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Feather mercury increases with feeding at higher trophic levels in two species of migrant raptors, Merlin (Falco columbarius) and Sharp-shinned Hawk (Accipiter striatus)
Citation:
Keyel, E., M. Etterson, G. Niemi, D. Evers, C. DeSorbo, J. Hoffman, J. Nichols, Y. Li, AND F. Nicoletti. Feather mercury increases with feeding at higher trophic levels in two species of migrant raptors, Merlin (Falco columbarius) and Sharp-shinned Hawk (Accipiter striatus). The Condor. The Cooper Ornithological Society, Bend, OR, 122:1-17, (2020). https://doi.org/10.1093/condor/duz069
Impact/Purpose:
This manuscript provides data on exposure to Hg in terrestrial raptors migrating past Hawk Ridge, Duluth MN. Analysis and comparison of feather Hg residues suggest most species, on average, are at relatively low risk, though some individuals of two species, Sharp-shinned Hawk (Accipiter striatus) and Merlin (Falco columbarius) had residues high enough to place them in the moderate risk category. This is the first community wide assessment of terrestrial raptors in Minnesota and will be of considerable interest to the ornithological community statewide and regionally.
Description:
Mercury (Hg) is a toxic heavy metal that, when methylated to form methylmercury (MeHg), biomagnifies through food webs and bioaccumulates in exposed animals. The purpose of this study was to assess Hg concentrations in migrating raptors migrating through in the upper Midwestern US. From 2009-2012, 966 raptors of 11 species were captured at Hawk Ridge, Duluth, MN, USA. Breast feathers were sampled to determine the concentration of total Hg, which is comprised primarily of MeHg (>95%). Mean Hg concentrations ranged from 0.11 3.46 ìg/g 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, Merlins (Falco columbarius) and Sharp-shinned Hawks (Accipiter striatus). Measured ä13C values were similar in both species and indicated a primarily terrestrial-derived diet, whereas ä15N values suggested that individual Merlins and Sharp-shinned Hawks feeding at higher trophic levels accumulated higher concentrations of Hg. The risk to birds associated with measured levels of feather Hg was evaluated by calculating blood-equivalent values using an established algorithm. These blood-equivalent values were then compared to effect thresholds available in the literature. The results suggest that raptors sampled at Hawk Ridge are, on average, at negligible to low risk, though some individual adult Merlins and Sharp-shinned Hawks had blood-equivalent concentrations that placed them in a moderate risk category.
URLs/Downloads:
DOI: Feather mercury increases with feeding at higher trophic levels in two species of migrant raptors, Merlin (Falco columbarius) and Sharp-shinned Hawk (Accipiter striatus)