The fossil record
indicates that birds evolved from feathered ancestors within the theropod group, which are traditionally placed within
the saurischian dinosaurs, though a 2017 paper has put them in a proposed
clade Ornithoscelida, along with the Ornithischia. True birds first appeared during the Cretaceous period, around 120 million years
ago. DNA-based evidence finds that birds diversified dramatically around
the time of the Cretaceous–Palaeogene
extinction event 66 million years ago,
which reduced the Pterosaurs, and killed off all the non-avian dinosaur lineages.
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, around 170 million years ago. 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 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 flightless birds, including ratites, penguins, and diverse endemic island species
of birds. The digestiveand 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.
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 nest and 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 as food for
human consumption and raw material in manufacturing, with domesticated and
undomesticated birds (poultry and game) 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 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.
