Marine predator on land: terrestrial foraging of lesser black-backed gulls (Larus fuscus) during the breeding season on Gotland, Sweden

Abstract The Baltic sub-species of the lesser black-backed gull (Larus fuscus fuscus) occurs within a relatively small region in the Palearctic. It is a monomorphic marine predator that winters in the Mediterranean and Africa. It is listed as ‘near threatened’ in Sweden and the reasons for its decline are not known. The largest breeding colony in Sweden, located on the island of Stora Karlso, Gotland, has had very low breeding success in recent years. Their foraging pattern on land might be an explanatory factor. The aim of this study was to characterize the gulls’ land-based foraging during the breeding season on Gotland using both visual observations and GPS tracking data collected between 2011-2013. This included determining which agricultural fields the gulls foraged on and how vegetation, prey availability, time of day and season, and abiotic features influenced this. I found that lesser black-backed gulls were more likely to be on fields when vegetation height and air temperature were lower and wind speeds were higher. The gulls were also present more often on fields in the morning and earlier in the breeding season. Additionally, I investigated how time of day and season, sex, gull identity and various abiotic factors affect the gulls’ choice to forage on land. The gulls were more likely to make foraging trips to Gotland when wind speeds, precipitation, cloud cover, and sea level pressure were higher, when air temperature was lower, and when thermal conditions were absent. I found that this population of lesser black-backed gull forages more often on land than at sea in May. This study contributes to our understanding of the links between food availability, foraging behavior, and population dynamics, which are important to management and conservation efforts of marine predators. (Less)