About Birds
Birds (class Aves) are winged, bipedal, endothermic (warm-blooded), egg-laying, vertebrate animals. There are around 10,000 living species, making them the most varied of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from the 5 cm (2 in) Bee Hummingbird to the 2.75 m (9 ft) Ostrich. The fossil record indicates that birds 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 150–145 Ma. Most paleontologists regard birds as the only clade of dinosaurs to have survived the Cretaceous–Tertiary extinction event approximately 65.5 Ma.
Modern birds are characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. All living species of birds have wings - the now extinct flightless Moa of New Zealand were the only exceptions. Wings are evolved forelimbs, and most bird species 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. Some birds, especially corvids and parrots, are among the most intelligent animal species; a number of bird species have been observed manufacturing and using tools, and many social species exhibit cultural transmission of knowledge across generations.
Many species undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social; they communicate using visual signals and through calls and songs, and participate in social behaviors 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 fertilizer. 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.
Distribution
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 kilometers (270 mi) inland in Antarctica. The highest bird diversity occurs in tropical regions. It was earlier thought that this high diversity was the result of higher speciation rates in the tropics, however recent studies found higher speciation rates in the high latitudes that were offset by greater extinction rates than in the tropics. Several families of birds have adapted to life both on the world's oceans and in them, with some seabird species coming ashore only to breed and some penguins have been recorded diving up to 300 meters (980 ft).
Many bird species have established breeding populations in areas to which they have been introduced by humans. Some of these introductions have been deliberate; the Ring-necked Pheasant, for example, has been introduced around the world as a game bird. Others have been accidental, such as the establishment of wild Monk Parakeets in several North American cities after their escape from captivity. Some species, including Cattle Egret, Yellow-headed Caracara and Galah, have spread naturally far beyond their original ranges as agricultural practices created suitable new habitat.
Ecology
Birds occupy a wide range of ecological positions. While some birds are generalists, others are highly specialized in their habitat or food requirements. Even within a single habitat, such as a forest, the niches occupied by different species of birds vary, with some species feeding in the forest canopy, others beneath the canopy, and still others on the forest floor. Forest birds may be insectivores, frugivores, and nectarivores. Aquatic birds generally feed by fishing, plant eating, and piracy or kleptoparasitism. Birds of prey specialize in hunting mammals or other birds, while vultures are specialized scavengers. Avivores are animals that are specialized at predating birds.
Some nectar-feeding birds are important pollinators, and many frugivores play a key role in seed dispersal. Plants and pollinating birds often coevolve, and in some cases a flower's primary pollinator is the only species capable of reaching its nectar.
Birds are often important to island ecology. Birds have frequently reached islands that mammals have not; on those islands, birds may fulfill ecological roles typically played by larger animals. For example, in New Zealand the moas were important browsers, as are the Kereru and Kokako today. Today the plants of New Zealand retain the defensive adaptations evolved to protect them from the extinct moa. Nesting seabirds may also affect the ecology of islands and surrounding seas, principally through the concentration of large quantities of guano, which may enrich the local soil and the surrounding seas.
A wide variety of Avian ecology field methods, including counts, nest monitoring, and capturing and marking, are used for researching avian ecology.
Conservation
Though human activities have allowed the expansion of a few species, such as the Barn Swallow and European Starling, they have caused population decreases or extinction in many other species. Over a hundred bird species have gone extinct in historical times, although the most dramatic human-caused avian extinctions, eradicating an estimated 750–1800 species, occurred during the human colonization of Melanesian, Polynesian, and Micronesian islands. Many bird populations are declining worldwide, with 1,227 species listed as threatened by Birdlife International and the IUCN in 2009.
The most commonly cited human threat to birds is habitat loss. Other threats include overhunting, accidental mortality due to structural collisions or long-line fishing bycatch, pollution (including oil spills and pesticide use), competition and predation from nonnative invasive species, and climate change.
Governments and conservation groups work to protect birds, either by passing laws that preserve and restore bird habitat or by establishing captive populations for reintroductions. Such projects have produced some successes; one study estimated that conservation efforts saved 16 species of bird that would otherwise have gone extinct between 1994 and 2004, including the California Condor and Norfolk Parakeet.
Evolution and taxonomy
The first classification of birds was developed by Francis Willughby and John Ray in their 1676 volume Ornithologiae. Carolus Linnaeus modified that work in 1758 to devise the taxonomic classification system currently in use. Birds are categorized as the biological class Aves in Linnaean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria. Phylogenetically, Aves is commonly defined as all descendants of the most recent common ancestor of modern birds and Archaeopteryx lithographica.
Archaeopteryx, from the Tithonian stage of the Late Jurassic (some 150–145 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, the crown group. This has been done by excluding most groups known only from fossils, and assigning them, instead, to the Avialae in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.
All modern birds lie within the subclass Neornithes, which has two subdivisions: the Palaeognathae, containing birds that are flightless (like ostriches) or weak fliers, and the wildly diverse Neognathae, containing all other birds. These two subdivisions are often given the rank of superorder, although Livezey and Zusi assigned them "cohort" rank. Depending on the taxonomic viewpoint, the number of known living bird species varies anywhere from 9,800 to 10,050.
Dinosaurs and the origin of birds
Based on fossil and biological evidence, most scientists accept that birds are a specialized sub-group of theropod dinosaurs. More specifically, they are members of Maniraptora, a group of theropods which includes dromaeosaurs and oviraptorids, among others. 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.
The consensus view in contemporary paleontology is that the birds, Aves, are the closest relatives of the deinonychosaurs, which include dromaeosaurids and troodontids. Together, these three form a group called Paraves. The basal dromaeosaur Microraptor has features which may have enabled it to glide or fly. The most basal deinonychosaurs are very small. This evidence raises the possibility that the ancestor of all paravians may have been arboreal, may have been able to glide, or both.
The Late Jurassic Archaeopteryx is well-known as one of the first transitional fossils to be found and it provided support for the theory of evolution in the late 19th century. Archaeopteryx has clearly reptilian characteristics: teeth, clawed fingers, and a long, lizard-like tail, but it has finely preserved wings with flight feathers identical to those of modern birds. It is not considered a direct ancestor of modern birds, but is the oldest and most primitive known member of Aves or Avialae, and it is probably closely related to the real ancestor.
Alternative theories and controversies
There have been many controversies in the study of the origin of birds. Early disagreements included whether birds evolved from dinosaurs or more primitive archosaurs. Within the dinosaur camp there were disagreements as to whether ornithischian or theropod dinosaurs were the more likely ancestors. 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. In fact, a bird-like hip structure evolved a third time among a peculiar group of theropods known as the Therizinosauridae. A few scientists suggest that birds are not dinosaurs, but evolved from early archosaurs like Longisquama.
Early evolution of birds
Birds diversified into a wide variety of forms during the Cretaceous Period. Many groups retained primitive characteristics, such as clawed wings and teeth, though the latter were lost independently in a number of bird groups, including modern birds (Neornithes). While the earliest forms, such as Archaeopteryx and Jeholornis, retained the long bony tails of their ancestors, 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 was 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. More advanced lineages also specialized in eating fish, like the superficially gull-like subclass of Ichthyornithes ("fish birds").
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.
Diversification of modern birds
Containing all modern birds, the subclass Neornithes is, due to the discovery of Vegavis, now known to have evolved into some basic lineages by the end of the Cretaceous and is split into two superorders, the Palaeognathae and Neognathae. The paleognaths include the tinamous of Central and South America and the ratites. The basal divergence from the remaining Neognathes was that of the Galloanserae, the superorder containing the Anseriformes (ducks, geese, swans and screamers) and the Galliformes (the pheasants, grouse, and their allies, together with the mound builders and the guans and their allies). The dates for the splits are much debated by scientists. 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. 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.
The classification of birds is a contentious issue. Sibley and Ahlquist's Phylogeny and Classification of Birds (1990) are a landmark work on the classification of birds, although it is frequently debated and constantly revised. Most evidence seems to suggest that 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.
Anatomy and Physiology
Compared with other vertebrates, birds have a body plan that shows many unusual adaptations, mostly to facilitate flight.
The skeleton consists of very lightweight bones. They have large air-filled cavities (called pneumatic cavities) which connect with the respiratory system. The skull bones in adults are fused and do not show cranial sutures. The orbits are large and separated by a bony septum. The spine has cervical, thoracic, lumbar and caudal regions with the number of cervical (neck) vertebrae highly variable and especially flexible, but movement is reduced in the anterior thoracic vertebrae and absent in the later vertebrae. The last few are fused with the pelvis to form the synsacrum. The ribs are flattened and the sternum is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings.
Like the reptiles, birds are primarily uricotelic, that is, their kidneys extract nitrogenous wastes from their bloodhe Ostrich) uric acid is excreted along with feces as a semisolid waste. However, birds such as hummstream and excrete it as uric acid instead of urea or ammonia via the ureters into the intestine. Birds do not have a urinary bladder or external urethral opening and (with exception of tingbirds can be facultatively ammonotelic, excreting most of the nitrogenous wastes as ammonia. They also excrete creatine, rather than creatinine like mammals. This material, as well as the output of the intestines, emerges from the bird's cloaca. The cloaca is a multi-purpose opening: waste is expelled through it, birds mate by joining cloaca, and females lay eggs from it. In addition, many species of birds regurgitate pellets. The digestive system of birds is unique, with a crop for storage and a gizzard that contains swallowed stones for grinding food to compensate for the lack of teeth. Most birds are highly adapted for rapid digestion to aid with flight. Some migratory birds have adapted to use protein from many parts of their bodies, including protein from the intestines, as additional energy during migration.
Birds have one of the most complex respiratory systems of all animal groups. Upon inhalation, 75% of the fresh air bypasses the lungs and flows directly into a posterior air sac which extends from the lungs and connects with air spaces in the bones and fills them with air. The other 25% of the air goes directly into the lungs. When the bird exhales, the used air flows out of the 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. Sound production is achieved using the syrinx, a muscular chamber incorporating multiple tympanic membranes which diverges from the lower end of the trachea; the trachea being elongated in some species, increasing the volume of vocalizations and the perception of the bird's size. 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). The postcava receives blood from the limbs via the renal portal system. Unlike in mammals, the red blood cells in birds have a nucleus.
Chromosomes
Birds have two sexes: male and female. The sex of birds is determined by the Z and W sex chromosomes, rather than by the X and Y chromosomes present in mammals. Male birds have two Z chromosomes (ZZ), and female birds have a W chromosome and a Z chromosome (WZ).
In nearly all species of birds, an individual's sex is determined at fertilization. However, one recent study demonstrated temperature-dependent sex determination among Australian Brush-turkeys, for which higher temperatures during incubation resulted in a higher female-to-male sex ratio.
Flight
Most birds can fly, which distinguishes them from almost all other vertebrate classes. Flight is the primary means of locomotion for most bird species and is used for breeding, feeding, and predator avoidance and escape. Birds have various adaptations for flight, including a lightweight skeleton, two large flight muscles, the pectoralis (which accounts for 15% of the total mass of the bird) and the supracoracoideus, as well as a modified forelimb (wing) that serves as an aerofoil. Wing shape and size generally determine a bird species' type of flight; many birds combine powered, flapping flight with less energy-intensive soaring flight. About 60 extant bird species are flightless, as were many extinct birds. Flightlessness often arises in birds on isolated islands, probably due to limited resources and the absence of land predators. Though flightless, penguins use similar musculature and movements to "fly" through the water, as do auks, shearwaters and dippers.