Birds have several key characteristics including being warm-blooded, having feathers, laying eggs, and having a four chambered heart. They are able to fly through generating lift as air passes over their wings with the top side having lower air pressure than the bottom. While most birds fly, some like penguins, ostriches and emus do not. Birds vary greatly in size from the smallest hummingbird to the largest ostrich and many migrate regularly for food, warmth or reproduction.