Feather mercury increases with feeding at higher trophic levels in two species of migrant raptors, Merlin (Falco columbarius) and Sharp-shinned Hawk (Accipiter striatus)

Mercury (Hg) is a toxic heavy metal that, when methylated to form methylmercury (MeHg), bioaccumulates in exposed animals and biomagnifies through food webs. The purpose of this study was to assess Hg concentrations in raptors migrating through the upper Midwestern USA. From 2009-2012, 966 raptors of 11 species were captured at Hawk Ridge, Duluth, MN. Breast feathers were sampled to determine the concentration of total Hg. Mean Hg concentrations ranged from 0.11 – 3.46 μg g-1 fresh weight across species and were generally higher in raptors that feed on birds in comparison with those that feed on mammals. To evaluate the effect of dietary sources on Hg biomagnification, carbon and nitrogen stable isotope ratios were measured in feathers of the two species with the highest Hg concentrations, Merlin (Falco columbarius) and Sharp-shinned Hawk (Accipiter striatus). Measured δ13C values were similar in both species and indicated a primarily terrestrial-derived diet, whereas δ15N values sugges...