Evolution and classificationThe first of birds was developed by and in their 1676 volume ''Ornithologiae''. modified that work in 1758 to devise the system currently in use. Birds are categorised as the Aves in . places Aves in the dinosaur .
DefinitionAves and a sister group, the order , contain the only living representatives of the reptile clade . During the late 1990s, Aves was most commonly defined as all descendants of the of modern birds and '. However, an earlier definition proposed by gained wide currency in the 21st century, and is used by many scientists including adherents of the system. Gauthier defined Aves to include only the of the set of modern birds. This was done by excluding most groups known only from , and assigning them, instead, to the broader group Avialae, in part to avoid the uncertainties about the placement of ''Archaeopteryx'' in relation to animals traditionally thought of as theropod dinosaurs. Gauthier and de Queiroz identified four different definitions for the same biological name "Aves", which is a problem. The authors proposed to reserve the term Aves only for the crown group consisting of the last common ancestor of all living birds and all of its descendants, which corresponds to meaning number 4 below. He assigned other names to the other groups. # Aves can mean all s closer to birds than to s (alternately ) # Aves can mean those advanced archosaurs with feathers (alternately ) # Aves can mean those feathered dinosaurs that fly (alternately ) # Aves can mean the last common ancestor of all the currently living birds and all of its descendants (a "", in this sense synonymous with Neornithes) Under the fourth definition ''Archaeopteryx'', traditionally considered one of the earliest members of Aves, is removed from this group, becoming a non-avian dinosaur instead. These proposals have been adopted by many researchers in the field of palaeontology and , though the exact definitions applied have been inconsistent. Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers. Most researchers define Avialae as branch-based clade, though definitions vary. Many authors have used a definition similar to "all s closer to birds than to '",Weishampel, David B.; Dodson, Peter; Osmólska, Halszka (eds.) (2004). ''The Dinosauria'', Second Edition. University of California Press., 861 pp. with ' being sometimes added as a second external specifier in case it is closer to birds than to ''Deinonychus''. Avialae is also occasionally defined as an (that is, one based on physical characteristics). , who named Avialae in 1986, re-defined it in 2001 as all dinosaurs that possessed feathered s used in flapping , and the birds that descended from them.Gauthier, J., and de Queiroz, K. (2001). "Feathered dinosaurs, flying dinosaurs, crown dinosaurs, and the name Aves." pp. 7–41 in ''New perspectives on the origin and early evolution of birds: proceedings of the International Symposium in Honor of John H. Ostrom'' (J.A. Gauthier and L.F. Gall, eds.). Peabody Museum of Natural History, Yale University, New Haven, CTGauthier, J. (1986). "Saurischian monophyly and the origin of birds." In: K. Padian, ed. ''The origin of birds and the evolution of flight.'' San Francisco: California, Acad. Sci. pp. 1–55. (Mem. Calif. Acad. Sci.8.) Despite being currently one of the most widely used, the crown-group definition of Aves has been criticised by some researchers. Lee and Spencer (1997) argued that, contrary to what Gauthier defended, this definition would not increase the stability of the clade and the exact content of Aves will always be uncertain because any defined clade (either crown or not) will have few synapomorphies distinguishing it from its closest relatives. Their alternative definition is synonymous to Avifilopluma.
Dinosaurs and the origin of birdsBased on fossil and biological evidence, most scientists accept that birds are a specialised subgroup of s, and more specifically, they are members of , a group of theropods which includes and , 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 Province of northeast China, which demonstrate many small theropod , contribute to this ambiguity. The consensus view in contemporary is that the flying theropods, or , are the closest relatives of the s, which include dromaeosaurids and s. Together, these form a group called . Some members of Deinonychosauria, such as ', 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 , 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 s. The ''Archaeopteryx'' is well known as one of the first s to be found, and it provided support for the 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.
Early evolutionOver 40% of key traits found in modern birds evolved during the 60 million year transition from the earliest to the first , i.e. the first dinosaurs closer to living birds than to '. The loss of osteoderms otherwise common in archosaurs and acquisition of primitive feathers might have occurred early during this phase. After the appearance of Maniraptoromorpha, the next 40 million years marked a continuous reduction of body size and the accumulation of (juvenile-like) characteristics. became increasingly less common while braincases enlarged and forelimbs became longer. The evolved into complex, pennaceous feathers. The oldest known paravian (and probably the earliest avialan) fossils come from the of China, which has been dated to the late period ( stage), about 160 million years ago. The avialan species from this time period include ', ', and '. The well-known probable early avialan, ''Archaeopteryx'', dates from slightly later Jurassic rocks (about 155 million years old) from . Many of these early avialans shared unusual anatomical features that may be ancestral to modern birds, but were later lost during bird evolution. These features include enlarged claws on the second toe which may have been held clear of the ground in life, and long feathers or "hind wings" covering the hind limbs and feet, which may have been used in aerial manoeuvreing. Avialans diversified into a wide variety of forms during the . Many groups retained , such as clawed wings and teeth, though the latter were lost independently in a number of avialan groups, including modern birds (Aves). Increasingly stiff tails (especially the outermost half) can be seen in the evolution of maniraptoromorphs, and this process culminated in the appearance of the , an ossification of fused tail vertebrae. In the late Cretaceous, about 100 million years ago, the ancestors of all modern birds evolved a more open pelvis, allowing them to lay larger eggs compared to body size. Around 95 million years ago, they evolved a better sense of smell. A third stage of bird evolution starting with (the "bird-chested" avialans) can be associated with the refining of aerodynamics and flight capabilities, and the loss or co-ossification of several skeletal features. Particularly significant are the development of an enlarged, sternum and the , and the loss of grasping hands.
Early diversity of bird ancestorsThe first large, diverse lineage of short-tailed avialans to evolve were the , 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 s, 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 (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 adaptations and seem to have included shorebird-like species, waders, and swimming and diving species. The latter included the superficially -like ' and the , 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 and a fully mobile fan of tail feathers, which may have replaced the "hind wing" as the primary mode of aerial maneuverability and braking in flight. A study on in the avian skull found that the of all Neornithes might have had a beak similar to that of the modern and a skull similar to that of the . As both species are small aerial and canopy foraging s, a similar ecological niche was inferred for this hypothetical ancestor.
Diversification of modern birdsAll modern birds lie within the Aves (alternately Neornithes), which has two subdivisions: the , which includes the flightless s (such as the es) and the weak-flying s, and the extremely diverse , containing all other birds. These two subdivisions are often given the of , although and Zusi assigned them "cohort" rank. Depending on the viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,758. The discovery of ', a late Cretaceous member of the , proved that the diversification of modern birds started before the . The affinities of an earlier fossil, the possible '' lentus'', dated to about 85 million years ago, are still too controversial to provide a fossil evidence of modern bird diversification. Most studies agree on a Cretaceous age for the most recent common ancestor of modern birds but estimates range from the to the latest . Similarly, there is no agreement on whether most of the early diversification of modern birds occurred before or after the . This disagreement is in part caused by a divergence in the evidence; most molecular dating studies suggests a Cretaceous , 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 , showed that while modern birds originated early in the Late Cretaceous in Western Gondwana, a pulse of diversification in all major groups occurred around the Cretaceous–Palaeogene extinction event. Modern birds expanded from West Gondwana to the Laurasia through two routes. One route was an Antarctic interchange in the Paleogene. This can be confirmed with the presence of multiple avian groups in Australia and New Zealand. The other route was probably through North American, via land bridges during the Paleocene. This allowed the expansion and diversification of Neornithes into the Holarctic and Paleotropics.
Classification of bird ordersof modern bird relationships based on Kuhl, H. ''et al.'' (2020) The classification of birds is a contentious issue. and '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.
Genomics, the has been sequenced for at least one species in about 90% of extant avian families (218 out of 236 families recognised by the ''Howard and Moore Checklist'').
DistributionBirds live and breed in most terrestrial habitats and on all seven continents, reaching their southern extreme in the 's breeding colonies up to inland in . The highest bird occurs in tropical regions. It was earlier thought that this high diversity was the result of higher rates in the tropics; however recent studies found higher speciation rates in the high latitudes that were offset by greater rates than in the tropics. Many species migrate annually over great distances and across oceans; several families of birds have adapted to life both on the world's oceans and in them, and some species come ashore only to breed, while some s have been recorded diving up to deep. Many bird species have established breeding populations in areas to which they have been by humans. Some of these introductions have been deliberate; the , for example, has been introduced around the world as a . Others have been accidental, such as the establishment of wild s in several North American cities after their escape from captivity. Some species, including , and , have far beyond their original ranges as created suitable new habitat.
Anatomy and physiologyCompared with other vertebrates, birds have a that shows many unusual adaptations, mostly to facilitate .
Skeletal systemThe skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the . The skull bones in adults are fused and do not show . The that house the eyeballs are large and separated from each other by a bony (partition). The 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 and absent in the later vertebrae. The last few are fused with the to form the . The ribs are flattened and the is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings. The wings are more or less developed depending on the species; the only known groups that lost their wings are the and s.
Excretory systemLike the s, birds are primarily uricotelic, that is, their s extract from their bloodstream and excrete it as , instead of or , through the ureters into the intestine. Birds do not have a or external urethral opening and (with exception of the ) 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 , rather than like mammals. This material, as well as the output of the intestines, emerges from the bird's . The cloaca is a multi-purpose opening: waste is expelled through it, most birds mate by , and females lay eggs from it. In addition, many species of birds regurgitate . It is a common but not universal feature of nestlings (born helpless, under constant parental care) that instead of excreting directly into the nest, they produce a . This is a mucus-covered pouch that allows parents to either dispose of the waste outside the nest or to recycle the waste through their own digestive system.
Reproductive systemMales within (with the exception of the s), the (with the exception of s), and in rudimentary forms in (but fully developed in ) possess a , which is never present in . The length is thought to be related to . When not copulating, it is hidden within the compartment within the cloaca, just inside the vent. Female birds have tubules that allow sperm to remain viable long after copulation, a hundred days in some species. Sperm from multiple males may through this mechanism. Most female birds have a single and a single , both on the left side, but there are exceptions: species in at least 16 different orders of birds have two ovaries. Even these species, however, tend to have a single oviduct. It has been speculated that this might be an adaptation to flight, but males have two testes, and it is also observed that the gonads in both sexes decrease dramatically in size outside the breeding season. Also terrestrial birds generally have a single ovary, as does the , an egg-laying mammal. A more likely explanation is that the egg develops a shell while passing through the oviduct over a period of about a day, so that if two eggs were to develop at the same time, there would be a risk to survival. Birds have two sexes: either or . The sex of birds is determined by the , rather than by the present in s. 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 claimed to demonstrate among the , for which higher temperatures during incubation resulted in a higher female-to-male . This, however, was later proven to not be the case. These birds do not exhibit temperature-dependent sex determination, but temperature-dependent sex mortality.
Respiratory and circulatory systemsBirds have one of the most complex s of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior 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 , 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 (or pharyngeal arch), unlike in the mammals where the left aortic arch forms this part of the . The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the circulating in birds retain their .
Heart type and featuresThe avian circulatory system is driven by a four-chambered, myogenic heart contained in a fibrous pericardial sac. This pericardial sac is filled with a for lubrication.Whittow, G. (2000). Sturkie's Avian Physiology/ edited by G. Causey Whittow. San Diego : Academic Press, 2000. The heart itself is divided into a right and left half, each with an and . The atrium and ventricles of each side are separated by 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 uses calcium to cause a 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 , and 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.Hoagstrom, C.W. (2002). "Vertebrate Circulation". ''Magill's Encyclopedia of Science: Animal Life''. Vol 1, pp. 217–219. Pasadena, California, Salem Press.
OrganisationBirds have a very efficient system for diffusing oxygen into the blood; birds have a ten times greater surface area to 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 contractions, and become more rigid as they move away from the heart. Blood moves through the arteries, which undergo , and into arterioles which act as a transportation system to distribute primarily oxygen as well as nutrients to all tissues of the body.Hill, Richard W. (2012) Animal Physiology/ Richard W. Hill, Gordon A. Wyse, Margaret Anderson. Third Edition pp. 647–678. Sinauer Associates, Sunderland, MA As the arterioles move away from the heart and into individual organs and tissues they are further divided to increase surface area and slow blood flow. Blood travels through the arterioles and moves into the capillaries where gas exchange can occur. Capillaries are organised into capillary beds in tissues; it is here that blood exchanges oxygen for carbon dioxide waste. In the capillary beds, blood flow is slowed to allow maximum 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 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.
Nervous systemThe 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 coordinates movement and the controls behaviour patterns, navigation, mating and building. Most birds have a poor with notable exceptions including s, s and . The avian is usually highly developed. Water birds have special flexible lenses, allowing for vision in air and water. Some species also have dual . Birds are , possessing (UV) sensitive s in the eye as well as green, red and blue ones. They also have , likely to mediate . Many birds show plumage patterns in 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 s 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—s 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 , a third eyelid that moves horizontally. The nictitating membrane also covers the eye and acts as a in many aquatic birds. The bird has a fan shaped blood supply system called the . Eyes of most birds are large, not very round and capable of only limited movement in the orbits, typically 10-20°. Birds with eyes on the sides of their heads have a wide , while birds with eyes on the front of their heads, such as owls, have and can estimate the . The avian lacks external but is covered by feathers, although in some birds, such as the ', ' and ' s, these feathers form tufts which resemble ears. The has a , but it is not spiral as in mammals.
Defence and intraspecific combatA few species are able to use chemical defences against predators; some can eject an unpleasant against an aggressor, and some species of s from have a powerful in their skin and feathers. A lack of field observations limit our knowledge, but intraspecific conflicts are known to sometimes result in injury or death. The screamers (), some jacanas (', '), the spur-winged goose ('), the torrent duck (') and nine species of lapwing (') use a sharp spur on the wing as a weapon. The steamer ducks ('), geese and swans ('), the solitaire ('), sheathbills ('), some guans (') and stone curlews (') use a bony knob on the r metacarpal to punch and hammer opponents. The jacanas ' and ' have an expanded, blade-like radius. The extinct ' was unique in having an elongate forelimb and massive hand which likely functioned in combat or defence as a jointed club or flail. , for instance, may strike with the bony spurs and bite when defending eggs or young.
Feathers, plumage, and scalesFeathers are a feature characteristic of birds (though also present in not currently considered to be true birds). They facilitate , provide insulation that aids in , 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 . The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. The arrangement and appearance of feathers on the body, called , may vary within species by age, , and . Plumage is regularly ed; the standard plumage of a bird that has moulted after breeding is known as the "" plumage, or—in the —"basic" plumage; breeding plumages or variations of the basic plumage are known under the Humphrey–Parkes system as "" plumages. Moulting is annual in most species, although may have two moults a year, and large birds of prey may moult only once every few years. Moulting patterns vary across species. In passerines, s are replaced one at a time with the innermost being the first. When the fifth of sixth primary is replaced, the outermost begin to drop. After the innermost tertiaries are moulted, the starting from the innermost begin to drop and this proceeds to the outer feathers (centrifugal moult). The greater primary 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.de Beer SJ, Lockwood GM, Raijmakers JHFS, Raijmakers JMH, Scott WA, Oschadleus HD, Underhill LG (2001).
FlightMost birds can , 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 () that serves as an . 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 , as were many extinct birds. Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators. Although flightless, penguins use similar musculature and movements to "fly" through the water, as do some flight-capable birds such as s, s and s.
BehaviourMost birds are , but some birds, such as many species of s and s, are or (active during twilight hours), and many coastal s feed when the tides are appropriate, by day or night.
Diet and feedingare varied and often include , fruit, plants, seeds, , and various small animals, including other birds. The is unique, with a for storage and a 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 stored in many parts of their bodies, including protein from the intestines, as additional energy during migration. (Erratum in ''Proceedings of the Royal Society B'' 267(1461):2567.) 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. strategies can vary widely by species. Many birds for insects, invertebrates, fruit, or seeds. Some hunt insects by suddenly attacking from a branch. Those species that seek s are considered beneficial 'biological control agents' and their presence encouraged in programmes. Combined, insectivorous birds eat 400–500 million metric tons of arthropods annually. Nectar feeders such as s, s, amongst others have specially adapted brushy tongues and in many cases bills designed to fit flowers. s and s with long bills probe for invertebrates; shorebirds' varied bill lengths and feeding methods result in the separation of s. s, s, s and pursue their prey underwater, using their wings or feet for propulsion, while aerial predators such as , s and s plunge dive after their prey. s, three species of , and some ducks are s. and s are primarily grazers. Some species, including s, s, and s, engage in , 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 s stealing from estimated that the frigatebirds stole at most 40% of their food and on average stole only 5%. Other birds are s; some of these, like s, are specialised carrion eaters, while others, like gulls, s, or other birds of prey, are opportunists.
Water and drinkingWater is needed by many birds although their mode of excretion and lack of s 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 s 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 , , , and families are capable of sucking up water without the need to tilt back their heads. Some desert birds depend on water sources and 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 that they provide to their chicks.
Feather careFeathers, being critical to the survival of a bird, require maintenance. Apart from physical wear and tear, feathers face the onslaught of fungi, feather mites and . The physical condition of feathers are maintained by often with the application of secretions from the . 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 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.
MigrationMany bird species migrate to take advantage of global differences of al temperatures, therefore optimising availability of food sources and breeding habitat. These migrations vary among the different groups. Many landbirds, s, and s 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 or s and a non-breeding season in the regions or opposite hemisphere. Before migration, birds substantially increase body fats and reserves and reduce the size of some of their organs. (Erratum in ''Proceedings of the Royal Society B'' 267(1461):2567.) Migration is highly demanding energetically, particularly as birds need to cross deserts and oceans without refuelling. Landbirds have a flight range of around and shorebirds can fly up to , although the is capable of non-stop flights of up to . s also undertake long migrations, the longest annual migration being those of s, which nest in and and spend the northern summer feeding in the North Pacific off Japan, and , an annual round trip of . Other seabirds disperse after breeding, travelling widely but having no set migration route. es nesting in the often undertake circumpolar trips between breeding seasons. Some bird species undertake shorter migrations, travelling only as far as is required to avoid bad weather or obtain food. species such as the boreal es 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 partially migratory. 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 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. as a 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 released in in the United States returned to its colony in , in Wales within 13 days, a distance of . Birds navigate during migration using a variety of methods. For migrants, the 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 . Orientation with the stellar compass depends on the position of the s surrounding . These are backed up in some species by their ability to sense the Earth's through specialised .
CommunicationBirds 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, to display breeding condition in sexually selected species, or to make threatening displays, as in the 's mimicry of a large predator to ward off s and protect young chicks. Variation in plumage also allows for the identification of birds, particularly between species. Visual communication among birds may also involve ritualised displays, which have developed from non-signalling actions such as preening, the adjustments of feather position, pecking, or other behaviour. These displays may signal aggression or submission or may contribute to the formation of pair-bonds. The most elaborate displays occur during courtship, where "dances" are often formed from complex combinations of many possible component movements; males' breeding success may depend on the quality of such displays. , which are produced in the , are the major means by which birds communicate with . 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 ' s of drive air through their feathers, s drum for long-distance communication, and s use tools to drum.
Flocking and other associationsWhile some birds are essentially territorial or live in small family groups, other birds may form large . The principal benefits of flocking are and increased foraging efficiency. Defence against predators is particularly important in closed habitats like forests, where is common and multiple eyes can provide a valuable early warning system. This has led to the development of many s, 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 cases. Birds sometimes also form associations with non-avian species. Plunge-diving s associate with s and , which push shoaling fish towards the surface. s have a with s, in which they forage together and warn each other of nearby and other predators.
Resting and roostingThe 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. s 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 one 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 s by viewing the outer margins of the flock. This adaptation is also known from s. is common because it lowers the 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 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. 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 s go into a nightly state of accompanied with a reduction of their metabolic rates. This shows in nearly a hundred other species, including s, s, and s. One species, the , even enters a state of . 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 to cool themselves.
Social systemsNinety-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 and , which is especially important for species in which females require males' assistance for successful brood-rearing. Among many socially monogamous species, (infidelity) is common. Such behaviour typically occurs between dominant males and females paired with subordinate males, but may also be the result of in ducks and other . 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 , , , , and , 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 circumstances. Breeding usually involves some form of courtship display, typically performed by the male. Most displays are rather simple and involve some type of . Some displays, however, are quite elaborate. Depending on the species, these may include wing or tail drumming, dancing, aerial flights, or communal . Females are generally the ones that drive partner selection, although in the polyandrous , this is reversed: plainer males choose brightly coloured females. , and are commonly performed between partners, generally after the birds have paired and mated. in males or females in numerous species of birds, including copulation, pair-bonding, and joint parenting of chicks. Over 130 avian species around the world engage in sexual interactions between the same sex or homosexual behaviours. "Same-sex courtship activities may involve elaborate displays, synchronized dances, gift-giving ceremonies, or behaviors at specific display areas including bowers, arenas, or leks."
Territories, nesting and incubationMany 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 s and s, often breed in instead; this is thought to offer protection from predators. Colonial breeders defend small nesting sites, and competition between and within species for nesting sites can be intense. All birds lay s with hard shells made mostly of . Hole and burrow nesting species tend to lay white or pale eggs, while open nesters lay d eggs. There are many exceptions to this pattern, however; the ground-nesting s have pale eggs, and camouflage is instead provided by their plumage. Species that are victims of have varying egg colours to improve the chances of spotting a parasite's egg, which forces female parasites to match their eggs to those of their hosts. Bird eggs are usually laid in a . Most species create somewhat elaborate nests, which can be cups, domes, plates, beds scrapes, mounds, or burrows.Hansell M (2000). ''Bird Nests and Construction Behaviour''. University of Cambridge Press Some bird nests, however, are extremely primitive; 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 lays its eggs on bare rock, and male s keep eggs between their body and feet. The absence of nests is especially prevalent in ground-nesting species where the newly hatched young are . , 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 es, 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 of body weight per day of incubation. The warmth for the incubation of the eggs of s comes from the sun, decaying vegetation or volcanic sources. Incubation periods range from 10 days (in s, s and birds) to over 80 days (in albatrosses and s). The diversity of characteristics of birds is great, sometimes even in closely related species. Several avian characteristics are compared in the table below.
Parental care and fledgingAt the time of their hatching, chicks range in development from helpless to independent, depending on their species. Helpless chicks are termed ', and tend to be born small, , immobile and naked; chicks that are mobile and feathered upon hatching are termed '. Altricial chicks need help and must be brooded for longer than precocial chicks. The young of many bird species do not precisely fit into either the precocial or altricial category, having some aspects of each and thus fall somewhere on an "altricial-precocial spectrum". Chicks at neither extreme but favouring one or the other may be termed or . The length and nature of parental care varies widely amongst different orders and species. At one extreme, parental care in s 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 , whose chicks take up to six months to 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, of the same species—usually close relatives of the , such as offspring from previous broods—will help with the raising of the young. Such alloparenting is particularly common among the , which includes such birds as the true , and s, but has been observed in species as different as the and . Among most groups of animals, is rare. In birds, however, it is quite common—more so than in any other vertebrate class. Although territory and nest site defence, incubation, and chick feeding are often shared tasks, there is sometimes a in which one mate undertakes all or most of a particular duty. The point at which chicks varies dramatically. The chicks of the ' murrelets, like the , 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 .
Brood parasites, in which an egg-layer leaves her eggs with another individual's brood, is more common among birds than any other type of organism.Davies N (2000). ''Cuckoos, Cowbirds and other Cheats''. : London 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 s to increase their reproductive output even though they could have raised their own young. One hundred bird species, including s, s, and , are obligate parasites, though the most famous are the s. 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.
Sexual selectionBirds have a variety of behaviours, with the tail being perhaps the most famous example of and the . Commonly occurring s such as size and colour differences are energetically costly attributes that signal competitive breeding situations. Many types of avian 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 colouration and are "honest" traits. Signals must be costly to ensure that only good-quality individuals can present these exaggerated sexual ornaments and behaviours.
Inbreeding depressionInbreeding causes early death () in the ''Taeniopygia guttata''. Embryo survival (that is, hatching success of fertile eggs) was significantly lower for mating pairs than for unrelated pairs. ''Geospiza scandens'' experiences (reduced survival of offspring) and the magnitude of this effect is influenced by environmental conditions such as low food availability.
Inbreeding avoidanceIncestuous matings by the ''Malurus coronatus'' result in severe fitness costs due to (greater than 30% reduction in hatchability of eggs). Females paired with related males may undertake extra pair matings (see 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 (''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. s ''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. in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin. Female offspring rarely stay at home, dispersing over distances that allow them to breed independently, or to join unrelated groups. In general, inbreeding is avoided because it leads to a reduction in progeny fitness () due largely to the homozygous expression of deleterious recessive alleles. between unrelated individuals ordinarily leads to the masking of deleterious recessive alleles in progeny.
EcologyBirds 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 occupied by different species of birds vary, with some species feeding in the , others beneath the canopy, and still others on the forest floor. Forest birds may be s, s, and s. Aquatic birds generally feed by fishing, plant eating, and piracy or . Birds of prey specialise in hunting mammals or other birds, while vultures are specialised s. s 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 , 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 example, in New Zealand nine species of were important browsers, as are the and today. Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa. Nesting s may also affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of , which may enrich the local soil and the surrounding seas. A wide variety of , including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.
Relationship with humansSince birds are highly visible and common animals, humans have had a relationship with them since the dawn of man. Sometimes, these relationships are , like the cooperative honey-gathering among s and African peoples such as the . Other times, they may be , as when species such as the have benefited from human activities. Several bird species have become commercially significant agricultural pests, and some pose an . Human activities can also be detrimental, and have threatened numerous bird species with extinction (, , s, , kills and predation by pet s and s are common causes of death for birds). Birds can act as vectors for spreading diseases such as , , , mycobacteriosis (avian ), (bird flu), , and over long distances. Some of these are that can also be transmitted to humans.
Economic importanceDomesticated birds raised for meat and eggs, called , are the largest source of animal protein eaten by humans; in 2003, tons of poultry and tons of eggs were produced worldwide. s account for much of human poultry consumption, though domesticated , , and 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 s, s, quail, s, , , , and . is also popular in Australia and New Zealand. Although 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 of geese and ducks), which are used as insulation in clothing and bedding, and seabird faeces (), which is a valuable source of phosphorus and nitrogen. The , sometimes called the Guano War, was fought in part over the control of guano deposits. Birds have been domesticated by humans both as pets and for practical purposes. Colourful birds, such as and s, are bred in or kept as pets, a practice that has led to the illegal trafficking of some . s and s have long been used for and , respectively. s, used since at least 1 AD, remained important as recently as . Today, such activities are more common either as hobbies, for entertainment and tourism, or for sports such as . Amateur bird enthusiasts (called birdwatchers, twitchers or, more commonly, ) number in the millions. Many homeowners erect s 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 mythologyBirds play prominent and diverse roles in religion and mythology. In religion, birds may serve as either messengers or priests and leaders for a , such as in the Cult of , in which the of served as chiefs or as attendants, as in the case of , the two s who whispered news into the ears of the . In several civilisations of , particularly and , priests were involved in y, 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 , as when (, ) embodied the fright, passivity, mourning, and beauty traditionally associated with doves. Birds have themselves been deified, as in the case of the , which is perceived as Mother Earth by the people of southern India. In the ancient world, doves were used as symbols of the (later known as Ishtar), the mother goddess , Dorothy D. Resig
In culture and folkloreBirds have featured in culture and art since prehistoric times, when they were represented in early s. Some birds have been perceived as monsters, including the mythological and the 's legendary , a giant bird capable of snatching humans. Birds were later used as symbols of power, as in the magnificent of the and 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 , whose paintings of were a great commercial success in Europe and who later lent his name to the . Birds are also important figures in poetry; for example, incorporated s into his ', and used a as an erotic symbol in his . The relationship between an and a sailor is the central theme of 's ', which led to the use of the . Other metaphors derive from birds; s and vulture investors, for instance, take their name from the scavenging vulture. Perceptions of bird species vary across cultures. s are associated with bad luck, , and death in parts of Africa, but are regarded as wise across much of Europe. s were considered sacred in and symbols of virtue in , but were thought of as thieves across much of Europe and harbingers of war in . In , birds, especially , often appear in .
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 , , and did, along with many later composers; they can incorporate recordings of birds into their works, as first did; or like and , they can duet with birds.
ConservationAlthough human activities have allowed the expansion of a few species, such as the and , they have caused population decreases or 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 n, n, and n islands. Many bird populations are declining worldwide, with 1,227 species listed as by and the in 2009. The most commonly cited human threat to birds is . Other threats include overhunting, accidental mortality due to collisions with or , , pollution (including s and pesticide use), competition and predation from nonnative , and climate change. Governments and groups work to protect birds, either by passing laws that and bird habitat or by establishing 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 and .
See also* * * * * *
Further reading* Roger Lederer und Carol Burr: ''Latein für Vogelbeobachter: über 3000 ornithologische Begriffe erklärt und erforscht'', aus dem Englischen übersetzt von Susanne Kuhlmannn-Krieg, Verlag DuMont, Köln 2014, . * del Hoyo, Josep; Elliott, Andrew; Sargatal, Jordi (eds.): ''Handbook of the Birds of the World'' (17-volume encyclopaedia), Lynx Edicions, Barcelona, 1992–2010. (''Vol. 1: Ostrich to Ducks'': , etc.). * ''All the Birds of the World'', Lynx Edicions, 2020. * ''National Geographic Field Guide to Birds of North America'', National Geographic, 7th edition, 2017. * ''National Audubon Society Field Guide to North American Birds: Eastern Region'', National Audubon Society, Knopf. * ''National Audubon Society Field Guide to North American Birds: Western Region'', National Audubon Society, Knopf. * Svensson, Lars: ''Birds of Europe'', Princeton University Press, second edition, 2010. * Svensson, Lars: ''Collins Bird Guide: The Most Complete Guide to the Birds of Britain and Europe'', Collins, 2nd edition, 2010.