Thursday, March 10, 2011

Diving Birds

Diving birds may enter the water from flight, as does the brown Pelican (Pelecanus occidentalis), or they may dive from the surface of the water. Diving birds are those which plunge into water to catch fish or other food. Most diving birds probably evolve from birds already adapted for swimming that were equipped with such adaptations as webbed or lobed feet for propulsion.

Some diving birds, for example, the extinct Hesperornithoformes of the Cretaceous Period, propelled them selves with their feet. They were large, streamlined, flightless birds with teeth for grasping slippery prey. Today the  cormorants (family Phalacrocoracidae) are the major groups of foot propelled diving birds. Their bodies are streamlined for underwater passage, with the hind limbs from the nondiving dabbling ducks to such well adapted divers as the ruddy duck of North America.

Other diving birds are wing propelled, like air, water is a fluid, through more dense; thus, underwater wing propulsion may be regarded as a specialized form of flight. The characteristic is found in the diving petrels (Pelecanoididae) of the Southern ocean and in the auks (Alcidae) of the Northern Hemipshere. The majority of these birds can fly in the air as well as underwater, although the recently extinct great auk (Pinguinus impenis) lost its ability to fly, as did the the penguins (spheniscidae) of the Southern Hemipshere.

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