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Birds are bipedal animals, which means they have two feet and can stand, walk, and run. They are warm-blooded animals that can lay eggs. Eggs are usually laid in a nest and are incubated by the parents. Most birds take care of their young for an extended length of time after hatching.
Birds have feathers and a beak with no teeth. Birds have a very lightweight but strong skeleton. They have forelimbs that are covered with feathers and are called wings. The wings and lightweight skeleton allows them to fly. Birds range in size from the 2 inch Bee Hummingbird to the Ostrich which can grow to 9 feet tall.
There are about 10,000 species of birds. Many of the species migrate long distances.
Birds are also social animals which participate in social behaviors. They are known to work together in breeding, flocking, hunting and mobbing predators that threaten them. They communicate using songs and calls. They can also communicate by using visual signals, such as quickly flapping their wings.
About 120–130 species have become extinct since the 17th century, and hundreds more before then. Their extinction is blamed on human activity. Currently about 1,200 species of birds are threatened with extinction, though efforts are underway to protect them.
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Subject Index 
