There are more than 9,800 known species of birds. They have a backbone and skeleton. Some of the bones are hollow to keep the bird light to fly. They are warm blooded, and are the only animals that have feathers. All birds lay eggs with hard, waterproof shells, which they create nests for. Birds incubate their eggs and care for their young, bringing food to the nest. The smallest bird is the male bee hummingbird, which is the size of a large bumblebee. The largest bird is the ostrich, which can weigh up to 340 pounds.