Prey Remains Analysis Report J Grant 2021 - 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 2021

Introduction

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 nests in Scotland. This activity has been ongoing on a regular basis since White-tailed eagles started nesting successfully in 1985.  In 2021 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.

White-tailed eagles 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 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 2021 are identified by territory numbers that have been allocated since the species started regaining its foothold in Scotland in the early 1980s.

Methods

As outlined in more detail in the 2020 report, nests were cleared during the autumn, and rebuilt at the time, in addition to which all remains on the ground in the immediate vicinity were collected. In a very few cases, some items were identified and noted at ringing time and added into the results of the main end of season analysis. Done in the laboratory, the aim is to separate out identifiable bones and feathers, and then identify as many as possible to the lowest taxonomic level (preferably species); this is possible for many bones due to variations in their size and structure. However, some closely related species can be hard to separate out from skeletal material (eg Herring gull and Lesser Black-backed gull or Common gull, Black-headed gull and Kittiwake), and the definitive presence of a particular species may only be certain if feathers representing it are recovered. It is also well known by now that fish are particularly poorly represented by skeletal remains in nest contents, and therefore evaluating the contributions of fish species to the diet of White-tailed eagle broods is fraught with uncertainty. Elasmobranches (cartilaginous fish) such as skates, rays and dogfish are even less likely to be identified by ‘hard tissue’. Desiccated uneaten tails however are often found. {However. significant numbers of uneaten dogfish tails have occasionally been counted in nests at ringing time in June, none of which were present to be recorded at the end of the year ‘clean out’.} Standard archaeological practice and terminology is to quantify numbers in terms of the ‘minimum number of individuals’ (MNI) that a group of bones from a species represents.

It is to be expected that by and large, the amount of prey remains recovered from an assemblage (a nest clearance) will be significantly influenced by the number of times that nest has been used (one year of use is regarded as one ‘occupation layer’) and the total number of young fledging from it during the years the same nest has been used. Unless territories are extremely well monitored, sometimes there can be some uncertainty with respect to both these parameters. It is felt, however, that the information in this report relating to the number of occupation layers and number of young fledged is likely to be correct.

Results

Tables 1 and 2 below show that the bone and feather prey remains seen in, and recovered from, the 14 White-tailed eagle nests in Lochaber, Sutherland, Skye, Raasay and Argyll show that over 400 prey items representing at least 42 different species were hunted and/or scavenged by the provisioning eagles. All nest contents except two showed remains of birds, mammals and fish, with the exception of territories 173 and 174 in Argyll from which no fish remains were recovered. By and large, the proportions (by numbers) of birds were greatest and fish least. Some bird species were represented by just one individual from one nest assemblage, Manx Shearwater, Shelduck and Black Grouse for example, whereas other species are represented by many individuals from different assemblages (e.g. Fulmar, Greylag goose and Herring gull). Mammals were represented in all 14 assemblages, although in four of them (Skye and Raasay Territories 60, 97, 98, and 99) lamb was the only mammal species found. In Territories 146 and 174 (both in Argyll) mammal numbers were greater than bird numbers; in both of these cases, lamb numbers were relatively high – particularly in the case of Territory 174. Mammal remains also include items that due to their size are assumed to be scavenged, such as red deer and adult sheep, with a few of these items showing features concomitant with butchery marks.

Table 1. Territory numbers for White-tailed eagle nests cleaned and analysed in four geographical areas in 2021

Year/Location	Lochaber	Sutherland	Skye1 and Raasay	Argyll
2021	40	106	57, 60, 97, 98, 99	119, 145, 146, 147,  173, 174, 175
Totals	1	1	5	7

 

Table 2a: Numbers of occupation layers and young fledged identified in White-tailed eagle nest contents by territory in 2021

Territory	40	57	60	97	98	99	106	119	145	146	147	173	174	175
Occupation layers	3	1	1+1	1+2	1	1	4	1	2	1	1	13	1	1
Young fledged	4	1	1	1	1	1	4	1	2	1	1	0	1	1

¹ failed on ~ 4 wk old young in 2019
² failed on small young of unknown age in 2020
³failed on ~ 4 wk old young in 2021

Table 2b: Numbers of bird prey species identified in White-tailed eagle nest contents by territory in 2021

Territory	40	57	60	97	98	99	106	119	145	146	147	173	174	175	Total
Black throated Diver	-	-	-	-	-	-	-	-	-	-	1	-	-	-	1
Fulmar	11	1	9	2	13	1	3	-	2	1	1	-	1	-	45
Manx Shearwater	-	-	-	-	1	-	-	-	-	-	-	-	-	-	1
Shag	-	1	-	-	1	1	1	-	-	-	1	-	-	-	5
Shag pullus	4	-	1	-	-	-	-	-	-	-	-	-	-	-	5
Grey heron	1	-	-	-	-	-	1	-	-	-	-	-	-	-	2
Greylag goose	6	-	-	-	-	1	4	3	-	1	3	-	1	1	20
Canada goose	1	-	-	-	-	-	-	1	-	-	-	-	8	-	10
Gosling	1	-	-	-	-	-	-	-	-	-	-	-	-	-	1
Shelduck	-	-	-	-	-	-	-	-	-	-	-	-	-	1	1
Mallard	2	-	-	-	-	1	-	3	-	2	1	-	1	1	11
Eider	3	-	-	-	-	1	-	-	2	-	1	-	-	-	7
Red br Merganser	-	-	-	-	-	-	-	1	-	1	1	-	-	-	3
Buzzard (pullus)	-	-	-	-	-	-	-	1	-	-	-	-	-	-	1
Red grouse	-	-	-	-	1	-	1	1	-	2	1	1	-	-	7
Black Grouse	-	-	-	-	-	-	-	-	-	-	1	-	-	-	1
Pheasant	-	-	-	-	-	-	-	-	-	4	-	1	-	5	10
Oystercatcher	1	-	-	-	1	-	-	-	-	-	-	-	1	-	3
Great Skua	-	-	-	1	-	-	-	-	-	-	-	-	-	-	1
Common Gull	1	-	-	-	-	-	-	-	-	-	1	-	-	1	3
Herring Gull	15	18	-	1	-	1	1	-	1	-	4	-	2	7	50
Herring gull pullus	-	-	-	3	-	-	-	-	-	-	-	-	-	-	3
Kittiwake	-	-	-	-	-	-	-	-	-	-	-	1	-	-	1
Puffin	-	-	-	-	1	-	-	-	-	-	-	-	-	-	1
Black Guillemot	2	-	-	-	-	-	-	-	-	-	-	-	-	-	2
Guillemot	2	3	-	6	3	1	-	-	-	1	-	-	1	1	18
Razorbill	-	-	1	-	-	-	-	-	-	-	-	-	-	-	1
Rock/Stock Dove	1	-	-	-	-	-	-	-	-	-	-	-	-	-	1
Barn owl	-	-	-	-	-	-	1	-	-	-	-	-	-	-	1
Jay	-	-	-	-	-	-	-	-	-	-	-	-	-	1	1
Carrion/Hooded crow	-	-	-	-	-	-	-	1	-	1	-	-	-	-	2
Bird spp (un-i.d.)	-	-	-	-	-	-	-	-	-	2	-	3	-	-	5
Bird pullus (un-i.d.)	1	-	-	1	-	-	-	1	-	2	6	-	1	-	12
Totals	52	23	11	14	21	7	12	12	5	17	22	6	16	18	236

 

Table 2c: Numbers of mammal prey species identified in White-tailed eagle nest contents by territory in 2021

Territory	40	57	60	97	98	99	106	119	145	146	147	173	174	175	Total
Rabbit	-	-	-	-	-	-	1	-	-	3	11	-	7	-	22
Mountain hare	-	-	-	-	-	-	1	-	-	2	-	-	-	-	3
Fox (cub)	-	-	-	-	-	-	-	-	1	-	-	-	1	-	2
Lamb	12	2	6	2	2	2	6	7	3	11	4	3	24	5	86
Sheep carrion	-	-	-	-	-	-	-	1	-	-	-	-	-	-	1
Red deer carrion	-	-	-	-	-	-	-	-	-	2	-	-	1	1	4
Sika carrion	-	-	-	-	-	-	-	-	-	1	-	-	-	-	1
Roe deer carrion	-	-	-	-	-	-	-	-	-	-	-	-	1	-	1
Roe deer fawn	2	1	-	-	-	-	1	1	-	-	-	-	-	-	5
Common Seal (pup)	1	-	-	-	-	-	-	-	-	-	-	-	-	-	1
Totals	15	3	6	2	2	2	9	9	4	19	15	3	34	6	129

 

Table 2d: Numbers of fish prey species identified in White-tailed eagle nest contents by territory in 2021

Territory	40	57	60	97	98	99	106	119	145	146	147	173	174	175	Total
Fish (Teleost sp)	1	1	3	5	-	1	1	-	1	1	1	-	-	3	18
Ling	-	-	-	-	-	-	-	-	-	1	-	-	-	-	1
Wrasse spp	-	-	-	-	-	-	-	-	-	2	-	-	-	-	2
Pike	-	-	-	-	-	-	-	8	-	-	-	-	-	-	8
Spotted dogfish	3	-	-	-	2	-	-	-	1	1	-	-	-	-	7
Thornback Ray	1	-	-	-	-	-	-	-	-	-	-	-	-	1	2
Spurdog	-	-	-	-	-	-	-	-	-	-	-	-	-	1	1
Totals	5	1	3	5	2	1	1	8	2	5	1	0	0	5	39
TOTALS	72	27	20	21	25	10	22	29	11	41	38	9	50	29	404

 

Table 2e: Numbers of total prey species identified in White-tailed eagle nest contents by territory in 2021

Territory	40	57	60	97	98	99	106	119	145	146	147	173	174	175	Total
TOTALS	72	27	20	21	25	10	22	29	11	41	38	9	50	29	404

Discussion

Despite the uncertainty pertaining to some aspects of investigating prey selection and provisioning of nestlings by White-tailed eagle adults, nest clearances still give a useful – although partial – insight into their ecology. The emphasis in this study is on lamb remains, their numbers and proportions. As mentioned in Methods, trying to elucidate the proportions of fish in the diet of White-tailed eagles is very difficult and will almost certainly be inaccurate. This also goes for species such as octopus – often recorded during ringing visits, but rarely represented in end of season assemblages. As well as being competent hunters of birds, mammals and fish, White-tailed eagles scavenge readily, practice kleptoparasitism - ‘mugging’ gulls, otters and occasionally, it has been suggested, golden eagles, and acclimatise readily to accepting handouts of fish from trawler crews and tourist boat operators. These different ‘income opportunities’ exploited by these eagles, which are clearly available to greater or lesser extents in different home ranges, can make the picture more complex when trying to work out how White-tailed eagles are fitting in to modern ecological systems – both marine and terrestrial - and all of which now include human activities. The diversity of bird prey seen in this study emphasises how versatile White-tailed eagles are, and gives some insight into the avian prey base within their home ranges. Numbers of different species will be a function of their population sizes, and the difficulty and risk to the eagles when hunting and tackling them. So for example, Black-throated divers, Great skuas, Rock doves and Jays are rarely encountered as prey, while Fulmars, Greylag geese and Herring gulls are frequently seen in significant numbers. Clearly, of value would be background survey data on the various species and numbers available to territorial eagles. The same, in some cases somewhat easier to obtain, would go for wild and domestic mammals such as lagomorphs (rabbits and hares), foxes, deer and sheep/lamb flocks.  These results show interesting differences in the overall number of prey items. Territory 106 (North Sutherland with a nest site approximately 20 kilometres from the nearest coast) had remains representing a minimum of 22 prey items including seven bird species, four mammal species and one teleost fish species. This nest was known to have been used over four years (four occupation layers) fledging four young. Likewise the Argyll Territory 145, with its coastal nest assemblage representing two occupation layers and fledging a total of two young, yielded remains representing a minimum of only eleven items from seven species. The probability that these relatively small numbers of prey items may be a function of low density prey bases could to some extent be supported by background ecological and agricultural knowledge. But the possibility that these low numbers of recovered items may be a function of higher proportions of poorly represented prey species such as small avian pulli, soft bodied prey such as octopus and fish (both teleosts and elasmobranches) is clearly more difficult to evaluate. In addition, the incidences of adults removing prey items from the nest after having fed their young to eat elsewhere, is challenging to assess.

Lamb remains were found in all assemblages not surprisingly, given that the sampling method was intentionally biased towards those territories where lamb predation is reported as an ongoing issue. Again, numbers of lambs represented varied considerably. Skye and Raasay Territories 97, 98 and 99 had lamb bones representing a minimum number of two each. Assemblages from Territories 98 and 99 were known to represent only one occupation layer each, while that from Territory 97 is less certain. It failed on young in 2020 and fledged one in 2021. Enquiries as to when it failed in 2020 suggest that the brood was small and therefore the assemblage from T97 is more representative of one occupation layer than two. At the other end of the scale, lamb bones representing larger numbers of animals were recovered from Argyll Territories 40, 146 and 174 (MNIs of 12, 11 and 24 animals respectively). Information from fieldworkers confirms that the remains from T40 represent three occupation layers fledging four young, so a mean lamb MNI per year would be four animals. The assemblages from Territories 146 and 174 are both considered to represent one occupation layer, so MNIs of 11 and 24 respectively are clearly more significant. With bones representing at least 24 lambs, numerically Territory 174 has clearly brought in more mammals than birds in 2021, although eight of the avian prey were Canada geese – substantial prey items.

The relationship between White-tailed eagles and lambs is an ongoing and sensitive topic of debate, with some very polarised views on both sides as to whether lambs that are included in eagles’ diets are predominantly healthy and killed, or already dead and scavenged. With respect to the study of lamb remains in nest contents analysis at the end of the breeding season, the ‘killed or scavenged’ question is impossible to answer, as the skeletal remains, occasionally including skin and other soft tissues, cannot really shed light on this. It is however very useful to know the extent to which lamb is present in the diet of particular territorial eagles.

A considerable amount of communication and effort has gone into the work detailed here. This includes everything from landowner/agent access permissions to tree climbing, safety support, nest clearance and prey remains analysis, plus other background information contributing to knowledge of the relevant eagles’ whereabouts and activities.    

Thanks, therefore, are due to the fieldworkers, farm owners, tenant farmers and managers, foresters and forest managers, ecological surveyors, gamekeepers, estate managers and members of private land management groups whose assistance made this work possible.

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