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Evolution and Diversity
Birds and mammals both evolved from reptiles, although they each evolved from a different type of reptile.
Mammals first appeared about 225 million years ago and underwent a dramatic increase in number, diversity and size, after the extinction of the dinosaurs and large reptiles 65 million years ago.
There are currently about 4,000 living species of mammals which range in size from a 2 gram shrew to a 160,000 kilogram blue whale.
Archaeopteryx, the oldest known bird fossil, lived about 150 million years ago. Birds also underwent an extensive evolutionary radiation, but did so 138 to 66 million years ago, much earlier than mammals. There are now about 8,600 living species of birds -- which range from a 2 gram bee hummingbird to 150 kilogram ostriches. Although there are so many species of birds, they are less structurally diverse than other vertebrate groups, such as mammals or fish, probably because of the physical restraints of flying.
External Structures
Feathers are an anatomical structure unique to birds. Bird feathers vary in length, color and texture -- some feathers are so fine and sleek that the resemble fur -- but all birds are covered with feathers, which evolved as highly modified scales.
Mammals are covered with fur which also varies greatly in color, length and texture. A few species, such as humans or hairless variations of pets have little or no fur, but these are the exception rather than the rule. Modern birds do not have teeth; instead they have hard beaks for biting and tearing food. Mammals, however, have a variety of teeth.
Locomotion
Another striking difference between mammals and birds is that birds have wings in place of the arms or forelegs of mammal species. Many types of birds use their wings to fly. Some types of birds, such as large ostriches or emu, are now flightless. Some flightless birds, such as penguins, have wings that are adapted to swimming and are used as paddles rather than for flight.
Mammals usually walk on four limbs, although some mammals in the primate family have adapted to walking upright part-time or full-time. As well, bats have evolved for flight and whales and dolphins have adapted to an entirely aquatic habitat.
Internal Structures
Birds have reduced their weight by evolving hollow bones, which are reinforced with internal struts for strength. Mammals however, have solid bones. Birds also have a large vertical keel on their sternum, which mammals lack, to which the robust breast muscles are attached. These muscles are used to pull the wings during flight.
Reproduction
Birds reproduce by laying eggs, while most mammals give birth to live young. There are, however, a few species of mammals of ancient lineage, called monotremes, that lay leathery, reptile-like eggs. Male and female monotremes secrete milk to nourish their young. Making milk is another characteristically mammalian feature that birds lack.