Birds (
class Aves) are
feathered,
winged,
bipedal,
endothermic (
warm-blooded),
egg-laying,
vertebrate animals. With around 10,000 living species, they are the most
speciose class of
tetrapod vertebrates. All present species belong to the subclass
Neornithes, and inhabit ecosystems across the globe, from the Arctic to the Antarctic.
Extant birds range in size from the 5 cm (2 in)
Bee Hummingbird to the 2.75 m (9 ft)
Ostrich. The
fossil record indicates that birds emerged within
theropod dinosaurs during the
Jurassic period, around 160 million years (Ma) ago.
Paleontologists regard birds as the only
clade of dinosaurs to have survived the
Cretaceous–Paleogene extinction event 65.5 Ma (million years) ago.
Modern birds are
characterised by
feathers, a
beak with no
teeth, the
laying of
hard-shelled eggs, a high
metabolic rate, a four-chambered
heart, and a lightweight but strong
skeleton. All living species of birds have wings- the most recent species without wings was the
moa, which is generally considered to have become extinct in the 1500s. Wings are evolved forelimbs, and most bird species can
fly. Flightless birds include
ratites,
penguins, and a number of diverse
endemic island species. Birds also have unique
digestive and
respiratory systems that are highly adapted for flight. Some birds, especially
corvids and
parrots, are among the most intelligent animal species; a number of bird species have been observed
manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.
Many species undertake long distance annual
migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and
songs, and participate in social behaviours, including
cooperative breeding and hunting,
flocking, and
mobbing of predators. The vast majority of bird species are
socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have
polygynous ("many females") or, rarely,
polyandrous ("many males") breeding systems. Eggs are usually laid in a nest and
incubated by the parents. Most birds have an extended period of parental care after hatching.
Many species are of economic importance, mostly as sources of food acquired through hunting or farming. Some species, particularly
songbirds and
parrots, are popular as pets. Other uses include the harvesting of
guano (droppings) for use as a
fertiliser. Birds
figure prominently in all aspects of human culture from religion to poetry to popular music. About 120–130 species have become
extinct as a result of human activity since the 17th century, and hundreds more before then. Currently about 1,200 species of birds are threatened with extinction by human activities, though efforts are underway to
protect them.
Evolution and taxonomy
|
|
| The birds' phylogenetic relationships to major living reptile groups. |
The first
classification of birds was developed by
Francis Willughby and
John Ray in their 1676 volume
Ornithologiae.
[2] Carolus Linnaeus modified that work in 1758 to devise the
taxonomic classification system currently in use.
[3] Birds are categorised as the
biological class Aves in
Linnaean taxonomy.
Phylogenetic taxonomy places Aves in the dinosaur
clade Theropoda.
[4] Aves and a sister group, the clade
Crocodilia, contain the only living representatives of the
reptile clade
Archosauria. During the late 1990s, Aves was most commonly defined
phylogenetically as all descendants of the
most recent common ancestor of modern birds and
Archaeopteryx lithographica.
[5] However, an earlier definition proposed by
Jacques Gauthier gained wide currency in the 21st century, and is used by many scientists including adherents of the
Phylocode system. Gauthier defined Aves to include only the modern bird groups, the
crown group. This was done by excluding most groups known only from fossils, and assigning them, instead, to the
Avialae,
[6] in part to avoid the uncertainties about the placement of
Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
All modern birds lie within the crown group
Neornithes, which has two subdivisions: the
Palaeognathae, which includes the flightless
ratites (such as the
ostriches) and the weak-flying
tinamous, and the extremely diverse
Neognathae, containing all other birds.
[4] These two subdivisions are often given the
rank of
superorder,
[7] although
Livezey and Zusi assigned them "cohort" rank.
[4] Depending on the
taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800
[8] to 10,050.
[9]
Dinosaurs and the origin of birds
Based on fossil and biological evidence, most scientists accept that birds are a specialized subgroup of
theropod dinosaurs.
[10] More specifically, they are members of
Maniraptora, a group of theropods which includes
dromaeosaurs and
oviraptorids, among others.
[11] As scientists have discovered more nonavian theropods closely related to birds, the previously clear distinction between nonbirds and birds has become blurred. Recent discoveries in the
Liaoning Province of northeast China, which demonstrate many small
theropod dinosaurs had feathers, contribute to this ambiguity.
[12]
The consensus view in contemporary
paleontology is that the birds, or
avialans, are the closest relatives of the
deinonychosaurs, which include
dromaeosaurids,
troodontids and possibly
archaeopterygids.
[13] Together, these three form a group called
Paraves. Some
basal members of this group, such as
Microraptor and
Archaeopteryx, have features which may have enabled them to glide or fly. The most basal deinonychosaurs are very small. This evidence raises the possibility that the ancestor of all paravians may have been
arboreal, may have been able to glide, or both.
[14][15] Unlike
Archaeopteryx and the feathered dinosaurs, who primarily ate meat, recent studies suggest that the first birds were
herbivores.
[16]
The
Late Jurassic Archaeopteryx is well known as one of the first
transitional fossils to be found, and it provided support for the theory of
evolution in the late 19th century.
Archaeopteryx was the first fossil to display both clearly reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, as well as wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, though it is possibly closely related to the real ancestor.
[17]
Alternative theories and controversies
Early disagreements on the origins of birds included whether birds evolved from
dinosaurs or more primitive
archosaurs. Within the dinosaur camp, there were disagreements as to whether
ornithischian or
theropod dinosaurs were the more likely ancestors.
[18] Although ornithischian (bird-hipped) dinosaurs share the hip structure of modern birds, birds are thought to have originated from the
saurischian (lizard-hipped) dinosaurs, and therefore evolved their hip structure
independently.
[19] In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the
Therizinosauridae.
A small minority of researchers, such as paleornithologist
Alan Feduccia of the
University of North Carolina, challenge the majority view, contending that birds are not dinosaurs, but evolved from early archosaurs like
Longisquama.
[20][21]
Early evolution of birds
|
|
| Basal bird phylogeny simplified after Chiappe, 2007[22] |
Birds diversified into a wide variety of forms during the
Cretaceous Period.
[22] Many groups retained
primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including
modern birds (Neornithes). While the earliest forms, such as
Archaeopteryx and
Jeholornis, retained the long bony tails of their ancestors,
[22] the tails of more advanced birds were shortened with the advent of the
pygostyle bone in the
clade Pygostylia. In the late Cretaceous, around 95 million years ago, the ancestor of all modern birds also evolved better olfactory senses.
[23]
The first large, diverse lineage of short-tailed birds to evolve were the
Enantiornithes, or "opposite birds", so named because the construction of their shoulder bones was in reverse to that of modern birds. Enantiornithes occupied a wide array of ecological niches, from sand-probing shorebirds and fish-eaters to tree-dwelling forms and seed-eaters.
[22]
Many species of the second major bird lineage to diversify, the
Ornithurae (including the ancestors of modern birds), specialised in eating fish, like the superficially
gull-like subclass
Ichthyornithes (fish birds).
[24] One order of Mesozoic seabirds, the
Hesperornithiformes, became so well adapted to hunting fish in marine environments, they lost the ability to fly and became primarily aquatic. Despite their extreme specializations, the Hesperornithiformes represent some of the closest relatives of modern birds.
[22]
Diversification of modern birds
Containing all modern birds, the subclass Neornithes is, due to the discovery of
Vegavis, now known to have evolved into some basic lineages by the end of the Cretaceous
[25] and is split into two superorders, the
Palaeognathae and
Neognathae. The paleognaths include the
tinamous of
Central and South America and the
ratites. The basal divergence from the remaining Neognathes was that of the
Galloanserae, the superorder containing the
Anseriformes (
ducks,
geese,
swans and
screamers) and the
Galliformes (the
pheasants,
grouse, and their allies, together with the
mound builders and the
guans and their allies). The dates for the splits are much debated by scientists. The Neornithes are agreed to have evolved in the Cretaceous, and the split between the Galloanseri from other Neognathes occurred before the
Cretaceous–Paleogene extinction event, but there are different opinions about whether the
radiation of the remaining Neognathes occurred before or after the extinction of the other dinosaurs.
[26] This disagreement is in part caused by a divergence in the evidence; molecular dating suggests a Cretaceous radiation, while
fossil evidence supports a
Tertiary radiation. Attempts to reconcile the molecular and fossil evidence have proved controversial.
[26][27]
The classification of birds is a contentious issue.
Sibley and
Ahlquist's
Phylogeny and Classification of Birds (1990) is a landmark work on the classification of birds,
[28] although it is frequently debated and constantly revised. Most evidence seems to suggest the assignment of orders is accurate,
[29] but scientists disagree about the relationships between the orders themselves; evidence from modern bird anatomy, fossils and DNA have all been brought to bear on the problem, but no strong consensus has emerged. More recently, new fossil and molecular evidence is providing an increasingly clear picture of the evolution of modern bird orders.
Classification of modern bird orders
Cladogram showing a 2012 classification of Neoaves, based on several phylogenetic studies.
[30]
This is a list of the taxonomic orders in the subclass Neornithes, or modern birds. This list uses the traditional classification (the so-called
Clements order), revised by the Sibley-Monroe classification. The
list of birds gives a more detailed summary of the orders, including families.
Subclass Neornithes
The subclass Neornithes has two extant
superorders –
Superorder
Palaeognathae:
The name of the superorder is derived from
paleognath, the ancient Greek for "old jaws" in reference to the skeletal anatomy of the palate, which is described as more primitive and reptilian than that in other birds. The Palaeognathae consists of two orders which comprise 49 existing species.
Superorder
Neognathae:
The superorder Neognathae comprises 27 orders which have a total of nearly ten thousand species. The Neognathae have undergone
adaptive radiation to produce the staggering diversity of form (especially of the bill and feet), function, and behaviour that are seen today.
The orders comprising the Neognathae are:
The radically different Sibley-Monroe classification (
Sibley-Ahlquist taxonomy), based on molecular data, found widespread adoption in a few aspects, as recent molecular, fossil, and anatomical evidence supported the
Galloanserae.
[26]
Distribution
The range of the
House Sparrow has expanded dramatically due to human activities.
[31]
Birds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the
Snow Petrel's breeding colonies up to 440 kilometres (270 mi) inland in
Antarctica.
[32] The highest bird
diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher
speciation rates in the tropics, however recent studies found higher speciation rates in the high latitudes that were offset by greater
extinction rates than in the tropics.
[33] Several families of birds have adapted to life both on the world's oceans and in them, with some
seabird species coming ashore only to breed
[34] and some
penguins have been recorded diving up to 300 metres (980 ft).
[35]
Many bird species have established breeding populations in areas to which they have been
introduced by humans. Some of these introductions have been deliberate; the
Ring-necked Pheasant, for example, has been introduced around the world as a
game bird.
[36] Others have been accidental, such as the establishment of wild
Monk Parakeets in several North American cities after their escape from captivity.
[37] Some species, including
Cattle Egret,
[38] Yellow-headed Caracara[39] and
Galah,
[40] have
spread naturally far beyond their original ranges as
agricultural practices created suitable new habitat.
Anatomy and physiology
External anatomy of a bird (example:
Yellow-wattled Lapwing): 1 Beak, 2 Head, 3 Iris, 4 Pupil, 5 Mantle, 6 Lesser
coverts, 7 Scapulars, 8 Median coverts, 9 Tertials, 10 Rump, 11 Primaries, 12 Vent, 13 Thigh, 14 Tibio-tarsal articulation, 15 Tarsus, 16 Foot, 17 Tibia, 18 Belly, 19 Flanks, 20 Breast, 21 Throat, 22 Wattle
Compared with other vertebrates, birds have a
body plan that shows many unusual adaptations, mostly to facilitate
flight.
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the
respiratory system.
[41] The skull bones in adults are fused and do not show
cranial sutures.
[42] The
orbits are large and separated by a bony
septum. The
spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior
thoracic vertebrae and absent in the later vertebrae.
[43] The last few are fused with the
pelvis to form the
synsacrum.
[42] The ribs are flattened and the
sternum is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings.
[44]
Like the
reptiles, birds are primarily uricotelic, that is, their
kidneys extract nitrogenous wastes from their bloodstream and excrete it as
uric acid instead of
urea or
ammonia via the ureters into the intestine. Birds do not have a
urinary bladder or external urethral opening and (with exception of the
Ostrich) uric acid is excreted along with feces as a semisolid waste.
[45][46][47] However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.
[48] They also excrete
creatine, rather than
creatinine like mammals.
[42] This material, as well as the output of the intestines, emerges from the bird's
cloaca.
[49][50] The cloaca is a multi-purpose opening: waste is expelled through it, birds mate by
joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate
pellets.
[51] The
digestive system of birds is unique, with a
crop for storage and a
gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth.
[52] Most birds are highly adapted for rapid digestion to aid with flight.
[53] Some migratory birds have adapted to use protein from many parts of their bodies, including protein from the intestines, as additional energy during migration.
[54]
Birds have one of the most complex
respiratory systems of all animal groups.
[42] Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior
air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the lung and the stored fresh air from the posterior air sac is simultaneously forced into the lungs. Thus, a bird's lungs receive a constant supply of fresh air during both inhalation and exhalation.
[55] Sound production is achieved using the
syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea;
[56] the trachea being elongated in some species, increasing the volume of vocalizations and the perception of the bird's size.
[57] The bird's heart has four chambers like a mammalian heart. In birds the main arteries taking blood away from the heart originate from the right
aortic arch (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the
aorta.
[42] The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating
red blood cells in birds retain their
nucleus.
[58]
The
nervous system is large relative to the bird's size.
[42] The most developed part of the brain is the one that controls the flight-related functions, while the
cerebellum coordinates movement and the
cerebrum controls behaviour patterns, navigation, mating and nest building. Most birds have a poor
sense of smell with notable exceptions including
kiwis,
[59] New World vultures[60] and
tubenoses.
[61] The avian
visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water.
[42] Some species also have dual
fovea. Birds are
tetrachromatic, possessing
ultraviolet (UV) sensitive
cone cells in the eye as well as green, red and blue ones.
[62] This allows them to perceive ultraviolet light, which is involved in courtship. Many birds show plumage patterns in ultraviolet that are invisible to the human eye; some birds whose sexes appear similar to the naked eye are distinguished by the presence of
ultraviolet reflective patches on their feathers. Male
Blue Tits have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.
[63] Ultraviolet light is also used in foraging—
kestrels have been shown to search for prey by detecting the UV reflective urine trail marks left on the ground by rodents.
[64] The eyelids of a bird are not used in blinking. Instead the eye is lubricated by the
nictitating membrane, a third eyelid that moves horizontally.
[65] The nictitating membrane also covers the eye and acts as a
contact lens in many aquatic birds.
[42] The bird
retina has a fan shaped blood supply system called the
pecten.
[42] Most birds cannot move their eyes, although there are exceptions, such as the
Great Cormorant.
[66] Birds with eyes on the sides of their heads have a wide
visual field, while birds with eyes on the front of their heads, such as owls, have
binocular vision and can estimate the
depth of field.
[67] The avian
ear lacks external
pinnae but is covered by feathers, although in some birds, such as the
Asio,
Bubo and
Otus owls, these feathers form tufts which resemble ears. The
inner ear has a
cochlea, but it is not spiral as in mammals.
[68]
A few species are able to use chemical defenses against predators; some
Procellariiformes can eject an unpleasant
oil against an aggressor,
[69] and some species of
pitohuis from
New Guinea have a powerful
neurotoxin in their skin and feathers.
[70]
Chromosomes
Birds have two sexes: male and female. The sex of birds is determined by the
Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).
[42]
In nearly all species of birds, an individual's sex is determined at fertilization. However, one recent study demonstrated
temperature-dependent sex determination among
Australian Brush-turkeys, for which higher temperatures during incubation resulted in a higher female-to-male
sex ratio.
[71]
Feathers, plumage, and scales
Feathers are a feature characteristic of birds (though also present in
some dinosaurs not currently considered to be true birds). They facilitate
flight, provide insulation that aids in
thermoregulation, and are used in display, camouflage, and signaling.
[42] There are several types of feathers, each serving its own set of purposes. Feathers are epidermal growths attached to the skin and arise only in specific tracts of skin called pterylae. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called
plumage, may vary within species by age,
social status,
[72] and
sex.
[73]
Plumage is regularly
moulted; the standard plumage of a bird that has moulted after breeding is known as the "non-breeding" plumage, or—in the
Humphrey-Parkes terminology—"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey-Parkes system as "alternate" plumages.
[74] Moulting is annual in most species, although some may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines,
flight feathers are replaced one at a time with the innermost primary being the first. When the fifth of sixth primary is replaced, the outermost tertiaries begin to drop. After the innermost tertiaries are moulted, the secondaries starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary coverts are moulted in synchrony with the primary that they overlap.
[75] A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.
[76] As a general rule, the tail feathers are moulted and replaced starting with the innermost pair.
[75] Centripetal moults of tail feathers are however seen in the
Phasianidae.
[77] The centrifugal moult is modified in the tail feathers of
woodpeckers and
treecreepers, in that it begins with the second innermost pair of feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail.
[75][78] The general pattern seen in
passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward.
[79] Before nesting, the females of most bird species gain a bare
brood patch by losing feathers close to the belly. The skin there is well supplied with blood vessels and helps the bird in incubation.
[80]
Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this.
[81] The bill is used to brush away foreign particles and to apply
waxy secretions from the
uropygial gland; these secretions protect the feathers' flexibility and act as an
antimicrobial agent, inhibiting the growth of feather-degrading
bacteria.
[82] This may be supplemented with the secretions of
formic acid from ants, which birds receive through a behaviour known as
anting, to remove feather parasites.
[83]
The
scales of birds are composed of the same keratin as beaks, claws, and spurs. They are found mainly on the toes and
metatarsus, but may be found further up on the ankle in some birds. Most bird scales do not overlap significantly, except in the cases of
kingfishers and
woodpeckers. The scales of birds are thought to be
homologous to those of reptiles and mammals.
[84]
Flight
Main article:
Bird flight
Most birds can
fly, which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for breeding, feeding, and predator avoidance and escape. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb (
wing) that serves as an
aerofoil.
[42] Wing shape and size generally determine a bird species' type of flight; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are
flightless, as were many extinct birds.
[85] Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators.
[86] Though flightless, penguins use similar musculature and movements to "fly" through the water, as do
auks,
shearwaters and
dippers.
[87]
Behaviour
Most birds are
diurnal, but some birds, such as many species of
owls and
nightjars, are
nocturnal or
crepuscular (active during twilight hours), and many coastal
waders feed when the tides are appropriate, by day or night.
[88]
Diet and feeding
Feeding adaptations in beaks
Birds' diets are varied and often include
nectar, fruit, plants, seeds,
carrion, and various small animals, including other birds.
[42] Because birds have no teeth, their
digestive system is adapted to process
unmasticated food items that are swallowed whole.
Birds that employ many strategies to obtain food or feed on a variety of food items are called generalists, while others that concentrate time and effort on specific food items or have a single strategy to obtain food are considered specialists.
[42] Birds' feeding strategies vary by species. Many birds
glean for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek
pest insects are considered beneficial 'biological control agents' and their presence encouraged in
biological pest control programs.
[89] Nectar feeders such as
hummingbirds,
sunbirds,
lories, and lorikeets amongst others have specially adapted brushy tongues and in many cases bills designed to fit
co-adapted flowers.
[90] Kiwis and
shorebirds with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of
ecological niches.
[42][91] Loons,
diving ducks,
penguins and
auks pursue their prey underwater, using their wings or feet for propulsion,
[34] while aerial predators such as
sulids,
kingfishers and
terns plunge dive after their prey.
Flamingos, three species of
prion, and some ducks are
filter feeders.
[92][93] Geese and
dabbling ducks are primarily grazers.
Some species, including
frigatebirds,
gulls,
[94] and
skuas,
[95] engage in
kleptoparasitism, stealing food items from other birds. Kleptoparasitism is thought to be a supplement to food obtained by hunting, rather than a significant part of any species' diet; a study of
Great Frigatebirds stealing from
Masked Boobies estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%.
[96] Other birds are
scavengers; some of these, like
vultures, are specialised carrion eaters, while others, like gulls,
corvids, or other birds of prey, are opportunists.
[97]
Water and drinking
Water is needed by many birds although their mode of excretion and lack of
sweat glands reduces the physiological demands.
[98] Some desert birds can obtain their water needs entirely from moisture in their food. They may also have other adaptations such as allowing their body temperature to rise, saving on moisture loss from evaporative cooling or panting.
[99] Seabirds can drink seawater and have
salt glands inside the head that eliminate excess salt out of the nostrils.
[100]
Most birds scoop water in their beaks and raise their head to let water run down the throat. Some species, especially of arid zones, belonging to the
pigeon,
finch,
mousebird,
button-quail and
bustard families are capable of sucking up water without the need to tilt back their heads.
[101] Some desert birds depend on water sources and
sandgrouse are particularly well known for their daily congregations at waterholes. Nesting sandgrouse and many plovers carry water to their young by wetting their belly feathers.
[102] Some birds carry water for chicks at the nest in their crop or regurgitate it along with food. The pigeon family, flamingos and penguins have adaptations to produce a nutritive fluid called
crop milk that they provide to their chicks.
[103]
Bathing and dusting
A bird bathes by wetting its feathers using any of a variety of techniques.
- Wading involves birds standing in water and fluffing their feathers and flicking their wings in and out of the water. The birds also roll their breasts on the surfacing of the water and then flick their heads back in order to throw water on the feathers of the back. Robins, thrushes, mockingbirds, jays, and titmice have been observed to use this bathing technique.
- Dipping – Some birds dip into water during flight, wetting their wings. The tail is used to direct the spray of the water unto the back. Swifts and swallows have been observed to use this bathing technique.
- Some birds dart repeated into water, quickly immersing and rolling through the water. They vibrate their feathers after each dart. Chickadees, yellowthroats, wrens, buntings, and waterthrushes have displayed this bathing behavior.
- In areas where there is a scarcity of water, birds such as the wrentit, wet their feathers with the dew from vegetation.
- Especially in arid areas birds use dusting instead of water bathing. Birds rake the dust from the ground and then cast the dust over their bodies. The dust is also to settle on the feathers and then shaken off. Wrens, House Sparrows, Wrentits, larks have been observed to use dusting.
- Other birds perform a more passive form of bathing, extending their wings and tails during light drizzles so that their feathers are wet. Woodpeckers and nuthatches practice this method of bathing.
Birds bath or dust primarily as a means of maintaining their plumage. For some birds such as the quail, dusting has been found to help maintain optimal lubrication on feathers. The dust absorbs the excess oil, dry skin and excess debris. Dusting is perhaps also used to prevent or remove parasites.
[104]
Migration
Main article:
Bird migration
Many bird species migrate to take advantage of global differences of
seasonal temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds,
shorebirds, and
waterbirds undertake annual long distance migrations, usually triggered by the length of daylight as well as weather conditions. These birds are characterised by a breeding season spent in the
temperate or
arctic/
antarctic regions and a non-breeding season in the
tropical regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs.
[54][105] 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),
[106] although the
Bar-tailed Godwit is capable of non-stop flights of up to 10,200 km (6,300 mi).
[107] Seabirds also undertake long migrations, the longest annual migration being those of
Sooty Shearwaters, which nest in
New Zealand and
Chile and spend the northern summer feeding in the North Pacific off Japan,
Alaska and
California, an annual round trip of 64,000 km (39,800 mi).
[108] 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.
[109]
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.
[110] 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.
[111] Partial migration can form a large percentage of the migration behaviour of birds in some regions; in Australia, surveys found that 44% of non-passerine birds and 32% of passerines were partially migratory.
[112] Altitudinal migration is a form of short distance migration in which birds spend the breeding season at higher altitudes elevations and move to lower ones during suboptimal conditions. It is most often triggered by temperature changes and usually occurs when
the normal territories also become inhospitable due to lack of food.
[113] 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.
[114]
The ability of birds to return to precise locations across vast distances has been known for some time; in an experiment conducted in the 1950s a
Manx Shearwater released in
Boston returned to its colony in
Skomer,
Wales, within 13 days, a distance of 5,150 km (3,200 mi).
[115] Birds navigate during migration using a variety of methods. For
diurnal migrants, the
sun is used to navigate by day, and a stellar compass is used at night. Birds that use the sun compensate for the changing position of the sun during the day by the use of an
internal clock.
[42] Orientation with the stellar compass depends on the position of the
constellations surrounding
Polaris.
[116] These are backed up in some species by their ability to sense the Earth's
geomagnetism through specialised
photoreceptors.
[117]
Communication
The startling display of the
Sunbittern mimics a large predator.
Birds
communicate using primarily visual and auditory signals. Signals can be interspecific (between species) and intraspecific (within species).
Birds sometimes use plumage to assess and assert social dominance,
[118] 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.
[119] Variation in plumage also allows for the identification of birds, particularly between species. Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds.
[42] The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements;
[120] males' breeding success may depend on the quality of such displays.
[121]
Call of the
House Wren, a common North American songbird
Bird calls and songs, which are produced in the
syrinx, are the major means by which birds communicate with
sound. This communication can be very complex; some species can operate the two sides of the syrinx independently, allowing the simultaneous production of two different songs.
[56] Calls are used for a variety of purposes, including mate attraction,
[42] evaluation of potential mates,
[122] bond formation, the claiming and maintenance of territories,
[42] the identification of other individuals (such as when parents look for chicks in colonies or when mates reunite at the start of breeding season),
[123] and the warning of other birds of potential predators, sometimes with specific information about the nature of the threat.
[124] Some birds also use mechanical sounds for auditory communication. The
Coenocorypha snipes of
New Zealand drive air through their feathers,
[125] woodpeckers drum territorially,
[53] and
Palm Cockatoos use tools to drum.
[126]
Red-billed Queleas, the most numerous species of bird,
[127] form enormous flocks—sometimes tens of thousands strong.
Flocking and other associations
While some birds are essentially territorial or live in small family groups, other birds may form large
flocks. The principal benefits of flocking are
safety in numbers and increased foraging efficiency.
[42] Defence against predators is particularly important in closed habitats like forests, where
ambush predation is common and multiple eyes can provide a valuable early warning system. This has led to the development of many
mixed-species feeding flocks, which are usually composed of small numbers of many species; these flocks provide safety in numbers but increase potential competition for resources.
[128] Costs of flocking include bullying of socially subordinate birds by more dominant birds and the reduction of feeding efficiency in certain cases.
[129]
Birds sometimes also form associations with non-avian species. Plunge-diving
seabirds associate with
dolphins and
tuna, which push shoaling fish towards the surface.
[130] 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.
[131]
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.
[132] 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.
[133] It has been suggested that there may be certain kinds of sleep which are possible even when in flight.
[134] Some birds have also demonstrated the capacity to fall into
slow-wave sleep one
hemisphere of the brain at a time. The birds tend to exercise this ability depending upon its position relative to the outside of the flock. This may allow the eye opposite the sleeping hemisphere to remain vigilant for
predators by viewing the outer margins of the flock. This adaptation is also known from
marine mammals.
[135] Communal roosting is common because it lowers the
loss of body heat and decreases the risks associated with predators.
[136] Roosting sites are often chosen with regard to thermoregulation and safety.
[137]
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.
[138] Some
hummingbirds go into a nightly state of
torpor accompanied with a reduction of their metabolic rates.
[139] 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.
[140] Birds do not have sweat glands, but they may cool themselves by moving to shade, standing in water, panting, increasing their surface area, fluttering their throat or by using special behaviours like
urohidrosis to cool themselves.
Breeding
Social systems
Ninety-five percent of bird species are socially monogamous. These species pair for at least the length of the breeding season or—in some cases—for several years or until the death of one mate.
[142] Monogamy allows for
biparental care, which is especially important for species in which females require males' assistance for successful brood-rearing.
[143] Among many socially monogamous species, extra-pair copulation (infidelity) is common.
[144] 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.
[145] 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.
[146] Males of species that engage in extra-pair copulations will closely guard their mates to ensure the parentage of the offspring that they raise.
[147]
Other mating systems, including
polygyny,
polyandry,
polygamy,
polygynandry, and
promiscuity, also occur.
[42] Polygamous breeding systems arise when females are able to raise broods without the help of males.
[42] Some species may use more than one system depending on the circumstances.
Breeding usually involves some form of courtship display, typically performed by the male.
[148] 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,
[149] although in the polyandrous
phalaropes, this is reversed: plainer males choose brightly coloured females.
[150] Courtship feeding,
billing and allopreening are commonly performed between partners, generally after the birds have paired and mated.
[53]
Homosexual behaviour has been observed in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks.
[151]
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.
[152]
All birds lay
amniotic eggs with hard shells made mostly of
calcium carbonate.
[42] Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay
camouflaged eggs. There are many exceptions to this pattern, however; the ground-nesting
nightjars have pale eggs, and camouflage is instead provided by their plumage. Species that are victims of
brood parasites have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts.
[153]
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.
[154] 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,
[155] and feathers are often used for nest insulation.
[154] 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.
Incubation, which optimises temperature for chick development, usually begins after the last egg has been laid.
[42] 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.
[156] The warmth for the incubation of the eggs of
megapodes comes from the sun, decaying vegetation or volcanic sources.
[157] Incubation periods range from 10 days (in
woodpeckers,
cuckoos and
passerine birds) to over 80 days (in albatrosses and
kiwis).
[42]
Parental care and fledging
At the time of their hatching, chicks range in development from helpless to independent, depending on their species. Helpless chicks are termed
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.
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.
[158] 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.
[159] 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.
[160]
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.
[161] Such alloparenting is particularly common among the
Corvida, which includes such birds as the true
crows,
Australian Magpie and
Fairy-wrens,
[162] 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.
[42] 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.
[163]
The point at which chicks
fledge varies dramatically. The chicks of the
Synthliboramphus murrelets, like the
Ancient Murrelet, leave the nest the night after they hatch, following their parents out to sea, where they are raised away from terrestrial predators.
[164] 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.
[165] Chicks may also follow their parents during their first
migration.
[166]
Brood parasites
Main article:
Brood parasite, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism.
[167] 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.
[168] One hundred bird species, including
honeyguides,
icterids,
estrildid finches and
ducks, are obligate parasites, though the most famous are the
cuckoos.
[167] 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.
[169]
Ecology
The
South Polar Skua (left) is a generalist predator, taking the eggs of other birds, fish, carrion and other animals. This skua is attempting to push an
Adelie Penguin (right) off its nest
Birds occupy a wide range of ecological positions.
[127] 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 specialized at predating birds.
Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal.
[170] Plants and pollinating birds often
coevolve,
[171] and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.
[172]
Birds are often important to island ecology. Birds have frequently reached islands that mammals have not; on those islands, birds may fulfill ecological roles typically played by larger animals. For example, in New Zealand the
moas were important browsers, as are the
Kereru and
Kokako today.
[170] Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa.
[173] 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
[174] and the surrounding seas.
[175]
A wide variety of
Avian ecology field methods, including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.
Relationship with humans
Since birds are highly visible and common animals, humans have had a relationship with them since the dawn of man.
[176] Sometimes, these relationships are
mutualistic, like the cooperative honey-gathering among
honeyguides and African peoples such as the
Borana.
[177] Other times, they may be
commensal, as when species such as the
House Sparrow[178] have benefited from human activities. Several bird species have become commercially significant agricultural pests,
[179] and some pose an
aviation hazard.
[180] Human activities can also be detrimental, and have threatened numerous bird species with extinction (
hunting,
avian lead poisoning,
pesticides,
roadkill, 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.
[181]
Economic importance
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.
[182] Chickens account for much of human poultry consumption, though 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.
[183] Muttonbirding is also popular in Australia and New Zealand.
[184] Though some hunting, such as that of muttonbirds, may be sustainable, hunting has led to the extinction or endangerment of dozens of species.
[185]
The use of cormorants by Asian fishermen is in steep decline but survives in some areas as a tourist attraction.
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 feces (
guano), which is a valuable source of phosphorus and nitrogen. The
War of the Pacific, sometimes called the Guano War, was fought in part over the control of guano deposits.
[186]
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.
[187] 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,
[188] or for sports such as
pigeon racing.
Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly,
birders) number in the millions.
[189] Many homeowners erect
bird feeders near their homes to attract various species.
Bird feeding has grown into a multimillion dollar industry; for example, an estimated 75% of households in Britain provide food for birds at some point during the winter.
[190]
Religion, folklore and culture
Birds play prominent and diverse roles in folklore, religion, and
popular culture. In religion, birds may serve as either messengers or priests and leaders for a
deity, such as in the Cult of
Makemake, in which the
Tangata manu of
Easter Island served as chiefs;
[191] or as the
rooster(cock) serves as a tangible vessel of Christ as in the gospel of –Matthew, Mark and Luke in the New Testament with Christ speaking through the cock;
[192] or as attendants, as in the case of
Hugin and Munin, two
Common Ravens who whispered news into the ears of the
Norse god Odin.
[193] In several civilizations 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.
[194] They may also serve as
religious symbols, as when
Jonah (Hebrew:
יוֹנָה,
dove) embodied the fright, passivity, mourning, and beauty traditionally associated with doves.
[195] Birds have themselves been deified, as in the case of the
Common Peacock, which is perceived as Mother Earth by the
Dravidians of India.
[196] Some birds have also been perceived as monsters, including the mythological
Roc and the
Māori's legendary
Pouākai, a giant bird capable of snatching humans.
[197]
Birds have been featured in culture and art since prehistoric times, when they were represented in early
cave paintings.
[198] Birds were later used in religious or symbolic art and design, such as the magnificent
Peacock Throne of the
Mughal and
Persian emperors.
[199] 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.
[200] 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.
[201] 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'.
[202] Other
English metaphors derive from birds;
vulture funds and vulture investors, for instance, take their name from the scavenging vulture.
[203]
Perceptions of various bird species often vary across cultures.
Owls are associated with bad luck,
witchcraft, and death in parts of Africa,
[204] but are regarded as wise across much of Europe.
[205] Hoopoes were considered sacred in
Ancient Egypt and symbols of virtue in
Persia, but were thought of as thieves across much of Europe and harbingers of war in
Scandinavia.
[206]
Conservation
The
California Condor once numbered only 22 birds, but conservation measures have raised that to over 300 today.
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,
[207] 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.
[208] Many bird populations are declining worldwide, with 1,227 species listed as
threatened by
Birdlife International and the
IUCN in 2009.
[209][210]
The most commonly cited human threat to birds is
habitat loss.
[211] Other threats include overhunting, accidental mortality due to
structural collisions or
long-line fishing bycatch,
[212] pollution (including
oil spills and pesticide use),
[213] competition and predation from nonnative
invasive species,
[214] 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.
[215]
Notes
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