I know what you ate last breeding season: anthropogenic food subsidies as a driver of the trophic ecology of nesting Kelp Gulls in a freshwater ecosystem in Patagonia, Argentina

Abstract Gulls are opportunistic birds that take advantage of anthropogenic food subsidies, often leading to conflicts with human populations. In Argentina, the Kelp Gull ( Larus dominicanus ) is distributed along the Atlantic coast and in some freshwater environments. We evaluated the trophic ecology of a population nesting in Nahuel Huapi Lake, with access to anthropogenic food subsidies. We used complementary methodologies (pellets, stomach contents, stable isotopes) to determine their diet during the incubation and chick‐rearing periods in 2019 and 2020, in two colonies (~261 and 314 nests) located close to urban areas. Pellets showed a relatively high percentage of anthropogenic food resources (>40%) and terrestrial arthropod remains (>40%) in all breeding stages and years, except during incubation in 2019, when >70% of rodent remains were recorded. Stomach contents also showed a high percentage of garbage in both years (40–80% incubating adults, 30–40% chicks) and of terrestrial arthropods (30–60%). Isotopic niche width differed between years, and the isotopic composition ( δ 13 C and δ 15 N) of adult and chick red blood cells showed a significant proportion of terrestrial resource (natural and anthropogenic items combined) consumption in all stages and years and a lower proportion of garbage in 2019. This study shows the generalist and opportunistic habits of Kelp Gulls at the Nahuel Huapi Lake colonies, evidenced by the wide variety of food consumed and the ability to take advantage of the food that becomes available in the natural environment (e.g., rodent outbreak throughout 2019). This study of the trophic ecology of the Kelp Gulls in the Nahuel Huapi Lake provides essential baseline information for future assessments of the gulls' demographic behaviour and their role within the urban‐lake‐terrestrial ecosystem. Further studies to understand the use of resources during the non‐breeding season are recommended.