National Bird Day

Birds (Aves) are a group of vertebrates that evolved from dinosaurs. They are endothermic, with feathers.

Modern birds are toothless: they have beaked jaws. They lay hard-shelled eggs. They have a high metabolic rate, a four-chambered heart, and a strong yet lightweight skeleton.

Birds live all over the world. They range in size from the 5 cm (2 in) bee hummingbird to the 2.70 m (9 ft) ostrich. They are the class of tetrapods with the most living species: about ten thousand. More than half of these are passerines, sometimes known as perching birds.

Birds are the closest living relatives of the Crocodilia. The fossil record shows that birds evolved from feathered theropod dinosaurs.

Some birds, especially crows and parrots, are among the most intelligent animals. Several bird species make and use tools. Many social species pass on knowledge across generations, a form of culture. Many species annually migrate great distances. Birds are social. They communicate with visual signals, calls, and bird songs.

Most bird species are socially monogamous, usually for one breeding season at a time, sometimes for years, but rarely for life. Other species are polygynous (one male with many females) or, rarely, polyandrous (one female with many males). Birds produce offspring by laying eggs which are fertilized by sexual reproduction. 

Any species of birds are eaten by humans. Domesticated and undomesticated birds (poultry and game) are sources of eggs, meat, and feathers. Songbirds, parrots, and other species are popular as pets. Guano is harvested for use as a fertilizer. Birds 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.