. Bird_sentence_329

Bird calls and songs, which are produced in the syrinx, are the major means by which birds communicate with sound. Bird_sentence_330

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. Bird_sentence_331

Calls are used for a variety of purposes, including mate attraction, evaluation of potential mates, bond formation, the claiming and maintenance of territories, the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season), and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat. Bird_sentence_332

Some birds also use mechanical sounds for auditory communication. Bird_sentence_333

The Coenocorypha snipes of New Zealand drive air through their feathers, woodpeckers drum for long-distance communication, and palm cockatoos use tools to drum. Bird_sentence_334

Flocking and other associations Bird_section_26

While some birds are essentially territorial or live in small family groups, other birds may form large flocks. Bird_sentence_335

The principal benefits of flocking are safety in numbers and increased foraging efficiency. Bird_sentence_336

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. Bird_sentence_337

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. Bird_sentence_338

Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases. Bird_sentence_339

Birds sometimes also form associations with non-avian species. Bird_sentence_340

Plunge-diving seabirds associate with dolphins and tuna, which push shoaling fish towards the surface. Bird_sentence_341

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. Bird_sentence_342

Resting and roosting Bird_section_27

The high metabolic rates of birds during the active part of the day is supplemented by rest at other times. Bird_sentence_343

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. Bird_sentence_344

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. Bird_sentence_345

It has been suggested that there may be certain kinds of sleep which are possible even when in flight. Bird_sentence_346

Some birds have also demonstrated the capacity to fall into slow-wave sleep one hemisphere of the brain at a time. Bird_sentence_347

The birds tend to exercise this ability depending upon its position relative to the outside of the flock. Bird_sentence_348

This may allow the eye opposite the sleeping hemisphere to remain vigilant for predators by viewing the outer margins of the flock. Bird_sentence_349

This adaptation is also known from marine mammals. Bird_sentence_350

Communal roosting is common because it lowers the loss of body heat and decreases the risks associated with predators. Bird_sentence_351

Roosting sites are often chosen with regard to thermoregulation and safety. Bird_sentence_352

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. Bird_sentence_353

Many birds rest on one leg, while some may pull up their legs into their feathers, especially in cold weather. Bird_sentence_354

Perching birds have a tendon locking mechanism that helps them hold on to the perch when they are asleep. Bird_sentence_355

Many ground birds, such as quails and pheasants, roost in trees. Bird_sentence_356

A few parrots of the genus Loriculus roost hanging upside down. Bird_sentence_357

Some hummingbirds go into a nightly state of torpor accompanied with a reduction of their metabolic rates. Bird_sentence_358

This physiological adaptation shows in nearly a hundred other species, including owlet-nightjars, nightjars, and woodswallows. Bird_sentence_359

One species, the common poorwill, even enters a state of hibernation. Bird_sentence_360

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. Bird_sentence_361

Breeding Bird_section_28

See also: :Category:Avian sexuality, Animal sexual behaviour § Birds, Seabird breeding behaviour, and Sexual selection in birds Bird_sentence_362

Social systems Bird_section_29

Ninety-five per cent of bird species are socially monogamous. Bird_sentence_363

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. Bird_sentence_364

Monogamy allows for both paternal care and biparental care, which is especially important for species in which females require males' assistance for successful brood-rearing. Bird_sentence_365

Among many socially monogamous species, extra-pair copulation (infidelity) is common. Bird_sentence_366

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. Bird_sentence_367

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. Bird_sentence_368

Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise. Bird_sentence_369

Other mating systems, including polygyny, polyandry, polygamy, polygynandry, and promiscuity, also occur. Bird_sentence_370

Polygamous breeding systems arise when females are able to raise broods without the help of males. Bird_sentence_371

Some species may use more than one system depending on the circumstances. Bird_sentence_372

Breeding usually involves some form of courtship display, typically performed by the male. Bird_sentence_373

Most displays are rather simple and involve some type of song. Bird_sentence_374

Some displays, however, are quite elaborate. Bird_sentence_375

Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal lekking. Bird_sentence_376

Females are generally the ones that drive partner selection, although in the polyandrous phalaropes, this is reversed: plainer males choose brightly coloured females. Bird_sentence_377

Courtship feeding, billing and allopreening are commonly performed between partners, generally after the birds have paired and mated. Bird_sentence_378

Homosexual behaviour has been observed in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks. Bird_sentence_379

Over 130 avian species around the world engage in sexual interactions between the same sex or homosexual behaviors. Bird_sentence_380

"Same-sex courtship activities may involve elaborate displays, synchronized dances, gift-giving ceremonies, or behaviors at specific display areas including bowers, arenas, or leks." Bird_sentence_381

Territories, nesting and incubation Bird_section_30

See also: Bird nest Bird_sentence_382

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. Bird_sentence_383

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. Bird_sentence_384

Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense. Bird_sentence_385

All birds lay amniotic eggs with hard shells made mostly of calcium carbonate. Bird_sentence_386

Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay camouflaged eggs. Bird_sentence_387

There are many exceptions to this pattern, however; the ground-nesting nightjars have pale eggs, and camouflage is instead provided by their plumage. Bird_sentence_388

Species that are victims of brood parasites have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts. Bird_sentence_389

Bird eggs are usually laid in a nest. Bird_sentence_390

Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows. Bird_sentence_391

Some bird nests, however, are extremely primitive; albatross nests are no more than a scrape on the ground. Bird_sentence_392

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. Bird_sentence_393

During nest construction, some species seek out plant matter from plants with parasite-reducing toxins to improve chick survival, and feathers are often used for nest insulation. Bird_sentence_394

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. Bird_sentence_395

The absence of nests is especially prevalent in ground-nesting species where the newly hatched young are precocial. Bird_sentence_396

Incubation, which optimises temperature for chick development, usually begins after the last egg has been laid. Bird_sentence_397

In monogamous species incubation duties are often shared, whereas in polygamous species one parent is wholly responsible for incubation. Bird_sentence_398

Warmth from parents passes to the eggs through brood patches, areas of bare skin on the abdomen or breast of the incubating birds. Bird_sentence_399

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. Bird_sentence_400

The warmth for the incubation of the eggs of megapodes comes from the sun, decaying vegetation or volcanic sources. Bird_sentence_401

Incubation periods range from 10 days (in woodpeckers, cuckoos and passerine birds) to over 80 days (in albatrosses and kiwis). Bird_sentence_402

The diversity of characteristics of birds is great, sometimes even in closely related species. Bird_sentence_403

Several avian characteristics are compared in the table below. Bird_sentence_404


SpeciesBird_header_cell_1_0_0 Adult weight





(per year)Bird_header_cell_1_0_3

Clutch sizeBird_header_cell_1_0_4
Ruby-throated hummingbird (Archilochus colubris)Bird_cell_1_1_0 3Bird_cell_1_1_1 13Bird_cell_1_1_2 2.0Bird_cell_1_1_3 2Bird_cell_1_1_4
House sparrow (Passer domesticus)Bird_cell_1_2_0 25Bird_cell_1_2_1 11Bird_cell_1_2_2 4.5Bird_cell_1_2_3 5Bird_cell_1_2_4
Greater roadrunner (Geococcyx californianus)Bird_cell_1_3_0 376Bird_cell_1_3_1 20Bird_cell_1_3_2 1.5Bird_cell_1_3_3 4Bird_cell_1_3_4
Turkey vulture (Cathartes aura)Bird_cell_1_4_0 2,200Bird_cell_1_4_1 39Bird_cell_1_4_2 1.0Bird_cell_1_4_3 2Bird_cell_1_4_4
Laysan albatross (Diomedea immutabilis)Bird_cell_1_5_0 3,150Bird_cell_1_5_1 64Bird_cell_1_5_2 1.0Bird_cell_1_5_3 1Bird_cell_1_5_4
Magellanic penguin (Spheniscus magellanicus)Bird_cell_1_6_0 4,000Bird_cell_1_6_1 40Bird_cell_1_6_2 1.0Bird_cell_1_6_3 1Bird_cell_1_6_4
Golden eagle (Aquila chrysaetos)Bird_cell_1_7_0 4,800Bird_cell_1_7_1 40Bird_cell_1_7_2 1.0Bird_cell_1_7_3 2Bird_cell_1_7_4
Wild turkey (Meleagris gallopavo)Bird_cell_1_8_0 6,050Bird_cell_1_8_1 28Bird_cell_1_8_2 1.0Bird_cell_1_8_3 11Bird_cell_1_8_4

Parental care and fledging Bird_section_31

Main article: Parental care in birds Bird_sentence_405

At the time of their hatching, chicks range in development from helpless to independent, depending on their species. Bird_sentence_406

Helpless chicks are termed altricial, and tend to be born small, blind, immobile and naked; chicks that are mobile and feathered upon hatching are termed precocial. Bird_sentence_407

Altricial chicks need help thermoregulating and must be brooded for longer than precocial chicks. Bird_sentence_408

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". Bird_sentence_409

Chicks at neither extreme but favoring one or the other may be termed semi-precocial or semi-altricial. Bird_sentence_410

The length and nature of parental care varies widely amongst different orders and species. Bird_sentence_411

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. Bird_sentence_412

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. Bird_sentence_413

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. Bird_sentence_414

The main purpose of the guard stage is to aid offspring to thermoregulate and protect them from predation. Bird_sentence_415

In some species, both parents care for nestlings and fledglings; in others, such care is the responsibility of only one sex. Bird_sentence_416

In some species, other members of the same species—usually close relatives of the breeding pair, such as offspring from previous broods—will help with the raising of the young. Bird_sentence_417

Such alloparenting is particularly common among the Corvida, which includes such birds as the true crows, Australian magpie and fairy-wrens, but has been observed in species as different as the rifleman and red kite. Bird_sentence_418

Among most groups of animals, male parental care is rare. Bird_sentence_419

In birds, however, it is quite common—more so than in any other vertebrate class. Bird_sentence_420

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. Bird_sentence_421

The point at which chicks fledge varies dramatically. Bird_sentence_422

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. Bird_sentence_423

Some other species, such as ducks, move their chicks away from the nest at an early age. Bird_sentence_424

In most species, chicks leave the nest just before, or soon after, they are able to fly. Bird_sentence_425

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. Bird_sentence_426

Chicks may also follow their parents during their first migration. Bird_sentence_427

Brood parasites Bird_section_32

Main article: Brood parasite Bird_sentence_428

Brood parasitism, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism. Bird_sentence_429

After a parasitic bird lays her eggs in another bird's nest, they are often accepted and raised by the host at the expense of the host's own brood. Bird_sentence_430

Brood parasites may be either obligate brood parasites, which must lay their eggs in the nests of other species because they are incapable of raising their own young, or non-obligate brood parasites, which sometimes lay eggs in the nests of conspecifics to increase their reproductive output even though they could have raised their own young. Bird_sentence_431

One hundred bird species, including honeyguides, icterids, and ducks, are obligate parasites, though the most famous are the cuckoos. Bird_sentence_432

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. Bird_sentence_433

Sexual selection Bird_section_33

Main article: Sexual selection in birds Bird_sentence_434

Birds have evolved a variety of mating behaviours, with the peacock tail being perhaps the most famous example of sexual selection and the Fisherian runaway. Bird_sentence_435

Commonly occurring sexual dimorphisms such as size and colour differences are energetically costly attributes that signal competitive breeding situations. Bird_sentence_436

Many types of avian sexual selection have been identified; intersexual selection, also known as female choice; and intrasexual competition, where individuals of the more abundant sex compete with each other for the privilege to mate. Bird_sentence_437

Sexually selected traits often evolve to become more pronounced in competitive breeding situations until the trait begins to limit the individual's fitness. Bird_sentence_438

Conflicts between an individual fitness and signalling adaptations ensure that sexually selected ornaments such as plumage coloration and courtship behaviour are "honest" traits. Bird_sentence_439

Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours. Bird_sentence_440

Inbreeding depression Bird_section_34

Main article: Inbreeding depression Bird_sentence_441

Inbreeding causes early death (inbreeding depression) in the zebra finch Taeniopygia guttata. Bird_sentence_442

Embryo survival (that is, hatching success of fertile eggs) was significantly lower for sib-sib mating pairs than for unrelated pairs. Bird_sentence_443

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. Bird_sentence_444

Inbreeding avoidance Bird_section_35

Main article: Inbreeding avoidance Bird_sentence_445

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). Bird_sentence_446

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. Bird_sentence_447

However, there are ecological and demographic constraints on extra pair matings. Bird_sentence_448

Nevertheless, 43% of broods produced by incestuously paired females contained extra pair young. Bird_sentence_449

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. Bird_sentence_450

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. Bird_sentence_451

Southern pied babblers Turdoides bicolor appear to avoid inbreeding in two ways. Bird_sentence_452

The first is through dispersal, and the second is by avoiding familiar group members as mates. Bird_sentence_453

Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Bird_sentence_454

Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated. Bird_sentence_455

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. Bird_sentence_456

Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups. Bird_sentence_457

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. Bird_sentence_458

Cross-fertilisation between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny. Bird_sentence_459

Ecology Bird_section_36

Birds occupy a wide range of ecological positions. Bird_sentence_460

While some birds are generalists, others are highly specialised in their habitat or food requirements. Bird_sentence_461

Even within a single habitat, such as a forest, the niches occupied by different species of birds vary, with some species feeding in the forest canopy, others beneath the canopy, and still others on the forest floor. Bird_sentence_462

Forest birds may be insectivores, frugivores, and nectarivores. Bird_sentence_463

Aquatic birds generally feed by fishing, plant eating, and piracy or kleptoparasitism. Bird_sentence_464

Birds of prey specialise in hunting mammals or other birds, while vultures are specialised scavengers. Bird_sentence_465

Avivores are animals that are specialised at preying on birds. Bird_sentence_466

Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal. Bird_sentence_467

Plants and pollinating birds often coevolve, and in some cases a flower's primary pollinator is the only species capable of reaching its nectar. Bird_sentence_468

Birds are often important to island ecology. Bird_sentence_469

Birds have frequently reached islands that mammals have not; on those islands, birds may fulfil ecological roles typically played by larger animals. Bird_sentence_470

For example, in New Zealand nine species of moa were important browsers, as are the kererū and kokako today. Bird_sentence_471

Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa. Bird_sentence_472

Nesting seabirds may also affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of guano, which may enrich the local soil and the surrounding seas. Bird_sentence_473

A wide variety of avian ecology field methods, including counts, nest monitoring, and capturing and marking, are used for researching avian ecology. Bird_sentence_474

Relationship with humans Bird_section_37

Main article: Birds in culture Bird_sentence_475

Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man. Bird_sentence_476

Sometimes, these relationships are mutualistic, like the cooperative honey-gathering among honeyguides and African peoples such as the Borana. Bird_sentence_477

Other times, they may be commensal, as when species such as the house sparrow have benefited from human activities. Bird_sentence_478

Several bird species have become commercially significant agricultural pests, and some pose an aviation hazard. Bird_sentence_479

Human activities can also be detrimental, and have threatened numerous bird species with extinction (hunting, avian lead poisoning, pesticides, roadkill, wind turbine kills and predation by pet cats and dogs are common causes of death for birds). Bird_sentence_480

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. Bird_sentence_481

Some of these are zoonotic diseases that can also be transmitted to humans. Bird_sentence_482

Economic importance Bird_section_38

See also: Pet § Birds Bird_sentence_483

Domesticated birds raised for meat and eggs, called poultry, are the largest source of animal protein eaten by humans; in 2003, 76 million tons of poultry and 61 million tons of eggs were produced worldwide. Bird_sentence_484

Chickens account for much of human poultry consumption, though domesticated turkeys, ducks, and geese are also relatively common. Bird_sentence_485

Many species of birds are also hunted for meat. Bird_sentence_486

Bird hunting is primarily a recreational activity except in extremely undeveloped areas. Bird_sentence_487

The most important birds hunted in North and South America are waterfowl; other widely hunted birds include pheasants, wild turkeys, quail, doves, partridge, grouse, snipe, and woodcock. Bird_sentence_488

Muttonbirding is also popular in Australia and New Zealand. Bird_sentence_489

Although some hunting, such as that of muttonbirds, may be sustainable, hunting has led to the extinction or endangerment of dozens of species. Bird_sentence_490

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. Bird_sentence_491

The War of the Pacific, sometimes called the Guano War, was fought in part over the control of guano deposits. Bird_sentence_492

Birds have been domesticated by humans both as pets and for practical purposes. Bird_sentence_493

Colourful birds, such as parrots and mynas, are bred in captivity or kept as pets, a practice that has led to the illegal trafficking of some endangered species. Bird_sentence_494

Falcons and cormorants have long been used for hunting and fishing, respectively. Bird_sentence_495

Messenger pigeons, used since at least 1 AD, remained important as recently as World War II. Bird_sentence_496

Today, such activities are more common either as hobbies, for entertainment and tourism, or for sports such as pigeon racing. Bird_sentence_497

Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly, birders) number in the millions. Bird_sentence_498

Many homeowners erect bird feeders near their homes to attract various species. Bird_sentence_499

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. Bird_sentence_500

In religion and mythology Bird_section_39

Birds play prominent and diverse roles in religion and mythology. Bird_sentence_501

In religion, birds may serve as either messengers or priests and leaders for a deity, such as in the Cult of Makemake, in which the Tangata manu of Easter Island served as chiefs or as attendants, as in the case of Hugin and Munin, the two common ravens who whispered news into the ears of the Norse god Odin. Bird_sentence_502

In several civilisations of ancient Italy, particularly Etruscan and Roman religion, priests were involved in augury, or interpreting the words of birds while the "auspex" (from which the word "auspicious" is derived) watched their activities to foretell events. Bird_sentence_503

They may also serve as religious symbols, as when Jonah (Hebrew: יוֹנָה‎, dove) embodied the fright, passivity, mourning, and beauty traditionally associated with doves. Bird_sentence_504

Birds have themselves been deified, as in the case of the common peacock, which is perceived as Mother Earth by the people of southern India. Bird_sentence_505

In the ancient world, doves were used as symbols of the Mesopotamian goddess Inanna (later known as Ishtar), the Canaanite mother goddess Asherah, and the Greek goddess Aphrodite. Bird_sentence_506

In ancient Greece, Athena, the goddess of wisdom and patron deity of the city of Athens, had a little owl as her symbol. Bird_sentence_507

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. Bird_sentence_508

Indigenous peoples of the central Andes maintain legends of birds passing to and from metaphysical worlds. Bird_sentence_509

In culture and folklore Bird_section_40

Birds have featured in culture and art since prehistoric times, when they were represented in early cave paintings. Bird_sentence_510

Some birds have been perceived as monsters, including the mythological Roc and the Māori's legendary Pouākai, a giant bird capable of snatching humans. Bird_sentence_511

Birds were later used as symbols of power, as in the magnificent Peacock Throne of the Mughal and Persian emperors. Bird_sentence_512

With the advent of scientific interest in birds, many paintings of birds were commissioned for books. Bird_sentence_513

Among the most famous of these bird artists was John James Audubon, whose paintings of North American birds were a great commercial success in Europe and who later lent his name to the National Audubon Society. Bird_sentence_514

Birds are also important figures in poetry; for example, Homer incorporated nightingales into his Odyssey, and Catullus used a sparrow as an erotic symbol in his Catullus 2. Bird_sentence_515

The relationship between an albatross and a sailor is the central theme of Samuel Taylor Coleridge's The Rime of the Ancient Mariner, which led to the use of the term as a metaphor for a 'burden'. Bird_sentence_516

Other English metaphors derive from birds; vulture funds and vulture investors, for instance, take their name from the scavenging vulture. Bird_sentence_517

Perceptions of bird species vary across cultures. Bird_sentence_518

Owls are associated with bad luck, witchcraft, and death in parts of Africa, but are regarded as wise across much of Europe. Bird_sentence_519

Hoopoes were considered sacred in Ancient Egypt and symbols of virtue in Persia, but were thought of as thieves across much of Europe and harbingers of war in Scandinavia. Bird_sentence_520

In heraldry, birds, especially eagles, often appear in coats of arms. Bird_sentence_521

In music Bird_section_41

Main article: Birds in music Bird_sentence_522

In music, birdsong has influenced composers and musicians in several ways: they can be inspired by birdsong; they can intentionally imitate bird song in a composition, as Vivaldi, Messiaen, and Beethoven did, along with many later composers; they can incorporate recordings of birds into their works, as Ottorino Respighi first did; or like Beatrice Harrison and David Rothenberg, they can duet with birds. Bird_sentence_523

Conservation Bird_section_42

Main article: Bird conservation Bird_sentence_524

See also: Late Quaternary prehistoric birds, List of extinct birds, and Raptor conservation Bird_sentence_525

Although human activities have allowed the expansion of a few species, such as the barn swallow and European starling, they have caused population decreases or extinction in many other species. Bird_sentence_526

Over a hundred bird species have gone extinct in historical times, although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonisation of Melanesian, Polynesian, and Micronesian islands. Bird_sentence_527

Many bird populations are declining worldwide, with 1,227 species listed as threatened by BirdLife International and the IUCN in 2009. Bird_sentence_528

The most commonly cited human threat to birds is habitat loss. Bird_sentence_529

Other threats include overhunting, accidental mortality due to collisions with buildings or vehicles, long-line fishing bycatch, pollution (including oil spills and pesticide use), competition and predation from nonnative invasive species, and climate change. Bird_sentence_530

Governments and conservation groups work to protect birds, either by passing laws that preserve and restore bird habitat or by establishing captive populations for reintroductions. Bird_sentence_531

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. Bird_sentence_532

See also Bird_section_43


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