Like mammals, birds are endothermic, or warm blooded, but they are the only animals that have feathers. The 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.
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. 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.