Birds are warm-blooded vertebrates with a strong yet lightweight skeleton.
Feathers, beaks, and claws are formed of keratin, also seen in reptile scales but different from that of mammals. Their flight feathers are waterproof.
Birds don't have heavy jaws and teeth but a keratin beak that helps them eat, preen, and hunt for prey.
Their forelimbs are modified to form wings for flight. A penguin uses its wing as a flipper rather than for flying.
The feet of the bird are generally covered with keratin scales and help in holding and killing prey or other functions such as swimming.
Birds have excellent sight, with the horizon and the ground in focus simultaneously.