• Birds have existed for more than 140 million years--far, far longer than human beings have.Birds of prey image by Warren Millar from Fotolia.com
  
  Birds are vertebrates since they have a backbone. Feathers cover the body of birds instead of skin or fur, as with some other animals. Bird species vary from small, such as the hummingbird, to large, as in the flamingo. As of 2005, approximately 8,600 species of birds are known to exist on the Earth.
  
  

Birds of Flight

• Birds of prey, which feed on other birds or mammals, including even monkeys, tend to be among the largest birds that fly. Carrion-eating Andean condors are one of the largest birds of flight in the world, with a weight of up to 33 lbs. Among non-carrion-eating birds of prey, eagles are the largest. The bald eagle, the symbol of the United States of America, has a wingspan of up to eight feet and can weigh up to 14 lbs. Kori bustards are another species of very large birds capable of flight, weighing in at an astonishing 40 lbs.
  
  

Birds without Flight

• Most birds fly as a means of transportation, yet some are incapable of flight. The ostrich is the largest bird in the world. Although they cannot fly, they are extremely strong runners. Ostriches weigh in at up to 350 lbs and can stand up to 9 feet tall. Emus and greater rheas are other large birds incapable of flight.
  
  

Water Birds

• Some birds prefer a marine habitat. The largest of such birds include loons, geese, and pelicans. Pelicans can have a wingspan of up to a whopping ten feet and can also weigh up to 33 lbs. Loons, among the oldest living birds, can be up to three feet long and weigh as much as 15 lbs.