Area utilization of gulls in a coastal farmland landscape: habitat mosaic supports niche segregation of opportunistic species

The intensively farmed coastal lowland landscape of Germany, adjacent to the North Sea, provides important foraging opportunities for Black-headed, Common, Herring and Lesser Black-backed gull (Larus ridibundus, L. canus, L. argentatus and L. fuscus). We expected that spatial and temporal utilization of the landscape mosaic as well as behavioural traits and utilization of food resources would differ between these closely related species, facilitating niche segregation. We recorded habitat types and their utilization by the four species over a whole year. Furthermore, we related species abundance to several abiotic parameters. Black-headed and Common gulls were the most numerous species in the study area throughout the year. In general, the former species preferred bare fields with recently prepared soils and was often associated with tractors in the fields, whereas the latter species was most often found on pastures. Black-headed gulls seem to have a higher ability to exploit ephemeral, food sources associated with human activities whereas common gulls prefer habitats with low human activity and with naturally distributed prey. The most prominent abiotic parameter influencing gull abundance was presence of tractors. Black-headed gulls have most likely benefited from recent changes in agricultural practice, particularly the increase in cropped land, while Common gulls may have suffered from a decline in pastures. At present, utilization of the farmland habitat mosaic leads to niche segregation and supports coexistence, as two of the four gull species mainly forage in the marine environment, while there is significant habitat partitioning between the other two temporally, spatially and behaviourally.