Vertebrate, warm-blooded animals that walk or stay only on the hind limbs, while the forelimbs are modified as wings that, like many other unique anatomical features, are adaptations to flight, although not all fly. They have a feather-covered body and current birds a toothless horny beak. To reproduce, they lay eggs, which hatch until hatching.
Its taxonomic group is called Aves class for classical systematics, but in current phylogenetic systematics this clade has no rank, and is in turn included successively within clades: Theropoda, Dinosauria, Archosauria, Sauropsida, Tetrapoda, etc., although there are more intermediate nesting with denomination.
Birds originated from bipedal carnivorous dinosaurs from the Jurassic, 150-200 million years ago. Its subsequent evolution gave rise, after strong radiation to the nearly 10,000 current species. Birds are the most diverse tetrapods, yet they have great morphological homogeneity compared to mammals.
The kinship relationships of bird families cannot always be defined by morphology, but with DNA analysis they begin to become clear. Birds inhabit all terrestrial biomes, and also all oceans. The size can be from 6,4 cm in the hummingbird to 2,74 meters in the ostrich. Behaviors are diverse and remarkable, such as in nesting, hatchling feeding, migrations, mating, and the tendency for group association. Communication between birds is variable and can involve visual cues, calls, and singing. Some emit great diversity of sounds, and stand out for their intelligence and for the ability to culturally transmit knowledge to new generations.
The human being has had an intense relationship with birds. In the human economy, poultry and game birds are food sources. Songbirds and parrots are popular as pets. Down duck and domestic geese are used to fill pillows, and many birds were previously hunted to decorate hats with their feathers. Bird guano is used in soil fertilization. Some birds are revered or repudiated for religious reasons, superstitions, or erroneous prejudices. Many are cultural symbols and a frequent reference for art. Since the 17th century, more than 120 species have become extinct as a result of human activities and, currently, more than 1,200 threatened birds need efforts to conserve them.