Study Area.

Our study areas were within LMNRA (36°0.6′N, 114°47.8′W) and an area of SNV encompassing nearly 8000 km² of arid lands bordering LMNRA and surrounding the metropolitan area of Las Vegas, Nevada. Both study areas are in the eastern Mojave Desert and receive an average of <14 cm/yr of precipitation (Hereford et al. 2004). Lake Mead National Recreation Area comprises 4025 km² of land surrounding Lakes Mead and Mohave overlapping the state border between southern Nevada and northwest Arizona. Our SNV study area has very little surface water except isolated low discharge springs. Elevations range from 192 to 1719 m in LMNRA and approximately 500 to 3632 m in SNV. Vegetation along slopes and canyons in LMNRA consists of Mojave Desert scrub dominated by creosote bush (Larrea tridentata) and white bursage (Ambrosia dumosa), with narrow strips of riparian vegetation lining the shores of both lakes. Vegetation within SNV is primarily Mojave Desert scrub at lower elevations, transitioning to pinyon-juniper woodlands (Pinus spp., and Juniperus spp.) along mountain slopes, and culminating in a treeless alpine zone on the higher peaks.

Peregrine Feather Collection.

We estimated age of peregrine nestlings using 10 × 42 binoculars, or a 20–60× spotting scope, by comparing feather growth with a photographic guide (Clum et al. 1996). Flight and body feathers are not completely grown by fledging, so we classified free-flying hatch-year (HY) peregrines as fledglings when a sheath was still present around the base of feathers. We classified peregrines as after-hatch-year (AHY) when we confirmed they had at least entered their second year or had attained full adult plumage (i.e., definitive basic plumage; Pyle 2008).

We collected feather samples from AHY and nestling peregrines during the 2012 and 2013 breeding seasons (19 March to 19 June 2012; 24 January to 13 June 2013). We captured AHY peregrines using noose-harnessed Rock Pigeons (Columba livia; Bloom et al. 2007) at the base of eyrie cliffs during the courtship and nestling breeding stages. Nestlings were sampled from the eyrie when they were between 21–30 d old to allow for adequate feather growth. We banded each peregrine with a standard U.S. Geological Survey aluminum band, and with a uniquely coded, alphanumeric black anodized color band (Acraft Sign and Nameplate Co. Ltd., Edmonton, Alberta, Canada) to aid future identification of individuals. We generally sexed individuals by tarsus width (male <6 mm; female ≥6 mm) and used published wing chord lengths for verification of adult sex when necessary (Pyle 2008).

We removed 1.5–2.0 cm of the distal end of the fourth primary flight feather (p4) from both wings using clean stainless steel scissors. We took care to not cut the follicle when sampling nestlings, which is vascularized and prone to bleeding in growing feathers (USFWS 2003). We also collected axillary feathers from four AHY peregrines in order to investigate differences in total Hg concentrations between feather types. Axillaries were cut near the base of the feather vane excluding the calamus. Additionally, we collected samples from as many feather types as possible from dead peregrine nestlings and fledglings discovered in the vicinity of eyries, and we collected molted feathers from adults at eyries from 2008–2013. All feather samples were stored in polyethylene collection bags at room temperature prior to analysis.

Collection of Prey Remains.

During the 2012 and 2013 breeding seasons, we collected feathers of prey remains from eyries, the base of eyrie cliffs, and from associated plucking perches when banding nestlings and again after young fledged. We also collected prey remains during a related project following the 2008–2011 breeding seasons. In collaboration with a regional expert (N.J. Schmitt), we identified feathers to the lowest taxonomical unit (i.e., family, genus, or species), which in some cases consisted of >1 similar species (i.e., “species group”). Our selection of feather types for Hg analysis and species identification was limited by the types of feathers available at the eyries and perches, but we preferentially tested remiges (i.e., primaries and secondaries) and retrices. We classified avian prey as “aquatic” or “terrestrial” based on their nesting and foraging habitat requirements (i.e., “bird type”). We also assigned them to a foraging guild (i.e., aquatic herbivore, aquatic invertivore, carnivore, frugivore, granivore, insectivore, omnivore, piscivore) based on the primary breeding season diet of each species, as determined by personal observations and from Ehrlich et al. (1988).

We estimated proportions by biomass of bird type consumed per territory using prey remains and direct observation during previous research within LMNRA from 2005–2010 (Barnes 2011), and from prey remains collected in both study areas from 2011–2013. We determined the number of individuals in prey remains by counting the minimum number of bills, legs, and diagnostic flight feathers per species, and estimated mass using published values of adults (Dunning 2007). We measured the straight line distance from the active eyrie in each territory to the nearest lake shoreline (i.e., Lake Mead or Lake Mohave) using a geographic information system (ArcGIS v. 9.3, Environmental Systems Research Institute, Redlands, California, U.S.A.), in order to assess the importance of distance to permanent water when considering prey composition and contaminant levels within territories.

Feather Analysis.

We tested total Hg in feathers using an AMA 254 atomic absorption spectrometer (method detection limit  =  2.5 ng/g [ppb]; Leco Corporation, St. Joseph, Michigan, U.S.A.) at the environmental and occupational health laboratory at the University of Nevada, Las Vegas. Results are presented as total Hg on a fresh weight (fw) basis in µg/g (equivalent to ppm). Total Hg levels are a surrogate for MeHg because virtually all total Hg in feathers has been found to be the organic methylated form (Thompson and Furness 1989, Bond and Diamond 2009). We limited analysis to the distal end of feather samples (1.5–4.0 cm) when dealing with large flight feathers (i.e., primaries, secondaries, rectrices) from relatively large-bodied species of prey (e.g., raptors, large aquatic birds, doves, etc.), but analyzed the entire feather, excluding the calamus, when presented with smaller body feathers or most feathers from small-bodied birds (e.g., passerines). Using stainless steel scissors, we cut each sample into small pieces to allow it to fit in a small nickel weigh boat. Analyzed portions of feathers weighed from 2.1–41.8 mg fw. We then placed the weigh boat with sample in a catalyst tube in the AMA 254 atomic absorption spectrometer for thermal decomposition (750°C for 320 sec). Ultra pure O₂ was used as a carrier gas to transport the gaseous sample to a gold-plated amalgamator where total Hg was determined using a wavelength of 253.65 nm. We ran quality control tests every 10–12 samples, consisting of a method blank and one or two samples of standard certified reference material (CRM; CRM 1633b, CRM 2709, CRM 2711; National Institute of Standards and Technology, Gaithersburg, Maryland, U.S.A.), to verify calibration. Accepted CRM recoveries ranged from 86–111% of the certified values of total Hg (mean  =  94.7 ± 5.2%, n  =  118). We report all samples with a total Hg level below our method detection limit as zero.

Statistical Analysis.

We report the individual with the highest level of total Hg when >1 AHY peregrine was tested in a territory in order to gauge maximum exposure in each territory. For peregrine nestlings, we report all brood means for total Hg, but use the nestling with the highest total Hg concentration within broods (six broods) when comparing among territories. In prey, when we verified >1 feather from the same individual, we relied on the feather sample with the highest total Hg level to account for maximum total Hg exposure. Our ability to confidently attribute multiple feathers to the same prey item was limited because of the haphazard nature of collecting plucked feathers from eyries; however, we analyzed >1 feather from 54 prey individuals representing 30 species.

Based on highly significant Shapiro-Wilk tests (P < 0.001), total Hg concentrations in peregrines and their prey were not normally distributed, so we used nonparametric tests to assess differences among groups. We used Mann-Whitney U-tests to compare overall difference of total Hg concentrations between peregrine age class (HY vs. AHY), and by age class between study areas. To assess differences between total Hg concentrations in peregrines in which we sampled both p4s (i.e., right and left fourth primary), we pooled by age class and compared means using a paired samples t-test because the difference between means of the two feathers was normally distributed.

We used Mann-Whitney U-tests to compare differences between mean total Hg concentration detected in prey remains collected per peregrine territory by study area, and mean total Hg concentration by bird type (i.e., aquatic vs. terrestrial). We used Kruskal-Wallis tests to assess differences among bird types by study area and differences of mean total Hg concentrations of prey foraging guilds, using stepwise step-down comparisons to look for significant differences. We performed Pearson's coefficient correlations to analyze relationships by territory between adult total Hg, brood total Hg, mean prey total Hg, distance of eyries to Lakes Mead or Mohave, and the proportion of aquatic birds taken as prey by biomass. We used independent samples t-tests to compare mean distance of eyries to permanent water by study area and the proportion of bird types taken as prey in the two study areas.

We report arithmetic means ± SE unless otherwise indicated, and consider results significant at P ≤ 0.05. All statistical tests were conducted in SPSS 21 (IBM SPSS, Armonk, New York, U.S.A.).

Peregrine Falcons.

We captured 14 AHY peregrines during the 2012 and 2013 breeding seasons, and collected an additional 11 molted AHY feathers, representing 12 territories within LMNRA and five territories from SNV. From only one territory did we capture both a resident male and female, and this was done in subsequent years. We sampled a total of 24 HY peregrines, representing six broods from LMNRA and 3 broods from SNV. All peregrine feather samples contained detectable levels of total Hg (range  =  0.12–42.54 µg/g, n  =  49). Overall, total Hg concentrations in AHY peregrines (mean  =  12.19 ± 2.5 µg/g, n  =  25) were significantly higher than in HY peregrines (mean  =  3.76 ± 0.8 µg/g, n  =  24; t₂₉  =  −3.213, P  =  0.003). There was not a significant correlation between total Hg detected in adult peregrines compared to their broods (r  =  .65, P  =  0.075; Table 1), although this comparison was limited by small sample size (i.e., only eight broods sampled with ≥1 attendant adult also sampled).

Table 1.

Pearson's coefficient correlation table of variables relating to total mercury contamination at Peregrine Falcon breeding territories in the Southern Nevada and Lake Mead National Recreation Area, Nevada and Arizona study areas, 2008–2013. Sample size is the number of territories assessed for each variable. Distance to water is the straight-line distance (km) of the eyrie to the nearest lake (i.e., Lakes Mead or Mohave), and aquatic prey biomass is the proportion of aquatic birds taken relative to all prey taken per territory.

Total Hg concentrations in p4s of AHY peregrines at territories in LMNRA (mean  =  17.24 ± 4.13 µg/g, range  =  1.46–42.54 µg/g, n  =  12) were significantly greater than in AHY peregrines in SNV (mean  =  2.7 ± 1.06 µg/g, range  =  0.93–6.8 µg/g, n  =  5; Mann-Whitney U  =  7.0, z  =  −2.424, P  =  0.014). We were precluded from conducting statistical analyses between sexes because we generally did not sample both sexes from the same territory; however, we detected total Hg means of 19.4 ± 5.28 µg/g in AHY females (range  =  1.39–42.54 µg/g, n  =  8) and 4.35 ± 1.53 µg/g in AHY males (range  =  1.46–10.77 µg/g, n  =  6). In LMNRA, AHY females had a mean of 21.97 ± 5.32 µg/g total Hg (range  =  7.15–42.54 µg/g, n  =  7) compared to 4.34 ± 2.2 µg/g in AHY males (range  =  1.46–10.77 µg/g, n  =  4), while in SNV a single AHY female had 1.39 µg/g and two AHY males had 1.94 µg/g and 6.8 µg/g respectively. The only territory in which we sampled both AHY sexes, albeit in subsequent years, was in LMNRA and the female had 16.82 µg/g of total Hg compared to the male with 10.77 µg/g.

Total Hg concentrations in p4s of peregrine broods in LMNRA (mean  =  5.82 ± 2.07 µg/g, range  =  0.75–13.05 µg/g, n  =  6) were significantly greater than in SNV (mean  =  0.67 ± 0.18 µg/g, range  =  0.37–1.0 µg/g, n  =  3; Mann-Whitney U  =  1.0, z  =  −2.066, P  =  0.048). We sampled broods from two LMNRA territories in consecutive years, with an increase of mean brood total Hg concentration of 0.86 µg/g between years at one territory (2012  =  1.84 µg/g, 2013  =  2.7 µg/g) and a decrease of 0.62 µg/g between years at a second territory (2012  =  4.42 µg/g, 2013  =  3.8 µg/g). Overall, we detected total Hg means of 3.05 ± 0.75 µg/g in female nestlings (range  =  0.6–11.53 µg/g, n  =  15) and 4.52 ± 2.06 µg/g in male nestlings (range  =  0.12–13.05 µg/g, n  =  7). In LMNRA, female nestlings had a mean of 3.9 ± 0.89 µg/g total Hg (range  =  0.75–11.53 µg/g, n  =  11) compared to 7.73 ± 2.62 µg/g for male nestlings (range  =  1.96–13.05 µg/g, n  =  4), while in SNV female nestlings had a mean of 0.71 ± 0.1 µg/g (range  =  0.6–1.0 µg/g, n  =  4) compared to a mean of 0.23 ± 0.07 µg/g in male nestlings (range  =  0.12–0.37 µg/g, n  =  3).

Eyries in LMNRA were significantly closer to water (mean  =  1.3 km, n  =  12) than those in SNV (mean  =  40.4 km, n  =  7; t₆  =  −2.54, P  =  0.044) and aquatic birds comprised a significantly greater percent of prey remains by biomass in peregrine territories in LMNRA (mean  =  64.3%, eight territories) than in SNV (14.7%, four territories; t₁₀  =  5.78, P < 0.001). Adult peregrine total Hg concentrations were positively correlated with mean prey total Hg (r  =  .65, P  =  0.042); however, brood total Hg concentrations were not significantly correlated with mean prey total Hg (r  =  .69, P  =  0.089; Table 1). The proportion by biomass of aquatic birds taken as prey was positively correlated with mean prey total Hg (r  =  .84, P  =  0.001) and negatively correlated with distance of eyries to water (r  =  −.72, P  =  0.008; Table 1).

Variation Within Individuals and Broods.

We did not detect a significant difference in total Hg between paired p4s within individuals (t₂₁  =  −1.113, n  =  22, P  =  0.278). We were hindered by small sample size from conducting statistical analysis on individuals from which we collected p4 and axillary feather samples (i.e., for HY n  =  3; for AHY n  =  4). However the difference in total Hg concentration between these two feather types within HY peregrines was relatively small (mean difference  =  1.22 µg/g, range  =  0.41–2.74 µg/g), whereas it was much greater in AHY peregrines (mean difference  =  14.69 µg/g, range  =  3.46–37.27 µg/g).

Variation of total Hg among nestlings within broods was generally low (Table 2), although small sample size precluded rigorous testing. Excluding one anomalous territory (L1), the mean difference in total Hg among individuals within broods was only 0.58 µg/g (n  =  5). Only a single nestling varied greatly from its brood-mates (nestling with highest total Hg in L1 was 126% greater than its siblings' mean) and development of this individual was stunted relative to its siblings; its tail length was only 61% as long as the mean for its siblings (41 mm vs. a mean of 67 mm) and the emerged portion of its p4 was just 50% of its siblings' mean (23 mm vs. a mean of 46 mm).

Table 2.

Total mercury concentrations (µg/g fresh weight) detected in Peregrine Falcon nestlings within the same brood in territories in the Southern Nevada (S1 and S2) and Lake Mead National Recreation Area (L1–L4), Nevada and Arizona study areas, 2012. All nestlings within broods were analyzed.

We analyzed various feathers and feather types from the remains of five HY peregrines (three nestlings, two fledglings) found in eyries or at the base of eyrie cliffs (4–25 feathers/individual; Table 3). Overall variation of total Hg detected between feathers was low, with 1.42 µg/g being the mean difference between highest and lowest concentration within individuals (range  =  0.15–4.73 µg/g, mean SD  =  0.43). The individual with the most variation (fledgling 1) also had the highest total Hg levels and survived longer than the others (i.e., survived ≥2 wk after fledging). We could not confirm cause of death in any instance; however, nestling 1 likely was predated as a nearly fledged nestling based on feather growth, and fledgling 1 had very low pectoral muscle mass as if from starvation.

Table 3.

Total mercury concentrations (µg/g fresh weight) detected in feathers of carcasses of hatch-year Peregrine Falcons in the Southern Nevada and Lake Mead National Recreation Area, Nevada and Arizona study areas, 2012.

Analysis of Prey.

We tested feathers from 337 individuals representing 94 species, or species types, from avian prey (1–26 individuals/prey type; Appendix 1), measuring detectable levels of total Hg in all but 39 individuals. Mean total Hg concentrations detected in prey remains collected per peregrine territory in LMNRA (mean  =  2.26 ± 0.46 µg/g, range  =  0.27–3.9 µg/g, n  =  7 territories) were significantly greater than in SNV (mean  =  0.23 ± 0.06 µg/g, range  =  0.09–0.4 µg/g, n  =  5 territories; Mann-Whitney U  =  33.0, z  =  2.517, P  =  0.01). Eared Grebes (Podiceps nigricollis) contained some of the highest total Hg concentrations (mean  =  12.27 ± 2.2 µg/g, range  =  0.2–30.93 µg/g, n  =  22), followed by Red-winged Blackbirds (Agelaius phoeniceus; mean  =  6.96 ± 6.83 µg/g, range  =  <0.01–34.29 µg/g, n  =  5) and Black-necked Stilts (Himantopus mexicanus; mean  =  6.57 ± 2.21 µg/g, range  =  0.13–13.8 µg/g, n  =  6). Some commonly taken species with relatively low concentrations of total Hg were American Coots (Fulica americana; mean  =  0.8 ± 0.42 µg/g, range  =  0.08–3.23 µg/g, n  =  7), Great-tailed Grackles (Quiscalus mexicanus; mean  =  0.43 ± 0.11 µg/g, range  =  <0.01–2.29 µg/g, n  =  25) and the four commonly taken Columbidae species as a whole (mean  =  0.11 ± 0.03 µg/g, range  =  <0.01–1.4 µg/g, n  =  51).

Overall, aquatic birds (mean  =  5.07 ± 0.84 µg/g, range  =  <0.01–30.93 µg/g, n  =  80) contained a significantly higher concentration of total Hg than terrestrial birds (mean  =  0.76 ± 0.18 µg/g, range  =  <0.01–34.29 µg/g, n  =  257; Mann-Whitney U  =  16886, z  =  8.69, P < 0.001). There were significant differences in mean total Hg found in bird type by study area (Fig. 1; H₃  =  96.4, n  =  337, P < 0.001). Stepwise step-down comparisons indicated aquatic birds from LMNRA (mean  =  5.55 ± 0.92 µg/g, range  =  <0.01–30.93 µg/g, n  =  72) contained significantly more total Hg than all other groups, whereas terrestrial birds from SNV were significantly lower than all other groups (mean  =  0.27 ± 0.05 µg/g, range  =  <0.01–2.19 µg/g, n  =  95). There were also significant differences in mean total Hg among foraging guilds pooled by study area (Fig. 2; H₆  =  134.6, n  =  337, P < 0.001). Although stepwise step-down comparisons indicated overlap among several of the guilds, aquatic invertivores yielded the highest mean total Hg level (mean  =  6.17 ± 1.11 µg/g, range  =  0.06–30.93 µg/g, n  =  58), followed by piscivores (mean  =  3.16 ± 1.08 µg/g, range  =  1.13–7.89 µg/g, n  =  6) and aquatic herbivores (mean  =  1.83 ± 0.73 µg/g, range  =  <0.01–9.92 µg/g, n  =  15). The total Hg concentration for the single individual we analyzed from the frugivore guild (Cedar Waxwing; Bombycilla cedrorum) was below our detection limit.

Figure 1.

Concentrations of mean total mercury (µg/g fresh weight) detected in avian prey by bird type collected from Peregrine Falcon territories in the Southern Nevada (SNV) and Lake Mead National Recreation Area (LMNRA), Nevada and Arizona study areas, 2008–2013. Avian prey is categorized by bird type (AB  =  aquatic bird, TB  =  terrestrial bird) based on general habitat type preferred by each species for foraging and breeding. Mean total Hg levels are shown with 95% confidence intervals. Sample size is indicated in parentheses for each group. Letters above each bar indicate significant difference among means, determined with a Kruskal-Wallis test using stepwise step-down comparisons.

Figure 2.

Concentrations of mean total mercury (µg/g fresh weight) detected in avian prey by foraging guild collected from Peregrine Falcon territories in the Southern Nevada and Lake Mead National Recreation Area, Nevada and Arizona study areas, 2008–2013. Avian prey is categorized by foraging guild based on each species' primary diet during the breeding season (AH  =  aquatic herbivore, AI  =  aquatic invertivore, C  =  carnivore, F  =  frugivore, I  =  insectivore, G  =  granivore, O  =  omnivore, P  =  piscivore), and study areas are pooled. Mean total Hg levels are shown with 95% confidence intervals. Sample size is indicated in parentheses for each guild. Letters above each bar indicate significant difference among means, determined with a Kruskal-Wallis test using stepwise step-down comparisons.

Prey Analysis.

In a synthesis of research on Hg concentrations of 38 species of birds between 1969–2003 in northeastern North America, Evers et al. (2005) indicated that trophic level largely influenced differences in Hg burdens, and availability of MeHg was based on general habitat type (e.g., aquatic > terrestrial). Although exposure was greater in aquatic systems, there was a gradient of increasing Hg based on the trophic level of foraging guilds such that herbivore < omnivore < insectivore < piscivore. Concentrations of total Hg in the eight foraging guilds we examined largely followed this progression, with the notable exception that aquatic invertivores exceeded piscivores as the guild with the highest mean total Hg concentration, followed by aquatic herbivores above all others (Fig. 2). Our ability to evaluate the Hg risk posed by piscivorous prey to raptors was limited by the small number of individuals we collected (n  =  6), but the large body size of many of the piscivores (e.g., cormorants, mergansers, Aechmophorus grebes, herons, and egrets, etc.) likely reduces the predation risk they face from peregrines. The high concentration of total Hg in aquatic herbivores was somewhat surprising, but many of these species also feed on aquatic insects and invertebrates (Ehrlich et al. 1988). The levels of total Hg we detected in terrestrial insectivores (mean  =  1.07 ± 0.34 µg/g) and omnivores (mean  =  0.77 ± 0.26 µg/g) were both higher than those Keller et al. (2014) documented for the same guilds in the southern Appalachian Mountains, an area with very high Hg deposition.

In LMNRA, Eared Grebes appear to be the single largest contributor of total Hg to peregrines, based on availability (i.e., they constituted 25.3% of all aquatic birds on Lake Mead during aquatic bird surveys from 2004–2009; Barnes and Jaeger 2012), contribution to diet (i.e., 14.9% of the peregrine diet by biomass; Barnes 2011), and high Hg concentrations (mean  =  12.27 µg/g, n  =  22). Seasonal patterns recorded in LMRNA from 2004–2009 indicated that Eared Grebes were generally present in large numbers during April and May (Barnes and Jaeger 2012), roughly corresponding with the incubation and early nestling stages of local peregrines (Barnes et al. 2015). Nearly half of the North American Eared Grebe population annually stage from August–January on the Great Salt Lake, where they forage on brine shrimp during their molt migration (Aldrich and Paul 2002). Water samples from the Great Salt Lake contained the highest MeHg concentrations recorded for surface water in the U.S. (Naftz et al. 2008), and Hg concentrations in the livers of several species of wintering waterfowl on the Great Salt Lake were among the highest in published literature for those species (Vest et al. 2009).

Although the total Hg concentrations we detected in terrestrial birds were fairly low for both study areas (mean LMNRA  =  1.04 µg/g, mean SNV  =  0.27 µg/g) relative to aquatic birds, it is interesting that terrestrial birds in LMNRA had significantly higher total Hg levels than those in SNV. Terrestrial birds foraging near shorelines in LMNRA were likely exposed to Hg levels more closely associated with aquatic systems (i.e., consuming insects with larval development in aquatic systems; see Keller et al. 2014). Based on recent modeling of reduced nest success associated with Hg concentration in tail feathers of Carolina Wrens (Thryothorus ludovicianus; nest success reductions of 10, 20, and 30% with levels of 3.0, 4.7, and 6.4 µg/g Hg; Jackson et al. 2011), we considered total Hg concentrations in six species of terrestrial passerines in LMNRA were potentially high enough to affect reproduction. Most alarming, were the high concentrations of total Hg we detected in Red-winged Blackbirds (mean  =  6.96 µg/g, range  =  0–34.29 µg/g), Brewer's Blackbirds (Euphagus cyanocephalus; mean  =  5.54 µg/g, range  =  0.02–23.51 µg/g), and Common Ravens (Corvus corax; mean  =  3.75 µg/g, range  =  1.41–7.54 µg/g; Appendix 1). Many of the terrestrial bird species we tested from both study areas breed locally (Floyd et al. 2007), whereas many aquatic birds in LMNRA were highly migratory, with very little local breeding (Barnes and Jaeger 2012). As such, it is possible that Hg found in terrestrial birds may be more indicative of local contamination than aquatic birds. Although we did not quantify it, a sizable proportion of our terrestrial bird prey remains consisted of feathers with retained sheaths. These feathers represent recently fledged birds not yet able to make long flights, thereby reflecting Hg contamination in the immediate vicinity of breeding territories.

Summary.

Our study represents the first assessment of Hg contaminant levels in peregrines and their avian prey in southern Nevada and northwest Arizona. We documented baseline total Hg concentrations within peregrines breeding in our two study areas and determined general dietary pathways of exposure through which they are vulnerable to bioaccumulation. We could not infer where sources of Hg contamination originated without knowing the details of species-specific and individual movements of prey prior to capture by peregrines. The large difference in Hg contamination between our study areas indicated a trend of increasing contamination with decreasing distance to water, likely driven by availability of aquatic birds. We did not assess breeding performance and therefore, in the absence of toxicological studies, we were unable to determine effects total Hg may have on breeding peregrines. However, we note that the breeding population of peregrines in LMNRA was expanding from 2006–2010 and reproductive output appeared to be healthy overall (Barnes et al. 2015).

Within North America, upper trophic level piscivorous birds such as the Common Loon (Gavia immer; Fevold et al. 2003, Evers et al. 2005), Osprey (DesGranges et al. 1998), and Bald Eagle (Bowerman et al. 2002, Bechard et al. 2009) have been used as biomonitors of Hg in the environment. The applicability of using a single species as a bioindicator of ecosystem health can be limited when the species is restricted to a specific habitat type (e.g., aquatic systems) or has a limited geographic breeding distribution. This is particularly apparent, in light of recent research on biomagnification of Hg in terrestrial systems (Rimmer et al. 2005, Cristol et al. 2008, Keller et al. 2014). We suggest that Peregrine Falcons, with their cosmopolitan breeding distribution, catholic diet, and use of diverse habitat types (e.g., marine and freshwater shorelines, Arctic tundra, alpine cliffs, temperate forest, open desert, urban areas, etc.; White et al. 2013), may be an ideal indicator species of Hg contamination globally. Additional research is needed to establish adverse effect thresholds of Hg on peregrines, and studies that incorporate prey contamination will be especially valuable for assessing ecosystem health by determining system-wide effects of Hg across a broad range of avian species.