Aves

Aves is a class of animals within the subphylum Vertebrata that comprises the birds. Birds are characterized by feathers, a beak without teeth (though some embryos develop tiny teeth), a high metabolic rate, a four-chambered heart, hollow pneumatic bones, and a sturdy flight skeleton in most species. They are endothermic and lay hard-shelled eggs. Most possess wings, although many species are flightless.

Taxonomy and diversity: Aves comprises roughly 10,000 living species organized into two major clades, Palaeognathae (ratites and tinamous) and Neognathae, which together cover most birds. Neognathae includes the majority of species in orders such as Passeriformes (perching birds), Anseriformes (ducks and swans), Accipitriformes (hawks and eagles), and Charadriiformes (waders and gulls).

Evolution and systematics: Birds evolved from small theropod dinosaurs of the clade Maniraptora during the Mesozoic Era, with birds appearing in the late Jurassic. Archaeopteryx is among the earliest well-known fossils bridging non-avian dinosaurs and modern birds. Modern birds are sometimes subdivided into the crown group Aves and various stem-lineages.

Ecology and biology: Birds occupy diverse habitats worldwide, from deserts to forests to oceans. Adaptations include flight in most species, an efficient respiratory system with air sacs, keen vision, acute hearing, and complex vocalizations. Reproduction typically involves oviparous egg-laying with incubation by parents. Many species migrate seasonally.

Conservation and human interactions: Birds face threats such as habitat loss, hunting, invasive species, and climate change. Conservation efforts focus on habitat protection, monitoring populations, and legal protections. Birds are widely studied as ecological indicators and model organisms in biology.