Resource partitioning in Atlantic puffins and razorbills facing declining food: an analysis of feeding areas and dive behaviour in relation to diet

Abstract

Multi-species communities of closely-related seabirds present opportunities to determine how such species coexist. Machias Seal Island (MSI), New Brunswick, Canada, is a migratory bird sanctuary where several seabird species breed, including the largest number of Atlantic puffins Fratercula arctica and razorbills Alca torda in the Gulf of Maine/Bay of Fundy ecosystem. The species differ in nest sites, body size and wing-loading, as well as life history (specifically post-natal care); they take different proportions of a similar range of prey species, and recent studies show limited overlap in foraging areas at other sites. We wished to expand our understanding of resource partitioning at MSI by measuring differences in foraging areas and behaviour, in the context of recent declines in availability of key prey species and concomitant decreasing breeding success of both species, which suggest that carrying capacity may have been reached. Using GPS loggers in 2 breeding seasons, and long-term chick-diet data collected over 20 yr, we investigated differences in horizontal and vertical foraging distributions and prey that allow these 2 species to breed sympatrically. Logger data collected from puffins (n = 7) and razorbills (n = 8) revealed that razorbills fed in shallower water than puffins and took shorter foraging trips. Prey brought to chicks at control nests showed higher proportions of high-energy fish in razorbill diet compared with puffins. Foraging behaviour is likely affected by declining availability of high-quality food and increasing temperature since 2010.