Understanding associations between nitrogen and carbon isotopes and mercury in three Ammodramus sparrows

We analyzed nitrogen and carbon stable isotope ratios and mercury (Hg) in breast feathers from three species of closely related sparrows, Saltmarsh, Seaside, and Nelson's Sparrows (Ammodramus caudacutus, A. maritimus, and A. nelsoni, respectively), to assess if trophic position and food web structure influence Hg exposure in these species. Sparrows were captured during the non-breeding season from 2006 to 2008 in North Carolina salt marshes near Wrightsville Beach, New Hanover County. Generalized linear models were used to test for the influence of species, δ(15)N, and δ(13)C on breast feather Hg. The most parsimonious model included species, δ(15)N, and their interaction term and explained 36% of the variation in breast feather Hg. Each species exhibited a different association between breast feather δ(15)N and Hg with Seaside Sparrows showing a positive correlation (r=0.27, P=0.03), Nelson's Sparrows a negative correlation (r=-0.28, P=0.01), and Saltmarsh Sparrows with no significant association. For Saltmarsh Sparrows, δ(15)N and Hg revealed decoupling between breast feather Hg and trophic position. Our results demonstrate that the influence of δ(15)N on breast feather Hg is likely indicative of geographic variation in δ(15)N baselines rather than trophic position.