Patterns in tern trophic diversity in a region experiencing rapid climate change

Abstract Foraging plasticity provides a mechanism for long‐lived species to adapt to rapidly changing environments. When shifts in food availability occur, individual variation in plasticity can lead to an increase in within‐species trophic diversity. We tested for drivers of trophic diversity in Arctic tern ( Sterna paradisaea ) and common tern ( Sterna hirundo ) chicks across three years (2017, 2018, 2021) and five seabird breeding islands in the Gulf of Maine. We measured tern chick trophic diversity using two approaches: (1) the Shannon‐Wiener diversity index of observed prey feeds and (2) the stable isotope standard ellipse area for blood cell and plasma samples. We hypothesized that individuals would vary in how they responded to declines in food availability and that individual‐level dietary responses would influence fitness. Across contexts (islands and years), we predicted that tern trophic diversity would be correlated with sea surface temperature and with the percentage of preferred prey in the diet of tern chicks (P1), which we used as two indicators of food availability. Furthermore, we predicted that individuals would vary in the magnitude and direction of shifts in isotope values over a two‐week period (P2) and that the magnitude of these shifts would be correlated with chick growth and survival (P3). Trophic diversity varied across islands, years, and species but was not correlated with either indicator of food availability (P1 Not Supported). Though tern chicks generally shifted to higher δ 15 N values and less‐enriched δ 13 C values later in the season, the magnitude of these shifts varied across individuals (P2 Supported). Chicks that shifted to relatively high δ 15 N values also showed greater magnitude shifts to less‐enriched δ 13 C values and, in Arctic terns, had a lower asymptotic mass (P3 Partially Supported). Our study suggests trade‐offs in individual‐level foraging and diet plasticity in seabirds that should be explored further.