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The numbers, breeding success and diet of golden eagles in southern Scotland in relation to changes in land use
Biological Conservation 34 ( 1985) 121 -140
The Numbers, Breeding Success and Diet of Golden Eagles in Southern Scotland in Relation to Changes in Land Use M. Marquiss Institute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian EH260QB, Great Britain
D. A. Ratcliffe Nature Conservancy Council, Northminster House, Peterborough PE1 1UA, Great Britain
& R. Roxburgh "Charlescot', 20 Townhead, Catrine, Ayrshire KA5 6SQ, Great Britain
ABSTRACT Golden eagles Aquila chrysaetos recolonised southwest Scotland in the early 1940s. By 1966jbur pairs were established and breeding success was good. In the early 1970s, coincident with large-scale afforestation, two pairs ceased to breed and a third pair rarely produced young. Good breeding perJormance was associated with spring diet; the most productive pair consuming more large birds and less carrion than the other pairs. Animals living in conifer plantations did not .[orm a substantial part oJ the diet at any time of year. It seemed that the afforestation of open country had removed much of the productive Joraging habitat for three o[ the./but" pairs of eagles, as the remaining open country was at higher altitude where live prey were scarce.
INTRODUCTION The golden eagle Aquila chrysaetos was only a sporadic breeder in southern Scotland from about 1870 until the early 1940s when a pair 121
Biol. Conserv. 0006-3207/85/$0330 c') Elsevier Applied Science Publishers Ltd, England, 1985, Printed in Great Britain
122
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
became established, first nesting in 1945. The 19th century disappearance of the species from the region has been attributed to persecution, and its return coincided with the decline of grouse preservation here, beginning in World War II. By 1965 four pairs were breeding but in the early 1970s two of these pairs stopped producing eggs. This paper summarises the history and breeding success of golden eagles in southern Scotland and assesses diet in recent years (1974-80). In an attempt to explain the recent decline in breeding we examine its coincidence with the afforestation of uplands in the vicinity. The predominant land use in the uplands of southern Scotland has changed dramatically in the last 35 years from sheep farming to forestry. In 1945 the southern uplands were grass or heather dominated, grazed by sheep, feral goats and a few red deer Cervus elaphus. Grouse moors were maintained in a few areas but even these carried some sheep as well. After 1945 there was an enormous expansion in the afforestation of unenclosed hill land, and the region was progressively planted with exotic conifers (mainly Norway spruce Picea abies and sitka spruce P. sitchensis) so that by 1980 little of the hill ground occupied by eagles held sheep (Fig. 1).
SOUTHWEST S(
Fig. 1.
The study area.
Golden eagles and land use in Scotland
123
Afforestation entails the removal of sheep stocks and the fencing of the forest margin from ground still carrying sheep. The ground is ploughed and the tree seedlings are planted in the upturned sod. In the absence of sheep, the original short-cropped vegetation soon becomes luxuriant. After about ten years the tree canopy closes and the ground vegetation decreases; herbaceous vegetation remains chiefly on roadsides and rides and on areas too wet, too sterile or too high for trees. Deer and goats survive in these areas, particularly in the 'islands' of open ground at high altitude, but their populations are ~culled' to prevent tree damage by browsing. During the period just after the removal of sheep, when herbaceous vegetation is luxuriant, there is sometimes an abundance of smaller grazing animals, short-tailed field voles Microtus agrestis, and blackgame Lyrurus tetrix and their predators, weasels Mustela nivalis, foxes Vulpes vulpes, short-eared owls Asio flammeus and kestrels Falco tinnunculus, but this abundance only lasts a few years before the tree canopy closes suppressing other vegetation. METHODS D. A. R. and R. R. collected most data on breeding success, their own information being supplemented by that from local ornithologists. Although the eagles favoured particular sites for both nesting and roosting, they also used adjacent crags or small blocks of old woodland as alternative roosts. These were well defined by accumulations of droppings, pellets and moulted feathers and were thus located easily. From spring 1974 to winter 1979/80 pellets were collected from the roosts of the four eagle pairs and their contents identified (where necessary hair and feather remains were examined microscopically, Day 1965). The altitude, and amounts of various habitats within the vicinity of each eagle territory were measured using 1 : 63 360 Ordnance Survey maps, updated to show all the afforested land and the dates that trees were planted. RESULTS
History of breeding in southern Scotland In Galloway and Carrick, at least five different 19th century breeding haunts are mentioned, but these gradually became deserted after 1850,
124
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
and the last occupied eyrie seems to have been in the Glen Trool area in 1876 (Harper, 1896; Service, 1903; Dick, 1916). The history of the golden eagle in the Moffat-Tweedsmuir Hills is even vaguer, though there would be sufficient space and nesting habitat for 3-4 pairs. Breeding here had evidently ceased by 1850 (Gladstone, 1910). Few ornithologists were active in southern Scotland even by 1900, and breeding attempts after supposed extinction could have gone unnoticed. The next recorded nesting was in 1905, when a pair laid 3 eggs in one of the old Galloway haunts, but this attempt failed (Maxwell, 1927). J. F. Peters (pers. comm.) saw six nests with eggs at the same locality in Carrick during 1910-1920, but only one of these certainly produced young. Paton & Pike (1929) recorded that a pair continued to attempt breeding in this territory until 1923, but without rearing young. Subsequently, there were scattered records of birds seen in various places, but, apart from an unsubstantiated report of a nest in the Nithsdale area of Dumfriesshire around 1938-39, no further instances of nesting were known until 1945. In that year, a pair reared two young in the Galloway Hills (this name is used henceforth to include the Carrick section which is topographically continuous with it). Thereafter, they nested annually, using four alternative cliffs (Area A). A second pair appeared in another area (Area B) in 1948, rearing two young, and also nested annually, resorting usually to the same cliffs, but occasionally moving to another. A third pair established a nesting territory (Area C) which overlapped with that of Area A in 1952. The contents of the nest were never determined that year, but successful breeding took place in 1953. This pair continued to breed regularly on the same cliff until 1975 but have since used two nesting places. An evidently unfinished nest with only a single bird in attendance was found in a fourth Galloway locality (Area D) in 1962, but it was not until 1965 that eggs were laid and a young bird reared. This pair moved to an adjoining nesting area in 1969 and have remained there ever since, using two nesting locations. Two of the four pairs re-occupied the old nesting haunts of 1905 and 1910-23. In another part of the Galloway Hills there was an unconfirmed nesting in the late 1950s. The cliff was then deserted, but a pair of golden eagles with an eyrie were seen 1966: it was never established whether eggs were laid, and this pair disappeared after frequenting another cliff in the vicinity during the following year. Golden eagles were seen only occasionally in the Moffat-Tweedsmuir Hills during the 1950s and 1960s, but a pair returned to nest in this district in 1972. Eggs laid that year and
Golden eagles and land use in Scotland
125
in 1973 at an adjoining cliff failed to hatch, but in autumn 1974 a nest which had obviously contained young was found at a third site in the area. A carcase found late in 1974, 6 km away was probably one of this pair. Single immature eagles were seen hereabouts in 1976 and 1980 but there has been no further breeding. Single immature golden eagles have also been recorded in summer in two additional Southern Upland localities, from 1975 onwards.
Breeding success in the Galloway Hills 1945-83 For all four areas A to D there has probably been continuous occupation since the initial establishment of pairs, with records of nests refurbished and eggs laid in each during almost every year until the mid-1970s (Table 1). In most breeding attempts two eggs were laid, and at least in the earlier years when only one egg was recorded, it was possible that part of the clutch has been robbed. Birds nesting at B laid three eggs in at least seven years during 1950-60. Four other three-egg clutches were recorded, at C in 1959, 1960, 1977 and 1983. At first, fledged broods of two young were not u n c o m m o n with six in the 13 recorded broods prior to 1960. Since then, all of the 25 recorded broods have been of single young. Overall only about 40 "//o of nesting attempts produced flying young. Forty-five nests were known to have had eggs but failed to produce young. The main recorded circumstance of failure (23 instances) was the desertion of clutches most of which had been incubated for the normal period but had failed to hatch. In recent years many of these eggs have been examined and have had little or no embryonic development. At least seven clutches were known to have been taken by egg collectors between 1953 and 1961, and one in 1971. In 1968 both eggs of the pair at A were found broken in the nest and in 1970 one of the two eggs at C was broken. From 1965 there have been five instances where young were known to hatch but they died whilst still small. In recent years breeding performance deteriorated with the cessation of annual breeding in two of the four areas. At A a pair last reared young in 1973; they deserted a clutch in 1974, did not lay in 1975 and only a single bird was seen thereafter until 1979 when a pair refurbished an eyrie but did not lay. In 1980 a single egg was laid but soon deserted. In March 1981, a male eagle was killed by poisoned bait put down for other predators on a neighbouring estate. Since then there has been no record of a pair or a nesting attempt. At B eggs were laid annually until 1972 when
of of of of of of
years years years years years years
examined occupied by a single bird occupied by a pair nest built/refurbished eggs laid young reared
3 0"75
0"29
0 3 0
2.7 (7) 1"5 (6)
12 0 12 12 12 6
6
0 3 0
1.9 (11) 1.7 (3)
17 0 17 17 14+ 3
B
TABLE 1
0.50
0
0 1 0
2.1 (7) 1.3 (4)
10 0 10 10 10 4
C
0.30
3
2 0 1
2.0 (9) 1-0 (3)
l0 0 10 10 10 3
A
0.10
2
0 1 0
2.0 (2) 1-0 (1)
8 2 6 7 6 1
B
1964-73
0.57
2
0 0 1
1.8 (5) 1-0 (4)
7 0 7 7 7 4
C
0.50
0
0 0 0
2.0 (3) 1-0 (5)
10 2 8 9 6+ 5
D
E a g l e s in S o u t h e r n
Period and territory
Territories of Golden
0.00
2
0 0 0
1-5 (2) --
10 3 5 3 2 0
A
Scotland
0.10
0
0 0 0
2"0 (1) 1"0 (1)
10 4 6 3 1 1
B
1974 83
0.30
4
3 0 0
2.2 (10) 1-0 (3)
10 0 10 10 10 3
C
0.80
0 0 1 (nest collapsed) 1
2.0 (10) 1.0 (8)
10 0 10 10 10 8
D
° Failures are minimum values because in some years the outcome, or the type of failure was anknown. Nesting failures from clutch desertion and addling of eggs are combined, because the two are often connected, but to an unknown degree. In one of the eyries with broken eggs, only a single egg was broken, the remaining egg was addled.
No. of years clutch deserted ~r No. of years eggs addled ) No. young produced/occupied territory
Nesting failures~ No. of years all young died No. of years clutch robbed No. of years eggs broken
Mean clutch size (n) Mean brood size (n)
No. No. No. No. No. No.
A
194543
T h e B r e e d i n g S u c c e s s in F o u r
Golden eagles and land use in Scotland
127
there was a new but empty nest. Eggs were never recorded in subsequent years though the site was still occupied by an adult pair in 1976. From 1977 there was only one adult and although single immatures were seen in the vicinity almost annually in winter and spring, none of these apparently stayed until 1982. In 1983 eggs were laid for the first time in over a decade and an eaglet fledged for the first time since 1968. In contrast, the pair at C has continued to produce eggs every year, though it has reared young only three times in the last decade. Site D has had consistent success producing flying young in every year but two since 1970. In 1978 the nest collapsed early in incubation; the pair produced another egg in an alternative eyrie, but failed to hatch it. In 1983 two eggs were laid but they failed to hatch. It was difficult to assess the causes of failure in the earlier years. Failures not attributed to robbing could have been due to high pesticide levels, human disturbance, food shortage, or a combination of these factors.
Organochlorine pesticide residues Lockie et al., 1969, showed that eagles suffered poor breeding success in west Scotland in the years 1963 65 and attributed this to high concentrations of pesticides (particularly dieldrin) in their eggs. Following the withdrawal of dieldrin sheep dips in 1966, eagles subsequently (1966-68) bred more successfully. Comparing the breeding success of our eagles in the five years prior to 1966 with the five years afterwards, there was no difference in the frequency of young reared (P = 0"7; Fisher Exact Probability Test) or in clutches deserted, broken or addled (P = 0.8). The mean shell thickness (expressed as mean shell index Ratcliffe, 1967) of eggs in five clutches from A between 1951 and 1964 was 2'87, about 9 % thinner than the average for uncontaminated Scottish eagles (a mean shell index of 3" 15 for 84 clutches taken before 1947; the first year that the use of organochlorines became widespread). This level of shell-thinning suggests these eggs contained less than 1/~g g- 1 fresh weight D D E (Newton, 1979). Two deserted clutches of eggs taken from southern Scotland in 1964 A and 1965 B contained relatively low levels of organochlorine pesticide residues (0.6-1.0 #g g i fresh weight) compared with the considerably greater levels in most western pairs (1 pg g- 1 was considered to be the lower limit at which residues affected breeding, Lockie & Ratcliffe, 1964). Thus, although some eagles in southern
128
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
Scotland were contaminated with organochlorine residues in these years, this alone was probably insufficient to explain the poor breeding success.
Three unhatched eggs from recent years were thicker shelled (shell indices of 2-97 to 3"37) and contained very small amounts of organochlorine pesticides (0.4-0.5 #g g - 1) and PCBs (0-5-1.1 #g g - 1). By analogy with other raptors (Newton & Bogan, 1974; Peakall, 1976) such contamination would have had no influence on breeding performance or population. Habitat composition in areas around the four eagle territories The eagles were seen most frequently (more than 80 ~o of sightings) within 2.5 km of their roosting places, so this distance was used to draw arbitrary boundaries within which we could assess habitat composition and change. For each eagle pair an area was thus delimited such that no point within its boundaries was more than 2.5 km from a known roost. The resulting four areas differed slightly in shape according to the number and proximity of roosts but were nevertheless of similar size (3550-3970 ha). The areas so mapped did not encompass the whole home ranges of pairs but at least outlined areas of highest use. We classified all habitats in these areas into tour broad categories, open water, old woodland (Scots pine and deciduous woodland mature by 1945), new coniferous plantations and open ground (unenclosed hill, moor and sheepwalk). For coniferous plantations, we measured the areas planted in various years from 1925 to 1979. In general for any particular habitat type natural prey was likely to be more abundant at lower altitudes so we estimated the average altitude of open ground and of forest around each eagle territory by taking the mean of altitudes at the centre of each kilometre square (of the Ordnance Survey maps) containing open ground or forest. Very little of the four areas consisted of open water and old woodland O/ (0"6 2"9/o, Table 2). By 1974 only 38 ~o of the area around territory A remained open and thisl at high altitude (418 m). A greater proportion of the area around B was open (54 ~o) but this was also at high altitude (408 m). In contrast, the pairs which continued to breed annually still had a high proportion of open country in their vicinity and this at lower altitudes. The most successful pair D, had a large area of very low altitude open country. Considering only ground below 305m, 90~o of the
Golden eagles and land use in Scotland
129
TABLE 2 The Habitat Composition (~o) within 2.5 km of the Roosting Places in Four Territories of Golden Eagles in Southern Scotland
Habitat
Open water Old woodland Coniferous plantation, planted 1925 29 1930 34 1935 39 1950 54 1955 59 1960 64 1965 69 1970 74 1975 79 Open country (unenclosed hill, moor and sheepwalk) Mean altitude of open country (m) Mean altitude of open country before afforestation (m) 70 of open country below 305 m afforested by 1979
Territor)" A
B
C
D
0-3 0.3
2.9
2.4 0.2
18 0.6 0.5
3.3 4.9 5.9 1-9 13.3 12.2 19.9
--
38.0
4.1 4.1 15.1 19.5
6.6 3-4
54.3
78.3
3.1 1.8 14.0 4.8 8.6
9.1 64.8
418
408
337
161
288
363
315
165
90.4
84.5
309
33.6
originally open country around area A and 85 ~o around B had become afforested by 1979. The respective values for pairs still breeding annually were 31 ~o C and 34 ~o D. Examining the planting dates more closely, about 25 i)o of the open country below 305 m around A was afforested during the years 1970/74 when the pair ceased breeding, only a single bird remaining. At the same time, B suffered similar large-scale planting on its lower ground (29 !I0) accompanied by the total removal of sheep from its vicinity. Thus, the deterioration in the breeding performance of eagles in southern Scotland coincided both spatially and temporally with the progressive afforestation of open country in their vicinity, particularly of open country at low altitude where natural prey was probably most abundant.
11 1.5
" From unpublished pellet analyses by the late E. Blezard.
11 1.5
No. of pellets examined Mean no. items/pellet
11 18 71 0
Sp
sp
13 44 44 0
Da
(1963 69)
A~
(1957)
Large mammals Lagomorphs Large birds (% pigeon) Various others
Prey remains A
16 1-3
35 35 25 5
Sp
43 1.3
29 48 20 4
w
(1974-80)
8 1.1
22 56 11 11
sp
22 1-5
22 28 41 9
Su
25 1-6
22 37 27 15
A
(1974-80)
B
34 1-3
29 24 42 4
w
15 1.7
23 35 35 8
sp
C
Su
77 1.6
20 1-3
44 20 32 4
A
(1974-80)
18 31 44 7
Territory (years) and season
S p r i n g (Sp), S u m m e r (Su), A u t u m n (A) a n d W i n t e r ( W )
76 1-4
30 39 25 7
W
27 1.3
17 31 46 6
Sp
92 1.6
8 40 45 7
Su
16 1"8
7 36 46 11
A
(1974-80)
D
39 1.4
24 45 24 7
W
TABLE 3 T he P e r c e n t F r e q u e n c y o f F o u r T y p e s o f P r e y R e m a i n s in S a m p l e s o f P e l l e t s c o l l e c t e d f r o m E a g l e T e r r i t o r i e s in S o u t h e r n S c o t l a n d in
0~
~"
.~
Golden eagles and land use in Scotland
131
The contents of pellets Neither the incidence nor the volume of various remains in eagle pellets were likely to be directly proportional to the quantities of various foods ingested, so we could not assess the exact diet from pellets alone. However, the contents of pellets did indicate the qualitative composition of the diet and enabled broad comparisons between the diets of different pairs at different seasons within our areas and with other pellet analyses. Four categories of prey remains were distinguished: (i)
large mammals; sheep, goat and deer (probably almost entirely taken as carrion, Lockie & Stephen, 1959); (ii) lagomorphs; rabbits Oryctolagus cuniculus and hares Lepus sp., some of which may have been taken as carrion; (iii) large birds; mainly galliform birds with some ducks and pigeons; (iv) various others; including smaller birds and other mammals. In all, 510 pellets were examined, sample sizes for individual years were too small to investigate annual differences, so we combined pellets from all years (1974/80) to assess the variation between territories. For individual territories, the contents of pellets varied significantly between seasons. (Summing ~2 values for the three territories for which there were pellets from all four seasons: Z2 = 52.6, df = 21, P < 0.001.) Overall, remains of lagomorphs and large birds each formed about one-third of the items identified and large mammals accounted for about one-quarter. Remains of large mammals were most frequent in winter pellets and those of large birds in summer. Lagomorph remains and ~others' varied between territories in their peak season (Table 3). There were no trends, consistent over all seasons, between prey type and territory except for D, where large mammals were less frequent than in the other areas, large bird or lagomorph remains predominating. However, the diet in spring was correlated with breeding performance; pellets from the more successful pairs contained a greater proportion of large bird items (r s-- 1, P = 0"05). The proportion of large mammal remains in spring pellets was related more to habitat, with higher proportions in areas with least open country at low altitude (rs = 1, P = 0.05). These relationships are further supported by data from pellets collected in springs prior to 1970 when breeding performance at A was better and afforestation less (pellet analyses by the late E. Blezard). Eleven pellets from A (1957) and eleven from D (1963-69) contained a high proportion of large birds remains and few large mammal remains (Table 3).
132
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
An examination of the specific prey brings out further differences. Pellets from area D were markedly different from the rest, consistent with the area's situation, at low altitude and incorporating a large amount of sheep walk. Pellets from A, B and C contained the remains of animals associated with high ground--goat, blue hare Lepus timidus and red grouse Lagopus lagopus, whereas pellets from D also contained some species typical of lower ground with richer soils--sheep, brown hare L. capensis, rabbit, mole Talpa europaea, ducks, pheasant Phasianus colchicus, lapwing Vanellus vanellus and curlew Numenius arquata (Appendix I). Remains of game bird (mainly red grouse) figured highly (48 ~o) in pellets collected in springs prior to extensive afforestation (Appendix II) contrasting with their significantly lower (26%) representation in recent springs ( i f = 7.4, P = 0.006). For all pairs and all seasons, the prey were largely of species belonging to open country (mountain, moorland and marginal agricultural land) and included few items characteristic of conifer plantations. There was no evidence that eagles exploited a temporary abundance of some animals following afforestation (e.g. blackgame, short-tailed field voles and their predators) as these species were infrequently recorded from pellets (only 6 ~/0 of all items identified). Area B was subject to the most extensive afforestation in the last decade and area D least, yet the frequency of prey found in young plantations was no different between these areas except in summer when D took slightly more of such prey (P = 0.04; Fisher Exact test) particularly blackgame (7.6 ~0 of identified items).
DISCUSSI ON In the earlier years the breeding success of golden eagles in southern Scotland was not poor. Prior to 1964 the average for pairs at A, B and C was 0.49 young reared/occupied territory, a figure close to the 0-47 estimated for the whole of Scotland in 1964-68 (Everett, 1971). Moreover, ignoring robbed clutches the potential production was 0.59, a success rate approaching that of Deeside (0.62), which at that time had the best production in Scotland. The success of pairs A, B and C has thus fallen steadily, to 0-33 young/occupied territory in the ten years 1964-73 and then to 0.14 in the last decade. This very low figure is in part due to the poor success at C but mainly the result of the cessation of annual breeding at A and B.
Golden eagles and land use in Scotland
133
Poor breeding in golden eagles has been attributed to a variety of causes including human interference (Everett, 1971), organochlorine pesticide poisoning (Lockie et al., 1969), territorial strife caused by intruding subadult eagles (Hailer, 1982) and poor food supply (Newton, 1979; Tjernberg, 1983). Human interference of Galloway eagles is far less now than formerly, organochlorine levels are low, and the problem with subadult eagles was in their failure to settle and replace adults lost from the population rather than any persistent intrusion on territories occupied by a pair. The most reasonable explanation for the varying fortunes of the four territories is that food supply has changed, becoming poorer at A and B, less so at C and least at D. The food of golden eagles has been studied extensively. In North America, Fennoscandia, the Alps and Scotland the diet consists of large mammal carrion and live prey including lagomorph-sized mammals and large birds. The relative proportions of these three food types varies according to availability, live prey predominating in summer and carrion in other seasons wherever live prey is scarce. In the Alps (Hailer, 1982) and in North America (review by Olendorff, 1976, and Bloom & Hawks, 1982) lagomorph-sized mammals have been recorded as forming up to 90 ~o of food brought to the nest, and large mammals are mainly eaten by immature eagles or adults outside the breeding season (particularly in areas where scrub clearance and overgrazing has reduced natural prey (Spofford, 1964; Murphy, 1977)). The same is true in western Scotland where extractive sheep farming practices have reduced natural prey, sheep carrion is abundant and eagles there feed mainly on rabbit in summer and carrion at other times (Lockie, 1964). This contrasts with eagles in the northeast highlands where hares, red grouse and ptarmigan (Lagopus mutus) constitute the main part of the diet (Brown & Watson, 1964). The summer diet in mountainous areas of northern Sweden is similar, mainly willow grouse and ptarmigan, but in the coniferous habitats in Fennoscandia, capercaillie Tetrao urogallus, blackgame and white hares predominate (Tjernberg, 1981; Huhtala & Sulkava, 1977, cited in Tjernberg, 1981). Currently in southern Scotland eagles have a varied diet, with pairs feeding on goat and sheep carrion as well as lagomorphs and large birds. In spring, large birds were least frequent in the diet of the non-breeding pairs and most frequent at D where most young were reared. The available evidence also suggests that large birds, particularly red grouse, were formerly a more important part of the diet of A and D when
134
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
afforestation was less and A was breeding annually. The decline of breeding at A and B was coincident with the large-scale afforestation of the low-altitude open ground in their vicinity. Animal species living in young plantations did not form a substantial part of the diet of even those eagles living in heavily afforested areas, presumably because eagles could not catch their prey easily amongst young trees and rank vegetation. This is consistent with the fact that elsewhere in their range golden eagles are birds of open country. They are absent from continuous closed forest, though they often inhabit terrain with open woodland or scattered trees. Even in the breeding areas in coniferous parts of Fennoscandia the pine/spruce woodland is interspersed with extensive open areas of bogs and 'clearcuts' as well as with higher ground (Tjernberg, 1981). In the Alps many pairs breed in trees but most of their foraging is in open country above the treeline (Hailer, 1982). It seems that the afforestation of open country in southern Scotland has forced eagles there to forage at higher altitudes where natural prey, particularly red grouse, is less abundant but where carrion is still available albeit in less quantity than formerly. It is likely that the consequent reduction in hunting habitat and food supply has resulted in a reduced production of young from C and the cessation of annual breeding at A and B. There is also a problem with recruitment at A and B as adults lost from these pairs were not rapidly replaced. The problem is not one of the paucity of potential recruits because at B a succession of young eagles failed to settle. It may be that the forage in these areas is now so poor it can sustain only the more experienced or skilled individuals. Were there alternative suitable nesting and feeding areas farther away, pairs could have abandoned their old home range and adopted a new one at some distance. This appears to have happened to pair D which, during 1963-68, first occupied a nesting territory that became about 90~o afforested within a 2.5 km radius of the nesting cliff. The pair moved in 1969 to a new breeaing place which at the time had only about 20 ~o afforestation within the same radius. Birds at A, B and C have shown no inclination to move their quarters in this way, and it may be that they are constrained by the lack of suitable new breeding and feeding areas to which they could move. Afforestation is widespread and the failure of eagles to establish themselves in other parts of southern Scotland is probably due to the lack of suitable tracts of open country large enough to provide sufficient food, yet free of persecution. With respect to the latter, eagles benefited
Golden eagles and land use in Scotland
135
temporarily from the afforestation programme because the forest authorities protected the nesting areas from human interference. No birds in these areas were known to have been deliberately killed, the last known robbing of an eyrie was in 1971 and casual human disturbance has been minimal in recent years. Despite such benevolent attitudes towards the birds themselves, the destruction of their habitat continues. The full impact of tree planting around B in 1979 may not yet be apparent as the vegetation was still short in the early 1980s. Other areas have been ploughed in preparation for planting in 1980 and a substantial part of the low ground around D is scheduled for afforestation before 1985. With the failure of other pairs of golden eagles to become established on grouse moors, on the few remaining sheep walks, or in other heavily afforested areas, the species in southern Scotland may eventually be reduced to one regularly breeding pair.
SUMMARY (1)
(2)
(3)
(4)
After a long period of absence, golden eagles recolonised southwest Scotland in the early 1940s. By 1966 there were four regularly breeding pairs but in the early 1970s two pairs ceased to breed and a third pair that nested annually, rarely produced young. Initially broods of two were not infrequent and breeding success was good with an average of 0.49 young produced/occupied territory. In the last two decades this has fallen successively to 0.33 and then 0.14. Most failures were probably not pesticide-induced but could be attributed formerly to human interference and, latterly, to food shortage. The land use of the uplands of southwest Scotland has changed dramatically in the last 35 years from sheep farming to forestry. All four eagle areas experienced some afforestation, but the cessation of breeding in two areas was coincident with large-scale afforestation of the most productive land in their vicinity, the only open ground remaining being at high altitude. The contents of pellets showed that in recent years (1974/80) the eagles fed mainly on large mammals, lagomorphs and large birds, the latter chiefly in summer. The proportion of large birds in the diet in spring was related to breeding performance. Pellets from the one
136
(5)
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
successful breeding territory contained the remains of animals associated with low ground and less large mammal remains than pellets from the other territories. Animals living in conifer plantations did not form a substantial part of the diet. Eagles probably benefited initially from afforestation because they were better protected from human interference. However, afforestation inevitably reduces the amount of habitat for eagles, and as areas of conifer plantations increase a further decline is anticipated.
ACKNOWLEDGEMENTS We thank the various landowners in southwest Scotland who gave us access to their ground to examine nests and to collect pellets. In particular we thank the Forestry Commission and various other forestry authorities for making available planting maps of the study area. For their contributions to the data on status and breeding success we are grateful to A. D. Watson, W. Murray, R. Nelson, R. Stokoe, E. Blezard and G. Hughes-Onslow. The contents of eagle eggs were analysed for organochlorines by the Government Chemist and Dr J. Bogan. Dr I. Newton, A. D. Watson, A. N. Lance and S. J. Petty gave helpful criticism of the manuscript.
REFERENCES Bloom, P. H. & Hawks, S. J. (1982). Food habits of nesting golden eagles in northeast California and northwest Nevada. Raptor Res., 16, 110-15. Brown, L. H. & Watson, A. (1964). The golden eagle in relation to its food supply. Ibis, 106, 78-100. Day, M. G. (1965). Identification of hair and feather remains in the gut and faeces of stoats and weasels. J. Zool., 148, 201-17. Dick, Rev. C. H. (1916). Highways and byways in Galloway and Carrick. London, Macmillan. Everett, M. J. (1971). The golden eagle survey in Scotland. Br. Birds, 64, 49-56. Gladstone, H. S. (1910). The birds of Dumj}'iesshire. Witherby, London. Hailer, H. (1982). Raumorganisation und Dynamik einer Population des Steinadlers Aquila ehrysaetas in den Zentralalpen. Orn. Beob., 79, 163-211. Harper, M. McL. (1896). Rambles in Galloway, 2nd edn. Dalbeattie, Fraser. Lockie, J. D. (1964). The breeding density of the golden eagle and fox in relation to food supply in Wester Ross, Scotland. Scot. Nat., 71, 66-77.
Golden eagles and land use in Scotland
137
Lockie, J. D. & Stephen, D. (1959). Eagles, lambs and land management on Lewis. J. Anita. Ecol., 28, 43-50. Lockie, J. D. & Ratcliffe, D. A. (1964). Insecticides and Scottish golden eagles. Br. Birds, 57, 89 102. Lockie, J. D., Ratcliffe, D. A. & Balharry, R. (1969). Breeding success and dieldrin contamination of golden eagles in West Scotland. J. appl. Ecol., 6, 381 9. Maxwell, Sir H. (1927). The golden eagle in Galloway. Scott. Nat., 164, 62. Murphy, J. R. (1977). Eagles and livestock--some management considerations. Proc. ICBP World Conj~ on birds of prey, Vienna, 1975, 307-14. Newton, I. (1979). Population ecology of raptors. Berkhamsted, T. & A. D. Poyser. Newton, I. & Bogan, J. (1974). Relationships between organochlorine residues, eggshell thinning and hatching success in British sparrowhawks. Nature, Lond., 249, 582-3. Olendorff, R. R. (1976). The food habits of North American golden eagles. A m. Midl. Nat., 95, 232-6. Paton, W. & Pike, O. G. (1929). The birds oJAyrshire. London, Witherby. Peakall, D. B. (1976). The peregrine falcon (Falco peregrinus) and pesticides. Can. Fld-Nat., 90, 301-7. Ratcliffe, D. A. (1967). Decrease in eggshell weight in certain birds of prey. Nature, Lond., 215, 208 10. Service, R. (1903). The diurnal and nocturnal raptorial birds of the Solway area. Trans. ,Dumj?ies & Galloway nat. Hist. and antiq. Soc., 17, 327. Spofford, W. R. (1964). The golden eagle in the Trans-Pecos and Edwards Plateau of Texas. Audubon Conserv. Rep., 1, 1 47. Tjernberg, M. (1981). Diet of the golden eagle Aquila chrysaetos during the breeding season in Sweden. Holarct. Ecol., 4, 12 19. Tjernberg, M. (1983). Prey abundance and reproductive success of the golden eagle, Aquila chrysaetos in Sweden. Holarct. Ecol., 6, 17 23.
APPENDIX
1
Sheep Goat Deer sp. Fox Vulpes vuipes Mink Mustela vison Stoat Mustela erminea or weasel M. nivalis Rabbit Oryctolagus cuniculus Blue hare Lepus timidus Brown hare L. capensis Hare (sp.?) Lagomorph (sp.?) Water vole Arvicola amphibius Short-tailed field vole Microtus agrestis
Pellet contents’
-
-2l-3-3-312
-2--2-l-l--
4 3
6 6
10 10
1
-
1
3 5
-
1
2 5
B
1 5 ~ 515236-128-222 1----_-_---11-l
-
1-2--._-l-11--
4 1 5 9 1 23--l_l3_4____~
A
1 4
-
-
5 8
3 6
1
5
-
-
3
6 28
-
4 15
c
1 2
-
-
1 10
Territory and season
1
7 25
-
5 22
6
4
-
4 2
The Contents of Eagle Pellets collected from Four Territories in Southern Scotland 19741980 (The numbers in columns are the number of pellets in which the item occurred)
-
3 38
14
1--
12 -
D
2 4
2
-
2 -
14
8
2
8 5
ss
$
?J
3
1. h
5
;s
.
.
.
.
16
.
--
.
--
.
.
l
.
.
.
--
.
1 .
--
43
. .
1
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--
.
.
.
1
1
--
.
.
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--
.
.
.
8
1
l
2
.
.
.
.
.
.
.
.
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.
.
.
.
.
.
.
.
8
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--
.
.
.
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1
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.
.
.
.
.
.
.
.
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.
.
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. 1
. 1
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8 --
.
.
22
.
.
.
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.
.
.
25
.
--
l
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3
.
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.
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1
1
5
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.
1
34
--
--
.
.
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--
.
15
2
.
.
.
.
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.
.
.
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--
--
--
.
1
.
.
1
.
1
7
--
.
--
--
.
.
. .
.
.
.
77
--
.
1
1
1
2
3
2
6
3
37
1
.
--
.
.
.
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.
.
--
--
--
20
--
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--
.
.
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7
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.
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---
76
.
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.
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19
--
1
.
.
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27
--
--
.
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.
.
1
.
1
3
1
2
4
1
3
.
.
.
92
--
.
1
1
6
4
19
12
9
10
2
5
1
.
.
16
--
--
--
2
--
l
5
--
--
5
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1
1
39
--
--
--
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--
- -
4
4
1
4
--
--
--
r
- -
a
In addition, most pellets examined contained vegetation, and some contained grit, insect chitin or eggshell. These remains were probably taken incidentally, either as the contents of the gut and oviduct of prey, or sticking to food as it was eaten.
Hedgehog Erinaceus europaeus Mole Talpa europaea Mallard Anas platyrhynchus Duck (sp. ?) Kestrel Falco tinnunculus Red grouse Lagopus lagopus Black grouse Lyrurus tetrix Pheasant Phasianus colchicus Galliforme (sp. ?) Lapwing Vanellus vanellus Curlew Numenius arquata Wader (sp. ?) Gull (sp. ?) Pigeon (mainly 'homing' pigeons) Short-eared owl Asio flammeus Skylark Alauda arvensis Raven Corvus corax Carrion crow C. corone Fieldfare Turdus pilaris Wheatear Oenanthe oenanthe Meadow pipit Anthus pratensis Passerine (sp. ?) Number of pellets examined
E-
e~
E"
140
M. Marquiss, D. A. Ratcl$e,
APPENDIX
R. Roxburgh
II
The Contents
of Eagle Pellets Collected from Two Territories in Southern Scotland 1957-69” (The numbers in columns are the number of pellets in which the item occurred) Territory (years) and season Pellet contents (I&) Spring
Sheep Blue hare Lepus timidus Buzzard Buteo buteo Red grouse Lugopus lagopus Black grouse Lyrurus tetrix Pheasant Phasianus colchicus Pigeon (sp.?) Number of pellets examined
2 7 7
11
D (1963, ‘66 and ‘69) Spring
2 3 1 8 1 1 1 11
” A summary of data from pellet analyses by the late E. Blezard.
The Numbers, Breeding Success and Diet of Golden Eagles in Southern Scotland in Relation to Changes in Land Use M. Marquiss Institute of Terrestrial Ecology, Bush Estate, Penicuik, Midlothian EH260QB, Great Britain
D. A. Ratcliffe Nature Conservancy Council, Northminster House, Peterborough PE1 1UA, Great Britain
& R. Roxburgh "Charlescot', 20 Townhead, Catrine, Ayrshire KA5 6SQ, Great Britain
ABSTRACT Golden eagles Aquila chrysaetos recolonised southwest Scotland in the early 1940s. By 1966jbur pairs were established and breeding success was good. In the early 1970s, coincident with large-scale afforestation, two pairs ceased to breed and a third pair rarely produced young. Good breeding perJormance was associated with spring diet; the most productive pair consuming more large birds and less carrion than the other pairs. Animals living in conifer plantations did not .[orm a substantial part oJ the diet at any time of year. It seemed that the afforestation of open country had removed much of the productive Joraging habitat for three o[ the./but" pairs of eagles, as the remaining open country was at higher altitude where live prey were scarce.
INTRODUCTION The golden eagle Aquila chrysaetos was only a sporadic breeder in southern Scotland from about 1870 until the early 1940s when a pair 121
Biol. Conserv. 0006-3207/85/$0330 c') Elsevier Applied Science Publishers Ltd, England, 1985, Printed in Great Britain
122
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
became established, first nesting in 1945. The 19th century disappearance of the species from the region has been attributed to persecution, and its return coincided with the decline of grouse preservation here, beginning in World War II. By 1965 four pairs were breeding but in the early 1970s two of these pairs stopped producing eggs. This paper summarises the history and breeding success of golden eagles in southern Scotland and assesses diet in recent years (1974-80). In an attempt to explain the recent decline in breeding we examine its coincidence with the afforestation of uplands in the vicinity. The predominant land use in the uplands of southern Scotland has changed dramatically in the last 35 years from sheep farming to forestry. In 1945 the southern uplands were grass or heather dominated, grazed by sheep, feral goats and a few red deer Cervus elaphus. Grouse moors were maintained in a few areas but even these carried some sheep as well. After 1945 there was an enormous expansion in the afforestation of unenclosed hill land, and the region was progressively planted with exotic conifers (mainly Norway spruce Picea abies and sitka spruce P. sitchensis) so that by 1980 little of the hill ground occupied by eagles held sheep (Fig. 1).
SOUTHWEST S(
Fig. 1.
The study area.
Golden eagles and land use in Scotland
123
Afforestation entails the removal of sheep stocks and the fencing of the forest margin from ground still carrying sheep. The ground is ploughed and the tree seedlings are planted in the upturned sod. In the absence of sheep, the original short-cropped vegetation soon becomes luxuriant. After about ten years the tree canopy closes and the ground vegetation decreases; herbaceous vegetation remains chiefly on roadsides and rides and on areas too wet, too sterile or too high for trees. Deer and goats survive in these areas, particularly in the 'islands' of open ground at high altitude, but their populations are ~culled' to prevent tree damage by browsing. During the period just after the removal of sheep, when herbaceous vegetation is luxuriant, there is sometimes an abundance of smaller grazing animals, short-tailed field voles Microtus agrestis, and blackgame Lyrurus tetrix and their predators, weasels Mustela nivalis, foxes Vulpes vulpes, short-eared owls Asio flammeus and kestrels Falco tinnunculus, but this abundance only lasts a few years before the tree canopy closes suppressing other vegetation. METHODS D. A. R. and R. R. collected most data on breeding success, their own information being supplemented by that from local ornithologists. Although the eagles favoured particular sites for both nesting and roosting, they also used adjacent crags or small blocks of old woodland as alternative roosts. These were well defined by accumulations of droppings, pellets and moulted feathers and were thus located easily. From spring 1974 to winter 1979/80 pellets were collected from the roosts of the four eagle pairs and their contents identified (where necessary hair and feather remains were examined microscopically, Day 1965). The altitude, and amounts of various habitats within the vicinity of each eagle territory were measured using 1 : 63 360 Ordnance Survey maps, updated to show all the afforested land and the dates that trees were planted. RESULTS
History of breeding in southern Scotland In Galloway and Carrick, at least five different 19th century breeding haunts are mentioned, but these gradually became deserted after 1850,
124
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
and the last occupied eyrie seems to have been in the Glen Trool area in 1876 (Harper, 1896; Service, 1903; Dick, 1916). The history of the golden eagle in the Moffat-Tweedsmuir Hills is even vaguer, though there would be sufficient space and nesting habitat for 3-4 pairs. Breeding here had evidently ceased by 1850 (Gladstone, 1910). Few ornithologists were active in southern Scotland even by 1900, and breeding attempts after supposed extinction could have gone unnoticed. The next recorded nesting was in 1905, when a pair laid 3 eggs in one of the old Galloway haunts, but this attempt failed (Maxwell, 1927). J. F. Peters (pers. comm.) saw six nests with eggs at the same locality in Carrick during 1910-1920, but only one of these certainly produced young. Paton & Pike (1929) recorded that a pair continued to attempt breeding in this territory until 1923, but without rearing young. Subsequently, there were scattered records of birds seen in various places, but, apart from an unsubstantiated report of a nest in the Nithsdale area of Dumfriesshire around 1938-39, no further instances of nesting were known until 1945. In that year, a pair reared two young in the Galloway Hills (this name is used henceforth to include the Carrick section which is topographically continuous with it). Thereafter, they nested annually, using four alternative cliffs (Area A). A second pair appeared in another area (Area B) in 1948, rearing two young, and also nested annually, resorting usually to the same cliffs, but occasionally moving to another. A third pair established a nesting territory (Area C) which overlapped with that of Area A in 1952. The contents of the nest were never determined that year, but successful breeding took place in 1953. This pair continued to breed regularly on the same cliff until 1975 but have since used two nesting places. An evidently unfinished nest with only a single bird in attendance was found in a fourth Galloway locality (Area D) in 1962, but it was not until 1965 that eggs were laid and a young bird reared. This pair moved to an adjoining nesting area in 1969 and have remained there ever since, using two nesting locations. Two of the four pairs re-occupied the old nesting haunts of 1905 and 1910-23. In another part of the Galloway Hills there was an unconfirmed nesting in the late 1950s. The cliff was then deserted, but a pair of golden eagles with an eyrie were seen 1966: it was never established whether eggs were laid, and this pair disappeared after frequenting another cliff in the vicinity during the following year. Golden eagles were seen only occasionally in the Moffat-Tweedsmuir Hills during the 1950s and 1960s, but a pair returned to nest in this district in 1972. Eggs laid that year and
Golden eagles and land use in Scotland
125
in 1973 at an adjoining cliff failed to hatch, but in autumn 1974 a nest which had obviously contained young was found at a third site in the area. A carcase found late in 1974, 6 km away was probably one of this pair. Single immature eagles were seen hereabouts in 1976 and 1980 but there has been no further breeding. Single immature golden eagles have also been recorded in summer in two additional Southern Upland localities, from 1975 onwards.
Breeding success in the Galloway Hills 1945-83 For all four areas A to D there has probably been continuous occupation since the initial establishment of pairs, with records of nests refurbished and eggs laid in each during almost every year until the mid-1970s (Table 1). In most breeding attempts two eggs were laid, and at least in the earlier years when only one egg was recorded, it was possible that part of the clutch has been robbed. Birds nesting at B laid three eggs in at least seven years during 1950-60. Four other three-egg clutches were recorded, at C in 1959, 1960, 1977 and 1983. At first, fledged broods of two young were not u n c o m m o n with six in the 13 recorded broods prior to 1960. Since then, all of the 25 recorded broods have been of single young. Overall only about 40 "//o of nesting attempts produced flying young. Forty-five nests were known to have had eggs but failed to produce young. The main recorded circumstance of failure (23 instances) was the desertion of clutches most of which had been incubated for the normal period but had failed to hatch. In recent years many of these eggs have been examined and have had little or no embryonic development. At least seven clutches were known to have been taken by egg collectors between 1953 and 1961, and one in 1971. In 1968 both eggs of the pair at A were found broken in the nest and in 1970 one of the two eggs at C was broken. From 1965 there have been five instances where young were known to hatch but they died whilst still small. In recent years breeding performance deteriorated with the cessation of annual breeding in two of the four areas. At A a pair last reared young in 1973; they deserted a clutch in 1974, did not lay in 1975 and only a single bird was seen thereafter until 1979 when a pair refurbished an eyrie but did not lay. In 1980 a single egg was laid but soon deserted. In March 1981, a male eagle was killed by poisoned bait put down for other predators on a neighbouring estate. Since then there has been no record of a pair or a nesting attempt. At B eggs were laid annually until 1972 when
of of of of of of
years years years years years years
examined occupied by a single bird occupied by a pair nest built/refurbished eggs laid young reared
3 0"75
0"29
0 3 0
2.7 (7) 1"5 (6)
12 0 12 12 12 6
6
0 3 0
1.9 (11) 1.7 (3)
17 0 17 17 14+ 3
B
TABLE 1
0.50
0
0 1 0
2.1 (7) 1.3 (4)
10 0 10 10 10 4
C
0.30
3
2 0 1
2.0 (9) 1-0 (3)
l0 0 10 10 10 3
A
0.10
2
0 1 0
2.0 (2) 1-0 (1)
8 2 6 7 6 1
B
1964-73
0.57
2
0 0 1
1.8 (5) 1-0 (4)
7 0 7 7 7 4
C
0.50
0
0 0 0
2.0 (3) 1-0 (5)
10 2 8 9 6+ 5
D
E a g l e s in S o u t h e r n
Period and territory
Territories of Golden
0.00
2
0 0 0
1-5 (2) --
10 3 5 3 2 0
A
Scotland
0.10
0
0 0 0
2"0 (1) 1"0 (1)
10 4 6 3 1 1
B
1974 83
0.30
4
3 0 0
2.2 (10) 1-0 (3)
10 0 10 10 10 3
C
0.80
0 0 1 (nest collapsed) 1
2.0 (10) 1.0 (8)
10 0 10 10 10 8
D
° Failures are minimum values because in some years the outcome, or the type of failure was anknown. Nesting failures from clutch desertion and addling of eggs are combined, because the two are often connected, but to an unknown degree. In one of the eyries with broken eggs, only a single egg was broken, the remaining egg was addled.
No. of years clutch deserted ~r No. of years eggs addled ) No. young produced/occupied territory
Nesting failures~ No. of years all young died No. of years clutch robbed No. of years eggs broken
Mean clutch size (n) Mean brood size (n)
No. No. No. No. No. No.
A
194543
T h e B r e e d i n g S u c c e s s in F o u r
Golden eagles and land use in Scotland
127
there was a new but empty nest. Eggs were never recorded in subsequent years though the site was still occupied by an adult pair in 1976. From 1977 there was only one adult and although single immatures were seen in the vicinity almost annually in winter and spring, none of these apparently stayed until 1982. In 1983 eggs were laid for the first time in over a decade and an eaglet fledged for the first time since 1968. In contrast, the pair at C has continued to produce eggs every year, though it has reared young only three times in the last decade. Site D has had consistent success producing flying young in every year but two since 1970. In 1978 the nest collapsed early in incubation; the pair produced another egg in an alternative eyrie, but failed to hatch it. In 1983 two eggs were laid but they failed to hatch. It was difficult to assess the causes of failure in the earlier years. Failures not attributed to robbing could have been due to high pesticide levels, human disturbance, food shortage, or a combination of these factors.
Organochlorine pesticide residues Lockie et al., 1969, showed that eagles suffered poor breeding success in west Scotland in the years 1963 65 and attributed this to high concentrations of pesticides (particularly dieldrin) in their eggs. Following the withdrawal of dieldrin sheep dips in 1966, eagles subsequently (1966-68) bred more successfully. Comparing the breeding success of our eagles in the five years prior to 1966 with the five years afterwards, there was no difference in the frequency of young reared (P = 0"7; Fisher Exact Probability Test) or in clutches deserted, broken or addled (P = 0.8). The mean shell thickness (expressed as mean shell index Ratcliffe, 1967) of eggs in five clutches from A between 1951 and 1964 was 2'87, about 9 % thinner than the average for uncontaminated Scottish eagles (a mean shell index of 3" 15 for 84 clutches taken before 1947; the first year that the use of organochlorines became widespread). This level of shell-thinning suggests these eggs contained less than 1/~g g- 1 fresh weight D D E (Newton, 1979). Two deserted clutches of eggs taken from southern Scotland in 1964 A and 1965 B contained relatively low levels of organochlorine pesticide residues (0.6-1.0 #g g i fresh weight) compared with the considerably greater levels in most western pairs (1 pg g- 1 was considered to be the lower limit at which residues affected breeding, Lockie & Ratcliffe, 1964). Thus, although some eagles in southern
128
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
Scotland were contaminated with organochlorine residues in these years, this alone was probably insufficient to explain the poor breeding success.
Three unhatched eggs from recent years were thicker shelled (shell indices of 2-97 to 3"37) and contained very small amounts of organochlorine pesticides (0.4-0.5 #g g - 1) and PCBs (0-5-1.1 #g g - 1). By analogy with other raptors (Newton & Bogan, 1974; Peakall, 1976) such contamination would have had no influence on breeding performance or population. Habitat composition in areas around the four eagle territories The eagles were seen most frequently (more than 80 ~o of sightings) within 2.5 km of their roosting places, so this distance was used to draw arbitrary boundaries within which we could assess habitat composition and change. For each eagle pair an area was thus delimited such that no point within its boundaries was more than 2.5 km from a known roost. The resulting four areas differed slightly in shape according to the number and proximity of roosts but were nevertheless of similar size (3550-3970 ha). The areas so mapped did not encompass the whole home ranges of pairs but at least outlined areas of highest use. We classified all habitats in these areas into tour broad categories, open water, old woodland (Scots pine and deciduous woodland mature by 1945), new coniferous plantations and open ground (unenclosed hill, moor and sheepwalk). For coniferous plantations, we measured the areas planted in various years from 1925 to 1979. In general for any particular habitat type natural prey was likely to be more abundant at lower altitudes so we estimated the average altitude of open ground and of forest around each eagle territory by taking the mean of altitudes at the centre of each kilometre square (of the Ordnance Survey maps) containing open ground or forest. Very little of the four areas consisted of open water and old woodland O/ (0"6 2"9/o, Table 2). By 1974 only 38 ~o of the area around territory A remained open and thisl at high altitude (418 m). A greater proportion of the area around B was open (54 ~o) but this was also at high altitude (408 m). In contrast, the pairs which continued to breed annually still had a high proportion of open country in their vicinity and this at lower altitudes. The most successful pair D, had a large area of very low altitude open country. Considering only ground below 305m, 90~o of the
Golden eagles and land use in Scotland
129
TABLE 2 The Habitat Composition (~o) within 2.5 km of the Roosting Places in Four Territories of Golden Eagles in Southern Scotland
Habitat
Open water Old woodland Coniferous plantation, planted 1925 29 1930 34 1935 39 1950 54 1955 59 1960 64 1965 69 1970 74 1975 79 Open country (unenclosed hill, moor and sheepwalk) Mean altitude of open country (m) Mean altitude of open country before afforestation (m) 70 of open country below 305 m afforested by 1979
Territor)" A
B
C
D
0-3 0.3
2.9
2.4 0.2
18 0.6 0.5
3.3 4.9 5.9 1-9 13.3 12.2 19.9
--
38.0
4.1 4.1 15.1 19.5
6.6 3-4
54.3
78.3
3.1 1.8 14.0 4.8 8.6
9.1 64.8
418
408
337
161
288
363
315
165
90.4
84.5
309
33.6
originally open country around area A and 85 ~o around B had become afforested by 1979. The respective values for pairs still breeding annually were 31 ~o C and 34 ~o D. Examining the planting dates more closely, about 25 i)o of the open country below 305 m around A was afforested during the years 1970/74 when the pair ceased breeding, only a single bird remaining. At the same time, B suffered similar large-scale planting on its lower ground (29 !I0) accompanied by the total removal of sheep from its vicinity. Thus, the deterioration in the breeding performance of eagles in southern Scotland coincided both spatially and temporally with the progressive afforestation of open country in their vicinity, particularly of open country at low altitude where natural prey was probably most abundant.
11 1.5
" From unpublished pellet analyses by the late E. Blezard.
11 1.5
No. of pellets examined Mean no. items/pellet
11 18 71 0
Sp
sp
13 44 44 0
Da
(1963 69)
A~
(1957)
Large mammals Lagomorphs Large birds (% pigeon) Various others
Prey remains A
16 1-3
35 35 25 5
Sp
43 1.3
29 48 20 4
w
(1974-80)
8 1.1
22 56 11 11
sp
22 1-5
22 28 41 9
Su
25 1-6
22 37 27 15
A
(1974-80)
B
34 1-3
29 24 42 4
w
15 1.7
23 35 35 8
sp
C
Su
77 1.6
20 1-3
44 20 32 4
A
(1974-80)
18 31 44 7
Territory (years) and season
S p r i n g (Sp), S u m m e r (Su), A u t u m n (A) a n d W i n t e r ( W )
76 1-4
30 39 25 7
W
27 1.3
17 31 46 6
Sp
92 1.6
8 40 45 7
Su
16 1"8
7 36 46 11
A
(1974-80)
D
39 1.4
24 45 24 7
W
TABLE 3 T he P e r c e n t F r e q u e n c y o f F o u r T y p e s o f P r e y R e m a i n s in S a m p l e s o f P e l l e t s c o l l e c t e d f r o m E a g l e T e r r i t o r i e s in S o u t h e r n S c o t l a n d in
0~
~"
.~
Golden eagles and land use in Scotland
131
The contents of pellets Neither the incidence nor the volume of various remains in eagle pellets were likely to be directly proportional to the quantities of various foods ingested, so we could not assess the exact diet from pellets alone. However, the contents of pellets did indicate the qualitative composition of the diet and enabled broad comparisons between the diets of different pairs at different seasons within our areas and with other pellet analyses. Four categories of prey remains were distinguished: (i)
large mammals; sheep, goat and deer (probably almost entirely taken as carrion, Lockie & Stephen, 1959); (ii) lagomorphs; rabbits Oryctolagus cuniculus and hares Lepus sp., some of which may have been taken as carrion; (iii) large birds; mainly galliform birds with some ducks and pigeons; (iv) various others; including smaller birds and other mammals. In all, 510 pellets were examined, sample sizes for individual years were too small to investigate annual differences, so we combined pellets from all years (1974/80) to assess the variation between territories. For individual territories, the contents of pellets varied significantly between seasons. (Summing ~2 values for the three territories for which there were pellets from all four seasons: Z2 = 52.6, df = 21, P < 0.001.) Overall, remains of lagomorphs and large birds each formed about one-third of the items identified and large mammals accounted for about one-quarter. Remains of large mammals were most frequent in winter pellets and those of large birds in summer. Lagomorph remains and ~others' varied between territories in their peak season (Table 3). There were no trends, consistent over all seasons, between prey type and territory except for D, where large mammals were less frequent than in the other areas, large bird or lagomorph remains predominating. However, the diet in spring was correlated with breeding performance; pellets from the more successful pairs contained a greater proportion of large bird items (r s-- 1, P = 0"05). The proportion of large mammal remains in spring pellets was related more to habitat, with higher proportions in areas with least open country at low altitude (rs = 1, P = 0.05). These relationships are further supported by data from pellets collected in springs prior to 1970 when breeding performance at A was better and afforestation less (pellet analyses by the late E. Blezard). Eleven pellets from A (1957) and eleven from D (1963-69) contained a high proportion of large birds remains and few large mammal remains (Table 3).
132
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
An examination of the specific prey brings out further differences. Pellets from area D were markedly different from the rest, consistent with the area's situation, at low altitude and incorporating a large amount of sheep walk. Pellets from A, B and C contained the remains of animals associated with high ground--goat, blue hare Lepus timidus and red grouse Lagopus lagopus, whereas pellets from D also contained some species typical of lower ground with richer soils--sheep, brown hare L. capensis, rabbit, mole Talpa europaea, ducks, pheasant Phasianus colchicus, lapwing Vanellus vanellus and curlew Numenius arquata (Appendix I). Remains of game bird (mainly red grouse) figured highly (48 ~o) in pellets collected in springs prior to extensive afforestation (Appendix II) contrasting with their significantly lower (26%) representation in recent springs ( i f = 7.4, P = 0.006). For all pairs and all seasons, the prey were largely of species belonging to open country (mountain, moorland and marginal agricultural land) and included few items characteristic of conifer plantations. There was no evidence that eagles exploited a temporary abundance of some animals following afforestation (e.g. blackgame, short-tailed field voles and their predators) as these species were infrequently recorded from pellets (only 6 ~/0 of all items identified). Area B was subject to the most extensive afforestation in the last decade and area D least, yet the frequency of prey found in young plantations was no different between these areas except in summer when D took slightly more of such prey (P = 0.04; Fisher Exact test) particularly blackgame (7.6 ~0 of identified items).
DISCUSSI ON In the earlier years the breeding success of golden eagles in southern Scotland was not poor. Prior to 1964 the average for pairs at A, B and C was 0.49 young reared/occupied territory, a figure close to the 0-47 estimated for the whole of Scotland in 1964-68 (Everett, 1971). Moreover, ignoring robbed clutches the potential production was 0.59, a success rate approaching that of Deeside (0.62), which at that time had the best production in Scotland. The success of pairs A, B and C has thus fallen steadily, to 0-33 young/occupied territory in the ten years 1964-73 and then to 0.14 in the last decade. This very low figure is in part due to the poor success at C but mainly the result of the cessation of annual breeding at A and B.
Golden eagles and land use in Scotland
133
Poor breeding in golden eagles has been attributed to a variety of causes including human interference (Everett, 1971), organochlorine pesticide poisoning (Lockie et al., 1969), territorial strife caused by intruding subadult eagles (Hailer, 1982) and poor food supply (Newton, 1979; Tjernberg, 1983). Human interference of Galloway eagles is far less now than formerly, organochlorine levels are low, and the problem with subadult eagles was in their failure to settle and replace adults lost from the population rather than any persistent intrusion on territories occupied by a pair. The most reasonable explanation for the varying fortunes of the four territories is that food supply has changed, becoming poorer at A and B, less so at C and least at D. The food of golden eagles has been studied extensively. In North America, Fennoscandia, the Alps and Scotland the diet consists of large mammal carrion and live prey including lagomorph-sized mammals and large birds. The relative proportions of these three food types varies according to availability, live prey predominating in summer and carrion in other seasons wherever live prey is scarce. In the Alps (Hailer, 1982) and in North America (review by Olendorff, 1976, and Bloom & Hawks, 1982) lagomorph-sized mammals have been recorded as forming up to 90 ~o of food brought to the nest, and large mammals are mainly eaten by immature eagles or adults outside the breeding season (particularly in areas where scrub clearance and overgrazing has reduced natural prey (Spofford, 1964; Murphy, 1977)). The same is true in western Scotland where extractive sheep farming practices have reduced natural prey, sheep carrion is abundant and eagles there feed mainly on rabbit in summer and carrion at other times (Lockie, 1964). This contrasts with eagles in the northeast highlands where hares, red grouse and ptarmigan (Lagopus mutus) constitute the main part of the diet (Brown & Watson, 1964). The summer diet in mountainous areas of northern Sweden is similar, mainly willow grouse and ptarmigan, but in the coniferous habitats in Fennoscandia, capercaillie Tetrao urogallus, blackgame and white hares predominate (Tjernberg, 1981; Huhtala & Sulkava, 1977, cited in Tjernberg, 1981). Currently in southern Scotland eagles have a varied diet, with pairs feeding on goat and sheep carrion as well as lagomorphs and large birds. In spring, large birds were least frequent in the diet of the non-breeding pairs and most frequent at D where most young were reared. The available evidence also suggests that large birds, particularly red grouse, were formerly a more important part of the diet of A and D when
134
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
afforestation was less and A was breeding annually. The decline of breeding at A and B was coincident with the large-scale afforestation of the low-altitude open ground in their vicinity. Animal species living in young plantations did not form a substantial part of the diet of even those eagles living in heavily afforested areas, presumably because eagles could not catch their prey easily amongst young trees and rank vegetation. This is consistent with the fact that elsewhere in their range golden eagles are birds of open country. They are absent from continuous closed forest, though they often inhabit terrain with open woodland or scattered trees. Even in the breeding areas in coniferous parts of Fennoscandia the pine/spruce woodland is interspersed with extensive open areas of bogs and 'clearcuts' as well as with higher ground (Tjernberg, 1981). In the Alps many pairs breed in trees but most of their foraging is in open country above the treeline (Hailer, 1982). It seems that the afforestation of open country in southern Scotland has forced eagles there to forage at higher altitudes where natural prey, particularly red grouse, is less abundant but where carrion is still available albeit in less quantity than formerly. It is likely that the consequent reduction in hunting habitat and food supply has resulted in a reduced production of young from C and the cessation of annual breeding at A and B. There is also a problem with recruitment at A and B as adults lost from these pairs were not rapidly replaced. The problem is not one of the paucity of potential recruits because at B a succession of young eagles failed to settle. It may be that the forage in these areas is now so poor it can sustain only the more experienced or skilled individuals. Were there alternative suitable nesting and feeding areas farther away, pairs could have abandoned their old home range and adopted a new one at some distance. This appears to have happened to pair D which, during 1963-68, first occupied a nesting territory that became about 90~o afforested within a 2.5 km radius of the nesting cliff. The pair moved in 1969 to a new breeaing place which at the time had only about 20 ~o afforestation within the same radius. Birds at A, B and C have shown no inclination to move their quarters in this way, and it may be that they are constrained by the lack of suitable new breeding and feeding areas to which they could move. Afforestation is widespread and the failure of eagles to establish themselves in other parts of southern Scotland is probably due to the lack of suitable tracts of open country large enough to provide sufficient food, yet free of persecution. With respect to the latter, eagles benefited
Golden eagles and land use in Scotland
135
temporarily from the afforestation programme because the forest authorities protected the nesting areas from human interference. No birds in these areas were known to have been deliberately killed, the last known robbing of an eyrie was in 1971 and casual human disturbance has been minimal in recent years. Despite such benevolent attitudes towards the birds themselves, the destruction of their habitat continues. The full impact of tree planting around B in 1979 may not yet be apparent as the vegetation was still short in the early 1980s. Other areas have been ploughed in preparation for planting in 1980 and a substantial part of the low ground around D is scheduled for afforestation before 1985. With the failure of other pairs of golden eagles to become established on grouse moors, on the few remaining sheep walks, or in other heavily afforested areas, the species in southern Scotland may eventually be reduced to one regularly breeding pair.
SUMMARY (1)
(2)
(3)
(4)
After a long period of absence, golden eagles recolonised southwest Scotland in the early 1940s. By 1966 there were four regularly breeding pairs but in the early 1970s two pairs ceased to breed and a third pair that nested annually, rarely produced young. Initially broods of two were not infrequent and breeding success was good with an average of 0.49 young produced/occupied territory. In the last two decades this has fallen successively to 0.33 and then 0.14. Most failures were probably not pesticide-induced but could be attributed formerly to human interference and, latterly, to food shortage. The land use of the uplands of southwest Scotland has changed dramatically in the last 35 years from sheep farming to forestry. All four eagle areas experienced some afforestation, but the cessation of breeding in two areas was coincident with large-scale afforestation of the most productive land in their vicinity, the only open ground remaining being at high altitude. The contents of pellets showed that in recent years (1974/80) the eagles fed mainly on large mammals, lagomorphs and large birds, the latter chiefly in summer. The proportion of large birds in the diet in spring was related to breeding performance. Pellets from the one
136
(5)
M. Marquiss, D. A. Ratcliffe, R. Roxburgh
successful breeding territory contained the remains of animals associated with low ground and less large mammal remains than pellets from the other territories. Animals living in conifer plantations did not form a substantial part of the diet. Eagles probably benefited initially from afforestation because they were better protected from human interference. However, afforestation inevitably reduces the amount of habitat for eagles, and as areas of conifer plantations increase a further decline is anticipated.
ACKNOWLEDGEMENTS We thank the various landowners in southwest Scotland who gave us access to their ground to examine nests and to collect pellets. In particular we thank the Forestry Commission and various other forestry authorities for making available planting maps of the study area. For their contributions to the data on status and breeding success we are grateful to A. D. Watson, W. Murray, R. Nelson, R. Stokoe, E. Blezard and G. Hughes-Onslow. The contents of eagle eggs were analysed for organochlorines by the Government Chemist and Dr J. Bogan. Dr I. Newton, A. D. Watson, A. N. Lance and S. J. Petty gave helpful criticism of the manuscript.
REFERENCES Bloom, P. H. & Hawks, S. J. (1982). Food habits of nesting golden eagles in northeast California and northwest Nevada. Raptor Res., 16, 110-15. Brown, L. H. & Watson, A. (1964). The golden eagle in relation to its food supply. Ibis, 106, 78-100. Day, M. G. (1965). Identification of hair and feather remains in the gut and faeces of stoats and weasels. J. Zool., 148, 201-17. Dick, Rev. C. H. (1916). Highways and byways in Galloway and Carrick. London, Macmillan. Everett, M. J. (1971). The golden eagle survey in Scotland. Br. Birds, 64, 49-56. Gladstone, H. S. (1910). The birds of Dumj}'iesshire. Witherby, London. Hailer, H. (1982). Raumorganisation und Dynamik einer Population des Steinadlers Aquila ehrysaetas in den Zentralalpen. Orn. Beob., 79, 163-211. Harper, M. McL. (1896). Rambles in Galloway, 2nd edn. Dalbeattie, Fraser. Lockie, J. D. (1964). The breeding density of the golden eagle and fox in relation to food supply in Wester Ross, Scotland. Scot. Nat., 71, 66-77.
Golden eagles and land use in Scotland
137
Lockie, J. D. & Stephen, D. (1959). Eagles, lambs and land management on Lewis. J. Anita. Ecol., 28, 43-50. Lockie, J. D. & Ratcliffe, D. A. (1964). Insecticides and Scottish golden eagles. Br. Birds, 57, 89 102. Lockie, J. D., Ratcliffe, D. A. & Balharry, R. (1969). Breeding success and dieldrin contamination of golden eagles in West Scotland. J. appl. Ecol., 6, 381 9. Maxwell, Sir H. (1927). The golden eagle in Galloway. Scott. Nat., 164, 62. Murphy, J. R. (1977). Eagles and livestock--some management considerations. Proc. ICBP World Conj~ on birds of prey, Vienna, 1975, 307-14. Newton, I. (1979). Population ecology of raptors. Berkhamsted, T. & A. D. Poyser. Newton, I. & Bogan, J. (1974). Relationships between organochlorine residues, eggshell thinning and hatching success in British sparrowhawks. Nature, Lond., 249, 582-3. Olendorff, R. R. (1976). The food habits of North American golden eagles. A m. Midl. Nat., 95, 232-6. Paton, W. & Pike, O. G. (1929). The birds oJAyrshire. London, Witherby. Peakall, D. B. (1976). The peregrine falcon (Falco peregrinus) and pesticides. Can. Fld-Nat., 90, 301-7. Ratcliffe, D. A. (1967). Decrease in eggshell weight in certain birds of prey. Nature, Lond., 215, 208 10. Service, R. (1903). The diurnal and nocturnal raptorial birds of the Solway area. Trans. ,Dumj?ies & Galloway nat. Hist. and antiq. Soc., 17, 327. Spofford, W. R. (1964). The golden eagle in the Trans-Pecos and Edwards Plateau of Texas. Audubon Conserv. Rep., 1, 1 47. Tjernberg, M. (1981). Diet of the golden eagle Aquila chrysaetos during the breeding season in Sweden. Holarct. Ecol., 4, 12 19. Tjernberg, M. (1983). Prey abundance and reproductive success of the golden eagle, Aquila chrysaetos in Sweden. Holarct. Ecol., 6, 17 23.
APPENDIX
1
Sheep Goat Deer sp. Fox Vulpes vuipes Mink Mustela vison Stoat Mustela erminea or weasel M. nivalis Rabbit Oryctolagus cuniculus Blue hare Lepus timidus Brown hare L. capensis Hare (sp.?) Lagomorph (sp.?) Water vole Arvicola amphibius Short-tailed field vole Microtus agrestis
Pellet contents’
-
-2l-3-3-312
-2--2-l-l--
4 3
6 6
10 10
1
-
1
3 5
-
1
2 5
B
1 5 ~ 515236-128-222 1----_-_---11-l
-
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4 1 5 9 1 23--l_l3_4____~
A
1 4
-
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5 8
3 6
1
5
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3
6 28
-
4 15
c
1 2
-
-
1 10
Territory and season
1
7 25
-
5 22
6
4
-
4 2
The Contents of Eagle Pellets collected from Four Territories in Southern Scotland 19741980 (The numbers in columns are the number of pellets in which the item occurred)
-
3 38
14
1--
12 -
D
2 4
2
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8
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a
In addition, most pellets examined contained vegetation, and some contained grit, insect chitin or eggshell. These remains were probably taken incidentally, either as the contents of the gut and oviduct of prey, or sticking to food as it was eaten.
Hedgehog Erinaceus europaeus Mole Talpa europaea Mallard Anas platyrhynchus Duck (sp. ?) Kestrel Falco tinnunculus Red grouse Lagopus lagopus Black grouse Lyrurus tetrix Pheasant Phasianus colchicus Galliforme (sp. ?) Lapwing Vanellus vanellus Curlew Numenius arquata Wader (sp. ?) Gull (sp. ?) Pigeon (mainly 'homing' pigeons) Short-eared owl Asio flammeus Skylark Alauda arvensis Raven Corvus corax Carrion crow C. corone Fieldfare Turdus pilaris Wheatear Oenanthe oenanthe Meadow pipit Anthus pratensis Passerine (sp. ?) Number of pellets examined
E-
e~
E"
140
M. Marquiss, D. A. Ratcl$e,
APPENDIX
R. Roxburgh
II
The Contents
of Eagle Pellets Collected from Two Territories in Southern Scotland 1957-69” (The numbers in columns are the number of pellets in which the item occurred) Territory (years) and season Pellet contents (I&) Spring
Sheep Blue hare Lepus timidus Buzzard Buteo buteo Red grouse Lugopus lagopus Black grouse Lyrurus tetrix Pheasant Phasianus colchicus Pigeon (sp.?) Number of pellets examined
2 7 7
11
D (1963, ‘66 and ‘69) Spring
2 3 1 8 1 1 1 11
” A summary of data from pellet analyses by the late E. Blezard.