What is a bird?
Birds are vertebrates, with a backbone and skeleton, although some of the bones are hollow to keep the bird light. Their forelimbs have the same bones as the human arm, but they are highly modified to form the structure for wings. Some of the bones in the wrist and fingers are fused together for extra strength.
Like mammals, birds are endothermic, but they are the only animals that have feathers. Feathers are made of keratin. Each feather has a stiff, hollow center shaft with hundreds of side branches called barbs. Each barb has two rows of side branches called barbules. This structure allows air to gather in the feathers, making them lightweight and keeping the bird’s body heat from escaping.
Life begins in a nest
All birds lay eggs with hard, waterproof shells, which they create nests for. A nest may be just a scrape in the sand or an elaborate structure of twigs, leaves, and other gathered materials. Birds incubate their eggs until they hatch.Then the parents continue to care for their young, bringing food to the nest site as needed. The chicks of some bird species, like chickens, are already covered with down and can start finding their own food. They are called precocial. Other chicks, like robins, hatch with no feathers and are helpless, depending on their parents to feed them. They are called altricial.
A variety of birds
There are more than 9,800 known species of birds. The smallest is the male bee hummingbird Mellisuga helenae, which is 2.25 inches (5.7 centimeters) long and weighs only 0.056 ounces (1.6 grams). Its body is the size of a large bumblebee! The largest bird is the ostrich Struthio camelus, which can weigh up to 340 pounds (154 kilograms). The bird with the most feathers is the tundra or whistling swan Cygnus columbianus, which has more than 25,000 feathers. The bird that flies the fastest is the peregrine falcon Falco peregrinus, at 117 miles per hour (188 kilometers per hour) in a steep dive.