A comparison of the diet of white-tailed eagles (Haliaeetus albicilla) and golden eagles (Aquila chrysaetos) breeding in adjacent ranges in western Scotland.
Phil Whitfield (Natural Research), Robin Reid (RSPB), Mike Madders, (Natural Research), Mick Marquiss, (CEH), Ruth Tingay, (Natural Research), Paul Haworth (Haworth Conservation)
Prior to their extirpation during the early part of the 20th Century, white-tailed eagles co-existed with golden eagles across much of Scotland (Love, 1983). It is likely that competition for food resources between the two species was minimised by niche differentiation. Thus, white-tailed eagles nested predominantly in coastal areas where they ate mostly seabirds, waterfowl and fish (Love, 1983), whereas golden eagles mostly nested inland where they ate medium-sized terrestrial birds and mammals, and carrion (Watson, 1977). During the period when white-tailed eagles were in decline, lack of competition for prey may have allowed golden eagles to expand into coastal habitats. Following the re-establishment of breeding white-tailed eagles during the 1980s and gradual expansion of the population during the 1990s, there has been speculation that white-tailed eagles will replace golden eagles in coastal habitats, leading to range contraction and a decline in the golden eagle population (Madders & Marquiss, 2003).
The diets of white-tailed and golden eagles in Scotland were compared during the 1980s by Watson et al. (1992). This important study analysed oral pellets and food remains collected at roosts and nest sites on the island of Mull, western Scotland. The results showed that the diet of both species varied with season, and that white-tailed eagles had a broader diet than golden eagles. White-tailed eagles ate more seabirds, waterfowl and fish, whereas golden eagles ate more lagomorphs (rabbits and hares). However, the limited number of territorial white-tailed eagles present on Mull at that time (only two pairs) placed restrictions on the way in which data were analysed. In particular, the study did not discriminate between material collected from breeding and non-breeding birds. This is important because virtually all material collected during the breeding period from golden eagles was from productive nests, whereas much of the material from white-tailed eagles came from roosts occupied by birds that did not rear young. Non-breeding eagles do not need to transport food to nests and are therefore less constrained by food item size and weight. As a result, the study may have been biased because of differences in food selection related to breeding status rather than eagle species. Moreover, the golden eagles studied occupied a much broader geographic range (most of Mull) than the white-tailed eagles. As a result, golden eagles may have been able to more easily access food types whose distribution was spatially restricted (Madders & Marquiss, 2003).
Since Watson’s study, the number of territorial white-tailed eagles on Mull has trebled and the species’ range has expanded to cover most of the area occupied by golden eagles. This has made it possible to investigate dietary differences more rigorously and a preliminary analysis of the diets of breeding eagles from 1998-2000 was recently conducted (Madders & Marquiss, 2003).
In the present study, we are analysing pellets and prey remains collected in western Scotland 1998 – present, and making comparisons between the diets of golden and white-tailed eagles. Prey remains have been identified to species. Pellet contents have been identified to the lowest practical taxonomic level. Where necessary, feather fragments were classified after examination of the downy barbules at 200x magnification, using a reference collection and the keys devised by Day (1966) and Brom (1986). Mammalian hair was classified after examination of the medullary pattern, using the key in Teerink (1991) and Cowell & Thomas (1999). We expect to publish our results in 2007.
Acknowledgements
This study is part of the much larger effort to conserve white-tailed eagles led by the Sea Eagle Project Team.
References
Brom, T.G. (1986). Microscopic identification of feathers and feather fragments of Palaearctic birds. Bijdragen tot de Dierkunde 56: 181-204.
Cowell, D. and Thomas, G. (1999). A key to the guard hairs of British canids and mustelids. British Wildlife 11(2): 118-121.
Day, M.G. (1966). Identification of hair and feather remains in the gut and faeces of stoats and weasels. Journal of Zoology 148: 201-217.
Love, J.A. (1983). The Return of the Sea Eagle. Cambridge University Press, Cambridge.
Madders, M. and Marquiss, M. (2003). A comparison of the diet of white-tailed eagles and golden eagles breeding in adjacent ranges in west Scotland. In Helander, B., Marquiss, M. and Bowerman, W. (Eds). Proceedings from the International Sea Eagle Conference, Björkö, Sweden, 13-17 September 2000. Swedish Society for Nature Conservation.
Teerink, B.J. (1991). Hair of West European Mammals. Cambridge University Press, Cambridge.
Watson, J. (1997). The Golden Eagle. T. & A.D. Poyser, London.
Watson, J., Leitch, A.F. and Broad, R.A. (1992). The diet of the sea eagle Haliaeetus albicilla and golden eagle Aquila chrysaetos in western Scotland. Ibis 134: 27-31.
For further information, please contact phil.whitfield@natural-research.org
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