White-tailed Eagle Population and Breed

White-tailed Eagle (Haliaeetus albicilla)

White-tailed Eagle (Haliaeetus albicilla)

The New Year sees adults already re-establishing territories and, in fine weather, indulging in courtship display and nest-building. The earliest recorded eggs in Scotland have appeared on 29 February, but most are laid in late March. Some are not produced until mid-April, by which time the first laid will be hatching. Normal clutch size is two, but three eggs have been recorded at least twice, supporting historic records of clutches of three eggs. Incubation is 38- 40 days. Broods may be one or two chicks, and there is one case of triplets which fledged in 1997, though one of them was found dead only a few months later. Unlike Golden Eagles, White-tailed Eagle siblings rarely exhibit any aggression in the nest. The fledgling period can vary from 10-14 weeks, occasionally longer, presumably depending upon food availability. The latest Scottish fledging date is 29 August 2000, coincidentally from the hundredth chick to be fledged following the reintroduction.

Young birds may remain dependent upon food supplied by adults for several more months, but observations from the reintroduction on Rum showed that some individuals, with access to food dumps, can develop on their own, without any parental example (Love 1983). Juveniles from Rum have ranged as far afield as Northern Ireland and Shetland, but most were reported within 100 km of the island. A similar pattern was shown by the birds later released at Letterewe.

During this second phase, about 300 sightings of young wing-tagged eagles were reported annually, totalling over 2,000 records between 1993 and 2002. Although most White-tailed Eagles are confined to the Inner and Outer Hebrides and the west coast mainland, young birds in their first two or three years may appear anywhere in Scotland. One was even seen over the centre of Glasgow (Clyde) in March 1996!

White-tailed Eagle

Tagged bird ‘Red 7’, set free at Letterewe in the summer of 1993, is a good example of ranging behaviour. It remained near the release site until November, was seen at Munlochy Bay (Ross & Cromarty) from 4-9 December, before heading north to be next reported in Orkney on 25-26 April 1994, at Sumburgh Head, Mainland (Shetland) the following day, and then back again on Orkney on 29 April.

This bird is unique amongst Scottish White-tailed Eagles, being sighted in Fraena, Norway in October 1994 where it began breeding in the summer of 2001. White-tailed Eagles normally breed for the first time at
about five years, but several Scottish birds have paired a year or two earlier in the absence of competition from existing pairs that would be normal in an established population. The youngest Scottish breeding pair was a three-year-old male with a four-year-old female in 2000, which even produced a chick. Sadly, the male was illegally poisoned in 2002, when one of his siblings was successfully fostered into another nest, and the female fell to an identical fate the following year.

Once mature, White-tailed Eagles tend to be faithful to the vicinity of their breeding territory where they may then live long and productive lives. ‘Blondie’ is a case in point: released in Rum in 1979, she moved south to Mull and established one of the first breeding pairs there; and she remained on the island until her death, 21 years later. Her mate disappeared two years later, aged 23 years. Such a lifespan is not unusual; before she finally disappeared in 1918, the last bird in Scotland had a distinctive white plumage and was known to the local Shetland Islanders for some 30 years (Love 1983, Birds of Shetland).

White-tailed Eagle

Population

Close monitoring of this small reintroduced population, a high proportion of which are individually marked, and of its nesting attempts, allows a better measure of its population size than for many other species in Scotland. In 2004, 32 territorial pairs were known with several others in the process of establishment. It is unlikely that more than one or two existing pairs have been overlooked. Since the minimum recruitment is known from imported birds and breeding success, the total population can be estimated at some 250-300 birds. This allows for a minimum 10% mortality over the years, although unreported deaths will augment this figure. Both current estimates of survival (75% for immatures; 97% for adults) and productivity (0.61
young fledged/territorial pair) in Scotland indicate the population to be as healthy as other populations in similar habitat elsewhere in Europe (Bainbridge et al. 2003).

Trends

From a zero population in 1975, the two-phased release of 140 fledglings, together with subsequent breeding success in the wild, has maintained a steady increase, first in the nonbreeding population, and then in the number of territorial pairs. Over this 30-year period the average annual increment in birds equates roughly to 10% of the population, and in breeding pairs to 1-2%. The injection of potential recruits has been erratic, and the establishment of new pairs was at first slow. The number of breeding territories trebled in the last decade, however, as more wildbred birds and extra second phase releases all came of age.

Multiple sightings of 5-8 birds, mainly immatures, are becoming regular in some areas, which is consistent with
the sociability of the species found in Norway and elsewhere. The population might now be expected to
increase exponentially as it becomes more self-sustaining; the rate of increase, of course, depends upon threats.

Threats
Eight unhatched eggs that have been analysed have shown significant traces of chemical pollutants, such as PCBs, DDT and mercury (Love 1988, RSPB/CEH data per R Broad). Slight eggshell thinning can be measured, but the pollutant levels do not seem sufficient to cause serious infertility (M Marquiss pers. comm.).
Continued nest security and surveillance is essential. Sympathetic local communities are increasingly involved in this, and two public viewing facilities are curre n t l y operating in Skye and Mull. Nonetheless, four clutches
have been taken by egg collectors in recent years; and amongst the 25 or so dead birds that were reported up to 2004, at least six were victims of persecution, notably illegal poisoning. It is particularly sad that two thirds of the poisoned birds were adults. To lose established breeders from this small pioneer population is especially damaging to its successful establishment.

Gaps in Current Knowledge
Given the remoteness of some breeding pairs, considerable effort is required to maintain a comprehensive level of census and monitoring, especially as more and more new pairs become established. The sensitivity of eagles to nest disturbance creates gaps in our knowledge about breeding failure, mortality and hence the population’s vulnerability.