Dating in plumtree
They rank as the class of tetrapods with 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.
It is not considered a direct ancestor of birds, though it is possibly closely related to the true ancestor.True birds first appeared during the Cretaceous period, around 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.Recreational birdwatching is an important part of the ecotourism industry.Aves and a sister group, the clade Crocodilia, contain the only living representatives of the reptile clade Archosauria.Avialae, initially proposed to replace the traditional fossil content of Aves, is often used synonymously with the vernacular term "bird" by these researchers.
Avialae is also occasionally defined as an apomorphy-based clade (that is, one based on physical characteristics).
The avialan species from this time period include Anchiornis huxleyi, Xiaotingia zhengi, and Aurornis xui.
The well-known early avialan, Archaeopteryx, dates from slightly later Jurassic rocks (about 155 million years old) from Germany.
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 moas and elephant birds.
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. 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.