Seasonal and annual differences in the foraging ecology of two gull species breeding in sympatry and their use of fishery discards

Niche segregation between similar species will result from an avoidance of competition but also from environmental variability, including nowadays anthropogenic activities. Gulls are among the seabirds with greater behavioural plasticity, being highly opportunistic and feeding on a wide range of prey, mostly from anthropogenic origin. Here, we analysed blood and feather stable isotopes combined with pellet analysis to investigate niche partitioning between Audouin’s gull Larus audouinii and yellow-legged gull Larus michahellis breeding in sympatry at Deserta Island, southern Portugal, during 2014 and 2015. During the breeding season there was considerable overlap in the adults’ diet, as their stable isotope values of blood and primary feather (P1) did not differ, and their pellets were comprised mainly by marine fish species. However, Audouin’s gulls presented higher occurrences of pelagic fish, while yellow-legged gulls fed more on demersal fish, insects, and refuse. SIAR mixing models...