A new study has shown that lamb is decreased as a food source of breeding White-tailed Eagles in Scotland, with seabirds and fish the main prey.
The research assessed the diet of eagles between 1998 and 2017, establishing that the occurrence of lamb in their diet has decreased as more pairs have become established.
The analysis of food remains showed an incredibly diverse diet during the breeding season. Some 11,375 different food items were recorded in 293 samples from nest sites in 92 White-tailed Eagle territories across Scotland. No fewer than 121 species were recorded, with 70 species of bird, 17 species of mammals and at least 30 species of fish.
Lamb has decreased as a food source for Scottish White-tailed Eagles, according to the study (Michael Taylor).
The study, published in the Scottish Ornithologists' Club journal Scottish Birds, began in 1998 when there were only 18 pairs of White-tailed Eagles in Scotland, mostly in the Inner Hebrides. By 2017 there were at least 122 pairs across the country, and 58 nests were sampled in that year.
The analysis showed that in that time the proportion of lamb remains in nests have decreased. At the start of the study period between 1998 and 2002, 15 nests were sampled and lambs accounted for more than 30% of items in five of those nests. However, as the White-tailed Eagle population expanded, pairs establishing in new territories tended to feed on fewer lambs, so by the end of the study in 2017 lambs accounted for 30% or more of items at five of 58 nests sampled.
The most common species found in nests is now Northern Fulmar – 30% of items per nest – with lamb now only accounting for 6% on average. As this study was based on prey remains, fish, young birds and smaller items are likely to be under-represented in it as these remains are more easily digested or decomposed. The result is that this study will have overestimated the importance of large mammals and birds in the diet.
The study also recorded differences in diet based on the geographical location of the nests, supporting previous studies that show White-tailed Eagles are generalist foragers and able to adapt their diet to what is readily available locally. At coastal territories seabirds and fish made up most of the food remains. At nests in territories further inland, foods such as wetland birds and rabbits dominated.
Robin Reid, one of the paper's authors, said: "This publication has been made possible through collaboration between many researchers and fieldworkers and with support from several organisations. As a result, the findings are based on large and robust dataset with prey remains collected from the majority of White-tailed Eagle territories occupied in Scotland during the study period.
"It has been fascinating to see how the diet varies between individual territories and across regions. The wide range of prey items recorded demonstrates that the White-tailed Eagle is an opportunistic and adaptable predator and scavenger, consistent with findings from studies elsewhere across the species range in Europe."
Reid, R, et al. 2023. The breeding season diet of White-tailed Eagles in Scotland. Scottish Birds, 43:4