Keep the chicks moving: how Sandwich terns can minimize kleptoparasitism by black-headed gulls.

Sandwich terns, Sterna sandvicensis, often nest in association with black-headed gulls, Larus ridibundus. The gulls provide protection against predators, but can also adversely affect the terns' reproductive success through predation and piracy of fish. To test whether leading the chicks away from the nest site is an evasive strategy used by the parents to reduce the incidence of robbery by the gulls, we kept one group of Sandwich tern chicks at their original breeding site, while, with a wire-netting enclosure, we moved another group away from the gulls. The rate of kleptoparasitism was greatly reduced when the tern chicks were moved away from the original nest site, resulting in faster growth and earlier fledging. The rate of food parasitism and chick condition were affected only during the first 5 days of the experiment. After that, the rate of kleptoparasitism no longer differed between chicks that we moved away and those remaining in the colony. A second shift of the chicks again led to less kleptoparasitism and better chick condition. In line with these findings, the condition of free-living chicks that were lured away from their nesting site by their parents also improved. In particular, chicks initially in poor condition, which apparently suffered from high rates of kleptoparasitism, left the colony site. Free-living chicks are often lured away from the robbing gulls. However, not all subcolonies provided suitable escape routes and subsequently chicks in such subcolonies suffered from high mortality rates. Copyright 1999 The Association for the Study of Animal Behaviour.