Monitoring and conserving Scotland's birds of prey
Scotland is fortunate in having over 400 pairs of golden eagles and many experienced, skilled and enthusiastic fieldworkers dedicated to their study. There are marked differences in the abundance of territorial pairs across the Highlands and Islands of Scotland and in the species' conservation status across regions. In the west, densities can be relatively high and most potential territories are occupied, and breeding productivity can be good, although in parts there is probably a shortage of live prey and few young are produced, which may be due to a history of overly extractive land use. In the Western Isles numbers have been expanding for several years.
By contrast, across large swathes of the central and eastern Highlands many historical territories have remained unoccupied for many years. Those territories that are occupied, however, are some of the most productive in Scotland, and so for many years have routinely produced many young eagles. On this basis, there should be many potential recruits to fill the large number of unoccupied territories, and the eastern and central Highlands of Scotland should be full of territorial golden eagles; or, at least, the trend should be that an increasing number of vacant territories are being filled. Neither expectation has been apparent or realised according to national survey and SRSG observational efforts. The continued presence of numerous vacant territories cannot be explained by a shortage of fledged young eagles. The explanation must be because in these parts of the Highlands eagles are not surviving as long as elsewhere.
Detailed analyses have considered many possible reasons for this poor survival in the central and eastern Highlands and the overriding explanation, from many strands of evidence, is that it is due to illegal persecution of eagles; and this persecution is concentrated in areas managed for driven grouse shoots. In essence, eagle survival is poor and so occupied territories are thin on the ground because eagles are probably being killed by humans, and most of this persecution probably has basis in the management of grouse moor.
There are very few territories south of the Central Belt, and a relatively high number of records of eagles being killed will certainly not be helping an expansion into unoccupied habitat. The potential for recruitment from the Highlands may be low as the lowlands may act as a barrier to dispersal and, with the southern Highlands not being at carrying capacity, the pressure to overcome the lowland barrier may be low. Arran and Kintyre may be the best source of recruits away from the Southern Uplands. The Minch, an area of sea between the mainland and the Western Isles, probably also acts as a barrier to dispersal. Golden eagles in the Western Isles are slightly different genetically to those on the mainland, being less diverse. It seems likely that the Minch is an obstacle to mixing of populations, and the current Western Isles population probably largely originated from a small number of pairs that survived in deer forests of Lewis and Harris during the late 19th and early 20th centuries.
In the absence of any interference by humans, golden eagles are typically long-lived (20 years is a reasonable expectation for a mature adult) and have a relatively low breeding output, with a maximum of two fledglings produced per year. Breeding activity starts in late winter with attention on nest building. Eggs are laid in spring and after about six weeks of incubation, chicks can hatch. After about a further ten weeks the young may leave the nest and take their first fledgling flights.
Like other large raptors, golden eagles have a period of several years between fledging and settling to breed: probably around four to five years is typical, although in areas where birds are persecuted they can attempt to breed when still in subadult plumage because there is less competition from adults. During the period between fledging and settling to breed, young birds disperse away from their parents' territory and can explore widely. Recent satellite tagging studies of the dispersal of fledgling eagles in Scotland have shown that young birds can be very different in when they leave their parents' territory and go out 'on their own'. Some leave after a period of a few short weeks after fledging, while others can stay put for many months - up to the time when their parents are becoming active in their next breeding attempt. Some young birds disperse with no prior exploration of the world beyond their parents' territory - others make repeated excursions away from their parents' territory before they finally go it alone.
Once dispersed, some young eagles can roam over hundreds of kilometres, taking them well beyond their home ground, while others do not move far. Several periodically return to their parents' territory. Some young eagles, fledged in areas where they are probably relatively safe from human persecution, disperse into areas where the threat of persecution is more likely. Such areas are probably attractive to young dispersing eagles, because they have abundant food supplies and (apparently through persecution) a low density of territorial pairs. These attractive areas, but that carry an increased risk of being killed, can create a 'black hole' for young eagles originating in 'safe areas', spreading the population effects of persecution across a wider area than it occurs.
The golden eagle is a generalist predator, and part-time scavenger, gathering most of its food in open habitats where hunting birds can fly unimpeded. In many areas scavenging on carcasses of sheep and red deer probably sustains eagles through the winter. A variety of live food sources are exploited, from the size of a nestling meadow pipit up to a red deer calf, and from a seabird on the coast of the west, to a mountain hare in the mountains of the east.
Golden eagles can depredate smaller raptors, given the opportunity, but the relationship with white-tailed eagles is probably one of 'armed neutrality'. While there are a few examples of birds of each species killing birds of the other species, and of nest sites changing between species, there is no systematic evidence that the rapid expansion of white-tailed eagles is having an adverse effect on golden eagles. Golden eagles are more likely to nest on higher altitude cliffs and take 'terrestrial' food, and white-tailed eagles are more likely to nest at lower altitude (more often in trees) and take 'aquatic' food. While there is some overlap in the two species' 'niches' (several pairs of golden eagles nest on sea cliffs in the west and take seabirds, more typical of white-tailed eagles) but historical studies indicate that sea cliffs were used by golden eagles in the past, before white-tailed eagles were made extinct. The current relationship between the two species in Scotland appears similar to that in other countries, such as Norway, and in Scotland in the past.
Territorial golden eagles in Scotland are often not the traditionalist birds as portrayed in some populist media. While it seems that they are usually faithful to their mate over many years, DNA studies suggest that 'divorce' may occur, albeit rarely. Traditional nest sites can be used repeatedly over decades, but changing nest sites and creating new nest sites can be common. This has implications for surveyors looking to find active nest sites. Shifting and creating nest sites is to be expected given that new birds occupying a territory may have different preferences; and as land use and sources of good food supplies can change, then golden eagles should react in the placement of nest sites to best exploit such change.
Although golden eagles are often considered one of the better-known Scottish raptors, there is still much to be learnt, especially on their behaviour. We still await signs that the biggest threat facing Scottish golden eagles, persecution, is disappearing; those signs being their restoration to the large numbers of unoccupied territories in the eastern and central Highlands. The efforts of the SRSG will no doubt be critical in seeking such signs as they have been fundamental to almost every scientific publication on Scottish golden eagles in recent years.
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Hen Harrier• Goshawk• Sparrowhawk• Common Buzzard• Golden Eagle
Osprey• Kestrel• Merlin• Hobby• Peregrine Falcon• Barn Owl
Tawny Owl• Long-eared Owl• Short-eared Owl• Raven