Grebes Nesting in Gull Colonies: Protective Associations and Early Warning

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Abstract
Colonies (10) of Rolland''s and silver grebes in tule marshes were studied in Argentina in 1972-1973 to compare the behavior and reproductive success of grebes nesting in association with brown-hooded gulls with those nesting only with grebes. Although the effect of protector species on predators has been noted, no one has examined the differences in reproductive success. A total of 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. The adaptive significance of these protective associations is discussed.