Grebes nesting in gull colonies: protective associations and early warning.

Ten colonies of Rolland's grebe Podiceps rolland and silver grebe P. occipitalis in tule marshes in Argentina in 1972-73 were studied to compare the behavior and reproductive success of grebes nesting in association with brown-hooded gulls Larus maculipennis with those nesting only with grebes. Some 55% of the grebes nested in gull colonies. Grebes nesting in gull colonies responded to the warning cries of gulls, always covered their eggs before departing from the nests, had lower predation rates on adults, had lower predation rates on eggs, and had higher hatching success than grebes nesting in colonies without nesting gulls.-from Author