Foraging strategies of a generalist seabird species, the yellow-legged gull, from GPS tracking and stable isotope analyses

Generalist and opportunistic species, such as the yellow-legged gull Larus michahellis, can feed on a wide variety of food from both marine and terrestrial origins. This work evaluates the potential foraging strategies (terrestrial, mix and marine) of the yellow-legged gull during the breeding season, in Berlenga (39 degrees 24'55 '' N, 9 degrees 30'28 '' W) and Deserta (36 degrees 57'45 '' N, 7 degrees 53'29 '' W) Islands, Portugal, across 2011-2016. Stable isotope analyses (delta C-13 and delta N-15) of plasma and blood cells were performed to estimate the proportion of individuals pursuing each strategy based on discrimination analyses. For that, GPS loggers were used to assess individual foraging destinations. Overall, this study discriminated well the foraging strategies adopted by gulls through stable isotope analyses (estimated error of 16.7%). Results indicated a variation in foraging strategies across years and between colonies. As expected, this variation was influenced by oceanographic conditions and availability of marine food sources nearby the colonies. The isotopic niche of yellow-legged gulls pursuing a marine strategy was much smaller than the niche of gulls with a terrestrial strategy, but surprisingly only slight smaller than the niche of gulls pursuing a mixed strategy. Gulls adopting a terrestrial strategy fed on a wide variety of foods, which greatly influenced the amplitude of the isotopic values, and respective isotopic niche width. On the other hand, gulls adopting a mix strategy might be very selective in the consumption of foods, taking great advantage of their potential plasticity on both marine and terrestrial environments. This study highlights an overall preference for the marine and mix strategies in yellow-legged gulls.