There are more than 10,000 species of birds throughout the world, from the tiny Anna’s hummingbird at 4.07 g to 4.31 g (.143 oz to .152 oz.) to the African ostrich 90 to 130 kilograms (198 to 287 pounds). Birds are endothermic or warm blooded, two-legged, egg-laying vertebrates with feathers and wings –though many species of birds are unable to fly such as emus and penguins. Some birds hatch and are covered in down and able eat on their own, such as chickens or geese. This is called precocial. Others, such as robins, are altricial, and are hatched without down and must be fed by one of both of the parents. return to main index