Diving birds are birds which plunge into the water to catch fish or other prey. Such birds may enter the water from flight, such as pelicans, gannets and tropicbirds; or they may dive from the surface of the water, such as the diving ducks, cormorants and penguins. It is theorized that they evolved from birds already adapted for swimming that were equipped with such adaptations as lobed or webbed feet for propulsion.