COS 205-5 - Intraspecific Variation in Seabird Foraging Responses to Rapid Environmental Change in the Gulf of Maine

Background/Question/Methods

Individual-level behavioral plasticity could provide a mechanism for population persistence in rapidly changing environments. Here, we explore the fitness consequences of intraspecific variation in foraging and diet plasticity in two species of alcids (Atlantic puffins Fratercula arctica and black guillemot Cepphus grylle) and two species of terns (common tern Sterna hirundo and Arctic tern Sterna paradisaea) breeding in the Gulf of Maine. The Gulf of Maine is warming incredibly fast, leading to reduced seabird prey availability, particularly late in the breeding season. Foraging seabirds have two options when food availability declines: they can work harder to catch preferred prey or switch to less preferred prey. Though dietary shifts in response to warming waters have been studied at the species level, there is likely to be intraspecific variation in this response. Here, we use stable isotope data (plasma and blood cells) collected in 2018 and 2021 from seabird chicks on five islands to test two hypotheses: H1) Individuals vary in how they alter foraging behavior/diet in response to increases in sea surface temperature, leading to greater intraspecific variation; H2) An individual’s response to changes in sea surface temperature will influence their fitness, as measured by chick growth and fledging success.

Results/Conclusions

The results of our analysis varied by island, year, and species. In 2021 on Petit Manan Island, where sample sizes were greatest, intraspecific variation increased later in the breeding season, as predicted. On all islands and for all species, isotope signatures shifted to less-enriched δ¹³C in late July, potentially indicating that individuals were foraging farther offshore. Though we expected a shift to lower δ¹⁵N due to a higher reliance on invertebrates when fish declined, most individuals shifted to higher δ¹⁵N signatures later in the season, which could indicate starvation. Black guillemots were the only species to show links between chick growth rates and isotope shifts; chicks fed a less enriched δ¹³C diet later in the season had lower growth rates. In Arctic terns and Atlantic puffins, isotope shifts correlated with chick survival. While Arctic tern chicks with a higher δ¹⁵N signature later in the season were more likely to survive, Atlantic puffin chicks were less likely to survive. Our results are among the first to examine intraspecific variation in dietary shifts across changing food landscapes and to link this variation to fitness. They provide an important step in understanding how individual-level responses drive population-level responses to rapid environmental change.