 A male greater crested tern establishes a small area of the colony in preparation for nesting , and initially pecks at any other tern entering his territory . If the intruder is another male , it retaliates in kind , and is normally vigorously repelled by the incumbent . A female entering the nest area reacts passively to the male 's aggression , enabling him to recognise her sex and initiate pair formation by display , including head raising and bowing ; this behaviour is frequently repeated during nesting to reinforce the bond between the pair . Terns also use fish as part of the courtship ritual . One bird flies around the colony with a fish in its beak , calling loudly ; its partner may also fly , but the pair eventually settle and the gift is exchanged . 
