Birds are not mammals. They don't have fur, and they don't nurse their young with milk as mammals do; however, birds share some traits with mammals. They are warm blooded and can eat a wide variety of food, from fruits and seeds to reptiles, fish, small mammals and other birds.
Like mammals, birds are widespread and adapted to all sorts of habitats, living in all parts of the world from the tropics to the poles. Depending on their habitat, birds have different types of beaks, feet and nests. Herons have beaks like spears to catch fish. Sparrows and grosbeaks have heavy beaks to crack the shells of seeds. Hawks have grasping feet with sharp talons to catch their prey, while ducks, loons and geese have webbed feet to help them swim. Eagles build large nests in the forks of trees or on cliffs, killdeers lay their eggs on the bare ground and cuckoos lay their eggs in other birds' nests.
All birds have feathers, but not all birds can fly. Flightless birds include the ostrich, the emu, the cassowary and the penguin. Flying requires a great expenditure of energy. Flightless birds have frequently evolved in places where they have few predators or little competition. Other birds, such as raptors and swifts, are excellent fliers.