At Anzali Lagoon, on the southwest coast of the Caspian Sea, Great Crested Grebes Podiceps cristatus were observed breeding both in monospecific colonies and in a mixed colony with Whiskered Terns Chlidonias hybridus. In the mixed colony, Great Crested Grebes bred later than in their monospecific colonies, while laying clutches of similar size, but significantly fewer of their nests disappeared during incubation and their chicks had higher survival rates up to at least 20 days old. Human disturbance and the resulting increases in predation likely caused greater losses at monospecific colonies of Great Crested Grebes where there is no common anti-predatory activity; while the strong anti-predatory behaviour of Whiskered Terns in the mixed colony might have offered enhanced protection to the chicks. Unfortunately, we could not separate the effects of the seasonal differences in human disturbance and of the protective presence of terns on the Great Crested Grebe's reproductive output.