Diving methods of Great Northern and Black-throated Divers

On inland waters, I have noticed that Great Northern Divers Gavia immer always slither under slowly when diving, whereas Black-throated G. arctica jump upwards slightly before they dive. Depth of water appears to be irrelevant: in February 1978, I watched a Black-throated Diver and a Great Northern Diver swimming and feeding together on

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