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

Foraging plasticity provides a mechanism for long-lived species to adapt to rapidly changing environments and, when individuals vary in their plasticity, can drive changes in trophic diversity. We use chick provisioning data and stable isotope values of blood cells and plasma to test for drivers of trophic diversity in the diet of common terns (Sterna hirundo) and Arctic terns (Sterna paradisaea) breeding in the rapidly changing Gulf of Maine. We hypothesized that individual adult terns would differ in their response to reduced food availability, driving higher diversity in tern chick diet in lower-resource contexts, and that individual-level responses would influence fitness. We used the Shannon-Wiener Diversity Index as our measure of trophic diversity for provisioning data and the size of the two-dimensional isotope ellipse as our measure of trophic diversity for stable isotope data. We compared these measures of trophic diversity to proxies for food availability, including the occurrence of preferred prey (herring and hake) in tern chick diets and the average sea surface temperature in terns’ foraging range. Additionally, we compared shifts in isotope values across time for individually paired blood cell and plasma samples and, on one island and year, for individually paired plasma samples taken 10 days apart. Trophic diversity measured using provisioning and stable isotope data were correlated across contexts. Although both measures of trophic diversity were highly variable, neither was correlated with measures of food availability. Arctic tern chicks had less-enriched δ13C values, lower δ15N values, and were fed smaller prey than common tern chicks, but did not differ in their trophic diversity. Isotope measures of trophic diversity were greater for plasma samples, which tended to show higher δ15N values and more-enriched δ13C values, than for blood cell samples. For the paired plasma samples collected on one island, chicks shifted to higher δ15N values and less-enriched δ13C values later in the season. Chicks that shifted to relatively higher δ15N values also showed greater magnitude shifts to less-enriched δ13C values and, in Arctic terns, fledged at a smaller mass. Our study suggests trade-offs in individual-level foraging and diet plasticity in terns, with possible implications for fitness.