Report to NatureScot on White-tailed Eagle work with respect to collection and analysis of prey remains from territories associated with, or possibly implicated in, lamb predation in 2023.

The prey remains analysis report 2020 outlined the reasons for, and methodology behind, cleaning and analysing the contents of a sample of White-tailed eagle (WTE) nests in Scotland. This activity has been ongoing on a regular basis since White-tailed eagles started nesting successfully in 1985. The particular nests targeted for examination were some of those where lamb predation is reported as an ongoing issue and the nearby crofters and farmers are participating in the Sea Eagle Management Scheme. In 2022, nest examinations were not carried out due to the Highly Pathogenic Avian Influenza (HPAI) outbreak, which appeared to have a significant effect on WTE breeding success in at least some parts of Scotland. However, in 2023 nest examinations were resumed, following risk assessments by NatureScot staff. This included the use of PPE, including face fitted masks, while climbing and collecting prey remains in order to avoid exposure to any HPAI virus in nest contents. Once analysed, prey remains were treated as hazardous waste and disposed, in accordance with guidelines. Although the title specifically identifies WTEs, in 2023 one Golden eagle (GE) nest was targeted as it was very close to one participating crofter and it was thought pertinent (and easy) to examine.

Both the White-tailed eagle and the Golden eagle are protected under Schedule 1, 1A and A1 of the Wildlife and Countryside Act 1981 (as amended in Scotland). With this level of protection, there are sensitivities with respect to knowledge of nest site locations. Therefore, as before, no nest site locations are named and no grid references are given in this report. Home ranges which have had nests cleared and analysed in 2023 are, as usual, identified by territory codes.

Methods

The methods were as outlined in detail in the 2020 and 2021 reports, with nests being cleared and rebuilt during the autumn and early winter 2023. During the last two years more reference skeletons were also obtained in order to aid the resolution of remains identification. One minor descriptive change in the Results tables is that grey geese such as Greylag and Pink-footed are combined under the heading ‘Grey Geese’. This is due to the uncertainty of discrimination between many of the species solely on skeletal remains, given the ranges of sizes both within and between

Discussion

Looking at the percentages of the three different prey categories (numerically), 70% were birds – including at least 30 different species, 26% were mammals – seven different species and 4% were fish. Fish are almost certainly significantly under-represented and the mammal component of the diet is clearly very biased towards lamb (71% of mammalian items), with remains being found in every nest. This is not surprising, given that the nests selected for examination are associated with areas where crofters and farmers have expressed concerns with respect to predation of lambs by eagles. Taking the golden eagle nest contents as representing two occupation layers (it is uncertain – but no more than two), the nest contents as a whole represent remains from probably 20 occupation layers from 14 nests (ie effectively 20 pairs provisioning young). Lamb remains represented a total of at least 97 individuals. With minimum numbers of lambs in these assemblages ranging from two (in Territories 144 and 159) to 26 (Territory 50) the mean minimum number per occupation layer is 4.8. Clearly, however the figures show that two pairs (Territories 50 and 174) have significantly more than the average, even allowing for the fact that those two nests contents both represented two occupation layers. It is of course important to remember that lamb remains collected in this way, from empty nests after fledging, cannot reflect the methods the eagles used to obtain them (ie predated healthy lambs, weak moribund lambs and/or scavenged fresh or old carcases.

The 2022 WTE breeding season was clearly influenced by the outbreak of Highly Pathogenic Avian Influenza (HPAI) with significant numbers of young eagles found dead in the nest at ringing time. Some were tested for HPAI and found to be positive. It is assumed that dead and moribund infected prey species such as some of the wildfowl would be easy pickings for eagles and a very likely infection route. Interestingly although in 2023 HPAI was still affecting some numbers of prey species, and it is known that some WTE nestlings were infected, the number of dead young was nothing like the year before.

Thanks are due to the following for help with nest cleaning, transport, access permission and identification. Shona Amos, Stephen Bentall, Bidwells estate management staff, John Cameron, David Carss, Phil Di-Duca, Rhian Evans, Mick Marquiss and Callum Strong.

Fieldwork, analysis and relevant office work was funded by NatureScot.