Swan

Swans were birds native to Earth. They mated for life and often appeared on greetings cards. (TV: Something Borrowed)

In Greek mythology, Castor and Pollux were born when Zeus took the form of a swan and seduced their mother. The Tenth Doctor thought that was why, on the colony world of Aractus, they still said "Never turn your back on a swan". (PROSE: Wooden Heart)