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Diving birds are birds which plunge into water to catch fish or other food. They may enter the water from flight, as does the brown pelican (Pelecanus occidentalis), or they may dive from the surface of the water. More than likely they evolved from birds already adapted for swimming that were equipped with such adaptations as lobed or webbed feet for propulsion.
Foot-propelled diving birds
Some diving birds - for example, the extinct Hesperornithes of the Cretaceous Period - propelled themselves with their feet. They were large, streamlined, flightless birds with teeth for grasping slippery prey. Today, cormorants (family Phalacrocoracidae), loons (Gaviidae), and grebes (Podicipedidae) are the major groups of foot propelled diving birds.