The influence of risk and vulnerability on predator mobbing by terns (Sterna spp.) and gulls (Larus spp.)

The extent to which anti-predator mobbing can be modified in response to ne w predators was investigated by examining the response of terns (Sterna hir undo and S. paradisaea) and gulls (Larus fuscus, L. argentatus and L. canus ) to introduced mink Mustela vison, native otter Lutra lutra and rabbit Ory ctolagus cuniculus models in areas inhabited by mink and areas uninhabited by mink. Gulls and terns typically flock over predators of adult birds (a f leeing response), whilst diving at predators of chicks and eggs (an attacki ng response). Tents in mink-inhabited areas flocked over a model of a mink, but did not dive at it as much as they dived at otter and rabbit models. A stronger fleeing rather than attacking response suggests that adult terns perceive mink as a threat to themselves as well as to their chicks and eggs . Gulls, in comparison, dived at the mink model, indicating that mink are n ot perceived as a threat to the adults of these considerably larger species . Both gulls and tents showed a much lower level of response to mink in min k-free areas, suggesting that the type and extent of mobbing is altered thr ough experience with predators and the level of risk associated with mobbin g the predator.