Birds
Birds are a group of warm-blooded vertebrates constituting
the class Aves, characterized by feathers,
toothless beaked jaws, the laying of hard-shelled eggs,
a high metabolic rate, a four-chambered heart, and a strong yet
lightweight skeleton. Birds live worldwide and range in
size from the 5 cm (2 in) bee hummingbird to
the 2.75 m (9 ft) ostrich.
There are about ten thousand living species,[3] more than
half of which are passerine, or "perching" birds. Birds have wings whose
development varies according to species; the only known groups without wings
are the extinct moa and elephant birds.
Wings, which evolved from forelimbs, gave birds the ability to fly, although further
evolution has led to the loss of flight in some birds, including ratites, penguins,
and diverse endemic island species. The digestive and respiratory
systems of birds are also uniquely adapted for flight. Some bird species of
aquatic environments, particularly seabirds and
some waterbirds,
have further evolved for swimming.
Birds are a group of feathered theropod dinosaurs,
and constitute the only living dinosaurs. Likewise, the closest
living relatives of birds are the crocodilians.
Birds are descendants of the primitive avialans (whose
members include Archaeopteryx) which first appeared about 160 million
years ago (mya) in China. According to DNA evidence, modern birds (Neornithes)
evolved in the Middle to Late Cretaceous,
and diversified dramatically around the time of the Cretaceous–Paleogene extinction event 66
mya, which killed off the pterosaurs and all non-avian dinosaurs.
Some birds, especially corvids and parrots,
are among the most intelligent animals; several bird species make and use tools, with certain birds' use of
tools rivaling the best performances of primates.[4][5][6] Many social species pass
on knowledge across generations, which is considered a form of culture. Birds are social,
communicating with visual signals, calls, and songs,
and participating in such behaviours as cooperative breeding and hunting, flocking, and mobbing of predators. The vast majority
of bird species are socially (but not necessarily sexually) monogamous, usually for one breeding season at
a time, sometimes for years, but rarely for life. Other species have breeding
systems that are polygynous (one male with many females)
or, rarely, polyandrous (one female with many males).
Birds produce offspring by laying eggs which are fertilised through sexual reproduction. They are usually laid in a
nest and incubated by the parents. Most birds have
an extended period of parental care after hatching.
Many species of birds are
economically important as food for human consumption and raw material in
manufacturing, with domesticated and undomesticated birds
being important sources of eggs, meat, and feathers. Songbirds,
parrots, and other species are popular as pets. Guano (bird
excrement) is harvested for use as a fertiliser. Birds figure throughout human
culture. About 120 to 130 species have become extinct due
to human activity since the 17th century, and hundreds more before then. Human
activity threatens about 1,200 bird species with extinction, though efforts are
underway to protect them. Recreational birdwatching is
an important part of the ecotourism industry.