Birds (Aves) are a group of endothermic vertebrates, characterised 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. Birds live worldwide and range in size from the
5 cm (2 in) bee hummingbird to the 2.75 m (9 ft)
ostrich. They rank as the world’s most numerically-successful class
of tetrapods, with approximately ten thousand living species, more
than half of these being passerines, sometimes known as perching
birds. Birds have wings which are more or less developed depending on
the species; the only known groups without wings are the extinct moa
and elephant birds. Wings, which evolved from forelimbs, gave birds
the ability to fly, although further evolution has led to the loss of
flight in flightless birds, including ratites, penguins, and diverse
endemic island species of birds. 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 fossil record indicates that birds evolved from earlier feathered
dinosaurs within the theropod group, which are traditionally placed
within the saurischian dinosaurs; their closest living relatives are
the crocodilians. Primitive bird-like dinosaurs that lie outside class
Aves proper, in the broader group Avialae, have been found dating back
to the mid-
Jurassic period, around 170 million years ago. Many of
these early "stem-birds", such as Archaeopteryx, were not yet capable
of fully powered flight, and many retained primitive characteristics
like toothy jaws in place of beaks, and long bony tails. DNA-based
evidence finds that birds diversified dramatically around the time of
the Cretaceous–Palaeogene extinction event 66 million years ago,
which killed off the pterosaurs and all the non-avian dinosaur
lineages. But birds, especially those in the southern continents,
survived this event and then migrated to other parts of the world
while diversifying during periods of global cooling. This makes
them the sole surviving dinosaurs according to cladistics.
Some birds, especially corvids and parrots, are among the most
intelligent animals; several bird species make and use tools, and many
social species pass on knowledge across generations, which is
considered a form of culture. Many species annually migrate great
distances. Birds are social, communicating with visual signals, calls,
and bird songs, and participating in such social behaviours as
cooperative breeding and hunting, flocking, and mobbing of predators.
The vast majority of bird species are socially monogamous (referring
to social living arrangement, distinct from genetic monogamy), usually
for one breeding season at a time, sometimes for years, but rarely for
life. Other species have breeding systems that are polygynous
(arrangement of one male with many females) or, rarely, polyandrous
(arrangement of one female with many males). Birds produce offspring
by laying eggs which are fertilised through sexual reproduction. They
are usually laid in a nest and incubated by the parents. Most birds
have an extended period of parental care after hatching. Some birds,
such as hens, lay eggs even when not fertilised, though unfertilised
eggs do not produce offspring.
Many species of birds are economically important as food for human
consumption and raw material in manufacturing, with domesticated and
undomesticated birds (poultry and game) 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 prominently figure throughout human culture. About
120–130 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
Evolution and classification
1.2 Dinosaurs and the origin of birds
1.3 Early evolution
1.4 Early diversity of bird ancestors
1.5 Diversification of modern birds
1.6 Classification of bird orders
Anatomy and physiology
3.1 Skeletal system
3.2 Excretory system
3.3 Respiratory and circulatory systems
Heart type and features
3.4 Nervous system
3.5 Defence and intraspecific combat
3.7 Feathers, plumage, and scales
4.1 Diet and feeding
4.2 Water and drinking
4.6 Flocking and other associations
4.7 Resting and roosting
4.8.1 Social systems
4.8.2 Territories, nesting and incubation
4.8.3 Parental care and fledging
4.8.4 Brood parasites
4.8.5 Sexual selection
4.8.6 Inbreeding depression
4.8.7 Inbreeding avoidance
6 Relationship with humans
6.1 Economic importance
6.2 In religion and mythology
6.3 In culture and folklore
6.4 In music
7 See also
9 External links
Evolution and classification
Evolution of birds
Archaeopteryx lithographica is often considered the oldest known true
The first classification of birds was developed by Francis Willughby
John Ray in their 1676 volume Ornithologiae. Carl Linnaeus
modified that work in 1758 to devise the taxonomic classification
system currently in use. Birds are categorised as the biological
class Aves in Linnaean taxonomy.
Phylogenetic taxonomy places Aves in
the dinosaur clade Theropoda.
Aves and a sister group, the clade Crocodilia, contain the only living
representatives of the reptile clade Archosauria. During the late
1990s, Aves was most commonly defined phylogenetically as all
descendants of the most recent common ancestor of modern birds and
Archaeopteryx lithographica. However, an earlier definition
Jacques Gauthier gained wide currency in the 21st century,
and is used by many scientists including adherents of the Phylocode
system. Gauthier defined Aves to include only the crown group of the
set of modern birds. This was done by excluding most groups known only
from fossils, and assigning them, instead, to the Avialae, in part
to avoid the uncertainties about the placement of
relation to animals traditionally thought of as theropod dinosaurs.
Gauthier identified four different definitions for the same
biological name "Aves", which is a problem. Gauthier proposed to
reserve the term Aves only for the crown group consisting of the last
common ancestor of all living birds and all of its descendants, which
corresponds to meaning number 4 below. He assigned other names to the
Lizards (including snakes)
The birds' phylogenetic relationships to major living reptile groups.
Aves can mean all archosaurs closer to birds than to crocodiles
Aves can mean those advanced archosaurs with feathers (alternately
Aves can mean those feathered dinosaurs that fly (alternately Avialae)
Aves can mean the last common ancestor of all the currently living
birds and all of its descendants (a "crown group", in this sense
synonymous with Neornithes)
Under the fourth definition
Archaeopteryx is an avialan, and not a
member of Aves. Gauthier's 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
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."
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
Dinosaurs and the origin of birds
Main article: Origin of birds
Anchiornis huxleyi is an important source of information on the early
evolution of birds in the Late
Cladogram following the results of a phylogenetic study by Cau et al.,
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 dromaeosaurs and oviraptorids, among others. As
scientists have discovered more theropods closely related to birds,
the previously clear distinction between non-birds and birds has
become blurred. Recent discoveries in the
Liaoning Province of
northeast China, which demonstrate many small theropod feathered
dinosaurs, contribute to this ambiguity.
The consensus view in contemporary palaeontology is that the flying
theropods, or avialans, are the closest relatives of the
deinonychosaurs, which include dromaeosaurids and troodontids.
Together, these form a group called Paraves. Some basal members of
this group, such as Microraptor, have features which may have enabled
them to glide or fly. The most basal deinonychosaurs were very small.
This evidence raises the possibility that the ancestor of all
paravians may have been arboreal, have been able to glide, or
Archaeopteryx and the non-avialan feathered
dinosaurs, who primarily ate meat, recent studies suggest that the
first avialans were omnivores.
Archaeopteryx is well known as one of the first
transitional fossils to be found, and it provided support for the
theory of evolution in the late 19th century.
Archaeopteryx was the
first fossil to display both clearly traditional reptilian
characteristics: teeth, clawed fingers, and a long, lizard-like tail,
as well as wings with flight feathers similar to those of modern
birds. It is not considered a direct ancestor of birds, though it is
possibly closely related to the true ancestor.
See also: List of fossil bird genera
Confuciusornis sanctus, a
Cretaceous bird from China that lived 125
million years ago, is the oldest known bird to have a beak.
Cladogram following the results of a phylogenetic study by Cau et al.,
The earliest known avialan fossils come from the Tiaojishan Formation
of China, which has been dated to the late
Jurassic period (Oxfordian
stage), about 160 million years ago. The avialan species from this
time period include
Xiaotingia zhengi, and
The well-known 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
Avialans diversified into a wide variety of forms during the
Cretaceous Period. Many groups retained primitive characteristics,
such as clawed wings and teeth, though the latter were lost
independently in a number of avialan groups, including modern birds
(Aves). While the earliest forms, such as
Jeholornis, retained the long bony tails of their ancestors, the
tails of more advanced avialans were shortened with the advent of the
pygostyle bone in the group Pygostylia. In the late Cretaceous, about
100 million years ago, the ancestors of all modern birds evolved a
more open pelvis, allowing them to lay larger eggs compared to body
size. Around 95 million years ago, they evolved a better sense of
Early diversity of bird ancestors
Ichthyornis, which lived 93 million years ago, was the first known
prehistoric bird relative preserved with teeth.
Mesozoic bird phylogeny simplified after Wang et al., 2015's
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
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
Many species of the second major avialan lineage to diversify, the
Euornithes (meaning "true birds", because they include the ancestors
of modern birds), were semi-aquatic and specialised in eating fish and
other small aquatic organisms. Unlike the enantiornithes, which
dominated land-based and arboreal habitats, most early euornithes
lacked perching adaptations and seem to have included shorebird-like
species, waders, and swimming and diving species.
The later included the superficially gull-like Ichthyornis, the
Hesperornithiformes, which became so well adapted to hunting fish in
marine environments that they lost the ability to fly and became
primarily aquatic. The early euornithes also saw the development
of many traits associated with modern birds, like strongly keeled
breastbones, toothless, beaked portions of their jaws (though most
non-avian euornithes retained teeth in other parts of the jaws).
Euornithes also included the first avialans to develop true pygostyle
and a fully mobile fan of tail feathers, which may have replaced
the "hind wing" as the primary mode of aerial maneuverability and
braking in flight.
Diversification of modern birds
Sibley–Ahlquist taxonomy of birds and dinosaur
Other birds (Neoaves)
Basal divergences of modern birds
based on Sibley-Ahlquist taxonomy
All modern birds lie within the crown group Aves (alternately
Neornithes), which has two subdivisions: the Palaeognathae, which
includes the flightless ratites (such as the ostriches) and the
weak-flying tinamous, and the extremely diverse Neognathae, containing
all other birds. These two subdivisions are often given the rank
of superorder, although Livezey and Zusi assigned them "cohort"
rank. Depending on the taxonomic viewpoint, the number of known
living bird species varies anywhere from 9,800 to 10,050.
The discovery of Vegavis, a late
Cretaceous member of the Anatidae,
proved that the diversification of modern birds started before the
Cenozoic. 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
Most studies agree on a
Cretaceous age for the most recent common
ancestor of modern birds but estimates range from the Middle
Cretaceous to the latest Late Cretaceous. Similarly, there is
no agreement on whether most of the early diversification of modern
birds occurred before or after the Cretaceous–Palaeogene extinction
event. This disagreement is in part caused by a divergence in the
evidence; most molecular dating studies suggests a Cretaceous
radiation, while fossil evidence points to a
Cenozoic radiation (the
so-called 'rocks' versus 'clocks' controversy). Previous attempts to
reconcile molecular and fossil evidence have proved
controversial, but more recent estimates, using a more
comprehensive sample of fossils and a new way of calibrating molecular
clocks, showed that while modern birds originated early in the Late
Cretaceous, a pulse of diversification in all major groups occurred
around the Cretaceous–Palaeogene extinction event.
Classification of bird orders
See also: List of birds
Cladogram of modern bird relationships based on Prum, R.O. et al.
(2015) with some clade names after Yuri, T. et al. (2013).
Cassowary & emus)
Aepyornithiformes (Elephant bird)
Galliformes (chickens and relatives)
Anseriformes (ducks and relatives)
Gruiformes (rails and cranes)
Charadriiformes (waders and relatives)
Eurypygiformes (sunbittern and kagu)
Procellariiformes (albatross and petrels)
Suliformes (boobies, cormorants, etc.)
Pelecaniformes (pelicans, herons & egrets)
Cathartiformes (New World vultures)
Accipitriformes (hawks and relatives)
Coliidae (mouse birds)
Leptosomatiformes (cuckoo roller)
Trogoniformes (trogons and quetzals)
Bucerotiformes (hornbills and relatives)
Coraciformes (kingfishers and relatives)
Piciformes (woodpeckers and relatives)
The classification of birds is a contentious issue. Sibley and
Ahlquist's Phylogeny and Classification of Birds (1990) is a landmark
work on the classification of birds, although it is frequently
debated and constantly revised. Most evidence seems to suggest the
assignment of orders is accurate, but scientists disagree about
the relationships between the orders themselves; evidence from modern
bird anatomy, fossils and DNA have all been brought to bear on the
problem, but no strong consensus has emerged. More recently, new
fossil and molecular evidence is providing an increasingly clear
picture of the evolution of modern bird orders.
Lists of birds by region and
List of birds
List of birds by population
The range of the house sparrow has expanded dramatically due to human
Birds live and breed in most terrestrial habitats and on all seven
continents, reaching their southern extreme in the snow petrel's
breeding colonies up to 440 kilometres (270 mi) inland in
Antarctica. The highest bird diversity occurs in tropical regions.
It was earlier thought that this high diversity was the result of
higher speciation rates in the tropics; however recent studies found
higher speciation rates in the high latitudes that were offset by
greater extinction rates than in the tropics. Several families of
birds have adapted to life both on the world's oceans and in them,
with some seabird species coming ashore only to breed and some
penguins have been recorded diving up to 300 metres (980 ft)
Many bird species have established breeding populations in areas to
which they have been introduced by humans. Some of these introductions
have been deliberate; the ring-necked pheasant, for example, has been
introduced around the world as a game bird. Others have been
accidental, such as the establishment of wild monk parakeets in
several North American cities after their escape from captivity.
Some species, including cattle egret, yellow-headed caracara
and galah, have spread naturally far beyond their original ranges
as agricultural practices created suitable new habitat.
Anatomy and physiology
Bird anatomy and
External anatomy of a bird (example: yellow-wattled lapwing): 1 Beak,
2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser coverts, 7 Scapulars, 8
Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 Vent, 13 Thigh,
14 Tibio-tarsal articulation, 15 Tarsus, 16 Foot, 17 Tibia, 18 Belly,
19 Flanks, 20 Breast, 21 Throat, 22 Wattle, 23 Eyestripe
Compared with other vertebrates, birds have a body plan that shows
many unusual adaptations, mostly to facilitate flight.
The skeleton consists of very lightweight bones. They have large
air-filled cavities (called pneumatic cavities) which connect with the
respiratory system. The skull bones in adults are fused and do not
show cranial sutures. The orbits are large and separated by a bony
septum. The spine has cervical, thoracic, lumbar and caudal regions
with the number of cervical (neck) vertebrae highly variable and
especially flexible, but movement is reduced in the anterior thoracic
vertebrae and absent in the later vertebrae. The last few are
fused with the pelvis to form the synsacrum. The ribs are
flattened and the sternum is keeled for the attachment of flight
muscles except in the flightless bird orders. The forelimbs are
modified into wings.
Like the reptiles, birds are primarily uricotelic, that is, their
kidneys extract nitrogenous waste from their bloodstream and excrete
it as uric acid instead of urea or ammonia through the ureters into
the intestine. Birds do not have a urinary bladder or external
urethral opening and (with exception of the ostrich) uric acid is
excreted along with faeces as a semisolid waste. However,
birds such as hummingbirds can be facultatively ammonotelic, excreting
most of the nitrogenous wastes as ammonia. They also excrete
creatine, rather than creatinine like mammals. This material, as
well as the output of the intestines, emerges from the bird's
cloaca. The cloaca is a multi-purpose opening: waste is
expelled through it, most birds mate by joining cloaca, and females
lay eggs from it. In addition, many species of birds regurgitate
Palaeognathae (with the exception of the kiwis), the
Anseriformes (with the exception of screamers), and in rudimentary
Galliformes (but fully developed in Cracidae) possess a
penis, which is never present in Neoaves. The length is
thought to be related to sperm competition. When not copulating,
it is hidden within the proctodeum compartment within the cloaca, just
inside the vent. 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. Most birds are highly
adapted for rapid digestion to aid with flight. Some migratory
birds have adapted to use protein from many parts of their bodies,
including protein from the intestines, as additional energy during
Respiratory and circulatory systems
Birds have one of the most complex respiratory systems of all animal
groups. Upon inhalation, 75% of the fresh air bypasses the lungs
and flows directly into a posterior air sac which extends from the
lungs and connects with air spaces in the bones and fills them with
air. The other 25% of the air goes directly into the lungs. When the
bird exhales, the used air flows out of the lungs and the stored fresh
air from the posterior air sac is simultaneously forced into the
lungs. Thus, a bird's lungs receive a constant supply of fresh air
during both inhalation and exhalation.
Sound production is
achieved using the syrinx, a muscular chamber incorporating multiple
tympanic membranes which diverges from the lower end of the
trachea; the trachea being elongated in some species, increasing
the volume of vocalisations and the perception of the bird's size.
In birds, the main arteries taking blood away from the heart originate
from the right aortic arch (or pharyngeal arch), unlike in the mammals
where the left aortic arch forms this part of the aorta. The
postcava receives blood from the limbs via the renal portal system.
Unlike in mammals, the circulating red blood cells in birds retain
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 serous fluid for lubrication. The heart itself is
divided into a right and left half, each with an atrium and ventricle.
The atrium and ventricles of each side are separated by
atrioventricular valves which prevent back flow from one chamber to
the next during contraction. Being myogenic, the heart's pace is
maintained by pacemaker cells found in the sinoatrial node, located on
the right atrium.
The sinoatrial node uses calcium to cause a depolarising signal
transduction pathway from the atrium through right and left
atrioventricular bundle which communicates contraction to the
ventricles. The avian heart also consists of muscular arches that are
made up of thick bundles of muscular layers. Much like a mammalian
heart, the avian heart is composed of endocardial, myocardial and
epicardial layers. The atrium walls tend to be thinner than the
ventricle walls, due to the intense ventricular contraction used to
pump oxygenated blood throughout the body. Avian hearts are generally
larger than mammalian hearts when compared to body mass. This
adaptation allows more blood to be pumped to meet the high metabolic
need associated with flight.
Birds have a very efficient system for diffusing oxygen into the
blood; birds have a ten times greater surface area to gas exchange
volume than mammals. As a result, birds have more blood in their
capillaries per unit of volume of lung than a mammal. The arteries
are composed of thick elastic muscles to withstand the pressure of the
ventricular constriction, and become more rigid as they move away from
the heart. Blood moves through the arteries, which undergo
vasoconstriction, and into arterioles which act as a transportation
system to distribute primarily oxygen as well as nutrients to all
tissues of the body. As the arterioles move away from the heart
and into individual organs and tissues they are further divided to
increase surface area and slow blood flow. Blood travels through the
arterioles and moves into the capillaries where gas exchange can
Capillaries are organized into capillary beds in tissues; it is here
that blood exchanges oxygen for carbon dioxide waste. In the capillary
beds blood flow is slowed to allow maximum diffusion of oxygen into
the tissues. Once the blood has become deoxygenated it travels through
venules then veins and back to the heart. Veins, unlike arteries, are
thin and rigid as they do not need to withstand extreme pressure. As
blood travels through the venules to the veins a funneling occurs
called vasodilation bringing blood back to the heart. Once the
blood reaches the heart it moves first into the right atrium, then the
right ventricle to be pumped through the lungs for further gas
exchange of carbon dioxide waste for oxygen. Oxygenated blood then
flows from the lungs through the left atrium to the left ventricle
where it is pumped out to the body.
The nictitating membrane as it covers the eye of a masked lapwing
The nervous system is large relative to the bird's size. The most
developed part of the brain is the one that controls the
flight-related functions, while the cerebellum coordinates movement
and the cerebrum controls behaviour patterns, navigation, mating and
nest building. Most birds have a poor sense of smell with notable
exceptions including kiwis, New World vultures and
tubenoses. The avian visual system is usually highly developed.
Water birds have special flexible lenses, allowing accommodation for
vision in air and water. Some species also have dual fovea. Birds
are tetrachromatic, possessing ultraviolet (UV) sensitive cone cells
in the eye as well as green, red and blue ones.
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
Male blue tits have an ultraviolet reflective crown
patch which is displayed in courtship by posturing and raising of
their nape feathers.
Ultraviolet light is also used in
foraging—kestrels have been shown to search for prey by detecting
the UV reflective urine trail marks left on the ground by rodents.
With the exception of pigeons and a few other species, the eyelids
of birds are not used in blinking. Instead the eye is lubricated by
the nictitating membrane, a third eyelid that moves horizontally.
The nictitating membrane also covers the eye and acts as a contact
lens in many aquatic birds. The bird retina has a fan shaped blood
supply system called the pecten.
Most birds cannot move their eyes, although there are exceptions, such
as the great cormorant. Birds with eyes on the sides of their
heads have a wide visual field, while birds with eyes on the front of
their heads, such as owls, have binocular vision and can estimate the
depth of field. The avian ear lacks external pinnae but is covered
by feathers, although in some birds, such as the Asio, Bubo and Otus
owls, these feathers form tufts which resemble ears. The inner ear has
a cochlea, but it is not spiral as in mammals.
Defence and intraspecific combat
A few species are able to use chemical defences against predators;
Procellariiformes can eject an unpleasant stomach oil against an
aggressor, and some species of pitohuis from
New Guinea have a
powerful neurotoxin in their skin and feathers.
A lack of field observations limit our knowledge, but intraspecific
conflicts are known to sometimes result in injury or death. The
screamers (Anhimidae), some jacanas (Jacana, Hydrophasianus), the
spur-winged goose (Plectropterus), the torrent duck (Merganetta) and
nine species of lapwing (Vanellus) use a sharp spur on the wing as a
weapon. The steamer ducks (Tachyeres), geese and swans (Anserinae),
the solitaire (Pezophaps), sheathbills (Chionis), some guans (Crax)
and stone curlews (Burhinus) use a bony knob on the alular metacarpal
to punch and hammer opponents. The jacanas
Irediparra have an expanded, blade-like radius. The extinct Xenicibis
was unique in having an elongate forelimb and massive hand which
likely functioned in combat or defence as a jointed club or flail.
Swans, for instance, may strike with the bony spurs and bite when
defending eggs or young.
Birds 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
In nearly all species of birds, an individual's sex is determined at
fertilisation. However, one recent study demonstrated
temperature-dependent sex determination among the Australian
brushturkey, for which higher temperatures during incubation resulted
in a higher female-to-male sex ratio. This, however, was later
proven to not be the case. These birds do not exhibit
temperature-dependent sex determination, but temperature-dependent sex
Feathers, plumage, and scales
Feather and Flight feather
The disruptively patterned plumage of the
African scops owl
African scops owl allows it
to blend in with its surroundings.
Feathers are a feature characteristic of birds (though also present in
some dinosaurs not currently considered to be true birds). They
facilitate flight, provide insulation that aids in thermoregulation,
and are used in display, camouflage, and signalling. There are
several types of feathers, each serving its own set of purposes.
Feathers are epidermal growths attached to the skin and arise only in
specific tracts of skin called pterylae. The distribution pattern of
these feather tracts (pterylosis) is used in taxonomy and systematics.
The arrangement and appearance of feathers on the body, called
plumage, may vary within species by age, social status, and
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. 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.
A small number of species, such as ducks and geese, lose all of their
flight feathers at once, temporarily becoming flightless. As a
general rule, the tail feathers are moulted and replaced starting with
the innermost pair. Centripetal moults of tail feathers are
however seen in the Phasianidae. The centrifugal moult is
modified in the tail feathers of woodpeckers and treecreepers, in that
it begins with the second innermost pair of feathers and finishes with
the central pair of feathers so that the bird maintains a functional
climbing tail. The general pattern seen in passerines is
that the primaries are replaced outward, secondaries inward, and the
tail from centre outward. Before nesting, the females of most
bird species gain a bare brood patch by losing feathers close to the
belly. The skin there is well supplied with blood vessels and helps
the bird in incubation.
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. The
bill is used to brush away foreign particles and to apply waxy
secretions from the uropygial gland; these secretions protect the
feathers' flexibility and act as an antimicrobial agent, inhibiting
the growth of feather-degrading bacteria. This may be
supplemented with the secretions of formic acid from ants, which birds
receive through a behaviour known as anting, to remove feather
The scales of birds are composed of the same keratin as beaks, claws,
and spurs. They are found mainly on the toes and metatarsus, but may
be found further up on the ankle in some birds. Most bird scales do
not overlap significantly, except in the cases of kingfishers and
woodpeckers. The scales of birds are thought to be homologous to those
of reptiles and mammals.
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
Wing shape and size generally determine a bird's flight style and
performance; many birds combine powered, flapping flight with less
energy-intensive soaring flight. About 60 extant bird species are
flightless, as were many extinct birds. Flightlessness often
arises in birds on isolated islands, probably due to limited resources
and the absence of land predators. Though flightless, penguins
use similar musculature and movements to "fly" through the water, as
do auks, shearwaters and dippers.
Most birds are diurnal, but some birds, such as many species of owls
and nightjars, are nocturnal or crepuscular (active during twilight
hours), and many coastal waders feed when the tides are appropriate,
by day or night.
Diet and feeding
Feeding adaptations in beaks
Birds' diets are varied and often include nectar, fruit, plants,
seeds, carrion, and various small animals, including other birds.
Because birds have no teeth, their digestive system is adapted to
process unmasticated food items that are swallowed whole.
Birds that employ many strategies to obtain food or feed on a variety
of food items are called generalists, while others that concentrate
time and effort on specific food items or have a single strategy to
obtain food are considered specialists. Birds' feeding strategies
vary by species. Many birds glean for insects, invertebrates, fruit,
or seeds. Some hunt insects by suddenly attacking from a branch. Those
species that seek pest insects are considered beneficial 'biological
control agents' and their presence encouraged in biological pest
Nectar feeders such as hummingbirds, sunbirds, lories, and lorikeets
amongst others have specially adapted brushy tongues and in many cases
bills designed to fit co-adapted flowers. Kiwis and shorebirds
with long bills probe for invertebrates; shorebirds' varied bill
lengths and feeding methods result in the separation of ecological
niches. Loons, diving ducks, penguins and auks pursue their
prey underwater, using their wings or feet for propulsion, while
aerial predators such as sulids, kingfishers and terns plunge dive
after their prey. Flamingos, three species of prion, and some ducks
are filter feeders.
Geese and dabbling ducks are primarily
Some species, including frigatebirds, gulls, and skuas,
engage in kleptoparasitism, stealing food items from other birds.
Kleptoparasitism is thought to be a supplement to food obtained by
hunting, rather than a significant part of any species' diet; a study
of great frigatebirds stealing from masked boobies estimated that the
frigatebirds stole at most 40% of their food and on average stole only
5%. Other birds are scavengers; some of these, like vultures, are
specialised carrion eaters, while others, like gulls, corvids, or
other birds of prey, are opportunists.
Water and drinking
Water is needed by many birds although their mode of excretion and
lack of sweat glands reduces the physiological demands. Some
desert birds can obtain their water needs entirely from moisture in
their food. They may also have other adaptations such as allowing
their body temperature to rise, saving on moisture loss from
evaporative cooling or panting. Seabirds can drink seawater and
have salt glands inside the head that eliminate excess salt out of the
Most birds scoop water in their beaks and raise their head to let
water run down the throat. Some species, especially of arid zones,
belonging to the pigeon, finch, mousebird, button-quail and bustard
families are capable of sucking up water without the need to tilt back
their heads. Some desert birds depend on water sources and
sandgrouse are particularly well known for their daily congregations
at waterholes. Nesting sandgrouse and many plovers carry water to
their young by wetting their belly feathers. Some birds carry
water for chicks at the nest in their crop or regurgitate it along
with food. The pigeon family, flamingos and penguins have adaptations
to produce a nutritive fluid called crop milk that they provide to
Feathers being critical to the survival of a bird, require
maintenance. Apart from physical wear and tear, feathers face the
onslaught of fungi, ectoparasitic feather mites and birdlice. The
physical condition of feathers are maintained by preening often with
the application of secretions from the preen gland. Birds also bathe
in water or dust themselves. While some birds dip into shallow water,
more aerial species may make aerial dips into water and arboreal
species often make use of dew or rain that collect on leaves. Birds of
arid regions make use of loose soil to dust-bathe. A behaviour termed
as anting in which the bird encourages ants to run through their
plumage is also thought to help them reduce the ectoparasite load in
feathers. Many species will spread out their wings and expose them to
direct sunlight and this too is thought to help in reducing fungal and
ectoparasitic activity that may lead to feather damage.
A flock of
Canada geese in V formation
Many bird species migrate to take advantage of global differences of
seasonal temperatures, therefore optimising availability of food
sources and breeding habitat. These migrations vary among the
different groups. Many landbirds, shorebirds, and waterbirds undertake
annual long distance migrations, usually triggered by the length of
daylight as well as weather conditions. These birds are characterised
by a breeding season spent in the temperate or polar regions and a
non-breeding season in the tropical regions or opposite hemisphere.
Before migration, birds substantially increase body fats and reserves
and reduce the size of some of their organs.
Migration is highly demanding energetically, particularly as birds
need to cross deserts and oceans without refuelling. Landbirds have a
flight range of around 2,500 km (1,600 mi) and shorebirds
can fly up to 4,000 km (2,500 mi), although the
bar-tailed godwit is capable of non-stop flights of up to
10,200 km (6,300 mi). Seabirds also undertake long
migrations, the longest annual migration being those of sooty
shearwaters, which nest in
New Zealand and
Chile and spend the
northern summer feeding in the North Pacific off Japan,
California, an annual round trip of 64,000 km
(39,800 mi). Other seabirds disperse after breeding,
travelling widely but having no set migration route. Albatrosses
nesting in the
Southern Ocean often undertake circumpolar trips
between breeding seasons.
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. Species
may also travel shorter distances over part of their range, with
individuals from higher latitudes travelling into the existing range
of conspecifics; others undertake partial migrations, where only a
fraction of the population, usually females and subdominant males,
migrates. Partial migration can form a large percentage of the
migration behaviour of birds in some regions; in Australia, surveys
found that 44% of non-passerine birds and 32% of passerines were
Altitudinal migration is a form of short distance migration in which
birds spend the breeding season at higher altitudes and move to lower
ones during suboptimal conditions. It is most often triggered by
temperature changes and usually occurs when the normal territories
also become inhospitable due to lack of food. Some species may
also be nomadic, holding no fixed territory and moving according to
weather and food availability. Parrots as a family are overwhelmingly
neither migratory nor sedentary but considered to either be
dispersive, irruptive, nomadic or undertake small and irregular
The ability of birds to return to precise locations across vast
distances has been known for some time; in an experiment conducted in
the 1950s, a
Manx shearwater released in
Boston in the United States
returned to its colony in Skomer, in Wales within 13 days, a distance
of 5,150 km (3,200 mi). Birds navigate during migration
using a variety of methods. For diurnal migrants, the sun is used to
navigate by day, and a stellar compass is used at night. Birds that
use the sun compensate for the changing position of the sun during the
day by the use of an internal clock. Orientation with the stellar
compass depends on the position of the constellations surrounding
Polaris. These are backed up in some species by their ability to
sense the Earth's geomagnetism through specialised
Song of the house wren, a common North American songbird
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The startling display of the sunbittern mimics a large predator.
Birds communicate using primarily visual and auditory signals. Signals
can be interspecific (between species) and intraspecific (within
Birds sometimes use plumage to assess and assert social
dominance, to display breeding condition in sexually selected
species, or to make threatening displays, as in the sunbittern's
mimicry of a large predator to ward off hawks and protect young
chicks. Variation in plumage also allows for the identification
of birds, particularly between species.
Visual communication among birds may also involve ritualised displays,
which have developed from non-signalling actions such as preening, the
adjustments of feather position, pecking, or other behaviour. These
displays may signal aggression or submission or may contribute to the
formation of pair-bonds. The most elaborate displays occur during
courtship, where "dances" are often formed from complex combinations
of many possible component movements; males' breeding success may
depend on the quality of such displays.
Bird calls and songs, which are produced in the syrinx, are the major
means by which birds communicate with sound. This communication can be
very complex; some species can operate the two sides of the syrinx
independently, allowing the simultaneous production of two different
songs. Calls are used for a variety of purposes, including mate
attraction, evaluation of potential mates, bond formation,
the claiming and maintenance of territories, the identification of
other individuals (such as when parents look for chicks in colonies or
when mates reunite at the start of breeding season), and the
warning of other birds of potential predators, sometimes with specific
information about the nature of the threat. Some birds also use
mechanical sounds for auditory communication. The
New Zealand drive air through their feathers, woodpeckers drum
territorially, and palm cockatoos use tools to drum.
Flocking and other associations
Red-billed queleas, the most numerous species of bird, 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 flocks. The principal benefits of
flocking are safety in numbers and increased foraging efficiency.
Defence against predators is particularly important in closed habitats
like forests, where ambush predation is common and multiple eyes can
provide a valuable early warning system. This has led to the
development of many mixed-species feeding flocks, which are usually
composed of small numbers of many species; these flocks provide safety
in numbers but increase potential competition for resources.
Costs of flocking include bullying of socially subordinate birds by
more dominant birds and the reduction of feeding efficiency in certain
Birds sometimes also form associations with non-avian species.
Plunge-diving seabirds associate with dolphins and tuna, which push
shoaling fish towards the surface. Hornbills have a mutualistic
relationship with dwarf mongooses, in which they forage together and
warn each other of nearby birds of prey and other predators.
Resting and roosting
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.
Swifts are believed to be able to sleep in flight and radar
observations suggest that they orient themselves to face the wind in
their roosting flight. It has been suggested that there may be
certain kinds of sleep which are possible even when in flight.
Some birds have also demonstrated the capacity to fall into slow-wave
sleep one hemisphere of the brain at a time. The birds tend to
exercise this ability depending upon its position relative to the
outside of the flock. This may allow the eye opposite the sleeping
hemisphere to remain vigilant for predators by viewing the outer
margins of the flock. This adaptation is also known from marine
Communal roosting is common because it lowers the loss
of body heat and decreases the risks associated with predators.
Roosting sites are often chosen with regard to thermoregulation and
Many sleeping birds bend their heads over their backs and tuck their
bills in their back feathers, although others place their beaks among
their breast feathers. Many birds rest on one leg, while some may pull
up their legs into their feathers, especially in cold weather.
Perching birds have a tendon locking mechanism that helps them hold on
to the perch when they are asleep. Many ground birds, such as quails
and pheasants, roost in trees. A few parrots of the genus Loriculus
roost hanging upside down. Some hummingbirds go into a nightly
state of torpor accompanied with a reduction of their metabolic
rates. This physiological adaptation shows in nearly a hundred
other species, including owlet-nightjars, nightjars, and woodswallows.
One species, the common poorwill, even enters a state of
hibernation. Birds do not have sweat glands, but they may cool
themselves by moving to shade, standing in water, panting, increasing
their surface area, fluttering their throat or by using special
behaviours like urohidrosis to cool themselves.
See also: Category:Avian sexuality,
Animal sexual behaviour
Seabird breeding behaviour, and
Sexual selection in
Like others of its family the male
Raggiana bird-of-paradise has
elaborate breeding plumage used to impress females.
Ninety-five per cent of bird species are socially monogamous. These
species pair for at least the length of the breeding season or—in
some cases—for several years or until the death of one mate.
Monogamy allows for both paternal care and biparental care, which is
especially important for species in which females require males'
assistance for successful brood-rearing. Among many socially
monogamous species, extra-pair copulation (infidelity) is common.
Such behaviour typically occurs between dominant males and females
paired with subordinate males, but may also be the result of forced
copulation in ducks and other anatids.
Female birds have sperm storage mechanisms that allow sperm from males
to remain viable long after copulation, a hundred days in some
species. Sperm from multiple males may compete through this
mechanism. For females, possible benefits of extra-pair copulation
include getting better genes for her offspring and insuring against
the possibility of infertility in her mate. Males of species that
engage in extra-pair copulations will closely guard their mates to
ensure the parentage of the offspring that they raise.
Other mating systems, including polygyny, polyandry, polygamy,
polygynandry, and promiscuity, also occur. Polygamous breeding
systems arise when females are able to raise broods without the help
of males. Some species may use more than one system depending on
Breeding usually involves some form of courtship display, typically
performed by the male. Most displays are rather simple and
involve some type of song. Some displays, however, are quite
elaborate. Depending on the species, these may include wing or tail
drumming, dancing, aerial flights, or communal lekking. Females are
generally the ones that drive partner selection, although in the
polyandrous phalaropes, this is reversed: plainer males choose
brightly coloured females. Courtship feeding, billing and
allopreening are commonly performed between partners, generally after
the birds have paired and mated.
Homosexual behaviour has been observed in males or females in numerous
species of birds, including copulation, pair-bonding, and joint
parenting of chicks.
Territories, nesting and incubation
Many birds actively defend a territory from others of the same species
during the breeding season; maintenance of territories protects the
food source for their chicks. Species that are unable to defend
feeding territories, such as seabirds and swifts, often breed in
colonies instead; this is thought to offer protection from predators.
Colonial breeders defend small nesting sites, and competition between
and within species for nesting sites can be intense.
All birds lay amniotic eggs with hard shells made mostly of calcium
carbonate. Hole and burrow nesting species tend to lay white or
pale eggs, while open nesters lay camouflaged eggs. There are many
exceptions to this pattern, however; the ground-nesting nightjars have
pale eggs, and camouflage is instead provided by their plumage.
Species that are victims of brood parasites have varying egg colours
to improve the chances of spotting a parasite's egg, which forces
female parasites to match their eggs to those of their hosts.
Male golden-backed weavers construct elaborate suspended nests out of
Bird eggs are usually laid in a nest. Most species create somewhat
elaborate nests, which can be cups, domes, plates, beds scrapes,
mounds, or burrows. Some bird nests, however, are extremely
primitive; albatross nests are no more than a scrape on the ground.
Most birds build nests in sheltered, hidden areas to avoid predation,
but large or colonial birds—which are more capable of defence—may
build more open nests. During nest construction, some species seek out
plant matter from plants with parasite-reducing toxins to improve
chick survival, and feathers are often used for nest
insulation. Some bird species have no nests; the cliff-nesting
common guillemot lays its eggs on bare rock, and male emperor penguins
keep eggs between their body and feet. The absence of nests is
especially prevalent in ground-nesting species where the newly hatched
young are precocial.
Nest of an eastern phoebe that has been parasitised by a brown-headed
Incubation, which optimises temperature for chick development, usually
begins after the last egg has been laid. In monogamous species
incubation duties are often shared, whereas in polygamous species one
parent is wholly responsible for incubation. Warmth from parents
passes to the eggs through brood patches, areas of bare skin on the
abdomen or breast of the incubating birds. Incubation can be an
energetically demanding process; adult albatrosses, for instance, lose
as much as 83 grams (2.9 oz) of body weight per day of
incubation. The warmth for the incubation of the eggs of
megapodes comes from the sun, decaying vegetation or volcanic
sources. Incubation periods range from 10 days (in woodpeckers,
cuckoos and passerine birds) to over 80 days (in albatrosses and
The diversity of characteristics of birds is great, sometimes even in
closely related species. Several avian characteristics are compared in
the table below.
Ruby-throated hummingbird (Archilochus colubris)
House sparrow (Passer domesticus)
Greater roadrunner (Geococcyx californianus)
Turkey vulture (Cathartes aura)
Laysan albatross (Diomedea immutabilis)
Magellanic penguin (Spheniscus magellanicus)
Golden eagle (Aquila chrysaetos)
Wild turkey (Meleagris gallopavo)
Parental care and fledging
Main article: Parental care in birds
At the time of their hatching, chicks range in development from
helpless to independent, depending on their species. Helpless chicks
are termed altricial, and tend to be born small, blind, immobile and
naked; chicks that are mobile and feathered upon hatching are termed
Altricial chicks need help thermoregulating and must be
brooded for longer than precocial chicks. The young of many bird
species do not precisely fit into either the precocial or altricial
category, having some aspects of each and thus fall somewhere on an
"altricial-precocial spectrum". Chicks at neither extreme but
favoring one or the other may be termed semi-precocial or
Calliope hummingbird feeding fully grown chicks.
Altricial chicks of a white-breasted woodswallow.
The length and nature of parental care varies widely amongst different
orders and species. At one extreme, parental care in megapodes ends at
hatching; the newly hatched chick digs itself out of the nest mound
without parental assistance and can fend for itself immediately.
At the other extreme, many seabirds have extended periods of parental
care, the longest being that of the great frigatebird, whose chicks
take up to six months to fledge and are fed by the parents for up to
an additional 14 months. The chick guard stage describes the
period of breeding during which one of the adult birds is permanently
present at the nest after chicks have hatched. The main purpose of the
guard stage is to aid offspring to thermoregulate and protect them
In some species, both parents care for nestlings and fledglings; in
others, such care is the responsibility of only one sex. In some
species, other members of the same species—usually close relatives
of the breeding pair, such as offspring from previous broods—will
help with the raising of the young. Such alloparenting is
particularly common among the Corvida, which includes such birds as
the true crows,
Australian magpie and fairy-wrens, but has been
observed in species as different as the rifleman and red kite. Among
most groups of animals, male parental care is rare. In birds, however,
it is quite common—more so than in any other vertebrate class.
Though territory and nest site defence, incubation, and chick feeding
are often shared tasks, there is sometimes a division of labour in
which one mate undertakes all or most of a particular duty.
The point at which chicks fledge varies dramatically. The chicks of
Synthliboramphus murrelets, like the ancient murrelet, leave the
nest the night after they hatch, following their parents out to sea,
where they are raised away from terrestrial predators. Some other
species, such as ducks, move their chicks away from the nest at an
early age. In most species, chicks leave the nest just before, or soon
after, they are able to fly. The amount of parental care after
fledging varies; albatross chicks leave the nest on their own and
receive no further help, while other species continue some
supplementary feeding after fledging. Chicks may also follow
their parents during their first migration.
Main article: Brood parasite
Reed warbler raising a common cuckoo, a brood parasite.
Brood parasitism, in which an egg-layer leaves her eggs with another
individual's brood, is more common among birds than any other type of
organism. After a parasitic bird lays her eggs in another bird's
nest, they are often accepted and raised by the host at the expense of
the host's own brood.
Brood parasites may be either obligate brood
parasites, which must lay their eggs in the nests of other species
because they are incapable of raising their own young, or non-obligate
brood parasites, which sometimes lay eggs in the nests of conspecifics
to increase their reproductive output even though they could have
raised their own young. One hundred bird species, including
honeyguides, icterids, and ducks, are obligate parasites, though the
most famous are the cuckoos. Some brood parasites are adapted to
hatch before their host's young, which allows them to destroy the
host's eggs by pushing them out of the nest or to kill the host's
chicks; this ensures that all food brought to the nest will be fed to
the parasitic chicks.
The peacock tail in flight, the classic example of a Fisherian runaway
Sexual selection in birds
Birds have evolved a variety of mating behaviours, with the peacock
tail being perhaps the most famous example of sexual selection and the
Fisherian runaway. Commonly occurring sexual dimorphisms such as size
and colour differences are energetically costly attributes that signal
competitive breeding situations. Many types of avian sexual
selection have been identified; intersexual selection, also known as
female choice; and intrasexual competition, where individuals of the
more abundant sex compete with each other for the privilege to mate.
Sexually selected traits often evolve to become more pronounced in
competitive breeding situations until the trait begins to limit the
individual’s fitness. Conflicts between an individual fitness and
signalling adaptations ensure that sexually selected ornaments such as
plumage coloration and courtship behaviour are "honest" traits.
Signals must be costly to ensure that only good-quality individuals
can present these exaggerated sexual ornaments and behaviours.
Main article: Inbreeding depression
Inbreeding causes early death (inbreeding depression) in the zebra
finch Taeniopygia guttata. Embryo survival (that is, hatching
success of fertile eggs) was significantly lower for sib-sib mating
pairs than for unrelated pairs.
Darwin’s finch Geospiza scandens experiences inbreeding depression
(reduced survival of offspring) and the magnitude of this effect is
influenced by environmental conditions such as low food
Main article: 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). 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
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
Southern pied babblers Turdoides bicolor appear to avoid inbreeding in
two ways. The first is through dispersal, and the second is by
avoiding familiar group members as mates. Although both males and
females disperse locally, they move outside the range where
genetically related individuals are likely to be encountered. Within
their group, individuals only acquire breeding positions when the
opposite-sex breeder is unrelated.
Cooperative breeding in birds typically occurs when offspring, usually
males, delay dispersal from their natal group in order to remain with
the family to help rear younger kin.
Female offspring rarely stay
at home, dispersing over distances that allow them to breed
independently, or to join unrelated groups. In general, inbreeding is
avoided because it leads to a reduction in progeny fitness (inbreeding
depression) due largely to the homozygous expression of deleterious
recessive alleles. Cross-fertilisation between unrelated
individuals ordinarily leads to the masking of deleterious recessive
alleles in progeny.
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. While some
birds are generalists, others are highly specialised in their habitat
or food requirements. Even within a single habitat, such as a forest,
the niches occupied by different species of birds vary, with some
species feeding in the forest canopy, others beneath the canopy, and
still others on the forest floor. Forest birds may be insectivores,
frugivores, and nectarivores. Aquatic birds generally feed by fishing,
plant eating, and piracy or kleptoparasitism.
Birds of prey
Birds of prey specialise
in hunting mammals or other birds, while vultures are specialised
scavengers. Avivores are animals that are specialised at preying on
Some nectar-feeding birds are important pollinators, and many
frugivores play a key role in seed dispersal. Plants and
pollinating birds often coevolve, and in some cases a flower's
primary pollinator is the only species capable of reaching its
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
New Zealand the moas were important browsers, as are the
kereru and kokako today. Today the plants of
New Zealand retain
the defensive adaptations evolved to protect them from the extinct
moa. Nesting seabirds may also affect the ecology of islands and
surrounding seas, principally through the concentration of large
quantities of guano, which may enrich the local soil and the
A wide variety of avian ecology field methods, including counts, nest
monitoring, and capturing and marking, are used for researching avian
Relationship with humans
Main article: Birds in culture
Industrial farming of chickens
Since birds are highly visible and common animals, humans have had a
relationship with them since the dawn of man. Sometimes, these
relationships are mutualistic, like the cooperative honey-gathering
among honeyguides and African peoples such as the Borana. Other
times, they may be commensal, as when species such as the house
sparrow have benefited from human activities. Several bird
species have become commercially significant agricultural pests,
and some pose an aviation hazard. Human activities can also be
detrimental, and have threatened numerous bird species with extinction
(hunting, avian lead poisoning, pesticides, roadkill, wind turbine
kills and predation by pet cats and dogs are common sources of
death for birds).
Birds can act as vectors for spreading diseases such as psittacosis,
salmonellosis, campylobacteriosis, mycobacteriosis (avian
tuberculosis), avian influenza (bird flu), giardiasis, and
cryptosporidiosis over long distances. Some of these are zoonotic
diseases that can also be transmitted to humans.
See also: Pet § Birds
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 poultry, are the
largest source of animal protein eaten by humans; in 2003, 76 million
tons of poultry and 61 million tons of eggs were produced
worldwide. Chickens account for much of human poultry
consumption, though domesticated turkeys, ducks, and geese are also
relatively common. Many species of birds are also hunted for meat.
Bird hunting is primarily a recreational activity except in extremely
undeveloped areas. The most important birds hunted in North and South
America are waterfowl; other widely hunted birds include pheasants,
wild turkeys, quail, doves, partridge, grouse, snipe, and
Muttonbirding is also popular in Australia and New
Zealand. Though some hunting, such as that of muttonbirds, may be
sustainable, hunting has led to the extinction or endangerment of
dozens of species.
Other commercially valuable products from birds include feathers
(especially the down of geese and ducks), which are used as insulation
in clothing and bedding, and seabird faeces (guano), which is a
valuable source of phosphorus and nitrogen. The War of the Pacific,
sometimes called the
Guano War, was fought in part over the control of
Birds have been domesticated by humans both as pets and for practical
purposes. Colourful birds, such as parrots and mynas, are bred in
captivity or kept as pets, a practice that has led to the illegal
trafficking of some endangered species. Falcons and cormorants
have long been used for hunting and fishing, respectively. Messenger
pigeons, used since at least 1 AD, remained important as recently as
World War II. Today, such activities are more common either as
hobbies, for entertainment and tourism, or for sports such as
Amateur bird enthusiasts (called birdwatchers, twitchers or, more
commonly, birders) number in the millions. Many homeowners erect
bird feeders near their homes to attract various species.
has grown into a multimillion-dollar industry; for example, an
estimated 75% of households in Britain provide food for birds at some
point during the winter.
In religion and mythology
"The 3 of Birds" by the Master of the Playing Cards, 15th-century
Birds play prominent and diverse roles in religion and mythology. In
religion, birds may serve as either messengers or priests and leaders
for a deity, such as in the Cult of Makemake, in which the Tangata
Easter Island served as chiefs or as attendants, as in
the case of Hugin and Munin, the two common ravens who whispered news
into the ears of the
Norse god Odin. In several civilisations of
ancient Italy, particularly Etruscan and Roman religion, priests were
involved in augury, or interpreting the words of birds while the
"auspex" (from which the word "auspicious" is derived) watched their
activities to foretell events.
They may also serve as religious symbols, as when
יוֹנָה, dove) embodied the fright, passivity, mourning, and
beauty traditionally associated with doves. Birds have themselves
been deified, as in the case of the common peacock, which is perceived
as Mother Earth by the Dravidians of India. In religious images
preserved from the Inca and Tiwanaku empires, birds are depicted in
the process of transgressing boundaries between earthly and
underground spiritual realms. Indigenous peoples of the central
Andes maintain legends of birds passing to and from metaphysical
In culture and folklore
Painted tiles with design of birds from Qajar dynasty
Birds have featured in culture and art since prehistoric times, when
they were represented in early cave paintings. Some birds have
been perceived as monsters, including the mythological Roc and the
Māori's legendary Pouākai, a giant bird capable of snatching
humans. Birds were later used as symbols of power, as in the
Peacock Throne of the Mughal and Persian emperors.
With the advent of scientific interest in birds, many paintings of
birds were commissioned for books.
Among the most famous of these bird artists was John James Audubon,
whose paintings of North American birds were a great commercial
success in Europe and who later lent his name to the National Audubon
Society. Birds are also important figures in poetry; for example,
Homer incorporated nightingales into his Odyssey, and
Catullus used a
sparrow as an erotic symbol in his
Catullus 2. The relationship
between an albatross and a sailor is the central theme of Samuel
Taylor Coleridge's The Rime of the Ancient Mariner, which led to the
use of the term as a metaphor for a 'burden'. Other English
metaphors derive from birds; vulture funds and vulture investors, for
instance, take their name from the scavenging vulture.
Perceptions of bird species vary across cultures. Owls are associated
with bad luck, witchcraft, and death in parts of Africa, but are
regarded as wise across much of Europe. Hoopoes were considered
Ancient Egypt and symbols of virtue in Persia, but were
thought of as thieves across much of Europe and harbingers of war in
Scandinavia. In heraldry, birds, especially eagles, often appear
in coats of arms.
Main article: Birds in music
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
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
California condor once numbered only 22 birds, but conservation
measures have raised that to over 300 today.
See also: Late Quaternary prehistoric birds, List of extinct birds,
and Raptor conservation
Though human activities have allowed the expansion of a few species,
such as the barn swallow and European starling, they have caused
population decreases or extinction in many other species. Over a
hundred bird species have gone extinct in historical times,
although the most dramatic human-caused avian extinctions, eradicating
an estimated 750–1800 species, occurred during the human
colonisation of Melanesian, Polynesian, and Micronesian islands.
Many bird populations are declining worldwide, with 1,227 species
listed as threatened by
BirdLife International and the
The most commonly cited human threat to birds is habitat loss.
Other threats include overhunting, accidental mortality due to
collisions with buildings or vehicles, long-line fishing bycatch,
pollution (including oil spills and pesticide use), competition
and predation from nonnative invasive species, and climate
Governments and conservation groups work to protect birds, either by
passing laws that preserve and restore bird habitat or by establishing
captive populations for reintroductions. Such projects have produced
some successes; one study estimated that conservation efforts saved 16
species of bird that would otherwise have gone extinct between 1994
and 2004, including the
California condor and Norfolk parakeet.
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Extant chordate classes
Ascidiacea (sea squirts)
Thaliacea (pyrosomes, salps, doliolids)
(Vertebrates + Myxini)
(fish + Tetrapods)
Agnatha (jawless fish)
Chondrichthyes (cartilaginous fish: sharks, rays, chimaeras)
Actinopterygii (ray-finned fish)
Squamata (scaled reptiles)²
¹subclasses of Sarcopterygii
²orders of class Reptilia (reptiles)
³traditionally placed in Anapsida
italic are paraphyletic groups
Birds (class: Aves)
Origin of birds
Origin of flight
Evolution of birds
Families and orders
Glossary of bird terms
List by population
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Recently extinct birds
Late Quaternary prehistoric birds
Casuariiformes (emus and cassowaries)
Phasianinae (pheasants and relatives)
Columbiformes (doves and pigeons)
Caprimulgiformes (nightjars and relatives)
Apodiformes (swifts and hummingbirds)
Charadriiformes (gulls and relatives)
Gruiformes (cranes and relatives)
Eurypygiformes (kagu and sunbittern)
Gaviiformes (loons or divers)
Procellariiformes (albatrosses and petrels)
Suliformes (cormorants and relatives)
Pelecaniformes (pelicans and relatives)
Cariamiformes (seriemas and relatives)
Falconiformes (falcons and relatives)
Passeriformes (perching birds)
Cathartiformes (New World vultures and condors)
Accipitriformes (eagles and hawks)
Trogoniformes (trogons and quetzals)
Leptosomatiformes (cuckoo roller)
Bucerotiformes (hornbills and hoopoes)
Coraciiformes (kingfishers and rollers)
Piciformes (woodpeckers and relatives)
Birds in culture
In mythology and religion
Driven grouse shooting
In the arts
The Conference of the Birds
Ode to a Nightingale
To a Skylark
A History of British Birds
The Tale of Jemima Puddle-Duck
The Ugly Duckling
In theatre and ballet
of the Tower of London
John James Audubon
John James Audubon (The Birds of America)
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Roger Tory Peterson
Henry Constantine Richter
Royal Society for the Protection of Birds
Wildfowl & Wetlands Trust
Category:Birds and humans
Dinosaurs in culture
Living things in culture
Fish in culture
Insects in culture
Mammals in culture
Reptiles in culture
Fauna Europaea: 10699
BNF: cb11932889r (d