Birds

Birds (class Aves) are bipedal, warm-blooded, egg-laying vertebrate animals. Around 10,000 living and recently (after 1500) extinct species of birds compose the class Aves, making them the most diverse tetrapod vertebrates. more...

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They inhabit ecosystems across the globe, from Arctic terns to Antarctic penguins. Birds range in size from the tiny hummingbirds to the huge Ostrich. The fossil record indicates that birds evolved from theropod dinosaurs during the Jurassic period, c 200 to 150 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, though the ratites and several others, particularly endemic island species, have lost the ability to fly. Birds also have unique digestive and respiratory systems that are highly adapted for flight.

Many species of bird undertake long distance annual migrations, and many more perform shorter irregular movements. Birds are social and communicate using visual signals and through calls and song, and participate in social behaviours including cooperative hunting, cooperative breeding, flocking and mobbing of predators. The vast majority of bird species are socially monogamous, usually one breeding season at a time, sometimes for years, and rarely for life. Other species have breeding systems that are polygynous ("many females") or, rarely, polyandrous ("many males"). Among some monogamous species, extra-pair copulations are common. Eggs are usually laid in a nest and incubated and most birds have an extended period of parental care after hatching.

Birds are economically important to humans: many are important sources of food, acquired either through hunting or farming, and they provide other products. Some species, particularly songbirds and parrots, are popular as pets. Birds figure prominently in all aspects of human culture from religion to poetry and popular music. About 120–130 species have become extinct as a result of human activity since 1600, and hundreds more before this. Currently around 1,200 species of birds are threatened with extinction by human activities and efforts are underway to protect them.

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 still in use. Birds are categorised as the biological class Aves in Linnean taxonomy. Phylogenetic taxonomy places Aves in the dinosaur clade Theropoda. Aves and a sister group, the order 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. 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 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 in part to avoid the uncertainties about the placement of Archaeopteryx in relation to animals traditionally thought of as theropod dinosaurs.

Read more at Wikipedia.org