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 class of tetrapods with the most living species, at
approximately ten thousand, with more than half of these being
passerines , sometimes known as perching birds. Birds are the closest
living relatives of crocodilians . Birds are descendants of extinct
dinosaurs with feathers , making them the only surviving dinosaurs
according to cladistics .
The fossil record indicates that birds evolved from feathered
ancestors within the theropod group, which are traditionally placed
within the saurischian dinosaurs , though a 2017 paper has put them
in a proposed clade
Ornithoscelida , along with the
True birds first appeared during the
Cretaceous period, around 120
million years ago. DNA-based evidence finds that birds diversified
dramatically around the time of the Cretaceous–Palaeogene extinction
event 66 million years ago, which reduced the Pterosaurs , and killed
off all the non-avian dinosaur lineages. 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. Primitive bird-like dinosaurs that lie outside class Aves
proper, in the broader group
Avialae , have been found dating back to
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.
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 .
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
* 1.1 Definition
* 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
* 2 Distribution
Anatomy and physiology
* 3.1 Skeletal system
* 3.2 Excretory system
* 3.3 Respiratory and circulatory systems
Heart type and features
* 3.3.2 Organisation
* 3.5 Defence and intraspecific combat
* 3.6 Chromosomes
* 3.7 Feathers, plumage, and scales
* 3.8 Flight
* 4 Behaviour
* 4.1 Diet and feeding
* 4.2 Water and drinking
* 4.4 Migration
* 4.5 Communication
* 4.6 Flocking and other associations
* 4.7 Resting and roosting
* 4.8 Breeding
* 4.8.1 Social systems
* 4.8.2 Territories, nesting and incubation
* 4.8.3 Parental care and fledging
* 5 Ecology
* 6 Relationship with humans
* 6.1 Economic importance
* 6.2 In religion and mythology
* 6.3 In culture and folklore
* 6.4 In music
* 6.5 Conservation
* 7 See also
* 8 Notes
* 9 External links
EVOLUTION AND CLASSIFICATION
Evolution of birds _
Archaeopteryx lithographica _
is often considered the oldest known true bird.
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
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 _Archaeopteryx_ in
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
* Aves can mean those advanced archosaurs with feathers (alternately
* Aves can mean those that fly (alternately Avialae)
* Aves can mean all reptiles closer to birds than to crocodiles
* 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 _
Avialae is also
occasionally defined as an apomorphy-based clade (that is, one based
on physical characteristics).
Jacques Gauthier , who named
1986, re-defined it in 2001 as all dinosaurs that possessed feathered
wings used in flapping flight , and the birds that descended from
DINOSAURS AND THE ORIGIN OF BIRDS
Origin of birds
Origin of birds _
Anchiornis huxleyi _, which
means "Huxley's nearby bird", 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
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 both.
Unlike _Archaeopteryx_ and the non-avialan feathered dinosaurs, who
primarily ate meat, recent studies suggest that the first avialans
were omnivores .
Jurassic _Archaeopteryx_ is well known as one of the first
transitional fossils to be found, and it provided support for the
theory of evolution in the late 19th century. _Archaeopteryx_ was the
first fossil to display both clearly traditional reptilian
characteristics: teeth, clawed fingers, and a long, lizard-like tail,
as well as wings with flight feathers similar to those of modern
birds. It is not considered a direct ancestor of birds, though it is
possibly closely related to the true ancestor.
List of fossil bird genera _
Confuciusornis sanctus _,
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
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 _
Anchiornis huxleyi _, _
Xiaotingia zhengi _, and
Aurornis xui _.
The well-known early avialan, _Archaeopteryx_, dates from slightly
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 _Archaeopteryx_ and
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,
around 95 million years ago, the ancestor of all modern birds also
evolved a better sense of smell.
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
DIVERSIFICATION OF MODERN BIRDS
Sibley–Ahlquist taxonomy of birds and dinosaur
Other birds (
Basal divergences of modern birds
All modern birds lie within the crown group Aves (alternately
Neornithes), which has two subdivisions: the
Palaeognathae , which
includes the flightless ratites (such as the ostriches ) and the
weak-flying tinamous , and the extremely diverse
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
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).
Galliformes (chickens and relatives)
Anseriformes (ducks and relatives)
Strisores (hummingbirds and relatives)
Gruiformes (rails and cranes)
Charadriiformes (waders and relatives)
Eurypygiformes (sunbittern and kagu)
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 by population
The range of the house sparrow has expanded dramatically due to
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
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) deep.
Many bird species have established breeding populations in areas to
which they have been introduced by humans. Some of these introductions
have been deliberate; the ring-necked pheasant , for example, has been
introduced around the world as a game bird . Others have been
accidental, such as the establishment of wild monk parakeets in
several North American cities after their escape from captivity. Some
species, including cattle egret , yellow-headed caracara and galah
, have spread naturally far beyond their original ranges as
agricultural practices created suitable new habitat.
ANATOMY AND PHYSIOLOGY
Bird anatomy and
Bird vision See also:
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
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 pellets .
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 migration.
RESPIRATORY AND CIRCULATORY SYSTEMS
Birds have one of the most complex respiratory systems of all animal
groups. Upon inhalation, 75% of the fresh air bypasses the lungs and
flows directly into a posterior air sac which extends from the lungs
and connects with air spaces in the bones and fills them with air. The
other 25% of the air goes directly into the lungs. When the bird
exhales, the used air flows out of the lungs and the stored fresh air
from the posterior air sac is simultaneously forced into the lungs.
Thus, a bird's lungs receive a constant supply of fresh air during
both inhalation and exhalation.
Sound production is achieved using
the syrinx , a muscular chamber incorporating multiple tympanic
membranes which diverges from the lower end of the trachea; the
trachea being elongated in some species, increasing the volume of
vocalisations and the perception of the bird's size.
In birds, the main arteries taking blood away from the heart
originate from the right aortic arch (or pharyngeal arch), unlike in
the mammals where the left aortic arch forms this part of the aorta .
The postcava receives blood from the limbs via the renal portal
system. Unlike in mammals, the circulating red blood cells in birds
retain their nucleus .
Heart Type And Features
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. This allows them to perceive ultraviolet
light, which is involved in courtship. Birds have specialised
light-sensing cells deep in their brains that respond to light without
input from eyes or other sensory neurons. These photo-receptive cells
in the hypothalamus are involved in detecting the longer days of
spring, and thus regulate breeding activities.
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
Anhimidae ), some jacanas (_
Jacana _, _
the spur-winged goose (_
Plectropterus _), the torrent duck
Merganetta _) and nine species of lapwing (_
Vanellus _) use a sharp
spur on the wing as a weapon. The steamer ducks (_
Tachyeres _), geese
and swans (_
Anserinae _), the solitaire (_
Pezophaps _), sheathbills
Chionis _), some guans (_
Crax _) and stone curlews (_
Burhinus _) use
a bony knob on the alular metacarpal to punch and hammer opponents.
The jacanas _
Actophilornis _ and _
Irediparra _ have an expanded,
blade-like radius. The extinct _
Xenicibis _ was unique in having an
elongate forelimb and massive hand which likely functioned in combat
or defence as a jointed club or flail. Swans , for instance, may
strike with the bony spurs and bite when defending eggs or young.
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 (WZ).
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
Flight feather The 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 sex .
Plumage is regularly moulted ; the standard plumage of a bird that
has moulted after breeding is known as the "non-breeding" plumage,
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 parasites.
The scales of birds are composed of the same keratin as beaks, claws,
and spurs. They are found mainly on the toes and metatarsus , but may
be found further up on the ankle in some birds. Most bird scales do
not overlap significantly, except in the cases of kingfishers and
woodpeckers . The scales of birds are thought to be homologous to
those of reptiles and mammals.
Restless flycatcher in the downstroke
of flapping flight
Most birds can fly , which distinguishes them from almost all other
vertebrate classes. Flight is the primary means of locomotion for most
bird species and is used for searching for food and for escaping from
predators. Birds have various adaptations for flight, including a
lightweight skeleton, two large flight muscles, the pectoralis (which
accounts for 15% of the total mass of the bird) and the
supracoracoideus, as well as a modified forelimb (wing ) that serves
as an aerofoil .
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 nostrils.
Most birds scoop water in their beaks and raise their head to let
water run down the throat. Some species, especially of arid zones,
belonging to the pigeon , finch , mousebird , button-quail and bustard
families are capable of sucking up water without the need to tilt back
their heads. Some desert birds depend on water sources and sandgrouse
are particularly well known for their daily congregations at
waterholes. Nesting sandgrouse and many plovers carry water to their
young by wetting their belly feathers. Some birds carry water for
chicks at the nest in their crop or regurgitate it along with food.
The pigeon family, flamingos and penguins have adaptations to produce
a nutritive fluid called crop milk that they provide to their chicks.
Feathers being critical to the survival of a bird, require
maintenance. Apart from physical wear and tear, feathers face the
onslaught of fungi, ectoparasitic feather mites and 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.
Bird migration A flock of
Canada geese in V
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 migrations.
The ability of birds to return to precise locations across vast
distances has been known for some time; in an experiment conducted in
the 1950s, a
Manx shearwater released in
Boston in the United States
returned to its colony in
Skomer , in Wales within 13 days, a distance
of 5,150 km (3,200 mi). Birds navigate during migration using a
variety of methods. For diurnal migrants, the sun is used to navigate
by day, and a stellar compass is used at night. Birds that use the sun
compensate for the changing position of the sun during the day by the
use of an internal clock . Orientation with the stellar compass
depends on the position of the constellations surrounding
These are backed up in some species by their ability to sense the
Earth's geomagnetism through specialised photoreceptors .
Bird song Song of the house wren , a common North American
Problems playing this file? See media help ._
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
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
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 safety.
Many sleeping birds bend their heads over their backs and tuck their
bills in their back feathers, although others place their beaks among
their breast feathers. Many birds rest on one leg, while some may pull
up their legs into their feathers, especially in cold weather.
Perching birds have a tendon locking mechanism that helps them hold on
to the perch when they are asleep. Many ground birds, such as quails
and pheasants, roost in trees. A few parrots of the genus _
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 § Birds
Seabird breeding behaviour , and
Sexual selection in birds
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 the
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
Courtship feeding , billing and allopreening are commonly
performed between partners, generally after the birds have paired and
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
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 cowbird .
Incubation, which optimises temperature for chick development,
usually begins after the last egg has been laid. In monogamous
species incubation duties are often shared, whereas in polygamous
species one parent is wholly responsible for incubation. Warmth from
parents passes to the eggs through brood patches , areas of bare skin
on the abdomen or breast of the incubating birds. Incubation can be an
energetically demanding process; adult albatrosses, for instance, lose
as much as 83 grams (2.9 oz) of body weight per day of incubation.
The warmth for the incubation of the eggs of megapodes comes from the
sun, decaying vegetation or volcanic sources. Incubation periods
range from 10 days (in woodpeckers , cuckoos and passerine birds) to
over 80 days (in albatrosses and kiwis ).
The diversity of characteristics of birds is great, sometimes even in
closely related species. Several avian characteristics are compared in
the table below.
(per year) CLUTCH SIZE
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_)
COLUMNS ARE SORTABLE
Parental Care And Fledging
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 _precocial _.
Altricial chicks need help thermoregulating and
must be brooded for longer than precocial chicks. Chicks at neither of
these extremes can be semi-precocial or semi-altricial. A female
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 from predation.
In some species, both parents care for nestlings and fledglings; in
others, such care is the responsibility of only one sex. In some
species, other members of the same species—usually close relatives
of the breeding pair , such as offspring from previous broods—will
help with the raising of the young. Such alloparenting is
particularly common among the
Corvida , which includes such birds as
the true crows ,
Australian magpie and fairy-wrens , but has been
observed in species as different as the rifleman and red kite . Among
most groups of animals, male parental care is rare. In birds, however,
it is quite common—more so than in any other vertebrate class.
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 .
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.
Great spotted cuckoo
* _Pungtungia herzi _
* _Bombus (Psithyrus) _
Ammophila sabulosa _
Phengaris alcon _
* _Phengaris arion _
Phengaris rebeli _
The peacock tail in flight, the classic example of a Fisherian
runaway Main article:
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.
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
Incestuous matings by the purple-crowned fairy wren _Malurus
coronatus_ result in severe fitness costs due to inbreeding depression
(greater than 30% reduction in hatchability of eggs). Females paired
with related males may undertake extra pair matings (see
Promiscuity#Other animals for 90% frequency in avian species) that can
reduce the negative effects of inbreeding. However, there are
ecological and demographic constraints on extra pair matings.
Nevertheless, 43% of broods produced by incestuously paired females
contained extra pair young.
Inbreeding depression occurs in the great tit (_Parus major_) when
the offspring produced as a result of a mating between close relatives
show reduced fitness. In natural populations of _Parus major_,
inbreeding is avoided by dispersal of individuals from their
birthplace, which reduces the chance of mating with a close relative.
Southern pied babblers _Turdoides bicolor_ appear to avoid inbreeding
in two ways. The first is through dispersal, and the second is by
avoiding familiar group members as mates. Although both males and
females disperse locally, they move outside the range where
genetically related individuals are likely to be encountered. Within
their group, individuals only acquire breeding positions when the
opposite-sex breeder is unrelated.
Cooperative breeding in birds typically occurs when offspring,
usually males, delay dispersal from their natal group in order to
remain with the family to help rear younger kin.
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
specialise in hunting mammals or other birds, while vultures are
specialised scavengers . Avivores are animals that are specialised at
preying on birds.
Some nectar-feeding birds are important pollinators, and many
frugivores play a key role in seed dispersal. Plants and pollinating
birds often coevolve , and in some cases a flower's primary
pollinator is the only species capable of reaching its nectar.
Birds are often important to island ecology. Birds have frequently
reached islands that mammals have not; on those islands, birds may
fulfil ecological roles typically played by larger animals. For
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
RELATIONSHIP WITH HUMANS
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
Domesticated birds raised for meat and eggs, called poultry , are the
largest source of animal protein eaten by humans; in 2003, 76 million
tons of poultry and 61 million tons of eggs were produced worldwide.
Chickens account for much of human poultry consumption, though
domesticated turkeys , ducks , and geese are also relatively common.
Many species of birds are also hunted for meat.
Bird hunting is
primarily a recreational activity except in extremely undeveloped
areas. The most important birds hunted in North and South America are
waterfowl; other widely hunted birds include pheasants , wild turkeys
, quail, doves , partridge , grouse , snipe , and woodcock .
Muttonbirding is also popular in Australia and New Zealand. 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
World War II . Today, such activities are more common either as
hobbies, for entertainment and tourism, or for sports such as pigeon
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 ,
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
Hugin and Munin , the two common ravens who whispered news
into the ears of the
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
Birds have featured in culture and art since prehistoric times, when
they were represented in early cave paintings . Some birds have been
perceived as monsters, including the mythological Roc and the Māori
's legendary _Pouākai _, a giant bird capable of snatching humans.
Birds were later used as symbols of power, as in the magnificent
Peacock Throne of the Mughal and Persian emperors. With the advent of
scientific interest in birds, many paintings of birds were
commissioned for books.
Among the most famous of these bird artists was
John James Audubon ,
whose paintings of North American birds were a great commercial
success in Europe and who later lent his name to the National Audubon
Society . Birds are also important figures in poetry; for example,
Homer incorporated nightingales into his _
Odyssey _, and
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 .
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 birds.
California condor once numbered only 22 birds, but
conservation measures have raised that to over 300 today. Main
Bird conservation See also: Late Quaternary prehistoric
List of extinct birds , and
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
BirdLife International and the
IUCN in 2009.
The most commonly cited human threat to birds is habitat loss .
Other threats include overhunting, accidental mortality due to
collisions with buildings or vehicles , long-line fishing bycatch ,
pollution (including oil spills and pesticide use), competition and
predation from nonnative invasive species , and climate change.
Governments and conservation groups work to protect birds, either by
passing laws that preserve and restore bird habitat or by establishing
captive populations for reintroductions. Such projects have produced
some successes; one study estimated that conservation efforts saved 16
species of bird that would otherwise have gone extinct between 1994
and 2004, including the
California condor and
Norfolk parakeet .
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