Use of stable isotopes to investigate individual differences in diets and mercury exposures among common terns Sterna hirundo in breeding and wintering grounds

We measured variations in stable isotope signatures (delta(13)C and delta(15)N) and concentrations of mercury (Hg) in breast feathers from pairs of common terns Sterna hirundo and their chicks at a breeding site in Buzzards Bay, northwestern Atlantic Ocean. By collecting 2 sets of feathers from the same adult birds, we compared values of delta(13)C, delta(15)N and Hg in feathers grown in the wintering area in the South Atlantic Ocean ('southern' feathers) and in the breeding area ('regrown' feathers). Regrown feathers had lower delta(13)C, higher delta(15)N and higher Hg than southern feathers. Values of delta(13)C, delta(15)N and Hg were much more variable in adults than chicks. Within families, delta(13)C and delta(15)N were correlated between parents and chicks; Hg was correlated between male and female parents. Among regrown feathers, Hg was positively correlated with delta(13)C and negatively correlated with delta(15)N. These findings suggest that high individual exposure of common terns to Hg results from consumption of inshore prey at low trophic levels in restricted parts of Buzzards Bay and that members of pairs have similar diets in the breeding season but not in winter. They demonstrate the power of stable isotope analyses in revealing individual differences in foraging habits, diet and contaminant exposure in generalist predators.