Birds are vertebrates with bodies that are adapted for flying. However, some birds, like ostriches and penguins, cannot fly.
Birds live in nests which are either located on the ground or in bushes or trees. Some bird species build nests within holes that they burrow in the sides of trees while other birds favour making nests in the tree tops.
Birds, like mammals, have a constant body temperature. Their feathers help them to maintain their body heat. They are oviparous.
Birds have air sacks within their lungs. When they breathe in, fresh air provides oxygen. When they breathe out, carbon dioxide is expelled from the air sacks with the air. Matilda Afán de Rivera, 4° de primaria.