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 suggested that individual Merlin and Sharp-shinned Hawk 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. Predicted blood values were then compared to heuristic risk categories synthesized across avian orders. This analysis suggested that while some Merlin and Sharp-shinned Hawk were at moderate risk to adverse effects of MeHg, most of the sampled birds were at negligible or low risk.

Study Area

All samples were collected between 2009-2012 during fall migration (September through November) from the Hawk Ridge Bird Observatory banding station at Duluth, MN, USA (46.85^oN, -92.03^oW) located near the westernmost point of Lake Superior. Situated at ~300 m above sea level, Hawk Ridge is positioned on basalt rock that extends along the north shore of the lake. This unique location concentrates large numbers of birds of prey, and on average, the banding station bands over 2,600 raptors each fall (Evans et al. 2012). Limited band recovery data suggest that diurnal raptors migrating past Duluth likely originate from breeding grounds in Western Canada and northern Minnesota (Evans et al. 2012).

Raptor Sampling 

Birds were captured using mist, bow, and dho gaza nets (Evans et al. 2012, Table 1). Captured birds were identified, aged, and sexed using methods described by Pyle (2008). Two to four breast feathers were plucked from each sampled bird, inspected for external debris, and placed into envelopes. Feathers were chosen because they represent a reliable index of Hg exposure during the time of feather growth (Bearhop et al. 2000, Kenow et al. 2007, Condon and Cristol 2009) and because they can be sampled non-invasively and easily preserved at room temperature for long periods.

Hg Analysis

One feather per individual was analyzed for total Hg concentration by thermal decomposition spectrophotometry (EPA method 7473) using a DMA-80 Direct Mercury Analyzer (Milestone) at the BRI Toxicology Lab at Biodiversity Research Institute in Portland, ME, following methods described by Evers et al. (2005). Feathers were prepared for analysis using a protocol employed by U.S. Fish and Wildlife Service contract laboratories (e.g., Texas A & M University; R. Taylor, pers. comm.). Only feathers with no visual evidence of external debris were analyzed. Feathers were not washed, as this protocol does not recommend washing unless external contamination is of concern. Quality control methods including the use of analytical blanks, sample replicates, and certified reference materials DOLT-4, DORM-3 and DORM-4, were employed to evaluate analytical precision and accuracy. Total Hg concentrations in all feathers were above the method detection limit (0.001 µg g^-1). Measured Hg concentrations in Certified Reference Materials (CRMs, National Research Council, Canada and Joint Research Centre, European Union) incorporated into each sample run averaged 100% (DOLT-4), 103.7% (DORM-3) and 97.9% (DORM-4) of published values. Feather Hg concentrations are presented in μg g^-1 on a fresh weight (fw) basis. Previous work has shown that Hg concentrations can vary among feathers in different parts of the plumage of individual birds (Furness et al. 1986, Braune and Gaskin 1987, Peterson et al. 2019). Although we did not sample multiple feather tracts, we controlled for variation among feathers within birds and standardized comparisons across birds by limiting our sampling to only breast feathers.

We also analyzed limited duplicates (e.g., a separate breast feather from the same individual) to verify consistency in Hg concentrations among breast feathers within the same individuals. For duplicates we calculated percent relative difference and Pearson’s correlation coefficient. Because HY birds were known to have grown all feathers within a recent and well-defined timeframe (i.e., nestling development) we also analyzed duplicates and calculated the same statistics within HY and AHY age class groups. The fw of a feather is nearly equivalent to dry weight (dw; R. Taylor, TERL, Texas A & M University, mean % feather moisture <1%, n = 490, reported in DeSorbo et al. 2018). We therefore considered fw = dw for the purposes of literature comparisons.

Stable Isotope Analysis 

Ratios of carbon (δ¹³C) and nitrogen (δ¹⁵N) stable isotopes in feathers were measured for the two species of raptors with the highest mean Hg concentrations: Merlin and Sharp-shinned Hawk. This was done to evaluate the potential relationship between feather Hg and trophic status (as measured by δ¹⁵N), and aquatic versus terrestrial carbon source in the raptor diet (as measured by δ¹³C). Samples from 20 individuals within each age and sex class were selected for stable isotope analysis, with 10 feathers each from the lowest and highest Hg results for each class. Each feather sample was cleaned with a 2:1 chloroform:methanol solution (Hobson 1999), placed in a pre-combusted scintillation vial, and dried at 50^oC for at least 24 hours. After drying, the sample was minced and 0.7 μg analyzed with a Costec 4010 EA and Thermo Delta Plus XP isotope ratio mass spectrometer. Stable isotope ratios are reported in standard δ notation, wherein Vienna Pee Dee Belemnite and air are standards for δ¹³C and δ¹⁵N, respectively. Analytical error, calculated as the mean standard deviation of replicate reference material, was < 0.1‰ for δ¹³C and δ¹⁵N. 

Statistical Analyses

Measured feather Hg concentrations were natural log-transformed, and geometric mean concentrations (± SE) were calculated for all 11 species. For a subset of six species with greater sample sizes (n ≥20), analysis of variance was used to compare ln-transformed Hg concentrations, ln(feather Hg), by sex within species, age within species, and their interaction (cohort = all combinations of age and sex) within species.

Additional statistical analyses were performed for Merlin and Sharp-shinned Hawk, our two largest samples. A set of 18 general linear models was developed to compare ln(feather Hg) to the year in which birds were captured, ordinal day on which birds were captured, bird age class, sex, and all bivariate interaction terms (Table 2). Year was modeled as a quantitative covariate to test whether feather Hg concentrations were changing over the course of the four years of study. Date was also modeled as a quantitative covariate to explore seasonal variation in feather Hg. Age and sex were modeled as factors, each with 2 levels. Variable importance was calculated based on Akaike’s Information Criterion, corrected for small sample size (AIC_c), as the sum of AIC_c weights for each model in which the variable occurred (Burnham and Anderson 2002). A list of all models considered for Merlin and Sharp-shinned Hawk is provided in Table 2.

Correlations between ln(feather Hg) and δ¹³C, and between ln(feather Hg) and δ¹⁵N by sex, age, and cohort within each species were evaluated using Pearson’s correlation coefficient. All analyses were conducted in R (version 2.14.0, R Development Core Team 2013). 

There are three worksheets within the Excel file: Hg, Hg_Dups, and Isotope. Hg contains the methylmercury data and should be self-explanatory except perhaps for the 4-letter alphanumeric species codes, which can be easily looked up, and age codes (HY = Hatch Year, AHY = After Hatch Year, SY = Second Year, ASY = After Second Year). Hg_Dups contains the Hg lab duplicate data. Isotope contains the C and N stable isotope data, which should also be self-explanatory, although there are only two species (SS = Sharp-shinned Hawk and ML = Merlin).