Stamps from United States of America with the theme Birds

Birds are feathered, winged, bipedal, warm-blooded vertebrates. There are about 10,000 different species of birds, more species than any other vertebrate. Many species are economically important, especially as a source of food acquired through hunting or breeding. Some species, particularly songbirds and parrots, are popular as pets. Since the 17th century over 120 species have become extinct because of human activities. Currently about 1,200 species of birds are threatened with extinction due to human activities, although efforts are made to protect them.