December 8, 2021

Birds are a class of endothermic vertebrate living beings characterized by the presence of feathers, a toothless beak, rigid shell oviparity, high metabolism, a four-chambered heart, and a strong, lightweight pneumatic skeleton. Birds are present in all regions of the world and vary significantly in size, from the 5 cm of the hummingbird to the 2.75 m of the ostrich. They are the class of tetrapods with the largest number of living species, approximately ten thousand, of which more than half are passerines. Birds have wings, which are more or less developed depending on the species. The only known groups without wings are the moas and elephant birds, both extinct. The wings, which evolved from the forelegs, give birds the ability to fly, although speciation has produced non-flying birds such as ostriches, penguins, and several island endemic birds. The digestive and respiratory systems of birds are adapted to flight. Some species of birds that inhabit aquatic ecosystems, such as penguins and the duck family, have developed the ability to swim. Some birds, especially crows and parrots, are among the smartest animals on the planet. Some species build and use tools and pass knowledge between generations. Many species migrate over great distances. Birds are social animals that communicate with each other with visual signals, calls and songs, and carry out community activities such as breeding and cooperative hunting, flight in groups and groups to defend against predators. The vast majority of bird species are monogamous, usually during a mating season and sometimes for several years, but rarely for a lifetime. Other species are polygamous or, more rarely, polyandrous. Birds reproduce through eggs, which are fertilized for sexual reproduction and usually placed in a nest where they are incubated by the parents. Most birds have a prolonged period of parental care after hatching. Some birds, such as chickens, lay eggs even if they are not fertilized, although those eggs do not produce offspring. Birds, and in particular Darwin's finches, played an important role in the development of Darwin's theory of evolution by natural selection. The fossil record indicates that birds are the last dinosaur survivors, having evolved from feathered dinosaurs within the saurischian theropod group. The first birds appeared during the Cretaceous period, about 100 million years ago, and the last common ancestor is estimated to have lived 95 million years ago. DNA evidence indicates that birds evolved extensively during the Cretaceous-Paleogene extinction that killed non-avian dinosaurs. Birds in South America survived this event, then migrated to various parts of the world through various land passages, while diversifying into species during periods of global cooling. Some primitive birds within the Avialae group date back to the Jurassic period. Many of these bird ancestors, such as Archeopteryx, did not have full flight capability and many still had primitive features such as a jaw rather than a beak and vertebrate tail. Many bird species are economically important. Domesticated (poultry) and non-domesticated (game) birds are important sources of eggs, meat and feathers. Songbirds and parrots are popular pets. Guano is used as a fertilizer. Birds are a prominent element in the culture. However, since the 17th century, about 120 to 130 species have gone extinct due to human action and several hundred have gone extinct in previous centuries. There are currently 1 375 species of birds threatened by and

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