Common tern. High-speed photograph of a common tern, Sterna hirundo, in flight. Common terns are seabirds reaching 37 centimetres in length. They hover over seas or lakes and then plunge-dive to catch and feed on fish and crustaceans. Common terns nest in large colonies, laying eggs in a hollow in the ground. They are found in most of the world's marine environments and migrate south during the winters of the Northern Hemisphere.