Birds are vertebrate animals that have feathers, wings, and beaks. Like all vertebrates, they have bony skeleton. Most birds are able to fly, but some (like ostriches and penguins) cannot fly even though they still have wings. Other kinds of animals like insects and bats can fly too, but birds are the only animals with feathers. Birds have scales on their lower legs and feet, like reptiles. All birds reproduce by laying eggs with hard shells, and most build nests to help protect the eggs from weather and predators. Adult birds almost always sit on the eggs to keep them warm until they hatch.
Like mammals, birds are warm-blooded which means that they make their own body heat and can stay warm even when the sun is not out. Birds mostly eat high-energy foods like seeds and fruits, insects and other animals, nectar, or meat from dead animals. Very few birds eat other plant parts, like leaves or roots, which are more difficult to digest. Many birds feed insects to their babies to help them grow fast even if the adult birds eat seeds or fruits. The shape and size of bird beaks are adapted to the kinds of food they eat.
Birds can be found in all habitats above ground and there are even some species that make nests in underground burrows. Most birds try to lay their eggs in places that predators will not be able to reach easily.