Aves, the Latin word for birds, is a class of warm-blooded, toothless, feathered, winged vertebrates with horny beaks that are characterized by a number of features that facilitate flight. The Aves' high metabolic rate, four-chambered heart, lightweight but strong skeleton, and unique digestive and respiratory systems are well-suited to their flying needs. All birds lay hard-shelled eggs with a fatty yolk to support their embryos.

Birds are the most widespread flying vertebrates, though the Aves class also includes some non-flying species, including ostriches, emus and amphibious penguins.

Aves are a diverse group with over 10,000 known species, although their structural variations are less than those of many other vertebrate groups. Birds vary in size from tiny hummingbirds that weigh a mere fraction of an ounce to ostriches that can weigh in excess of 400 pounds.

All birds are bipedal, with their forelimbs evolved into wings. In some non-flying species such as kiwis, these wings have been reduced to vestiges. Other birds, namely penguins, still use their wings to great effect, but to swim rather than fly.

Birds play many important roles in the ecosystem. Bird species include pollinators such as hummingbirds, predators such as owls and scavengers like Marabou storks.