Wiggidy Wednesday
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and not to be left out, birds...
Birds (class Aves) are bipedal, warm-blooded, vertebrate animals that lay eggs. There are around 10,000 living species, making them the most diverse tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.7 m (9 ft) Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma.
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 birds have forelimbs modified as wings and most can fly, with some exceptions including ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight.
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 breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). 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.
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[1] Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[2] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.[3] Aves and a sister group, the clade Crocodilia, together are the sole living members of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[4] Archaeopteryx, from the Kimmeridgian stage of the Late Jurassic (some 155–150 million years ago), is the earliest known bird under this definition. Others, including Jacques Gauthier and adherents of the Phylocode system, have defined Aves to include only the modern bird groups, excluding most groups known only from fossils, and assigning them, instead, to the Avialae[5] 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 subclass Neornithes, which is divided into two superorders: the Paleognathae, containing mostly flightless birds like ostriches, and the wildly diverse Neognathae, containing all other birds.[3] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[6] to 10,050.[7]
There is substantial evidence that birds are theropod dinosaurs—more specifically, that they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.[8] As scientists discover more non-avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate that many small theropod dinosaurs had feathers, contribute to this ambiguity.[9]
The oldest known bird, the Late Jurassic Archaeopteryx, is well-known as one of the first transitional fossils to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Protoavis texensis may be even older, although the fossil's fragmentation leaves considerable doubt regarding whether it was a bird ancestor.[10]
The dromaeosaurids Cryptovolans and Microraptor may have been capable of powered flight to a similar or greater extent than that of Archaeopteryx. Cryptovolans had a sternal keel, and ribs with uncinate processes, and in fact makes a better "bird" than Archaeopteryx, which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds, and that the larger members of the family are secondarily flightless. This theory suggests that dromaeosaurs evolved from birds and not the other way around.[11] Evidence for this theory is currently inconclusive, as the exact relationship between the most advanced maniraptoran dinosaurs and the most primitive true birds is not well understood.
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.[12] In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae.
An alternate theory to the dinosaurian origin of birds, proposed by a few scientists (most notably Larry Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama;[13] this theory is contested by most paleontologists.[14]
Birds diversified into a wide variety of forms during the Cretaceous Period.[15] 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 such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors,[15] the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia.
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.[15] More advanced lineages also specialized in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds").[16] One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that 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.[15]
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[17] and is split into two superorders, the Paleognathae 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. It is agreed that the Neornithes evolved in the Cretaceous, and that the split between the Galloanseri from other Neognathes occurred before the K–T 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.[18] 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.[18][19]
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,[20] although it is frequently debated and constantly revised. Most evidence seems to suggest that the modern bird orders constitute accurate taxa,[21] 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.
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.[23] The highest bird diversity occurs in tropical regions, which may result from higher speciation rates in the tropics or greater extinction rates at higher latitudes.[24] 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[25] and some penguins recorded diving up to 300 metres (980 ft).[26]
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.[27] Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity.[28] Some species, including Cattle Egret,[29] Yellow-headed Caracara[30] and Galah,[31] have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.
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.[32] The skull bones are fused and do not show cranial sutures.[33] 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.[34] The last few are fused with the pelvis to form the synsacrum.[33] 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.[35]
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. Uric acid is excreted along with feces as a semisolid waste since birds do not have a separate bladder or uretral opening.[36][37] However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.[38] They also excrete creatine, rather than creatinine like mammals.[33] This material, as well as the output of the intestines, emerges from the bird's cloaca.[39][40] 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.[41] 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.[42] Most birds are highly adapted for rapid digestion to aid with flight.[43] Some migratory birds have the additional ability to reduce parts of the intestines prior to migration.[44]
Birds have one of the most complex respiratory systems of all animal groups.[33] 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.[45] Sound production is achieved using the syrinx, a muscular chamber with several tympanic membranes which is situated at the lower end of the trachea, from where it separates.[46] The bird's heart has four chambers and the right aortic arch gives rise to systemic circulation (unlike in the mammals where the left arch is involved).[33] The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the red blood cells in birds have a nucleus.[47]
The nervous system is large relative to the bird's size.[33] 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,[48] New World vultures[49] and tubenoses.[50] The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water.[33] 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.[51] 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 **** have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.[52] 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.[53] 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.[54] The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.[33] The bird retina has a fan shaped blood supply system called the pecten.[33] Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant.[55] 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.[56] 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.[57]
A few species are able to use chemical defenses against predators; some Procellariiformes can eject an unpleasant oil against an aggressor,[58] and some species of pitohuis from New Guinea secrete a powerful neurotoxin in their skin and feathers.[59]
A feature unique to birds are feathers – epidermal growths attached to the skin that serve a variety of functions: they aid in thermoregulation by providing insulation in cold weather and water, are essential to flight, and are also used in display, camouflage and signaling.[33] There are several types of feathers, each serving its own set of purposes. Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this.[60] 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.[61] 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.[62]
Plumage is the term given to the arrangement and appearance of feathers on the body; within species this can vary with age, social status,[63] or most commonly by biological gender.[64] 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.[65] 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. Some drop and regrow wing flight feathers, starting sequentially from the outermost feathers and progressing inwards (centripetal), while others replace feathers starting from the innermost ones (centrifugal). A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.[66] Centripetal moults of tail feathers are seen for example in the Phasianidae.[67] Centrifugal moult is seen, for instance, in the tail feathers of woodpeckers and treecreepers, although it begins with the second innermost pair of tail-feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail.[68] The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward.[69]
Feathers do not arise from all parts of a bird's skin, but grow in specific tracts, called pterylae. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. 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.[70]
Flight characterises most birds, distinguishing them from almost all other vertebrates with the exception of mammalian bats and the extinct pterosaurs. As the main means of locomotion for most bird species, flight is used for breeding, feeding, and predator avoidance and escape. Birds have a variety of adaptations to flight, including a lightweight skeleton, two large flight muscles, the pectoralis (accounting for 15% of the total mass of the bird) and the supracoracoideus, and a modified forelimb (the wing) serving as an aerofoil.[33] Wing shape and size are the main determiners of the type of flight a species is capable of. Many birds combine powered or flapping flight with less energy intensive soaring flight. About 60 species of extant birds are flightless, and many extinct birds were also flightless.[71] Flightlessness often arises in birds on isolated islands, probably due to the lack of land predators and limited resources, which are conditions that reward the loss of costly unnecessary adaptations.[72] Though flightless, penguins use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.[73]
Birds (class Aves) are bipedal, warm-blooded, vertebrate animals that lay eggs. There are around 10,000 living species, making them the most diverse tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.7 m (9 ft) Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, around 150–200 Ma (million years ago), and the earliest known bird is the Late Jurassic Archaeopteryx, c 155–150 Ma.
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 birds have forelimbs modified as wings and most can fly, with some exceptions including ratites, penguins, and a number of diverse endemic island species. Birds also have unique digestive and respiratory systems that are highly adapted for flight.
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 breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). 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.
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae.[1] Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use.[2] Birds are categorised as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda.[3] Aves and a sister group, the clade Crocodilia, together are the sole living members of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.[4] Archaeopteryx, from the Kimmeridgian stage of the Late Jurassic (some 155–150 million years ago), is the earliest known bird under this definition. Others, including Jacques Gauthier and adherents of the Phylocode system, have defined Aves to include only the modern bird groups, excluding most groups known only from fossils, and assigning them, instead, to the Avialae[5] 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 subclass Neornithes, which is divided into two superorders: the Paleognathae, containing mostly flightless birds like ostriches, and the wildly diverse Neognathae, containing all other birds.[3] Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800[6] to 10,050.[7]
There is substantial evidence that birds are theropod dinosaurs—more specifically, that they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others.[8] As scientists discover more non-avian theropods that are closely related to birds, the previously clear distinction between non-birds and birds has become blurred. Recent discoveries in the Liaoning Province of northeast China, which demonstrate that many small theropod dinosaurs had feathers, contribute to this ambiguity.[9]
The oldest known bird, the Late Jurassic Archaeopteryx, is well-known as one of the first transitional fossils to be found in support of evolution in the late 19th century, though it is not considered a direct ancestor of modern birds. Protoavis texensis may be even older, although the fossil's fragmentation leaves considerable doubt regarding whether it was a bird ancestor.[10]
The dromaeosaurids Cryptovolans and Microraptor may have been capable of powered flight to a similar or greater extent than that of Archaeopteryx. Cryptovolans had a sternal keel, and ribs with uncinate processes, and in fact makes a better "bird" than Archaeopteryx, which lacks some of these modern bird features. Because of this, some paleontologists have suggested that dromaeosaurs are actually basal birds, and that the larger members of the family are secondarily flightless. This theory suggests that dromaeosaurs evolved from birds and not the other way around.[11] Evidence for this theory is currently inconclusive, as the exact relationship between the most advanced maniraptoran dinosaurs and the most primitive true birds is not well understood.
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.[12] In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae.
An alternate theory to the dinosaurian origin of birds, proposed by a few scientists (most notably Larry Martin and Alan Feduccia), states that birds (including maniraptoran "dinosaurs") evolved from early archosaurs like Longisquama;[13] this theory is contested by most paleontologists.[14]
Birds diversified into a wide variety of forms during the Cretaceous Period.[15] 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 such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors,[15] the tails of more advanced birds were shortened with the advent of the pygostyle bone in the clade Pygostylia.
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.[15] More advanced lineages also specialized in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds").[16] One order of Mesozoic seabirds, the Hesperornithiformes, became so well adapted to hunting fish in marine environments that 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.[15]
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[17] and is split into two superorders, the Paleognathae 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. It is agreed that the Neornithes evolved in the Cretaceous, and that the split between the Galloanseri from other Neognathes occurred before the K–T 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.[18] 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.[18][19]
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,[20] although it is frequently debated and constantly revised. Most evidence seems to suggest that the modern bird orders constitute accurate taxa,[21] 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.
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.[23] The highest bird diversity occurs in tropical regions, which may result from higher speciation rates in the tropics or greater extinction rates at higher latitudes.[24] 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[25] and some penguins recorded diving up to 300 metres (980 ft).[26]
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.[27] Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity.[28] Some species, including Cattle Egret,[29] Yellow-headed Caracara[30] and Galah,[31] have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.
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.[32] The skull bones are fused and do not show cranial sutures.[33] 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.[34] The last few are fused with the pelvis to form the synsacrum.[33] 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.[35]
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. Uric acid is excreted along with feces as a semisolid waste since birds do not have a separate bladder or uretral opening.[36][37] However, birds such as hummingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia.[38] They also excrete creatine, rather than creatinine like mammals.[33] This material, as well as the output of the intestines, emerges from the bird's cloaca.[39][40] 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.[41] 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.[42] Most birds are highly adapted for rapid digestion to aid with flight.[43] Some migratory birds have the additional ability to reduce parts of the intestines prior to migration.[44]
Birds have one of the most complex respiratory systems of all animal groups.[33] 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.[45] Sound production is achieved using the syrinx, a muscular chamber with several tympanic membranes which is situated at the lower end of the trachea, from where it separates.[46] The bird's heart has four chambers and the right aortic arch gives rise to systemic circulation (unlike in the mammals where the left arch is involved).[33] The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the red blood cells in birds have a nucleus.[47]
The nervous system is large relative to the bird's size.[33] 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,[48] New World vultures[49] and tubenoses.[50] The avian visual system is usually highly developed. Water birds have special flexible lenses, allowing accommodation for vision in air and water.[33] 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.[51] 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 **** have an ultraviolet reflective crown patch which is displayed in courtship by posturing and raising of their nape feathers.[52] 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.[53] 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.[54] The nictitating membrane also covers the eye and acts as a contact lens in many aquatic birds.[33] The bird retina has a fan shaped blood supply system called the pecten.[33] Most birds cannot move their eyes, although there are exceptions, such as the Great Cormorant.[55] 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.[56] 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.[57]
A few species are able to use chemical defenses against predators; some Procellariiformes can eject an unpleasant oil against an aggressor,[58] and some species of pitohuis from New Guinea secrete a powerful neurotoxin in their skin and feathers.[59]
A feature unique to birds are feathers – epidermal growths attached to the skin that serve a variety of functions: they aid in thermoregulation by providing insulation in cold weather and water, are essential to flight, and are also used in display, camouflage and signaling.[33] There are several types of feathers, each serving its own set of purposes. Feathers require maintenance and birds preen or groom them daily, spending an average of around 9% of their daily time on this.[60] 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.[61] 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.[62]
Plumage is the term given to the arrangement and appearance of feathers on the body; within species this can vary with age, social status,[63] or most commonly by biological gender.[64] 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.[65] 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. Some drop and regrow wing flight feathers, starting sequentially from the outermost feathers and progressing inwards (centripetal), while others replace feathers starting from the innermost ones (centrifugal). A small number of species, such as ducks and geese, lose all of their flight feathers at once, temporarily becoming flightless.[66] Centripetal moults of tail feathers are seen for example in the Phasianidae.[67] Centrifugal moult is seen, for instance, in the tail feathers of woodpeckers and treecreepers, although it begins with the second innermost pair of tail-feathers and finishes with the central pair of feathers so that the bird maintains a functional climbing tail.[68] The general pattern seen in passerines is that the primaries are replaced outward, secondaries inward, and the tail from center outward.[69]
Feathers do not arise from all parts of a bird's skin, but grow in specific tracts, called pterylae. The distribution pattern of these feather tracts (pterylosis) is used in taxonomy and systematics. 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.[70]
Flight characterises most birds, distinguishing them from almost all other vertebrates with the exception of mammalian bats and the extinct pterosaurs. As the main means of locomotion for most bird species, flight is used for breeding, feeding, and predator avoidance and escape. Birds have a variety of adaptations to flight, including a lightweight skeleton, two large flight muscles, the pectoralis (accounting for 15% of the total mass of the bird) and the supracoracoideus, and a modified forelimb (the wing) serving as an aerofoil.[33] Wing shape and size are the main determiners of the type of flight a species is capable of. Many birds combine powered or flapping flight with less energy intensive soaring flight. About 60 species of extant birds are flightless, and many extinct birds were also flightless.[71] Flightlessness often arises in birds on isolated islands, probably due to the lack of land predators and limited resources, which are conditions that reward the loss of costly unnecessary adaptations.[72] Though flightless, penguins use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.[73]
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