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We monitored population size from 1996 to 2003 and studied behavioural interactions (in 2001) between the native Black-headed Gull Chroicocephalus ridibundus and an expansive, opportunistic predator, the Caspian Gull Larus cachinnans, at water reservoirs in Poland. The expansive species caused a population decline in the native species and affected its choice of nest sites. The Black-headed Gulls perceived the risk of predation on the part of the larger Caspian Gulls. When both species occurred in close proximity, the native gull breeding pairs built nests where the vegetation was higher and its cover greater than at the sites chosen by pairs breeding far away from the expansive species. The native gulls in proximity to the expansive species spent more time guarding their nests. However, this was not compensatory, as egg losses were higher and breeding success much lower in pairs breeding near the Caspian Gulls than in those breeding far from the latter. Such a low breeding performance in the Black-headed Gulls was probably caused either by predation on the part of Caspian Gulls or by aggressive interactions among Black-headed Gulls. In fact, the rate of intraspecific aggression in native gulls was higher in pairs breeding in proximity to the expansive species than in those breeding far away from it. These intraspecific fights, caused by the presence of the expansive species were, at least partially, responsible for egg and chick losses. We did not find the presence of native gulls to have any effect on the behaviour and breeding performance of the expansive gull. These results indicate that the expansive predatory Caspian Gull negatively affects local population size and alters the behaviour of the native Black-headed Gull, and may, both directly and indirectly, affect its reproductive performance.

Numerical and behavioural response of Black-headed Gull Chroicocephalus ridibundus on population growth of the expansive Caspian Gull Larus cachinnans