Invading occupied territory? Foraging whereabouts and prey of the Caspian Gull (Larus cachinnans) colonising The Netherlands

Caspian Gulls (Larus cachinnans) have expanded their breeding range over around 2500 km westward, from the Black Sea to Western Europe, within decades. As a colonising species, it invaded a region with two established guild members that faced poor breeding results from declining resources. A study of breeding performance and diet of the Caspian Gulls, in combination with a tracking study to document foraging behaviour and habitat choice, meant to examine the possible occurrence of interspecific competition. The study revealed a multi-dimensional ecological separation between taxa, resulting from differences in prey choice, spatial segregation in micro- and macro-habitats plus an essential difference in seasonal timing. Caspian gulls focused on freshwater habitats, ditches and canals in agricultural areas, while avoiding offshore marine and intertidal areas, and took different prey in major areas where other taxa co-occurred. On top of that, Caspian Gulls commenced breeding a month earlier than its two sympatric guild members, such that their energetic demands peaked (during chick rearing) well before that in the established taxa. Caspian Gulls, in their early days of colonisation, have not invaded occupied territory but found an open niche.