References Birds are two-legged vertebrates with feathers, a characteristic that distinguishes them from all other animals.
Modern birds lack teeth and acquire food with a rigid protein structure known as a bill.
Some fossil birds, however, had teeth and other characteristics of bipedal reptiles, the group from which birds clearly evolved. The precise ancestry of birds has been a subject of considerable debate. Songbirds represent a major group within this order. Although several new species have been discovered during the past decade most of the world's bird species are known and, next to mammals, birds are one of the best studied groups of organisms. Because it encompasses a transition between prairie and savanna regions of the Central Plains and mixed conifer-hardwood forests of the Noorthern Lake States, Wisconsin supports particularly diverse opportunities for birdwatchers.