Golden eagles (Aquila chrysaetos) that nest and are raised in Denali National Park and Preserve, Alaska, (Denali) are among some of the most traveled individuals of this species in the world. Within just six weeks of fledging (leaving their nest), some of Denali’s juvenile eagles fly over 4,000 miles to spend the winter in central Mexico. Here they are in the company of other migratory golden eagles from interior and northern Alaska and northwest Canada. In spring, these eagles fly back to Alaska with some spending their first independent summer ranging hundreds of miles in search of food across Alaska’s North Slope (McIntyre et al. 2008). Come autumn, they leave Alaska and fly south to their wintering grounds.
If a golden eagle raised in Denali lives to age 20 and repeats similar migrations, showing some fidelity to both its natal range in interior and northern Alaska and its winter range in central Mexico, it will travel over 250,000 miles in its life. Over the nearly 30 years that I have studied golden eagles in Denali there have been noticeable changes in the landscapes used by these eagles both in Denali and across their vast year-round range (from Alaska’s North Slope to central Mexico). These changes create new challenges, and perhaps new opportunities, for conserving Denali’s migratory golden eagles.
Golden eagles have soared over Denali for thousands of years, but recently we started to have concerns over their future in a rapidly changing world (McIntyre et al. 2002; McIntyre et al. 2006). Present and future generations of Denali’s golden eagles face many new challenges to their survival. Understanding what factors affect their population persistence becomes more urgent as the spread of human-activities across western North America is transforming the landscapes used by these eagles.
Concern over the conservation status of golden eagles in North America has recently increased as threats to populations, including the direct and indirect effects of human activities, become better understood (Katzner et al. 2012; Millsap et al. 2013). Here, I review some of those threats and discuss how the National Park Service (NPS) can play an important role in the conservation of this species.
Multiple Challenges at Multiple Scales
Golden eagles prefer terrain that is at “odds with the horizon” (Figure 1) (Dunne et al. 1989), but the horizon is being obscured by the rapid construction of new power lines, power poles, wind turbines, other human-made objects, and by habitat alteration. Many different types of human activities result in both direct and indirect threats to migratory golden eagles from Denali and other Alaska national parks. Direct threats, which occur primarily during migration and winter when eagles are away from Denali, include death from electrocution, poisoning, shooting, and collisions with human-made structures. While much recent attention has focused on the direct threats associated with wind farms (i.e., collisions with turbines), perhaps bigger threats to migratory golden eagles are the increased risk of electrocution from power poles (Millsap, pers. comm.; Kemper et al. 2013) and the loss of suitable foraging habitat (Katzner et al. 2012) across their winter ranges.
Habitat loss, viewed as an indirect threat, has been substantial across the wintering range of Denali’s migratory golden eagles. Sagebrush communities that once covered nearly 63 million hectares (155,700,000 acres) of western North America have decreased by nearly 60 percent (Knick et al. 2003) primarily due to human activities (West and Young 2000). Important prey species for golden eagles, such as black-tailed jackrabbits (Lepus californicus), are associated with these shrub habitats (Kochert et al. 1999). The loss of prey habitat can result in decreased prey availability for golden eagles, leading to decreased survival and reproduction.
Landscape-scale changes are also evident on the breeding grounds in Denali and across much of interior and northern Alaska in areas where Denali’s golden eagles spend much of their early years before they enter the breeding population (McIntyre et al. 2008). Here, rapid expansion of woody vegetation across open landscapes and into higher elevation is transforming once open landscapes into more closed landscapes (Sturm et al. 2001; Tape et al. 2012; Roland and Stehn 2014). In Denali, trees are expanding into once treeless areas and woody vegetation is expanding across terraces and floodplains (Roland and Stehn 2014).
The expansion of shrubs and trees across the breeding grounds in Denali could present challenges for golden eagles that often forage and capture prey in more open landscapes. Further, increased vegetation cover is likely to have a negative effect on important prey species, such as Arctic ground squirrel (Urocitellus parryii) (Wheeler and Hik 2014; Wheeler et al. 2015). Sturm et al. (2001), Tape et al. (2012), and Roland and Stehn (2014) postulate that vegetation expansion in interior and northern Alaska is associated with elevated temperatures during the twentieth century (Hinzman et al. 2005).
It is beyond the scope of this article to hypothesize about the responses of golden eagles and their prey to a warming climate. However, it would be remiss not to at least touch on this subject and offer a few examples of some direct and indirect effects of a warming climate on golden eagles and their prey. Direct effects include reduced golden eagle nestling survival due to increases in rainfall during the breeding season (Anctil et al. 2014).
Further, warmer temperatures could provide new avenues for infectious disease and parasites that may kill golden eagle nestlings and adults (Van Hemert et al. 2014). Indirect effects include changes in snowshoe hare (Lepus americanus) demography caused by a warming climate (Kielland et al. 2010) and higher overwinter mortality of Arctic ground squirrel caused by mid-winter rains flooding their hibernacula (Werner et al. 2015).
This could lead to decreases in the number of hares and squirrels available as prey for golden eagles during the breeding season and lead to reductions in golden eagle reproductive success. Further, there could be climate-induced seasonal variation in the life history strategies of Arctic grounds squirrels that may result in loss of breeding opportunities and lead to declines in population size (Sheriff et al. 2013). Additionally, extended drought on the golden eagle wintering areas could result in reduced prey availability and decreased survival. These are just a few of the many examples of how human-related activities could both directly and indirectly affect Denali’s golden eagles.
It is unknown how golden eagles and their prey species will respond to long-term directional change such as global warming (Boonstra 2004). Some golden eagles in Idaho have exhibited resiliency to large-scale changes in prey availability in southwest Idaho (M. Kochert, pers. comm.). But, it is currently unknown if Alaska’s migratory golden eagles have the demographic resiliency to absorb additional mortality from increasing threats or if their environment is changing at a rate that exceeds their ability to adapt (Millsap et al. 2013).
Long-term Data Shows Decline in Reproductive Success of Golden Eagles in Denali
Golden eagles have been the focus of study at Denali National Park and Preserve since 1987 (McIntyre and Adams 1999; McIntyre et al. 2006; McIntyre and Schmidt 2012). In the northern foothills of the Alaska Range, we have documented territory occupancy and reproductive success at nearly 80 territories annually for 27 consecutive years, resulting in one of the longest-running studies of a migratory population of golden eagles in North America. Audubon designated this area as an Important Bird Area specifically because of the high density of nesting golden eagles. Monitoring of territory occupancy and a series of reproductive metrics including nesting rate, nesting success, and fledgling production began in Denali in 1988. In 2002, the species was selected as one of the vital signs for the National Park Service Central Alaska Monitoring Network, providing a means to continue to monitor golden eagles in Denali into the future. Currently, the Denali and Snake River programs are among the longest-running monitoring studies of golden eagles in the world. These long-term data sets provide unique opportunities to study how golden eagles respond to changes in their environment that many shorter-term studies would miss (Steenhof et al. 1997).
Golden eagles are relatively long-lived and do not attempt to raise offspring every year (Kochert et al. 2002; Watson 2010). In Denali, there is a close link between snowshoe hare abundance and golden eagle reproduction, with more female eagles laying eggs in years when hares are in the high phase of their population cycle than when hare populations crash (McIntyre and Adams 1999; McIntyre and Schmidt 2012). This link was expected since snowshoe hare are one of the only sources of live prey in Denali early in the breeding season. However, the probability of a female eagle laying an egg and raising a fledgling in Denali has decreased by about 25 percent over our study period (McIntyre and Schmidt 2012) (Figure 2).
These declines were not expected and were not explained by conditions on the breeding grounds (McIntyre and Schmidt 2012). This suggests that conditions on the wintering grounds, where Denali’s golden eagles spend up to 40 percent of the year, maybe be influencing their reproduction. This is known as a carry-over effect (Harrison et al. 2011), where conditions and events before the breeding season (i.e., during migration and winter) influence reproduction (Steenhof et al. 1997; Harrison et al. 2011).
Conserving Denali’s Golden Eagles Requires a Collaborative Approach Across Multiple Scales
Declines in reproduction may eventually lead to a reduction in the number of golden eagles nesting in Denali and perhaps to a situation where management actions cannot reverse a decline. The future of Denali’s golden eagles hinges on our ability to collaborate with others to reduce mortality by human-driven causes, and conserve habitat across their year-round range, not just in Denali.
Our success also depends on making sure our park managers are clearly aware of how conditions and events outside of Denali’s boundaries can negatively affect Denali’s golden eagles. For instance, park managers need to understand how the construction of new power poles in Canada (Kemper et al. 2013) or northern Mexico, that were not designed to avoid raptor electrocutions, could reduce survival rates of Denali’s golden eagles and perhaps lead to a reduction in the number of golden eagles nesting in Denali.
To identify the factors affecting golden eagle reproduction in Denali, NPS scientists are collaborating with other scientists on local, regional, and continental scales. On the local scale, we are continuing to monitor territory occupancy and a series of reproductive metrics of golden eagles on their breeding grounds. We are also investigating if the age structure of the territory holders is changing, if intrusions by non-breeding eagles into territories interfere with reproduction and, in collaboration with the U.S. Geological Survey (USGS), nesting territory fidelity. To link reproduction with conditions and events during migration and winter, we are collaborating with U.S. Fish and Wildlife Service (FWS) scientists to track the movements of Denali’s golden eagles, identify the resources they use across the year, and identify sources of mortality.
Further, starting in 2016, we will be collaborating with USGS scientists to expand these studies to assess conditions of wintering ranges and how they influence reproduction. These studies use cutting-edge Global System for Mobile communications-Global Positioning System (GSM-GPS) tracking technology that will provide us with high-resolution data on eagle movements and behavior. We are also supporting a graduate study at West Virginia University to quantify landscape-scale change in areas used by Denali’s juvenile golden eagles. Results of this graduate study will provide new insight into the decadal change on the wintering grounds. In addition to Denali-specific studies, we are also collaborating with FWS and Alaska Department of Fish and Game (ADFG) scientists on studies of the year-round movements of Alaska’s migratory golden eagles. We are also collaborating with scientists from the FWS, USGS, Bureau of Land Management (BLM), universities, state agencies, and many non-governmental organizations to describe the temporal and spatial movement patterns of golden eagles across North America. In addition to these studies, NPS scientists are collaborating with FWS scientists to conduct surveys across Alaska to provide an estimate of the size of Alaska’s golden eagle population. It is likely that Alaska national parks contain a substantial proportion of the breeding population of migratory golden eagles in Alaska. For instance, there are at least 150 pairs of nesting golden eagles in Denali (McIntyre, unpublished data). Based on preliminary survey data and availability of habitat, there may be similar numbers in Lake Clark National Park and Preserve and perhaps more in Gates of the Arctic National Park and Preserve and Wrangell-St. Elias National Park and Preserve (NPS, unpublished data).
Overall, more than 600 pairs of golden eagles may be nesting in Alaska’s national parks. In addition to these studies, NPS scientists continue to seek support to expand studies to understand the climate-induced responses by important golden eagle prey species. For instance, despite their broad range and considerable functional role as drivers and indicators of environmental change (Wheeler and Hik 2013), little is known about the ecology of Arctic ground squirrels in Alaska’s national parks, including Denali.
Why are Alaska’s National Parks Important for Golden Eagle Conservation?
Alaska’s national parks encompass over 54 million acres (21,850,000 hectares), including many that protect important nesting and foraging habitat for migratory golden eagles. Alaska national parks hold in trust the closest approximation to complete ecosystems left on this planet—a protected land base unsurpassed anywhere (Brown 2005; Brown and Elder 2005). As such, they also hold in trust present and future generations of migratory golden eagles. As the NPS prepares to celebrate its 100th anniversary it has renewed its commitment for large landscape conservation (NPS 2014)—a model that should increase our ability to conserve golden eagles and other migratory birds.
Thanks to Maggie MacCluskie (NPS) for reading an earlier version of this manuscript and providing constructive suggestions for improving it, Brian Millsap (FWS) and Mike Kochert (retired USGS) for information on golden eagles in Mexico and Idaho, and the National Park Service, Denali National Park and Preserve, and the Central Alaska Monitoring Network for providing support for our long-term studies.
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Series: Alaska Park Science - Volume 14 Issue 2: Birds of Alaska's National Parks