Search

Estimating species density and abundance is challenging but important for establishing conservation and management strategies. Significant progress has been made toward estimating Golden Eagle (Aquila chrysaetos) abundance in the conterminous United States of America (USA) but much less is known about eagle abundance in Alaska. Here, we paired migration count and GPS-tracking data collected near Gunsight Mountain, Alaska, in a Bayesian framework to estimate the number of Golden Eagles in south-central Alaska. We estimated 1204 (95% credible interval: 866, 1526) potentially breeding (≥4 yr old) Golden Eagles annually moved through the Gunsight Mountain migration corridor and summered over an area of 150,325 km2 in south-central Alaska, equating to a density of 0.80 potentially breeding eagles/100 km2. By extrapolating across the species' nesting range in Alaska (1,180,489 km2) and incorporating published productivity and age-specific survival rates for eagles <4 yr old into our hierarchical model, we estimated 12,717 (95% credible interval: 9043, 16,349) Golden Eagles of all ages occur in Alaska, annually. We propose this as a conservative statewide population estimate because we used methods that likely underestimated population size. Even so, our estimate is three to five times larger than previous estimates and likely represents about one quarter of the USA's population. [ABSTRACT FROM AUTHOR]

We investigated Golden Eagle (Aquila chrysaetos) diet on the Seward Peninsula, Alaska, at 35 and 30 occupied eagle nest sites in July 2014 and 2015, respectively. We used 52 prey remains and 456 regurgitated pellets collected from nest and perch sites to identify 727 individual prey items representing 18 prey categories. Golden Eagles in our study area primarily consumed Arctic ground squirrels (Urocitellus parryii; 52% of total biomass), ptarmigan (Lagopus spp.; 20%), and waterfowl (15%). Dietary breadth differed between years. We detected 13% more ptarmigan and 12% fewer ground squirrels at perch sites compared to nest sites. Our results suggest that Golden Eagles have a relatively broad dietary niche in western Alaska during the first half of the breeding season. [ABSTRACT FROM AUTHOR]

The conservation of golden eagles (Aquila chrysaetos) requires a thorough understanding of their demographic parameters. Productivity, commonly measured as the number of nestlings or fledglings per pair, is the parameter reported by most studies of nesting golden eagles and is often used as a measure of their population status. Survival may be an equally or more important parameter to measure; however, survival rates of golden eagles are not well documented. We used satellite telemetry to estimate the probability of first-year survival for migratory golden eagles raised in Denali National Park and Preserve, Alaska, USA. We calculated the probability of first-year survival using program MARK. Based on the best approximating model, monthly survival probability was 0.88 ± 0.04 (mean ± SE) during the autumn migration and early winter period for golden eagles marked in 1997 and 0. 78 ± 0.05 during the same period for golden eagles marked in 1999. Monthly survival during the remaining 3 periods (i.e., late winter, spring migration, summer) was 0.94 ± 0.03 for both cohorts. Survival during the entire 11- month period was 0.34 ± 0. 10 for the 1997 cohort and 0. 19 ± 0.07 for the 1999 cohort. Causes of mortality included starvation, electrocution, and poaching. Our results indicate that low first-year survival may limit recruitment and we recommend that golden eagle monitoring programs include survival estimates. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms by which populations are regulated is critical for predicting the effects of large-scale perturbations. While discrete mortality events provide clear evidence of direct impacts, indirect pathways are more difficult to assess but may play important roles in population and ecosystem dynamics. Here, we use multi-state occupancy models to analyze a long-term dataset on nesting bald eagles in south-central Alaska with the goal of identifying both direct and indirect mechanisms influencing reproductive output in this apex predator. We found that the probabilities of both nest occupancy and success were higher in the portion of the study area where water turbidity was low, supporting the hypothesis that access to aquatic prey is a critical factor limiting the reproductive output of eagles in this system. As expected, nest success was also positively related to salmon abundance; however, the negative effect of spring warmth suggested that access to salmon resources is indirectly diminished in warm springs as a consequence of increased glacial melt. Together, these findings reveal complex interrelationships between a critical prey resource and large-scale weather and climate processes which likely alter the accessibility of resources rather than directly affecting resource abundance. While important for understanding bald eagle reproductive dynamics in this system specifically, our results have broader implications that suggest complex interrelationships among system components.

Unintended effects of recreational activities in protected areas are of growing concern. We used an adaptive-management framework to develop guidelines for optimally managing hiking activities to maintain desired levels of territory occupancy and reproductive success of Golden Eagles (Aquila chrysaetos) in Denali National Park (Alaska, U.S.A.). The management decision was to restrict human access (hikers) to particular nesting territories to reduce disturbance. The management objective was to minimize restrictions on hikers while maintaining reproductive performance of eagles above some specified level. We based our decision analysis on predictive models of site occupancy of eagles developed using a combination of expert opinion and data collected from 93 eagle territories over 20 years. The best predictive model showed that restricting human access to eagle territories had little effect on occupancy dynamics. However, when considering important sources of uncertainty in the models, including environmental stochasticity, imperfect detection of hares on which eagles prey, and model uncertainty, restricting access of territories to hikers improved eagle reproduction substantially. An adaptive management framework such as ours may help reduce uncertainty of the effects of hiking activities on Golden Eagles., (©2011 Society for Conservation Biology.)

There is an abundance of field data for levels of metals from a range of places, but relatively few from the North Pacific Ocean and Bering Sea. In this paper we examine the levels of arsenic, cadmium, chromium, lead, manganese, mercury and selenium in feathers from common eiders (Somateria mollissima), glaucous-winged gulls (Larus glaucescens), pigeon guillemots (Cepphus columba), tufted puffins (Fratercula cirrhata) and bald eagles (Haliaeetus leucocephalus) from the Aleutian Chain of Alaska. Our primary objective was to test the hypothesis that there are no trophic levels relationships for arsenic, cadmium, chromium, lead, manganese, mercury and selenium among these five species of birds breeding in the marine environment of the Aleutians. There were significant interspecific differences in all metal levels. As predicted bald eagles had the highest levels of arsenic, chromium, lead, and manganese, but puffins had the highest levels of selenium, and pigeon guillemot had higher levels of mercury than eagles (although the differences were not significant). Common eiders, at the lowest trophic level had the lowest levels of some metals (chromium, mercury and selenium). However, eiders had higher levels than all other species (except eagles) for arsenic, cadmium, lead, and manganese. Levels of lead were higher in breast than in wing feathers of bald eagles. Except for lead, there were no significant differences in metal levels in feathers of bald eagles nesting on Adak and Amchitka Island; lead was higher on Adak than Amchitka. Eagle chicks tended to have lower levels of manganese than older eagles.

We collected 136 fresh and unhatched eggs from bald eagle (Haliaeetus leucocephalus) nests and assessed productivity on eight islands in the Aleutian archipelago, 2000 to 2002. Egg contents were analyzed for a broad spectrum of organochlorine (OC) contaminants, mercury (Hg), and stable isotopes of carbon (delta13C) and nitrogen (delta15N). Concentrations of polychlorinated biphenyls (SigmaPCBs), p,p'-dichlorodiphenyldichloroethylene (DDE), and Hg in bald eagle eggs were elevated throughout the archipelago, but the patterns of distribution differed among the various contaminants. Total PCBs were highest in areas of past military activities on Adak and Amchitka Islands, indicating local point sources of these compounds. Concentrations of DDE and Hg were higher on Amchitka Island, which was subjected to much military activity during World War II and the middle of the 20th century. Concentrations of SigmaPCBs also were elevated on islands with little history of military activity (e.g., Amlia, Tanaga, Buldir), suggesting non-point sources of PCBs in addition to point sources. Concentrations of DDE and Hg were highest in eagle eggs from the most western Aleutian Islands (e.g., Buldir, Kiska) and decreased eastward along the Aleutian chain. This east-to-west increase suggested a Eurasian source of contamination, possibly through global transport and atmospheric distillation and/or from migratory seabirds. Eggshell thickness and productivity of bald eagles were normal and indicative of healthy populations because concentrations of most contaminants were below threshold levels for effects on reproduction. Contrary to our predictions, contaminant concentrations were not correlated with stable isotopes of carbon (delta13C) or nitrogen (delta15N) in eggs. These latter findings indicate that contaminant concentrations were influenced more by point sources and geographic location than trophic status of eagles among the different islands.