Sex difference in natal dispersal distances of Golden Eagles Aquila chrysaetos in Scotland

Natal dispersal distance (NDD) is critical in understanding and defining populations and their conservation. It is defined as the linear distance between the natal location and first reproductive (‘effective NDD’) or potential reproductive (‘gross NDD’) location. It is a measure of gene flow and the functional connectivity across generations between individuals breeding in the same or different geographies. NDD is difficult to record in large raptors. GPS-satellite telemetry has facilitated its recording. Previous Scottish studies showed that gross and effective NDD were apparently equivalent, and an algorithm based on telemetric data could identify the territory settlement timing and location of birds originally tagged as nestlings. We analysed natal dispersal data from 39 Golden Eagles GPS-tagged in Scotland to estimate NDD. Raw median estimates were 29.8 km for males (n = 22) and 58.6 km for females (n = 17), 38.1 km averaged across sexes. Males had significantly shorter NDD, as theoretically predicted. Our NDD estimates were shorter but not grossly dissimilar to those from the USA, where sex differences in NDD had not been confirmed. Respective sample sizes may underly the latter contrast in confirmation. We also showed that in the absence of data from sexed birds, NDD estimates can be different. Natal dispersal duration was not related to NDD, suggesting that time to prospect a territory opportunity was not associated with the selected territory's distance from the natal site. The previous status of the subsequent settled territory (occupied or vacant) was also not related to NDD. We conclude that sex differences in NDD are important in application to population demography and conservation. Although we found no support for two other potential drivers of NDD (natal dispersal duration and previous territory status), identifying additional influences on NDD is in its infancy in large raptors and deserves more study.     

Organochlorines and mercury in the eggs of Golden Eagles Aquila chrysaetos from Scotland

This paper documents the organochlorine levels found in unhatched eggs from 234 clutches of Golden Eagles Aquila chrysaetos obtained in Scotland during 1963-86. Organochlorine levels were highest in eggs from western coastal districts, somewhat lower in eggs from western inland districts, and lower still in eggs from eastern districts. These regional trends were associated with corresponding dietary differences.
Levels of HEOD declined in western districts during the period 1963-86, but not in eastern districts where they were low throughout. Levels of DDE declined generally, but significantly only in the east. In contrast, levels of PCBs increased generally, but significantly only in western inland districts.
No differences in organochlorine levels were found between clutches that produced no young and those that produced one young.
Mercury residues were examined in eggs from 66 clutches obtained during 1981-86. Low levels were detected in 10 out of 29 eggs from western coastal districts, in 2 out of 27 eggs from western inland districts, and in none out of 10 eggs from eastern districts.     

Complex effects of habitat loss on Golden Eagles Aquila chrysaetos

The development of commercial forests presents potential threats to large raptors that rely on prey caught in open country. We examined the effect of afforestation of breeding habitat used by a population of Golden Eagles Aquila chrysaetos in Scotland where, over the last 50 years, extensive stands of exotic conifers have been planted. Using data for 31 years on territory occupancy and breeding success, together with spatiotemporally dynamic mapping of forest cover and predicted areas of territory‐use in a Geographical Information System, we examined relationships between forest cover and Eagle ecology at landscape and individual territory scales. Several territories were abandoned during the earliest phases of forest planting, but relatively few were apparently lost to later plantings. Territories with poorer breeding productivity appeared to be more vulnerable to abandonment than territories with better breeding productivity. At the landscape scale, temporal differences in breeding productivity were negatively related to the extent of forest cover, although productivity of individual territories showed no clear relationship with forest cover. Several territories with less than a 5% increase in forest cover experienced reduced productivity; however, territories least constrained by neighbouring pairs of Eagles showed an increase in productivity. Territories experiencing the greatest increases in forest cover showed a greater use of spatially separated nest‐sites by occupying pairs. Hence, pairs that were less constrained by neighbours appeared to compensate for loss of open habitat by shifting their territory‐use, whereas pairs that were more constrained could not compensate for open habitat loss and suffered reduced productivity (and, probably in some cases, abandoned the territory). We suggest that simple guidelines based on the extent and locations of habitat loss are inadequate when predicting effects on large territorial raptors such as Golden Eagles. Consideration should also be given to the ‘quality’ of a territory or occupying pair, as well as the extent to which territory‐use is constrained by neighbouring pairs or other ‘unsuitable habitat’ which may have been affected by previous episodes of open habitat loss.     

Factors constraining the distribution of Golden Eagles Aquila chrysaetos in Scotland

Capsule Between 1992 and 2003 persecution appeared to be the main influential factor.

Aims To utilize temporal changes in the distribution and occupation of Golden Eagle territories in Scotland between the 1992 and 2003 national censuses to assess potential causes of regional and national population trends, by examining spatial associations with a number of potential constraints on the population.

Methods The distribution of occupied and vacant territories in the 1992 and 2003 censuses were entered as layers in a Geographical Information System (GIS), along with boundaries of biogeographical regions (Natural Heritage Zones) for regional analyses. Additional GIS layers were created for potential factors that may constrain the eagle population: the distribution and abundance of persecution incidents, new commercial conifer forests, popular mountains for hillwalkers (as surrogates for recreational activity), and the density of sheep and Red Deer (as surrogates for carrion abundance), drawn from comparable time periods to the national eagle censuses. Analyses then looked for spatial associations between eagle territory status and those constraint factors that may have influenced change in territory status.

Results We found little evidence to suggest that recreational disturbance was influential on the occupation of Golden Eagle territories, although some local effects may have occurred and further analyses are warranted. We could find evidence of only a limited number of territories having being abandoned recently due to the planting of commercial conifer forests. We also rejected the hypothesis that changes in territory occupation between national Golden Eagle censuses were influenced by change in carrion abundance. By contrast, results were consistent with the hypothesis that persecution was influential in the observed change in territory occupation between censuses, so that occupied eagle territories tended to decline where persecution was probably still influential and tended to increase where persecution had probably declined.

Conclusion In accordance with earlier predictions based on models of the demographic influence of persecution, in the central and eastern Highlands where grouse moor management predominates, the eagle population continued to decline to levels where increasingly large areas of suitable habitat are unoccupied by breeding pairs.     

Golden Eagles Aquila chrysaetos: land use and food in northeast Scotland

This paper contrasts changes in breeding numbers and breeding success of Golden Eagles Aquila chrysaetos on four areas with different land uses. On land primarily used for deer stalking but also for grouse shooting, and supporting abundant prey, the number of eagle pairs was steady in 1944-80. On deer land with fewer prey, the number of pairs declined greatly in the 1960s, when deer carrion became scarcer following increased shooting of red deer. On grouse moors incorporating deer stalking, the number of pairs declined in the 1950s due to persecution by gamekeepers, and then largely recovered as this lessened. On grouse moors with little or no stalking, the number of pairs fell after 1946 and remained low due to persecution, which has continued since then. Eagles on grouse moors bred poorly due to persecution. On deer land they were seldom persecuted by estate staff, and bred well. On an area of deer land, the mean annual number of young reared per undisturbed clutch in summer was related to the estimated weight of prey in the spring of the same year. The eagles have remained fairly pesticide-free, and bred well in 1963-65 when more contaminated birds in west Scotland bred poorly.     

Inter- and intra-specific dominance relationships and feeding behaviour of Golden Eagles Aquila chrysaetos and Sea Eagles Haliatetus albicilla at carcasses

The behaviour of Golden Eagles Aquila chrysaetos and Sea Eagles Haliceetus albicilla scavenging on artificially laid out carcasses in coastal Sør-Trøndelag Province, Norway, was studied during two winters (totalling 640 h of observations) and the intervening summer (430 h). Neither species fed at carcasses in summer. Although smaller, Golden Eagles were strongly dominant over Sea Eagles in direct competition for carcass access. In Sea Eagles, females dominated males, while in Golden Eagles, few conflicts between birds of known sex were observed. Age effects were weak and not statistically significant in both species. Conflicts for carcass access tended to be most escalated between Golden Eagles and least escalated between Sea Eagles, with interspecific conflicts intermediate. Most conflicts were won by the aggressor, suggesting that birds were generally able to assess relative dominance before launching an attack. Young eagles fed longer at a carcass than older individuals in both species, suggesting that young eagles may have been hungrier or less efficient feeders. Sea Eagles waited longer than Golden Eagles between arrival in the immediate carcass area and feeding at the carcass. This effect was greater when the carcass was already occupied but also occurred when no other eagle was already present. While interspecific competition for carrion did not appear to have important consequences for the two species in coastal Norway, in western Scotland, where Sea Eagles are currently re-establishing, carrion is important in the diet of both species all year round. Interspecific competition for this resource may therefore play a role in determining the ultimate realized niche (and therefore numbers) of the two species in Scotland in the longer term.     

Population structure and dispersal patterns in Scottish Golden Eagles Aquila chrysaetos revealed by molecular genetic analysis of territorial birds

Conservation management of species distributed across fragmented habitats requires consideration of population genetic structure and relative levels of genetic diversity throughout the relevant geographical range. The Golden Eagle Aquila chrysaetos is monitored within Scotland to ensure its survival in the face of land‐use pressure, persecution and future climate change. In this study we constructed DNA profiles for 271 individual birds using a collection of over 1600 moulted feathers collected from 148 territories, representing 34% of known Scottish territories in the largest population genetic study of Golden Eagles undertaken to date. The results, based on data from 10 nuclear microsatellite loci, revealed previously unreported genetic structure between the islands of the Outer Hebrides and the rest of Scotland (F_ST = 0.03), together with evidence of reduced genetic diversity in the Outer Hebridean population compared with mainland Scotland. Analysis of gene flow supports a hypothesis of limited, predominantly male‐mediated, dispersal from the Outer Hebrides to mainland Scotland. The persistence of this pattern is discussed with respect to variation in population density and persecution pressure across Scotland. A finding of non‐random mating within the Outer Hebrides is interpreted as evidence of natal philopatry that was revealed by more intensive sampling in these islands, and is likely to be accentuated by the apparent degree of isolation of the islands from the rest of Scotland.     

Impact of nest-trapping and radio-tagging on breeding Golden Eagles Aquila chrysaetos in Argyll, Scotland

Golden Eagles Aquila chrysaetos in South Argyll, Scotland, have been monitored annually since 1964. Other workers trapped and radiotagged breeding adults there for a separate study during 1991–1996. Our analysis of our monitoring data shows that the trapping and tagging was followed by reduced reproductive success (proportion of territories producing young each year). Also, nest-sites where adults had been trapped were used less frequently after the trapping.     

Spatial distribution of undulating flight displays of territorial Golden Eagles Aquila chrysaetos in Lewis,Scotland

ABSTRACT

Capsule: The undulating display flight of Golden Eagles Aquila chrysaetos has a territorial function on current consensus: we found that displays could occur at any location within a home range of territorial birds and were not more frequent at the territorial limits (as documented by a previous study) or around the nest site.

Aims: To test the null hypothesis that display activity does not spatially differ within the home range of territorial Golden Eagles.

Methods: We used 1488?h of observations of Golden Eagles between April 2003 and October 2004 (excluding November through January inclusive) on the Isle of Lewis, northwest Scotland involving primarily four territories. Territorial boundaries and nest sites were determined by the behaviour of the focal territorial birds. All flight activity was cast into three spatial categories (near-boundary, near-nest and elsewhere) and the relative occurrence of displays was referenced by overall flight activity recorded in these spatial categories.

Results: We failed to reject the null hypothesis. Prior findings of greater display rate at territory limits were not confirmed. Territorial Golden Eagles apparently displayed wherever they happened to be in their home range. There was no difference in display frequency between successful and failed breeding pairs and, within pairs, between successful and failed breeding seasons. Monthly changes in display activity did not differ from monthly changes in the presence of ‘intruding’ sub-adults. A peak in non-territorial bird activity in August and September was not reflected by a peak in display, however, we could not distinguish recently fledged ‘non-threatening’ birds in this period.

Conclusions: Undulating flight displays of Golden Eagles focused neither on territory boundaries nor the nest. Displays may be spontaneous or occur in response to the detection of potential trespassers or neighbours, however, we could not rule out any role of pair-bonding.     

Regional and temporal variations in prey selected by Golden Eagles Aquila chrysaetos during the nestling period in Japan

We examined regional and temporal variations in prey selection by Golden Eagles Aquila chrysaetos during the nestling period in Japan. We made direct video recordings of a pair of Golden Eagles in Akita prefecture as they delivered prey to the nest for two consecutive nestling periods. We also assembled data from previous studies in Japan, eventually obtaining 14 data sets with which we compared prey composition during nestling periods. Among them, four sets of data were recorded daily by video and used to investigate the temporal change in prey selection and the amount delivered to the nest. The prey item composition varied considerably among the data sets. Japanese Hares Lepus brachyurus were predominantly selected in three data sets, reflecting the lowest dietary breadths that were determined by prey composition. Data sets with higher dietary breadths consisted mainly of Japanese Hares, snakes and Copper Pheasants Syrmaticus soemmerringii . Temporal change in prey selection during nestling periods showed marked variation, but similarities were found in later deliveries of snakes and in total prey weights (83.7–89.9 kg) delivered to successfully fledged broods. Taken together, our results suggest that during nestling periods Golden Eagles in Japan specialized on Japanese Hare. Diet breadth increased through feeding predominantly on snakes, a temporarily available prey, to satisfy the breeding dietary requirement. Regionally varied temporal prey selection may be a key factor for sustaining Eagle populations in the forested mountain habitats of Japan, where prey and habitat conditions change dramatically during the breeding season.     

Responses of dispersing GPS-tagged Golden Eagles (Aquila chrysaetos) to multiple wind farms across Scotland

Wind farms may have two broad potential adverse effects on birds via antagonistic processes: displacement from the vicinity of turbines (avoidance), or death through collision with rotating turbine blades. Large raptors are often shown or presumed to be vulnerable to collision and are demographically sensitive to additional mortality, as exemplified by several studies of the Golden Eagle Aquila chrysaetos. Previous findings from Scottish Eagles, however, have suggested avoidance as the primary response. Our study used data from 59 GPS-tagged Golden Eagles with 28 284 records during natal dispersal before and after turbine operation < 1 km of 569 turbines at 80 wind farms across Scotland. We tested three hypotheses using measurements of tag records’ distance from the hub of turbine locations: (1) avoidance should be evident; (2) older birds should show less avoidance (i.e. habituate to turbines); and (3) rotor diameter should have no influence (smaller diameters are correlated with a turbine’s age, in examining possible habituation). Four generalized linear mixed models (GLMMs) were constructed with intrinsic habitat preference of a turbine location using Golden Eagle Topography (GET) model, turbine operation status (before/after), bird age and rotor diameter as fixed factors. The best GLMM was subsequently verified by k-fold cross-validation and involved only GET habitat preference and presence of an operational turbine. Eagles were eight times less likely to be within a rotor diameter’s distance of a hub location after turbine operation, and modelled displacement distance was 70 m. Our first hypothesis expecting avoidance was supported. Eagles were closer to turbine locations in preferred habitat but at greater distances after turbine operation. Results on bird age (no influence to 5+ years) rejected hypothesis 2, implying no habituation. Support for hypothesis 3 (no influence of rotor diameter) also tentatively inferred no habituation, but data indicated birds went slightly closer to longer rotor blades although not to the turbine tower. We proffer that understanding why avoidance or collision in large raptors may occur can be conceptually envisaged via variation in fear of humans as the ‘super predator’ with turbines as cues to this life-threatening agent.     

Ranging distance of resident Golden Eagles Aquila chrysaetos in western Scotland according to season and breeding status

Capsule Home-range of resident pairs of Golden Eagle was usually smaller during a successful breeding season than during winter and during an unsuccessful breeding season.

Aims To examine how Golden Eagles use space around their nests with respect to season and breeding status, and to compare home-range-use between a high and a low density region.

Methods Nine adults in six mainland Argyll ranges were radiotracked between 1991 and 1996. On the island of Mull visual observations of range-use were obtained for five ranges between 1994 and 1998.

Results Overall, Mull ranges were smaller than the Argyll ranges, reflecting the much higher range density on Mull. In both regions there were significant differences between ranging distances with season and breeding status. In general, ranging distances were smallest during breeding seasons when young were fledged.

Conclusions Studies of range-use in Golden Eagles must be conducted across a 12-month period, as a minimum.     

Spacing of Golden Eagle Aquila chrysaetos nests in relation to nest site and food availability

A regular nest spacing of Golden Eagle pairs was found in every carefully searched area within the species range in Sweden. The spacing regularity of nests used by pairs in forests (mostly tree-nesting) was similar to that in mountain areas (cliff-nesting), indicating that regular nest spacing is affected more by spacing behaviour than by shortage of nest sites. Distances to nearest neighbours were shorter in the mountain region (mean distance 102 km) than in the forests (170) of northern Sweden. Densities of the most important prey species for Golden Eagle were apparently higher in the mountains than in the forests. Differences in topography and habitat diversity between the two regions may also have contributed to the difference in breeding density.     

Long‐term breeding demography and density dependence in an increasing population of Golden Eagles Aquila chrysaetos

Few studies have quantified the dynamics of recovering populations of large raptors using long‐term, spatially explicit studies. Using data collected over 37 years in the western Italian Alps, we assessed the trends in distribution, abundance, fecundity and breeding population structure of Golden Eagles Aquila chrysaetos. Using the spatial distribution of territory centroids in 2007, we found that the spatial distribution of eagle territories was over‐dispersed up to 3 km. Although population size and total productivity increased from 1972 to 2008, the proportion of pairs that laid eggs showed a strong decline, falling to no more than 50% after 2003. On average, 15% of successful nests produced two fledglings, and productivity also declined over time. No significant relationship between population growth rate and total population size was detected, but the percentage of pairs that bred and breeding success showed evidence of density dependence, as they declined significantly with increasing density. Our results suggest that density dependence, operating across heterogeneous habitats, is currently regulating this population, while the carrying capacity may still be increasing. This may explain the apparent paradox of reduced breeding effort despite increasing total productivity.     

Habitat selection by adult Golden Eagles Aquila chrysaetos during the breeding season and implications for wind farm establishment

Capsule: Global Positioning System (GPS)-tagged adult Golden Eagles Aquila chrysaetos breeding in forests in northern Sweden selected clear-cuts, coniferous forests with lichens and steep slopes during the breeding season but avoided wetlands and mixed forest.

Aims: To investigate the habitat selection patterns of tree-nesting Golden Eagles, and identify how potential conflicts with wind farm development could be minimized.

Methods: The study is based on GPS tracking data from 22 adult eagles. We estimated home range sizes using a biased random bridge approach and habitat selection patterns using resource selection functions following a use-availability design.

Results: Core home range size among adults was variable during the breeding season (5–30?km²). Individual movement extents were variable, but sexes did not significantly differ in their scale of movement. At the landscape scale, individuals selected for clear-cuts and coniferous forest with ground lichens, whereas wetland, water bodies and mixed forest were avoided. Steeper and south facing slopes were selected for, whereas, north facing slopes were avoided.

Conclusions: Potential conflicts between eagles and wind energy establishment can be reduced if wind farms are placed away from steep slopes, minimizing areas that are clear-cut during construction, and locating turbines within dense, young and other less favoured forest habitats.     

Diet specificity is not associated with increased reproductive performance of Golden Eagles Aquila chrysaetos in Western Scotland

Amongst raptor species, individuals with specialized diets are commonly observed to have higher reproductive output than those with general diets. A suggested cause is that foraging efficiency benefits accrue to diet specialists. This diet specificity hypothesis thus predicts that diet breadth and reproductive success should be inversely related within species. We highlight, however, that a prey availability hypothesis also makes the same prediction in some circumstances. Hence, when high diet specificity results from high encounter rates with an abundant, preferred prey, then prey availability may affect reproductive success, with diet specialization as an incidental correlate. Using three insular study areas in western Scotland, we examine diet specificity and reproductive success in Golden Eagles Aquila chrysaetos. Diet breadth and breeding productivity were not negatively related in any of our study areas, even though birds with specific diets did tend to have a higher incidence of preferred prey (grouse and lagomorphs) in the diet. Indeed, in two study areas there was evidence that diet generalists had higher breeding productivity. Our results therefore failed to support the diet specificity hypothesis but were consistent with the prey availability hypothesis. We highlight that although many other studies are superficially consistent with the diet specificity hypothesis, our study is not alone in failing to provide support and that the hypothesis does not provide a generic explanation for all relevant results. Diet specificity in predators can be at least partially a response to prey diversity, availability and distribution, and benefits associated with different prey types, so that being a generalist is not necessarily intrinsically disadvantageous. We suggest that the available evidence is more consistent with variation in prey abundance and availability as a more influential factor explaining spatial and temporal variation in breeding productivity of ‘generalist’ species such as the Golden Eagle. Under this argument, prey abundance and availability are the main drivers of variation in reproductive output. Diet specificity is a consequence of variation in prey availability, rather than a substantial cause of variation in reproductive success.     

Ecological and environmental correlates of territory occupancy and breeding performance of migratory Golden Eagles Aquila chrysaetos in interior Alaska

Understanding relationships between environmental conditions and reproductive parameters is important when interpreting variation in animal population size. The northwestern North American population of Golden Eagles Aquila chrysaetos canadensis initiates courtship and nesting in early spring when prey diversity is low and weather conditions are severe. Snowshoe Hare Lepus americanus and Willow Ptarmigan Lagopus lagopus, the primary prey of Golden Eagles early in their nesting season in interior Alaska, both exhibit cyclical fluctuations in abundance, providing the opportunity to investigate such relationships. We used Bayesian hierarchical models to explore variation in territory occupancy, nesting rates, nesting success and productivity of Golden Eagles from 1988 to 2010 in Denali National Park and Preserve, Alaska, in relation to annual and site‐specific parameters including prey abundance, weather conditions, elevation and human activity. We also investigated the long‐term fluctuations of breeding performance over the course of the study. The abundance of Hares influenced both the number of Eagles that laid eggs and the number of Eagles that produced fledglings. The conditions on the breeding ground did not explain observed declines in nesting rates and fledgling production, suggesting that other factors such as change in the age structure of the population, increased intraspecific competition or deterioration of migration and wintering habitat are driving the long‐term trends of these parameters.     

Status of Golden Eagle Aquila chrysaetos in Britain in 2015

Capsule: A complete survey of Golden Eagles Aquila chrysaetos in Britain in 2015 found that the population had increased by 15% since 2003 to 508 territorial pairs.

Aims: The survey aimed to investigate the population size, distribution and breeding success of Golden Eagles in Britain, and to compare results with similar surveys since the early 1980s.

Methods: Every home range was visited on a minimum of three occasions between January and August 2015. First, to look for eagles or signs of their presence (January–March), then to look for evidence of breeding or further checks for occupation (April–June) and finally to record productivity of nesting pairs (July–August).

Results: The figure of 508 territorial pairs represents a 15% increase in the population from 442 pairs in 2003. The proportion of home ranges occupied was 70%. The largest increases in the proportion of occupied home ranges were in south-central Highlands (71%), northern moors and flows (38%) and northwest Highlands (29%), with modest increases of up to 10% in the other regions. Productivity was lower in 2015 than in 2003, and there was significant variation in breeding success between regions.

Conclusion: The British Golden Eagle population has increased since 2003, although the species is absent from England and Wales. The population now meets the abundance target identified to define favourable conservation status in Scotland, and while home range occupancy has increased there is regional variation, with some regions falling below the target levels. A combination of increased annual monitoring and tagging of eagles, as well as the introduction of new legislation, may serve as effective deterrents against persecution of eagles thus facilitating this population increase. However, concerns remain over low levels of home range occupancy particularly in the east Highlands, but also the proportion of sub-adult pairs holding territory in that region and in the south-central Highlands. Persecution associated with grouse moor management has been highlighted as a particular population constraint in both of these areas.