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Article

A Helicopter Survey for Cliff-Nesting Raptors Along the Dalton Highway in Northern Alaska, 2010

by
Erica H. Craig
*,†,
Jim D. Herriges
and
Tim H. Craig
†
Bureau of Land Management, 222 University Ave., Fairbanks, AK 99709, USA
*
Author to whom correspondence should be addressed.
†
Current address: Aquila Environmental, P.O. Box 81291, Fairbanks, AK 99708, USA.
Birds 2025, 6(1), 1; https://doi.org/10.3390/birds6010001
Submission received: 10 October 2024 / Revised: 17 November 2024 / Accepted: 5 December 2024 / Published: 2 January 2025
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Simple Summary

Rapid environmental changes are occurring globally and especially in Arctic regions. They are due to many factors, including climate change, resource extraction and energy development. Raptors are often considered important indicators of environmental health but limited data are often available for their occurrence in remote areas. We report the results of a helicopter survey conducted in 2010 in northern Alaska. The purpose of our survey was to identify occupied cliff-nesting raptor territories and distribution of historic nest sites along the Dalton Highway. We discuss how raptor distribution patterns in 2010 may be different from historic ones and provide baseline information for monitoring future change that may occur as the environment in the Arctic changes.

Abstract

We conducted a helicopter survey in 2010 for cliff-nesting raptors along the Dalton Highway in northern Alaska. The study area extended from the Yukon River northward ~395 km through the Brooks Range to the Arctic Plain. We documented 55 occupied raptor nesting territories, including 25 Golden Eagle (Aquila chrysaetos), 11 Gyrfalcon (Falco rusticolus), and 10 Peregrine Falcon (Falco peregrinus) territories. We also recorded vacant stick nests and raptor perch sites on cliffs. We identified more occupied eagle territories and hundreds more vacant eagle stick nests than were previously known in the study area. The average number of Golden Eagle nestlings/nesting pairs at the time of the survey was 1.5 ± 0.6. The most productive Golden Eagle pairs were located in the northern part of the study area. The number of occupied eagle territories in 2010 is far fewer than the number of eagle stick nests we recorded. Our data indicate that the distribution of nesting eagles in the 2010 study area is different than in the past. Whether this reflects changes in eagle density; a response to availability of prey, climate change, human activities, or a combination of these; or some other factor warrants further investigation. Intensive surveys along the Dalton Highway in the 1970s found no occupied nests for Peregrine Falcons or Gyrfalcons but we found 10 and 11, respectively. Therefore, the Peregrine Falcon and Gyrfalcon nests we recorded in 2010 reflect an increase in the number of known nesting pairs there.

1. Introduction

Rapid environmental changes are occurring in the Arctic due to many factors, including climate change, resource extraction, and energy development [1,2,3,4]. These changes have the potential to affect territory occupancy, productivity, nesting success, and distribution patterns of avian species, including raptors [5,6,7,8,9]. For instance, the Gyrfalcon (Falco rusticolus) is ranked as highly vulnerable to impending impacts related to climate change (The Climate Change Vulnerability Index—CCVI [10]) because of its “narrow ecological niche” in northern latitudes [11]. There is also concern about the population status of Golden Eagles (Aquila chrysaetos) in North America because demographic models show declines for populations in the contiguous western United States (US; [12,13]). In addition, multiple sources show declines in Golden Eagle counts throughout the western portion of their range in the US [14,15,16] and possibly in Alaska [6]. At the same time, alterations in extensive tracts of native shrub/grasslands are occurring in the western US [17,18] within the winter range of Arctic nesting eagles [9,17,19]. These changes have implications across a broad geographic scale for species that are residents, as well as winter migrants and the agencies responsible for making land-use decisions within a species’ range [13,20,21].
In summer 2010, we conducted a helicopter survey for cliff-nesting raptors, primarily on the Bureau of Land Management (BLM)-managed lands that border the Dalton Highway north of the Yukon River. Our survey was centered on the highway, which traverses northern Interior Alaska and is the only all-weather road in the US that accesses the Arctic Ocean. Because the highway provides human access, many existing and proposed development activities occur along the road. Most notable of these is the Trans-Alaska Pipeline (TAPS), which roughly parallels the highway. However, additional existing and proposed human perturbations are concentrated along the highway, including other pipeline rights-of-way, mines, residences, and research and recreational activities.
Golden Eagles, Gyrfalcons, and Peregrine Falcons (Falco peregrinus) are the primary cliff-nesting raptors that occur in our study area, with the greatest numbers in the more mountainous regions. The BLM-funded survey effort was driven by agency policy to fulfill obligations under the Federal Land Policy and Management Act of 1976, the National Environmental Policy Act, the Migratory Bird Treaty Act, and the Bald and Golden Eagle Protection Act (BGEPA; 16 U.S.C. 668-668d). The survey was of particular importance to the BLM because of increases in proposed development along the Dalton Highway related to pipelines and mineral extraction. In addition, there are regulatory changes in the US, including guidelines related to the disturbance or “take” of Golden Eagles [13,20,21]. The BGEPA defines “take” as “pursue, shoot, shoot at, poison, wound, kill, capture, trap, collect, molest or disturb”. “Disturb” is defined by US regulation as “to agitate or bother a Bald or Golden Eagle to a degree that causes, or is likely to cause, based on the best scientific information available, (1) injury to an eagle, (2) a decrease in its productivity, by substantially interfering with normal breeding, feeding, or sheltering behavior, or (3) nest abandonment, by substantially interfering with normal breeding, feeding, or sheltering behavior” (50 CFR 22.6). Our objectives were to (1) inventory as many of the occupied cliff-nesting raptor territories as possible in the survey area and (2) document other important raptor use areas and their characteristics, as evidenced by stick nests (occupied and vacant), nest ledges, and perch sites with whitewash (feces) on them. Our results will (1) help BLM personnel in making resource management decisions for lands along the Dalton Highway; (2) provide information for designing a long-term monitoring protocol; (3) provide additional data for statewide population estimates; and (4) help identify the parameters for developing predictive raptor distribution models across northern Alaska.

2. Methods

2.1. Study Area

The study area borders the Dalton Highway and encompasses ~10,130 km2 and extends from the Yukon River (~66° north latitude), northward ~395 km to Slope Mountain (~69° north latitude; Figure 1). It includes the foothills and mountains of the Brooks Range bordered by rolling tundra hills (elevation range: 125 to ≥1700 m). The boundary of the survey area was determined by the availability of cliffs for nesting raptors bordering the highway. We primarily surveyed on BLM-administered lands, but occasionally flew cliffs that were continuous across administrative boundaries or adjacent to BLM land (see Figure S1).
Glacier-carved mountain valleys and rugged peaks occur in the Brooks Range in the north-central part of the study area. This montane region includes barren, rugged slopes covered with sparse vegetation typical of an alpine tundra ecosystem, and subtended by patches of coniferous forests at lower elevations [22]. In contrast, rolling hills occur in the extreme southern and northern parts of the study area. While the topography in these extents is similar, they host different vegetation communities. Mesic and moist tundra vegetation communities predominate in both, but shrub-dominated riparian areas and patchy coniferous and deciduous forests occur in the south, while only tundra covers the northern foothills [22]. Cliffs with potential raptor nesting sites occur throughout the region, but most are found in the Koyukuk, Dietrich, and Atigun River drainages, which bisect the Brooks Range. Potential nest sites range from isolated tors or small outcrops less than 100 m high in the rolling tundra areas to massive escarpments (>500 m) in the Brooks Range. Other cliff-nest sites include cliffs that border drainages, and eroded alluvial bluffs along water courses.
We refer to five separate segments of the study area (Figure 1) when interpreting the results on occupied raptor territories, eagle stick nests, and perch site distribution. These segments include (1) the rolling tundra hills north of the Brooks Range (Tundra N: 901 km2); (2) the Atigun River drainage (AR: 885 km2); (3) the Dietrich River (DR: 816 km2); (4) the Middle Fork of the Koyukuk River (MFK: 1382 km2) from its confluence with the Dietrich River, south to where it leaves the mountains (~14 km south of the village of Coldfoot, Alaska); and (5) the rolling tundra hills south of the Brooks Range (Tundra S: 6143 km2) and north of the Yukon River.
The weather in the Arctic study area is characterized by continuous daylight and relatively moderate temperatures during the raptor nesting season. July temperatures in the central part of the study area (Wiseman, Alaska) average 14.1 °C [23], and precipitation averages about 35.7 cm/year [24].

2.2. Terminology

Occupied Territory—We considered a territory occupied if we observed an adult incubating, an adult(s) exhibiting defensive behavior (e.g., flying back and forth in front of the cliff), eggs, nestlings (see Figures S2–S4), or a family group of recently fledged young perched near a nest site, [25]. In addition, for Golden Eagles, we considered a territory occupied if a nest contained fresh greenery, even if we observed no eggs, nestlings, or adults. We classified falcon ledges with apparent evidence (white feces, also known as “whitewash”) of recent nesting but no nestlings, eggs, or adults exhibiting defensive behavior as likely territories that we were unable to verify; an adult could be observed in an unverified territory.
Occupied Nest—A stick nest or ledge within an occupied territory that contains eggs or nestlings or an adult in an incubation posture. Occupied Golden Eagle territories can contain more than one stick nest but only one occupied nest in any given year [26].
Unoccupied Nest—A nest in an occupied territory that does not contain nestlings, eggs or an adult in incubation posture, but does contain fresh greenery.
Vacant Nest—A stick nest or ledge where we did not observe adults, nestlings, eggs, fresh greenery, or fresh whitewash evidence of nesting in 2010. Unused nest ledges were identified by old whitewash build-up from previous years’ use.
Alternative Nest—For Golden Eagles, an unoccupied or vacant stick nest in an occupied territory [27]. Most alternative nests in a Golden Eagle territory are within 1 km of the occupied nest, with none reported more than 6 km away [26,27,28].
Budget constraints prevented more than one survey during the nesting season. Consequently, we report productivity values for the pairs that produced nestlings as the number of nestlings/nesting pairs at the time of the survey, regardless of nestling age. These values could be biased high because not all the nestlings we recorded were at 80% of fledging age [25]. Further, we may not have detected (1) early nesters that had failed; (2) territories that already fledged young prior to our survey; (3) late nesting raptors such as Peregrine Falcons that were still incubating eggs, as raptors are typically more secretive and difficult to detect then; and (4) post-inventory nestling mortality prior to fledging [25]. These factors may have biased our results by underestimating the total number of occupied cliff-nesting raptor territories in the study area and overestimating productivity [25]. Nonetheless, we report the nestlings/nesting pairs for Golden Eagles and Gyrfalcons because they allow a comparison among segments of the study area at the time of the survey and a relative comparison with other similar surveys that have been conducted in the Arctic.

2.3. Survey Methods

Our survey focused on Golden Eagles, Gyrfalcons, and Peregrine Falcons, but we recorded all the raptors we observed. We also recorded Common Ravens (Corvus corax) because they build stick nests on cliffs that can also provide nesting substrates for raptors that do not build their own nests [11,29]. We spent a total of eight days surveying (25 June through 2 July 2010) and ~50 h of helicopter time, including ferry time between survey points, within the study area. The surveys coincided with the late incubation/early nestling period for most Peregrine Falcons, during the mid to late nestling period for most Golden Eagles and Gyrfalcons, and after Common Raven young had typically fledged. Our single survey precluded estimating detection probability, which we assume is <1.0 (see: [30,31]). However, because we used the same experienced surveyors and pilot throughout the surveys, we feel confident in comparing our results among the segments of the study area.
The potential nest cliffs that occur on BLM lands bordering the Dalton Highway exceeded our budgeted flight time. Consequently, we determined likely cliff nesting areas to be surveyed through pre-flight analyses of historic data (e.g., [32,33,34,35]; agency databases), prior knowledge of the area, and Geographic Information System (GIS; ArcMapTM10.1; ESRI Inc., Redland, CA, USA) examination of the topography within the study area. During our survey of the AR, DR, and MFK segments in the Brooks Range with their extensive cliffs, we focused our efforts on those that were closest to the Dalton Highway where impacts by future or current development activities were most likely to occur. Consequently, the area covered, and study area boundary is smaller in these three segments in the Brooks Range than in Tundra N and S where cliffs and rock outcrops were more widely scattered. In the latter, we flew up to 35 km from the highway and at higher altitudes while searching for and ferrying between isolated cliffs, bluffs, and outcroppings.
The complexity of the mountainous terrain in the study area required the use of a helicopter to effectively conduct our surveys. We used a Robinson R-44 II helicopter (Robinson Helicopter Company, Torrance, CA, USA) and two experienced observers; the left rear seat observer also served as a recorder, and the right front seat pilot functioned as a third observer when flying conditions permitted. The flights were conducted when winds were calm to moderate and when there was little to no precipitation. On large slopes with multiple outcrops, we made two or more passes, loosely following slope contours. Large cliffs required multiple passes and long narrow outcrops were flown from top to bottom.
The recorder collected GPS waypoints and track logs using a Garmin 76CSx GPS (Garmin, Lenexa, KS, USA), and recorded data related to each waypoint on a field inventory form. Waypoints for the nest and perch locations were generally recorded from the point of sighting—slightly above and away from the nest site on the cliff. The GPS waypoint elevation was assumed to be the elevation of the nest site or perch, resulting in an error of ~10–20 m in addition to the inherent elevation error associated with the GPS unit [36].
We hovered near occupied nest sites only long enough to determine occupancy because helicopters can disturb nesting raptors [37]. We generally flew no closer than ~30 m from a cliff face and farther away in windy conditions. We used binoculars to examine nests and/or photographed them from a distance using a telephoto lens to subsequently verify the number of young or eggs in the lab. When we found an occupied large falcon (Gyrfalcon or Peregrine Falcon) territory, we spent little time attempting to identify the actual nest site if it was not immediately apparent. At sites where no adults were present and only young were observed, the species was determined based on nestling plumage and other physical characteristics (e.g., head and beak shape), as well as phenology, nest site location, and characteristics.

2.4. Analysis

We recorded stick nest locations, ledges or cavities, and perch sites that showed evidence (e.g., whitewash) of recent or historic raptor activity. We report the number of vacant stick nests within 1 km, 2 km, and then >6 km of the occupied Golden Eagle nests we located. While subjective, this allowed us to make a conservative estimate of the distribution of nests relative to territories occupied during the survey.
When we encountered multiple nests very close together (~30 m) in a single cliff complex, we did not record each one as an individual waypoint, so the reported total numbers of stick nests are underestimates. We assigned vacant stick nests to different species based on their size, physical appearance, and setting. In general, we attributed large nests comprising large sticks to Golden Eagles and smaller or sloppily built nests to Buteo spp. or ravens. We recorded raptor perch sites on cliffs as evidenced by more than a few dribbles of whitewash. However, the number of raptor perch sites we recorded are conservative estimates because we recorded a single waypoint when multiple perches were clustered together (e.g., we counted each occupied nest as a single perch site).
We report the distribution of perches relative to occupied Golden Eagle and large falcon territories in increments (within 1 km, 2 km, and >6 km). We examined the spatial distribution of occupied raptor territories, stick nests, and raptor perches in a GIS (ArcMap 10.1; [38]; coordinate system: Albers North American Datum 1983). Means are reported ± standard deviation (SD). We used a single-factor ANOVA to test for differences in the mean number of nestlings/nesting pairs among the three segments (AR, DR, and MFK) of the study area that we surveyed in the Brooks Range. Data from occupied territories in Tundra N and S (n = 2) were not included in this analysis because nest cliff availability was limited there and not comparable to the Brooks Range segments. However, the tundra segments were included in the calculations of the average number of nestlings/nesting pairs in the study area. The small sample size for Gyrfalcon and Peregrine Falcon territories precluded making similar comparisons among segments for those species.

3. Results

3.1. Overview

We identified 11 species of raptors, 55 occupied raptor territories, and two Common Raven family groups during our survey (Table 1). We also located 316 stick nests (3 of which were in trees) at 266 locations, and 389 raptor perch sites on cliffs. Golden Eagles, Gyrfalcons, and Peregrine Falcons were the most common nesters in the study area, with more than twice as many occupied Golden Eagle territories detected as territories of either large falcon.

3.2. Golden Eagles

We located 25 occupied Golden Eagle territories, 19 of which contained nestlings (29 total nestlings; range: 0–3/nest; Table 2). Six eagle nests had evidence of occupancy but no nestlings and we assumed that they were unsuccessful nesting attempts. Four of the six were in the MFK and contained fresh greenery but no nestlings, nor did we observe adults nearby. The remaining two territories were in Tundra S and the DR. The former had an adult eagle present at a nest that contained greenery but no eggs or nestlings. No adults were present at the DR nest, but there was an egg in the nest that we presumed was abandoned.
There was a significant difference in the number of nestlings/nesting pairs in the three segments in the Brooks Range (MFK, DR, and AR: F = 9.6778, df = 2, p ≤ 0.01; Table 2). There were fewer nestlings/nesting pairs produced in the MFK than in the DR and AR, but almost as many territories as in the other two segments. The DR and AR had an equal number of occupied territories, but more nestlings/nesting pairs were produced in the AR than in the DR. The northern end of the DR was the only segment in the study area with a nest containing three nestlings. Eagle pairs in the DR produced more than three times, and in the AR, more than five times the number of nestlings than pairs in the MFK (Table 2). We detected only two occupied eagle territories in the Tundra N and S sections of the study area (one in each).
The nestlings recorded in the study area ranged in age from ~25–65 days with a marked difference in age between the southern and northern segments. Eaglets in the MFK were still downy with barely emerging feathers. Most of the nestlings in the DR and AR were older, although they varied in age from downy with pin feathers, to fully feathered and near-fledging.
We found more Golden Eagle stick nests (occupied and vacant) in the mountainous terrain of the MFK and DR than in the AR (Table 2). Eagle stick nests spanned the widest elevational gradient of any of the raptor species’ nests we recorded (mean: 879 ± 238 m; range: 257−1535 m). Perch sites were not restricted to the vicinity of occupied territories but were scattered throughout the study area. Unlike stick nests, more of the perches were in the AR on the north slope of the Brooks Range than in the other segments. The MFK had the second largest number of raptor perch sites. The mean elevations of the raptor perch sites (978 ± 319 m) and occupied eagle nests in the study area (Table 3) were similar, but the perches covered a wider geographic and elevational range (126–1718 m). Only 16% of the unoccupied eagle stick nests were within 1 km of an eagle nest occupied in 2010 and 23.4% were within 2 km; 44.7% were farther than 6 km away. A total of 28.0% of the 389 perch sites were more than 6.0 km from an occupied Golden Eagle or large falcon nest in the study area; 13.4% of the perches were more than 10 km away.
The shortest distance between occupied eagle nests in our study area was 2.2 km (Table 3). One occupied eagle nest was 770 m from a Gyrfalcon nest that contained three nestlings. The closest Peregrine Falcon nest to an eagle nest was 5 km away and the next closest was 15 km away.
Golden Eagles comprised more than half of the 71 raptor sightings that were not associated with a known occupied territory (Table 1). All but two of those sightings were of adult eagles. Most of these sightings occurred in the DR and AR, the same general areas in which 64% of the occupied territories were located.

3.3. Gyrfalcons

We found 11 occupied Gyrfalcon territories that contained nestlings (Table 1). Four additional areas may have been occupied in 2010 based on fresh whitewash on perches and ledges on cliffs but for which we were unable to verify occupancy. The number of nestlings/nesting pairs ranged from 1 to 4 (2.4 ± 0.9); they varied in age from ~2 weeks to fully feathered and near-fledging. Six of the eleven occupied nests were in old Golden Eagle stick nests. Occupied Gyrfalcon territories were widespread across the study area, occurring in all five survey segments, but usually at the higher elevations (Table 3). The average elevation of the raptor perches was higher than for the occupied Gyrfalcon nests (Table 3).

3.4. Peregrine Falcons

We found 10 occupied Peregrine Falcon territories in the study area. Three contained eggs or an adult in an incubation posture, five contained nestlings less than a week old, and two contained birds that exhibited defensive behavior at a cliff site but where we did not locate a nest (Table 1). Of the eight occupied territories where we located the nest, five were on ledges and three were in old eagle stick nests. In addition, there were three other sites that had a single or pair present along with whitewash evidence on a ledge and/or nearby perches. Although we could not verify occupancy; we considered these as likely occupied territories. The number of nestlings in a nest ranged from 2 to 4. Occupied nests were generally at lower elevations in the study area (Table 3); only two (20%) were at elevations higher than 610 m. All but one of the occupied territories were in the MFK and Tundra S in the southern half of the study area, and most (n = 8) were on bluffs or cliffs bordering rivers or streams. The territory at the northern end of the study area was occupied by a defensive pair but we observed no eggs or nestlings.

3.5. Other Raptor Species

We found two Rough-legged Hawk (Buteo lagopus) nests on small rock outcrops in Tundra N at the northern boundary of the study area and saw Rough-legged Hawks in four other areas in Tundra N where we did not find nests. The elevation of the Rough-legged Hawk sightings ranged from 730 to 1521 m (1017 ± 250 m). We observed nine American Kestrels (Falco sparverius) during the survey, the farthest north of which was a female in flight at ~68° N latitude. We observed a male defending against a Golden Eagle in the southern DR just north of the junction of the MFK and DR. We found three occupied Harlan’s Hawk (Buteo jamaicensis harlani) territories along rivers in MFK and in the northern part of Tundra S. Two nests were in trees but the third observation was of two adult Harlan’s Hawks that defended a vacant stick nest on a low cliff bordering a river. Common Ravens had already fledged when we conducted our survey. However, we observed two family groups that were near nests and five other nests that likely had fledged young based on the amount of fresh whitewash at the nest site. The other incidental raptor sightings we made during the survey are presented in Table 1. They included a Bald Eagle (Haliaeetus leucocephalus) nest with one nestling in a cottonwood tree (Populus spp.) at the edge of a river (~66.8° N latitude).

4. Discussion

4.1. Golden Eagles

Raptor surveys have been conducted along portions of the Dalton Highway in the past [32,33,34,35]. Most of these previous surveys were conducted in the 1970s prior to, or during, the construction of the TAPS and when Peregrine Falcons, the primary species of concern, were listed under the Endangered Species Act. We cannot make direct comparisons between our results for occupied Golden Eagle territories and these previous studies because of the differences in survey methods (e.g., focal species, survey intensity, area covered, survey timing, and pilot experience). However, these early surveys provide useful information on what was known about the broad-scale distribution of Golden Eagles and other raptors in the study area prior to our 2010 survey.
Roseneau and Bente [34] and Ritchie and Curatolo [39] recorded 11 occupied (or likely occupied) Golden Eagle territories between the Yukon River and the northern boundary of the study area. The latter authors did not record vacant nests, but Roseneau and Bente [34] reported 54 vacant stick nests in this area. White and Streater [32], who surveyed Peregrine Falcons in the Brooks Range, also reported that the area was used by nesting Golden Eagles, although they made no attempt to estimate numbers. All of these earlier surveys reported fewer nesting eagles in the AR in the northern part of the study area than in the DR and MFK south of the Continental Divide. In contrast, we found eight occupied territories and the greatest evidence of raptor perch activity in the AR in 2010. Raptor surveys conducted in conjunction with other resource work by Craig and Hamfler [35] from 1999 to 2002 identified five occupied eagle territories and 76 unoccupied Golden Eagle stick nests from the crest of the Brooks Range south to the Kanuti River. In 2010, our more intensive surveys located 16 occupied territories in the same area.
While we cannot draw conclusions about changes in the density of nesting eagles in the study area since the earlier surveys, we can make some broad-scale inferences. Most importantly, the early surveyors identified the mountains and foothills of the Brooks Range as “prime habitat for nesting Golden Eagles” [32,33]. Roseneau and Bente [34] reported that Golden Eagles were common nesters in the AR, but that more nested in the DR and MFK, which they described as “the most important Golden Eagle nesting habitat traversed” by the proposed TAPS. Similar to their findings, we also observed most of the occupied Golden Eagle territories in our survey area in the mountains of the Brooks Range. In addition, we recorded more than twice as many occupied eagle territories (n = 25) and more than three times the number of eagle stick nests (n = 269) than were previously known in the study area [32,33,34,35]. The nests built by Golden Eagles spanned a wide elevational range (257−1535 m) that was consistent with those reported by other researchers (e.g., [33,35]; maximum elevation: 1340 and 1281 m, respectively). The nests were generally at higher elevations than nesting Peregrine Falcons in the study area but at similar elevations to Gyrfalcons, comparable to the results reported by Ritchie and Shook [40]. Similar to the early observations on eagle distribution, we found most vacant stick nests south of the crest of the Brooks Range in the DR and MFK (n = 200; 11 km2/nest) and fewer on the north slope of the Brooks Range, in the AR (n = 31; 29 km2/nest). However, unlike the historic surveys, in 2010, we found that the farther north AR and DR contained most of the occupied eagle territories (n = 16) and nestlings (n = 25) compared to the more southern MFK (seven territories; n = 3 nestlings). We located only two occupied eagle territories and one nestling in the tundra areas (Tundra N, S), which probably reflects the limited availability of potential cliff-nest sites there.
The productivity of Golden Eagles in our study (1.5 ± 0.6 nestlings/nesting pairs; range: 1–3) is similar to surveys at northern latitudes elsewhere (1.2−1.6; [39,41,42]). However, the differences in productivity and distribution of raptor perches throughout all five segments within the study area in 2010 were striking. It is generally assumed that Golden Eagle productivity in Arctic regions declines with increasing latitude [42,43,44,45]. Consequently, we expected that nestlings/nesting pairs and phenology would be comparable, or perhaps that productivity would be lower and phenology later in the northernmost nests. To the contrary, we found higher productivity at the higher latitudes and earlier nesting by eagles. Productivity in the northern AR segment was 1.8 nestlings/nesting pairs (n = 8 territories) and higher than in the DR and MFK. Further, the number of occupied territories with nestlings was highest in the AR and Tundra N segments of the study area (100%) while it was much lower farther south in the MFK (43%) and Tundra S (0%) segments. In addition, the average nestling age and the most evidence of recent raptor use (perches) were the highest in the AR on the north slope of the Brooks Range. In contrast, nestlings in the southern Brooks Range (MFK) were still downy and younger than most nestlings farther north. These differences in phenology among survey segments could result in even greater latitudinal differences in productivity because less advanced nestlings detected in our single survey are likely to have suffered higher mortality than older nestlings (see e.g., [44,46]).
Golden Eagle nests are persistent in the environment. The hundreds of vacant nests we found probably represent decades or even centuries of past nesting attempts [27,47]. It is not uncommon for Golden Eagles to have multiple, unoccupied alternative nests within a nesting territory. For example, Watson [48] reported almost 1400 nests in the territories of 411 eagles in Scotland (1–13 nests/territory) in a survey performed in 1982. Kochert and Steenhof [27] reported 454 nests used in 66 eagle territories in southwestern Idaho over a period of 45 years. The alternative nests in the Idaho study area were relatively close together (range: <1–1822 m); 90% were <500 m apart [27]. Only 16% of the vacant nests we located were within 1 km of an occupied nest and almost half of the nests were farther than 6 km, (the maximum distance reported in the literature for an alternative nest; [28]). This suggests that many of the nests we located were not alternative nests for the territories occupied in 2010 but represent territories that were occupied at some time in the past. There are a few accounts in the literature of Golden Eagles switching from one territory to another, although this behavior is not thought to be common [26,49]. Nonetheless, it is possible that the superfluous nests in our study area could reflect similar changes in population distribution patterns over time, rather than changes in nesting densities. It is also important to note that the number of occupied territories we report is likely a conservative estimate. It is well known that breeding pairs that fail early are more easily missed during single, later-season surveys like ours [50]. Unsuccessful pairs may not remain on their territory like successful nesting pairs would, or leave greenery or other evidence in a nest to signal a nesting attempt [25]. We recorded 37 adult Golden Eagles during our survey that were not obviously associated with an occupied territory. It is possible that some of these birds represented territories that we missed or that failed early.
There are several possible explanations for the differences we observed in the distribution, phenology, productivity, and territory occupancy among the different segments of our study area. The distribution and availability of prey probably account for at least some of the latitudinal differences we observed in the Brooks Range. The location of more raptor perches in the northernmost AR than farther south in the DR and MFK supports the assumption that prey were more abundant there. Raptors often hunt in open habitats where perch sites are abundant and prey are available [48]. Hunting perches are generally near nest sites, but that can vary, as clusters of perches far from nest sites may indicate areas that host greater prey availability [50]. Arctic ground squirrels are more prevalent in the alpine-dominated upper DR and the AR (THC, EHC pers. observ.). However, snowshoe hares, (Lepus americanus), are more abundant in the southern Brooks Range (MFK and lower DR; THC, EHC pers. observ.) but these are cyclic populations. Eagle reproductive success, timing, and even distribution may be influenced by when and where their prey species are abundant [51,52,53]. McIntyre and Schmidt [6] suggested that eagles nesting at high latitudes may exhibit “temporary dietary specialization” when the density of their usual prey declines, and become reliant on a particular species when it is available. It is common for eagles to occupy territories during low prey years even though they do not attempt to nest [27,45,54,55] and this may have occurred in our study area. Although quantified prey data were unavailable for the study area, snowshoe hare numbers in the MFK were low [56] and ground squirrels appeared to be abundant farther north during our survey (THC, EHC, pers. observ.).
Golden Eagles sometimes switch between alternative nests within a territory from one year to the next [27], as well as occasionally switching from one territory to another [26,49]. Golden Eagles on the Seward Peninsula in western Alaska, for example, have been documented to use not only alternative nests in the same territory, but entirely different territories in successive years (P. Bente, Alaska Dept. of Fish and Game, retired, pers. comm.). It is possible that low hare density in the southern Brooks Range may result in more eagles remaining on territories but not nesting, or even possibly, shifting northward and establishing new territories where alternate prey is more abundant.
It must be acknowledged that anthropogenic activity can also negatively affect Golden Eagles and their nesting success [26,46,57]. White and Streater [32] anecdotally noted that Golden Eagle nests in the mouths of canyons along the TAPS corridor were vacant and that occupied nests were located farther back in the side canyons. They suggested this may have resulted from eagles attempting to avoid the human activity associated with the TAPS, which is located in the valley bottoms. Today there is more human activity in the MFK segment (e.g., mining, two villages) than in segments farther north; much of which was initiated subsequent to the surveys in the 1970s [32,33,34]. However, the areas of human influence in the study area today are localized, and so the differences we observed in distribution, nest occupancy, and productivity are more likely due to factors at a broader scale (e.g., prey availability or differences in habitat).
Lastly, a northward shift in many bird species has been associated with changes in weather patterns and concomitant changes in habitat related to climate change [7,8,58]. It is possible a similar association has contributed to the northward shift in nesting eagles in our study area and the differences we observed in nesting phenology.
Whether the patterns we observed in nesting Golden Eagles in 2010 represent a change in density, a short-term redistribution of eagle territories in response to changes in prey, or long-term changes in eagle distribution in response to climate change, anthropogenic influences, habitat, or a combination of factors, is not clear and warrants further investigation. Establishing a long-term monitoring strategy could help clarify important parameters of eagle ecology in the study area.

4.2. Gyrfalcons and Peregrine Falcons

Gyrfalcon nests generally occurred at higher elevations than Peregrines in the survey area, and were similar to Golden Eagles, over a wider range of elevations. This is likely because gyrfalcons commonly prefer open habitat, mountainous terrain above timberline [11]. Further, more than half of the occupied Gyrfalcon nests we located were in old eagle stick nests. Others have also reported that Arctic gyrfalcons often nest in old eagle nests [11]. Some Peregrine Falcons in the study area also nested in old Golden Eagle stick nests. However, nesting peregrines were mostly restricted to the lower latitudes and elevations in the study area. Although we found some birds nesting higher (maximum: 848 m) than the 610 m upper boundary suggested by Cade [59], most (80%) were lower. Ritchie and Shook [40] reported that 84% of peregrine nests in east-central Alaska were lower than 700 m and slightly higher than what we found (mean: 512 m, maximum: 1067 m). White and Streater [32] assumed that Peregrine Falcons did not nest at higher elevations in the Brooks Range because they avoided Golden Eagle nesting territories. Others have also noted that there is little overlap in the distribution of the two species [40,41]. Similarly, we found Peregrine Falcons nesting primarily where the number of nesting eagles was lowest; the nearest occupied eagle nest we found was ~5 km from an occupied peregrine nest. The onset of nesting for Peregrine Falcons is later than for Golden Eagles in northern Alaska. It is possible that the presence of Golden Eagles, which were already on territories influenced peregrine nest site selection in the study area. However, peregrines also utilize a much wider range of nesting substrates and more avian prey than Golden Eagles [60]. This could contribute to some of the differences we observed in distribution between the two species.
We recorded more Gyrfalcons and Peregrine Falcons nesting in the study area than were recorded in earlier surveys [32,33,34,35]. Further, the number of territories occupied by peregrines in our study area is likely even higher because we conducted our survey when most peregrines were incubating and were difficult to detect [25]. In addition, our survey focused on cliff-nesting raptors. Peregrine Falcons in Interior Alaska nest on a wide variety of substrates in addition to cliffs [40,60,61,62,63]. As a result, we were more likely to have missed nesting pairs of peregrines than Golden Eagles and Gyrfalcons. Intensive Peregrine Falcon surveys by Roseneau and Bente [34] in the 1970s found both Gyrfalcons and Peregrine Falcons nesting north of our study area, but none in it. Craig and Hamfler [35] recorded three Gyrfalcon and two Peregrine Falcon nests in their 1999−2001 surveys of a portion of our study area. We do not know if the number of occupied Gyrfalcon nests we found represents an overall increase in the Gyrfalcon population at a landscape scale because they rely on irruptive prey [11,64]. However, because historic surveys in the 1970s focused on large falcons in the vicinity of the TAPS, it is likely that the 11 occupied territories we found indicate an increase in nesting Gyrfalcons within the study area. Similarly, the 10 Peregrine Falcon territories we recorded document an increase in the number of nesting peregrines in the study area and probably reflect the increase observed across Alaska and other parts of their range since pesticide restrictions were implemented in the early 1970s [40,60,65].

4.3. Other Raptor Species

The single occupied Bald Eagle nest that we encountered next to a river at ~66.8° N latitude is among the farthest north records of nesting Bald Eagles in North America (US Fish and Wildlife Service, Migratory Bird Management-Raptors, Juneau, Alaska; Bald Eagle nest database; [66]). The Harlan’s Hawks we recorded are farther north than the currently published breeding distribution in central Alaska [67]. The Rough-legged Hawks we recorded were at the southern edge of their breeding range in north-central Alaska [68]. Roseneau and Bente [34] found only one nesting pair of Rough-legged Hawks at the extreme northern extent of our study area. We located two pairs there in 2010. The American Kestrels we observed were in the upper reaches of the MFK and in the DR. It was believed until recently that the range of the American Kestrel was south of the Brooks Range [69]. However, there is a historic sighting of a kestrel reported in Anaktuvuk Pass, Alaska [70]. The farthest north observation we recorded was at about the same latitude (68.1° N, elevation: 1381 m) and may be one of the farthest north records for this species. Recent research documents kestrels nesting just south of this location [71]. Shifts in species distribution related to changing climatic conditions have been described for other avian species [7,8,58,72]. It is possible that the incidental raptor sightings in our study area represent preliminary evidence of similar northward range expansions in response to climate change.

4.4. Future Direction

We recommend that the lands along the Dalton Highway between the Yukon River and Slope Mountain be identified as an area for the long-term monitoring of cliff-nesting raptors in the Arctic. There are several reasons that this area should be considered. First, it is accessible by highway and supports a relatively large number of nesting Golden Eagles and other cliff-nesting raptors at a high latitude. Therefore, monitoring in this area is easier and more cost-effective than more remote study sites in the Arctic, increasing the likelihood of consistent funding over time. Second, the anthropogenic influences that are concentrated along the Dalton are accelerating with increasing demands for energy, minerals, and recreational activities. The area is an ideal location to monitor the effects of these changes in land use. Lastly, climate change is expected to have a profound impact on Arctic regions [2,10,72,73,74,75]. Monitoring is an important component of conservation management for these species; the unique characteristics of the study area are ideal for tracking potential responses of Arctic cliff-nesting raptors to changes in their environment.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/birds6010001/s1, Figure S1. Land status in the vicinity of the study area; Figure S2. A typical Golden Eagle nest in the study area with nestlings close to fledging age; Figure S3. A typical example of a Peregrine Falcon nest in the study area; Figure S4. A typical example of a Gyrfalcon eyrie in an old Golden Eagle stick nest.

Author Contributions

Conceptualization, T.H.C.; methodology, T.H.C., E.H.C., and J.D.H.; analysis, E.H.C.; investigation, E.H.C. and J.D.H.; writing—original draft preparation, E.H.C., T.H.C., and J.D.H.; writing—review and editing, E.H.C., T.H.C., and J.D.H.; project administration, E.H.C.; funding acquisition, T.H.C. All authors have read and agreed to the published version of the manuscript.

Funding

This project was funded by the Bureau of Land Management; special thanks to C. Staab, formerly BLM-Alaska State Office, for helping to obtain funding. Many of the personnel from the BLM-Central Yukon Field Office (CYFO), U.S. Fish and Wildlife Service, National Park Service, and Alaska Department of Fish and Game provided logistical support during this survey. Aquila Environmental subsidized data analysis and preparation of the manuscript.

Institutional Review Board Statement

Aerial surveys followed protocols described by Pagel [37] and were in compliance with guidelines outlined by the Ornithological Council [76].

Data Availability Statement

Data are archived by the Bureau of Land Management (BLM), Fairbanks, Alaska. Because these raptor species are federally protected in the United States under the Bald and Golden Eagle Protection Act and the Migratory Bird Treaty Act, and because of the sensitivity of raptor nest site locations, requests by researchers for data access must be made to the BLM office with justification for access and will be evaluated on a case by case basis.

Acknowledgments

We acknowledge our pilot, Mark Shelton, of Quicksilver Air for his expert flying and excellent observational skills. M. Henderson, The Peregrine Fund, two anonymous reviewers, T. Hammond, Fairbanks District BLM Office and J. McMillan, National Park Service, provided helpful editorial comments. The views presented in this paper are those of the authors and do not necessarily represent those of the BLM. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Area surveyed for cliff-nesting raptors in northern Alaska, 2010. Study area segments are defined as follows: Tundra N = tundra north, AR = Atigun River drainage, DR = Dietrich River drainage, MFK = Middle Fork of the Koyokuk River drainage, Tundra S = tundra south.
Figure 1. Area surveyed for cliff-nesting raptors in northern Alaska, 2010. Study area segments are defined as follows: Tundra N = tundra north, AR = Atigun River drainage, DR = Dietrich River drainage, MFK = Middle Fork of the Koyokuk River drainage, Tundra S = tundra south.
Birds 06 00001 g001
Table 1. Occupied raptor and Common Raven territories, number of stick nests recorded on cliffs or in trees, and all raptors observed during a survey of cliff-nesting raptors bordering the Dalton Highway in northern Alaska in 2010. Stick nests were categorized according to the species that built the nest. Birds reported as flying or perched were not obviously associated with an occupied territory. Numbers in parentheses indicate likely occupied territories that we were unable to verify. Raven numbers reflect family groups observed near recently used nests.
Table 1. Occupied raptor and Common Raven territories, number of stick nests recorded on cliffs or in trees, and all raptors observed during a survey of cliff-nesting raptors bordering the Dalton Highway in northern Alaska in 2010. Stick nests were categorized according to the species that built the nest. Birds reported as flying or perched were not obviously associated with an occupied territory. Numbers in parentheses indicate likely occupied territories that we were unable to verify. Raven numbers reflect family groups observed near recently used nests.
SpeciesStick NestsOccupied TerritoriesBird Observed Flying or Perched
Greenery in Nest but No Nestlings or EggsTerritorial Behavior/Evidence of OccupancyIncubating AdultOne or More Nestlings PresentOne or More Eggs PresentTotal
Golden Eagle2695 11 1 191 225 139
Gyrfalcon (4) 11 11 (4)6
Peregrine Falcon 2 (3)15210 (3)1
Rough-legged Hawk 2 24
Harlan’s Hawk 3 31
Bald Eagle1 1 11
American Kestrel 1 19
Merlin 2 2
Northern Harrier 5
Unidentified Raptor (Northern Goshawk?) 1
Short-eared Owl 1
Great Horned Owl 1
Common Raven 2 (5) 2 (5)2
Small Stick Nests 3 46
1 one nest had an adult present and contained greenery but no eggs or nestlings. 2 a single egg was lying exposed in the nest with no adults present; we assumed it was abandoned based on the phenology of eagles in the study area and the appearance of the egg. 3 Buteo hawks or Common Raven nests.
Table 2. Comparison of Golden Eagle territories, nestlings, and occupied and vacant stick nests and perch distribution in five segments of a study area bordering the Dalton Highway in northern Alaska in 2010.
Table 2. Comparison of Golden Eagle territories, nestlings, and occupied and vacant stick nests and perch distribution in five segments of a study area bordering the Dalton Highway in northern Alaska in 2010.
Study Area Segment
(Acronym; km2/Segment)		Number of Occupied Territories
(Territories with Nestlings)		Number of Nestlings/Nesting Pairs at the Time of the SurveyNumber of Nestlings/
Segment		Number of Golden Eagle Stick Nests
(km2/nest)		Number of Raptor Perches/Segments
(km2/Perch)
Northern Tundra Hills (Tundra N;
901)		1 (1)1.013 (300)56 (16)
Atigun River Valley—north Brooks Range (AR; 885)8 (8)1.8 ± 0.5 *1431 (29)111 (8)
Dietrich River Valley—south Brooks Range (DR; 816)8 (7)1.6 ± 0.8 *11102 (8)61 (13)
Middle Fork Koyukuk River Valley—(MFK; 1382)7 (3)1.0 ± 0 *398 (14)91 (15)
Southern Tundra Hills (Tundra S; 6143)1 (0)0.0035 (176)70 (88)
Total for Study Area (10,127)25 (19)1.5 ± 0.629269 (38)389 (26)
* asterisks indicate a significant difference in number of nestlings/nesting pairs among the three segments in the Brooks Range.
Table 3. Distances (km) between, and elevation (m) of, known occupied nests/ledges of conspecifics in the study area for the three most common cliff-nesting raptors located in the river valleys of the Brooks Range.
Table 3. Distances (km) between, and elevation (m) of, known occupied nests/ledges of conspecifics in the study area for the three most common cliff-nesting raptors located in the river valleys of the Brooks Range.
SpeciesMinimum Distance (n)Maximum DistanceMean DistanceMean ElevationElevation
Range
Golden Eagle2.2 (25)30.3 16.4 ± 4.0 1990 ± 220522−1333
Gyrfalcon6.2 (11)33.319.0 ± 2.9852 ± 223555−1202
Peregrine Falcon2.5 (10)121.314.3 ± 4.4416 ± 197120−848
1 excludes two isolated nests not in the cliff segments of the Brooks Range.
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Craig, E.H.; Herriges, J.D.; Craig, T.H. A Helicopter Survey for Cliff-Nesting Raptors Along the Dalton Highway in Northern Alaska, 2010. Birds 2025, 6, 1. https://doi.org/10.3390/birds6010001

AMA Style

Craig EH, Herriges JD, Craig TH. A Helicopter Survey for Cliff-Nesting Raptors Along the Dalton Highway in Northern Alaska, 2010. Birds. 2025; 6(1):1. https://doi.org/10.3390/birds6010001

Chicago/Turabian Style

Craig, Erica H., Jim D. Herriges, and Tim H. Craig. 2025. "A Helicopter Survey for Cliff-Nesting Raptors Along the Dalton Highway in Northern Alaska, 2010" Birds 6, no. 1: 1. https://doi.org/10.3390/birds6010001

APA Style

Craig, E. H., Herriges, J. D., & Craig, T. H. (2025). A Helicopter Survey for Cliff-Nesting Raptors Along the Dalton Highway in Northern Alaska, 2010. Birds, 6(1), 1. https://doi.org/10.3390/birds6010001

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