Seabird reproductive responses to changing climate and prey communities are mediated by prey packaging

Abstract

Climatic factors drive changes in forage fish communities and may influence the productivity of piscivorous predators, but specific mechanisms of response remain poorly known. Between 1984 and 2020, we studied the rhinoceros auklet Cerorhinca monocerata, a seabird breeding in the western North Pacific at Teuri Island, Japan. We tested the hypothesis that climate-mediated prey-switching affects ‘food packaging’ (i.e. the way energy is brought to dependent offspring) and breeding success by quantifying relationships between climate, prey energy density, amount of food delivered, and the growth and survival of chicks. Prey composition switched 4 times: 1988-1992, 1997-1998, 2013-2014, and 2017-2018. All but the last of these switches were associated with (lagged) shifts in seawater temperature/Pacific Decadal Oscillation. Rhinoceros auklets brought multiple fish in each meal-load to chicks, and numbers were inversely correlated with the size of the fish. These relationships varied between fish species. The heaviest meal-loads were achieved when diets were dominated by anchovy Engraulis japonicas, which occurred during warm phases (1992-2013). Chick production, growth rates, and mass at fledgling were also highest during the warm phases. This study shows that climate affects seabird reproduction by shifting the manner in which food is selected relative to changes in forage fish community structure and abundance.