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 2024 - J.R. Grant, 2024

https://nnr.nature.scot/doc/report-naturescot-white-tailed-eagle-work-respect-collection-and-analysis-prey-remains-territories-0

Introduction

Since white-tailed eagles started nesting successfully in Scotland in 1985 research workers have been studying their diets, primarily by examining prey remains in nests once the nestlings have fledged; the 2020 and 2023 reports outline the methodology and biases. More recently nest contents analyses for NatureScot have been primarily focussed on some of the territorial pairs that were on or near holdings participating in the Sea Eagle Management Scheme, or on or near some holdings that had raised concerns about predation of livestock.

As in previous years, due to sensitivities with respect to nest site locations and the legal protection accorded by Schedule 1, 1A and A1 of the Wildlife and Countryside Act 1981, no locations or map references are disclosed, with the individual home ranges identified by territory codes.

Methods

The methods were as outlined in detail in the 2020 and 2023 reports. Overall in 2024, 15 nests from five different geographical areas were targeted. At one nest in Wester Ross, landowner contact to request access permission was not possible and as a result this nest was not examined. The other 14 (see Table 1 for geographical details) were cleared, rebuilt (where necessary) and the contents analysed (see Table 2 for analysis results) during the autumn and early winter of 2024.

Table 1. Territory numbers for eagle nests cleaned and analysed in five geographical areas in 2024.
LocationLochaberRoss-shireSutherlandSkye (including Lochalsh) and RaasayArgyll
Territory No.25144, 2101068, 24, 43, 47, 60, 98, 99, 211174, 217
Totals12182

 

Table 2a. Numbers of occupation layers and young fledged in that period by territory number   
             Territory824254347609899106144174210211217
Occupation layers21?71?11½d11½ (single young died at about 6 weeks of age)11111?
Young fledged4 (One nestling taken for IoW project in 2023 and 20241?82 (One nestling taken for IoW project in 2024)111102 (One nestling taken for IoW project in 2024)1111½ (one young fell out and died after ringing)

 

Table 2b.  Bird prey species identified in eagle nest contents by territory in 2024.
             Territory824254347609899106144174210211217Totals
Fulmar94441913112--2-50
Gannet-------4------4
Manx Shearwater--3--13--1--1-9
Shag1133-11--1---112
Grey heron--2-1--1------4
‘Grey geese’--11103 11133-2-225
Canada goose----------144---14
Duck sp (probably Mallard)--1-1------4--6
Duck sp--11--------15--2
R-B Merganser--------1---1-2
Kestrel-----------1--1
Red grouse1-------11---15
Black grouse-----------1-12
Pheasant-----------1--1
Oystercatcher---------1----1
Common Gull (or Black-headed (or Kittiwake)-1--113-1-1---8
Kittiwake1-----------1-2
Great B-b Gull--32--3--10----18
Herring Gull23894133-8--22265
Puffin512--11-----1-11
Bl. Guillemot1-------------1
Guillemot1710615281411-7--142106
Razorbill1-3-1112--2---11
Woodpigeon1-----------1-1
Owl sp----------12---1
Raven----1-1--1--1-4
Hooded crow (or Carrion crow (or Rook))---1----------1
Bird pullus (un-ident.)---19-1-------11
Bird Totals392037363124452373518102429378
OVERALL TOTAL4924624655395232114246123340543

1not Mallard or Merganser?

2both Barn owl and Tawny owl known to be in the area

3two full grown and eight large goslings

4three fully grown and the rest large goslings

5unidentified but a dabbling duck (Anas sp) not Mallard or Teal – possibly Widgeon

 

Table 2c. – Mammal prey species in eagle nest contents by territory in 2024.
Mammal Prey Species824254347609899106144174210211217Total
Rabbit---117---1-231--43
Mink-------------11
Lamb2225751437-35-4986
Roe deer carrion---------1----1
Roe deer fawn------1121----5
Common Seal (pup)----1-1-------2
Mammal Totals2225823145835281410138

 

Table 2d. – Fish prey species in eagle nest contents by territory in 2024
Fish Prey Species824254347609899106144174210211217Total
Fish spp (Teleost)82-2112111-11-21
L-sp Catshark---------1--416
Fish Totals82-2112112-15127

 

Table 2e. – Minimum number of prey remains represented in eagle nest contents by territory in 2024
             Territory824254347609899106144174210211217TOTAL
OVERALL TOTAL4924624655395232114246123340543

Discussion

Regular WTE nest contents analysis contributes to increased knowledge with respect to this species’ ecology and possible effects on other prey species’ population sizes and distributions - including some domestic livestock. Resource availabilities limit the number of nests that can be examined in any one autumn, and the ‘candidates’ also change from year to year depending on breeding success and other factors. The ‘ideal’ research territory is one that breeds successfully every year, with a nest or nests that is/are reasonably easy to access, safe to work in and known to be only one ‘occupation layer’ (ie all remains are known to be from that breeding season alone). This is only the case when a nest is known to be new and used for the first time, or has been thoroughly cleaned out the last time it was successful. Territories 47, 98, 99, 144, 174, 210 and 211 come into this category; The new nest in Territory 106 is also only one occupation layer but as the single young died about half way through its development it has been logged as ‘half an occupation layer’. The numbers of prey items recovered from an assemblage can very a lot, as a function of factors such as geographical location modified by the availability of prey ‘hotspots’ like rabbit warrens, gull nesting colonies and human induced feeding opportunities such as fishing and tourist boats. It is therefore very risky to attempt to elucidate the number of occupation layers in an assemblage from the number of prey items. Nest contents from territories 24, 43 and 217 are thought probably each to represent one occupation layer but it is not certain. Out of the 14 examined nests, six of them (47, 98, 99, 106, 144 and 174) were in territories that had been cleaned and analysed in 2023 allowing some indication of dietary variation within individual territories between years.

Overall, remains representing a Minimum Number of Individuals (MNI) of 543 prey items from at least 34 different species of birds, mammals and fish were identified – the majority to species level. Combined percentages of the three main categories were as follows: 69.6% birds, 25.4% mammals and 4.9% fish, which is very similar to 2023 (70%, 26% and 4% respectively). With this study revolving around WTE pairs that are either already implicated in lamb losses or are being investigated routinely due to their proximity, for example, to lambing parks, it is not surprising that 62.3% of the mammalian component was lamb; only two nests had no lamb remains at all (106 and 210). With the MNI of lambs in any one nest ranging from zero to 25, totalling 86 and taking into account the (estimated) total number of occupation layers (21), the mean number of lambs per occupation layer is 4.1 This statistic is, however, of debatable value compared to those relating to individual pairs of eagles – and the concerns raised by their relevant farming neighbours. Taking territory 174 as an example, that pair’s provisioning of lamb (as shown by prey remains) appears to be fairly consistent between years 2022/3 (two occupation years - 8.5 per year) and 2024 (one occupation layer – 9 lambs). And of course it is always necessary to remember that a study of this sort retrieving bones from nests cannot shed light on a prey item’s status at the point of origin – ie was it live and healthy, moribund or already dead and therefore scavenged. Background observational fieldwork on WTEs throughout their distribution shows them to be competent hunters, even of relatively fast agile prey such as grouse, ducks, hares etc and it is reasonable to infer that a large proportion of prey items in this size category are live caught. Recently, of course outbreaks of HPAI will have increased the numbers of dead and moribund avian prey and changed the proportions of ‘healthy and killed’ versus ‘dead/dying and scavenged’. With larger prey such as deer calves, assumptions with respect to ‘healthy and killed’ or ‘dead and scavenged’ are riskier. However, when Red and Sika deer calves feature in significant numbers in an assemblage - as in some years in WTE Territories 53 and 155 (not a part of the NatureScot study) – it is felt unlikely that these were all scavenged. (WTEs have been observed by fieldworkers killing Sika calves.)

Thanks are due to the following for help with nest cleaning, transport, access permission and identification, Tom Amos, Troels Bavnhoj, Stephen Bentall, Bidwells estate management group, Emma Bryce, John Cameron, David Carss, Jamie Crowther, Phil Di-Duca, Alan Drever, Rhian Evans, Viv de Fresnes, Colin Leslie, Scott Mackenzie, Mick Marquiss, Charlie Miller, Mary Montgomery, Yvonne White, Callie MacLean, Andrew Aitchison.

 

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