Birds (
Aves), also known as
avian dinosaurs, alre a group of
endothermic vertebrates, characterised by
feathers,
toothless beakedjaws, the
laying of
hard-shelled eggs, a high
metabolic rate, a four-chambered
heart, and a lightweight but strong
skeleton. Birds live worldwide and range in size from the 5 cm (2 in)
bee hummingbird to the 2.75 m (9 ft)
ostrich. They rank as the
class of
tetrapodswith the most living species, at approximately ten thousand, with more than half of these being
passerines, sometimes known as perching birds or, less accurately, as
songbirds.
The
fossil record indicates that birds are the
last surviving group of
dinosaurs, having evolved from
feathered ancestors within the
theropod group of
saurischian dinosaurs. True birds first appeared during the
Cretaceous period, around
100 million years ago.
[4] DNA-based evidence finds that birds diversified dramatically around the time of the
Cretaceous–Palaeogene extinction event that killed off all other dinosaurs. Birds, especially those in the southern continents, survived this event and then migrated to other parts of the world while diversifying during periods of global cooling. Primitive bird-like dinosaurs that lie outside class Aves proper, in the broader group
Avialae, have been found dating back to the mid-
Jurassic period. Many of these early "stem-birds", such as
Archaeopteryx, were not yet capable of fully powered flight, and many retained primitive characteristics like toothy jaws in place of beaks, and long bony tails.
Birds have
wings which are more or less developed depending on the species; the only known groups without wings are the
extinctmoas and
elephant birds. Wings, which evolved from
forelimbs, gave birds the ability to
fly, although further
speciation has led to the loss of
flight in
flightless birds, including
ratites,
penguins, and diverse
endemic island species of birds. The
digestive and
respiratory systems of birds are also uniquely adapted for flight. Some bird species of aquatic environments, particularly the aforementioned flightless penguins, and also members of the
duck family, have further evolved for
swimming. Birds, specifically
Darwin's finches, played an important part in the
inception of Darwin's theory of
evolution by
natural selection.
Some birds, especially
corvids and
parrots, are among the most
intelligent animals; several bird species
make and use tools, and many
social species pass on knowledge across generations, which is considered a form of
culture. Many species annually
migrate great distances. Birds are social, communicating with visual signals, calls, and
bird songs, and participating in such social behaviours as
cooperative breeding and hunting,
flocking, and
mobbing of predators. The vast majority of bird species are
socially monogamous (referring to social living arrangement, distinct from genetic monogamy), usually for one breeding season at a time, sometimes for years, but rarely for life. Other species have breeding systems that are
polygynous (arrangement of one male with many females) or, rarely,
polyandrous(arrangement of one female with many males). Birds produce offspring by laying
eggs which are fertilised through
sexual reproduction. They are usually laid in a
nestand
incubated by the parents. Most birds have an extended period of parental care after hatching. Some birds, such as
hens, lay eggs even when not fertilised, though unfertilised eggs do not produce offspring.
Many species of birds are economically important. Domesticated and undomesticated birds (
poultry and
game) are 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
prominently figure throughout human culture. About 120–130 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.