Birds are hollow bodied, feather winged creatures. There are various kinds of birds in the bird kingdom and are identified by their feathers and body size. Birds do not have teeth but instead have a beak that allows them to prey at their food. They have two claws for feet and are warm blooded creatures. They reproduce by laying eggs in their nests. These nests are built or adopted depending on the breed of bird. Their ability to fly is related to the fact that they are hollow boned. Some birds do not fly but have the ability to swim, for example ducks. The smallest bird in the world is the hummingbird which measure approximately 2.2 inches and weighs about 1.6 kg.