Habitat and features of the external structure of birds.

Birds live and breed in most habitats around the world and are present on all seven continents. Certainly, some areas contain more species than others, with the greatest diversity found in tropical regions.
The structures associated with flight, even if they are rudimentary or specialized for land or water movement, easily distinguish birds from other animals. While different skeletal and internal features are specific to different birds, feathers are unique and are present on all birds.