Impacts of ski-development on ptarmigan (Lagopus mutus) at Cairn Gorm, Scotland

Biological Conservation 116 (2004) 267–275 www.elsevier.com/locate/biocon

Impacts of ski-development on ptarmigan (Lagopus mutus) at Cairn Gorm, Scotland Adam Watson*, Robert Moss Centre for Ecology and Hydrology, Banchory AB31 4BW, Scotland, UK Received 16 April 2002; received in revised form 27 March 2003; accepted 21 April 2003

Abstract This paper reports adverse impacts on numbers and breeding success of ptarmigan (Lagopus mutus) in 1967–1996 at a ski area in the Cairngorms massif, where ptarmigan normally show 10-year population cycles. An influx of carrion crows (Corvus corone), generalist predators, followed the development. On the most developed area near the main car park, ptarmigan occurred at high density but then lost nests to frequent crows, reared abnormally few broods, died flying into ski-lift wires and declined until none bred for many summers. On a nearby higher area with fewer wires, ptarmigan lost nests to frequent crows and reared abnormally few broods, but seldom died on wires. Adult numbers declined and then became unusually steady for over two decades, with no significant cycle. On a third area further from the car park, ptarmigan lost fewer nests to the less frequent crows but bred more poorly than in the massif’s centre, and showed cycles of lower amplitude than there. On a fourth area yet further away, with few or no crows, ptarmigan bred as well as in the massif’s centre and showed cycles of the same amplitude as there. # 2003 Elsevier Ltd. All rights reserved. Keywords: Tourism; Human impact; Ski development; Increased generalist predators; Crows; Lagopus mutus; Localised extirpation; Conserve population cycles

1. Introduction Ptarmigan occur over a vast circumpolar range (Holder and Montgomerie, 1993). In the fragmented range of southern races, however, tourist developments in Japan, the Alps and Pyrenees have caused habitat loss and mortality on ski-lift wires (Storch, 2000). Ptarmigan abound on Scottish arctic–alpine land, the biggest continuous block of which is in the Cairngorms massif. Here also is Scotland’s busiest ski centre and montane tourist facility at Cairn Gorm, 100 km west of Aberdeen. In a study of direct tourist disturbance on ptarmigan at Cairn Gorm, Watson (1979) found similar density and breeding success on a much-visited area and on a rarely visited one. In 1981, however, he realised that breeding on both had been abnormally poor for years, associated with unexpectedly wide-ranging egg-robbing by crows. Here we report data for 1967–1996 from these study areas and from less affected ground. The study * Corresponding author. Fax: +44-1330-82-3303. E-mail address: [email protected] (A. Watson). 0006-3207/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0006-3207(03)00197-6

involved human-induced mortality from nest predation by crows, and from adults flying into ski wires and snow fences. The centre of the massif had very few crows and no wires. Hence we could compare numbers with or without the abnormal mortality, from an area near the car park, through three successively more distant and less affected areas, to a remote pristine area. Ptarmigan show
10-year cycles in several countries (Holder and Montgomerie, 1993; Moss and Watson, 2001) including the Cairngorms (Watson et al., 1998). The present study provided an opportunity to find whether the abnormal mortality affected not only the birds’ abundance but also their natural cycles. When work began in 1967, numbers on the north side of the massif including Cairn Gorm had already fallen from a cyclic peak in 1962–1964 (Watson et al., 1998).

2. Study areas Scottish ski centres are in sheltered corries where windblown snow accumulates. Hence topography determines their siting, and thus areas for studying their

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impact (Fig. 1). Area A (Table 1) in such a corrie had Cairn Gorm’s first lifts in 1961 and later in the development the most lifts and snow fences. In contrast, the adjacent E had no lift or snow fence till 1970. Its south slope, along with N, formed the north section of a plateau from Cairn Gorm to Ben Macdui, while S formed the south section. We used principal component analysis to examine the dynamics of ptarmigan populations on S, N, and E simultaneously. Although counts were also done before the development on land that we later identified as A, E, and N after development, they were not mapped before then so we refer to this composite northern area as N*. Area DS, a big pristine tract in the centre of the massif (Fig. 1, Table 1), included D at 8–11 km from the car park, where ptarmigan were studied in more detail (Watson et al., 1998). Development began in 1960 with a road up to 640 m. In 1961 a chairlift and snow fences were built on A, and later more facilities were added there and elsewhere, including tows on E from 1970 onwards (Figs. 1 and 2). Skiers and walkers increased greatly on A, and to a successively lesser extent on E, N, and S (references in

Watson, 1991). Cairn Gorm attracted the most skiers of any Scottish ski centre (65 000–391 000 skier-days per winter), and the most summer visitors of any Scottish montane facility, with 250 000 cars per summer at the car park and 48 000–105 000 people per summer on the chairlift (Highlands and Islands Enterprise, 1995). Bared ground increased on A to a peak in 1968 and then declined, and rose far less on E (mainly near A), but almost all bared ground had re-vegetated by 1990 (Watson, 1985, 1994). Bedrock, climate, soils, vegetation, the bird and its food, predators and competitors have been described (Watson, 1965; Watson et al., 1998 and references). Territorial ptarmigan occur at high density only on Table 1 Study areas and types of ptarmigan data Area

Size (ha)

Altitude (m)a

Total counts (years)

Walk data (years)

Breeding success (years)

A E N S N* DSd Dd

73b 271c 356 526 700 6500 131

750–1100 850–1245 900–1180 960–1290 750–1245 760–1310 840–1150

67–96 67–96 – – – – 51–77

– 67–96 67–96 67–96 51–57 43–96 –

67–96 67–96 67–96 67–96 51–57 47–57 51–77

Annual effort (km per year) for walk data had a median of 90 and range 46–291 on N, 53 and 9–279 on S, 41 and 6–89 on N*, and 16 and 6–87 on E (no data for 10 years, so n=20). The coefficient of variation for an N or S walk datum (4  datum/boot-strapped 95% confidence band) was typically 30%. a Proportions of ground <1000 m on A, E, N, and S were 88, 8, 10, and 2% b Includes Watson’s (1979) ‘disturbed’ area of 60 ha, plus an adjacent disturbed 13 ha. c Includes the 45-ha south-east part of E, Watson’s (1979) ‘undisturbed’ rarely-visited area. d From Watson et al. (1998).

Fig. 1. Study areas. Straight lines in or north of areas A and E show ski lifts in 1991–1996, apart from a tow on A’s north-east boundary (dotted line). Grid lines marked 00 are junctions where Ordnance Survey squares, each denoted by a pair of letters, adjoin. The northwest quadrant is NH, north-east NJ, south-west NN and south-east NO. A large area of ptarmigan ground lying to the west of area DS is indicated by unenclosed boundaries. Hatching shows cliffs on boundaries. The 760-m contour, broadly the lower limit of nesting ptarmigan, was the boundary along the east and south of DS and the north corner of A, and outside study areas is shown as a dashed line.

Fig. 2. Cumulative lengths (km) of ski lifts on ptarmigan summer ground. Total wire length was twice the cumulative length, as each pylon carried a wire on either side, one going up and one down. ‘Other’ is outside study areas. In 1991–1996, A had 14.9 km of wire per km2 and E only 1.1, while A had 10.1 km of snow fences per km2 and E only 2.0.

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ground with much heath for food interspersed with boulders for cover. Such heath covered most of A and D, less of E, yet less of N, and little of S (Table 2). Being higher and more exposed than A or D, areas E, N, and S had, in that order, more snowfields, boulder fields, continuous grassland, and exposed ground dominated by bare soil. These habitats supported few or no territorial ptarmigan, for the last had very sparse heath and the others none. Low density at high altitude is typical in Scotland, the greatest densities being at lower altitudes where there is more heath (Watson, 1965, 1979). During intensive study on D, foxes (Vulpes vulpes) and golden eagles (Aquila chrysaetos) caused almost all deaths of adult ptarmigan and many of big chicks, and foxes almost all of the 20% loss of nests to predation (Watson, 1965). Chick loss, mostly soon after hatching and from non-specific causes linked to maternal nutrition, was the main factor affecting breeding success (Watson, 1965; Moss and Watson, 1984). Peregrines (Falco peregrinus) and stoats (Mustela erminea) killed a very few adults in the present study. Observers saw no fox or stoat on A, E, N, or S, but they did occur, as evident from tracks in snow, faeces, and prey remains, though fox sign was far scarcer than on D. Eagles and peregrines hunted all areas before development, but after 1965 no peregrine was seen on A, and after 1974 no eagle on A or E. No ptarmigan was shot in nearly all years on D (Watson et al., 1998) or after 1947 on other areas. Crows (Corvus corone) in Scotland inhabit low ground, seldom visiting high hills. After the development, however, they often frequented car parks, ski area, and beyond (Watson, 1979, 1996). So did gulls, nearly all black-headed (Larus ridibundus) but a few common (Larus canus). Observers often saw crows and gulls eating insects and tourists’ food scraps, and occasionally crows carrying ptarmigan eggs or hunting for nests, and twice crows taking chicks on area E. Common gulls on other hills took eggs, but there was no evidence that any gulls took eggs in this study. Table 2 Percentages of quadrats with different habitat features on five areas HeathOther Area Number of Damageda Boulder Snowb or water dominated field quadrats A E N S D

77c 273c 346c 522c 50d

12 1 0 0 0

0 2 0 10 2

0 0 0 1 0

61 36 5 0 52

27 61 95 89 44

a Bared by construction work or trampling, with an extra 18% of A partly damaged. b Proportion of ground covered by snowfields in May–June was S > N > E > D > A. c In August 1979 (Pitkin, 1979). d In July 1982 (Moss and Watson, 1984), 25 on D and 25 from similar adjacent ‘west Derry Cairngorm’.

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3. Methods 3.1. Bird counts On area A in 1967–1980 and on D in all years of its study, AW used dogs to count all adult ptarmigan in spring (late April–mid May), supplemented by a ‘territory census’ to check whether territories lay mainly within study areas (Watson, 1965). On A and E in spring 1981–1996, dispersed observers watched at the dawn or dusk chorus, to find numbers and locations of crowing cocks (Watson and O’Hare, 1979). Subsequent searches of these locations with dogs and territory census showed hen spring numbers. On A, D, and E, he counted all adults and well-grown young in late summer (late July–early August), using dogs. Breeding success, the number of young reared per hen, had two components: the proportion of hens with broods, and ‘brood size’ or number of young per brood. Areas DS, N, and S were too big for total counts, so we used adult numbers seen per 10 km of transect walks in May–August (Watson, 1965). These ‘walk data’ are closely correlated with spring density from dog counts (Watson et al., 1998, 2000, and unpublished data on N and S). We calculated breeding success on N and S from late-summer dog counts on at least half of each area. Crows and gulls are conspicuous on arctic–alpine land (Watson, 1979). From 1943, Watson (1996 and unpublished data) noted numbers seen per day (6–8 h) in May–early October. These provided annual indices of abundance on the whole plateau, numbers seen on each study area being too low and variable for valid annual indices. In most summers he found ptarmigan eggshells with signs of predation by crows. He followed the fate of some nests, though insufficient to estimate annual loss, and could not measure chick predation. Adult deaths were noted from carcasses or remains found during dog counts of live birds, supplemented in 1981–1996 by extra searches under wires, but the latter did not result in more deaths being found. Total search effort per km2 was A=E > N > S. Tests, with carcasses placed blind by a colleague in summer, showed that AW and his dog found most of them in two counts during the next fortnight (Watson et al., 1998). As he did no counts in winter, however, when feathers on snow soon blow away (Watson, 1965), most deaths went unrecorded (Appendix) and we use records to indicate the main causes of death, not to measure mortality rates. 3.2. Analyses To detect cycles we calculated autocorrelation coefficients (Chatfield, 1984; Watson et al., 1998). Adult densities, walk data, breeding success, and crow and

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gull numbers had more values small than big, so for analyses of variance and covariance, principal component analysis, regressions and correlation coefficients (SAS Institute, 1996), we approximately normalised data by natural logarithms (+ 0.1 if there were zeros). To test whether breeding success differed between periods or areas, we used Poisson or logistic regressions (SAS GENMOD procedure with log and logit links respectively, corrected for overdispersion as necessary), and estimated the significance of each effect after controlling for all others (SAS type 3 analyses). Analyses of year-to-year change in adult numbers rested on Nt ¼ Nt1 ð1 þ Bt1 ÞSt1 ;

ð1Þ

Fig. 3. Number of adults seen per 10 km of transect walks in summer. No pair was seen on A in summer 1979–1995. To make all data comparable for this Figure, we converted adult summer densities on A and E to adults per 10 km, by a no-intercept regression of walk data on density at E (slope=1.360.06, R2=0.96, n=20).

where Nt is adult number in summer t, Bt1 the number of young reared per adult in summer t1, and St1 the number of adults in summer t divided by the number in summer t1. We used summer counts because we had longer runs of them than of spring counts, which they closely resembled (Appendix). An even sex ratio was assumed in adults and young. Rt measures population growth: Rt1 ¼loge Nt  loge Nt1 ¼ loge ð1 þ Bt1 Þ þ loge St1 : ð2Þ

In closed or sufficiently large ptarmigan populations, Bt usually varies more than St (Bergerud, 1988; Moss and Watson, 2001), and Rt is positively correlated with loge(1+Bt). Movement, however, can be substantial (Watson et al., 1998; Martin et al., 2000). We therefore calculated Rt and also loge(1+Bt) for areas E, N, and S separately, and used principal component analysis to find linear dependencies among these two parameters for all three areas simultaneously.

4. Results 4.1. Adult density over the whole 30 years Adult density at DS as indicated by walk data, showed significant
10-year periodicity (Figs. 3 and 4), the unperturbed pattern for the massif (Watson et al., 1998, 2000). Density at N and S, though lower, showed a similar pattern. All three cycles on S had bigger amplitude than on N, density on S being below that on N in each trough, and above it at each peak. Peaks at DS, N, and S coincided in 1971, but 1981 and 1991 peaks at N and S came a year later than at DS. On E, the 1971 peak occurred also, but no clear peak followed, and density became fairly steady for over 20 years. On A, no pair was seen in the 17 summers 1979– 1995.

Fig. 4. Autocorrelation coefficients showing 10-year cycles at N, S and DS, but not at E.

4.2. Adult density before versus after development An analysis of variance (R2=0.94) tested whether walk data (Table 3, Fig. 3) differed among areas, and between ‘eras’ (before or after development) within areas. Two of the areas were DS and S. The third, N*/ N, we treated as one, with data from N* (i.e. N, E, and A combined) for the era before development, and from N for the era after. Explanatory variables were area, year nested in era, and area  era interaction. All effects were significant, owing to variation among years within eras (F17,34=22.77, P40.0001), higher mean density on DS than on N*/N or S (F2,34=54.29, P40.0001), and change in density between eras (F2,34=9.22, P=0.0006) due largely to a decrease on N*/N (Table 3). Hence mean density on DS exceeded that on N or S before development, probably because much of N and S was poor habitat (Table 2). After development, mean density did not change significantly on DS or S. On N, however, it became lower than on N* before development, suggesting a decline on N after development (Table 3).

A. Watson, R. Moss / Biological Conservation 116 (2004) 267–275 Table 3 Mean number of adults per 10 km (walk data SEM) and mean breeding success (young per henSEM) before development (1951– 1957), and after it (1967–1978 for numbers and 1967–1977 for breeding) Area

N*/N S DSa

Before

After

Adults

Young per hen

Adults

Young per hen

11.33.2 9.63.1 19.45.7

1.020.22 0.980.25 0.760.11

6.91.5 12.43.4 26.44.8

0.300.11 0.680.27 0.950.25

The mean number of adults per 10 km on N* (A, E, and N) before development exceeded that on N after development. This may have been due to pre-development densities on A and E exceeding those on N because A and E had higher proportions of heath (Table 2). However, N had a higher proportion of heath than S, yet its mean ptarmigan density after development was only about half that of S. Hence mean density on N was probably lower after than before development. Similar reasoning applies to lower breeding success on N after development than on N* before development. a Breeding success data from D (within DS).

4.3. Breeding success before versus after development Because we had insufficient data on breeding success over the very large DS, we used data from D within DS, to compare with N*/N and S (Table 3). A generalised linear model explained 67% of the total deviance (499.3) in terms of study area (F2,32=1.83, P=0.18), year nested in era (F16,32=2.23, P=0.026), and area  era interaction (F2,32=5.43, P=0.009). Comparisons within each area showed that breeding success at the pristine D after development did not differ significantly from that beforehand (F1,16=0.64), and likewise at S (F1,16=2.66). In contrast, breeding success at N* before development exceeded that at N afterwards (F1,16=9.56, P=0.007), suggesting poorer breeding on N after development. 4.4. Breeding success on different areas after development For 1967–96 there were data on breeding success at A, E, N, and S. This allowed comparisons at different distances from the development (Table 4), on a smaller geographical scale than in the foregoing section. A generalised linear model explained 70% of the total deviance (1078) in terms of year (F29,70=4.90, Table 4 Mean values for measures of breeding success ( SEM) after development (1967–1996) Area

Young per hen

Brood hensa

Brood size

A E N S

0.330.18 0.340.07 0.530.12 1.130.17

0.180.09 0.170.03 0.250.04 0.500.04

1.800.37 2.050.21 2.330.44 2.130.22

Hens were on A in only 13 summers; otherwise, sample size was 30 summers, except for brood size at A (6), E (22), and N (25). a Proportion with a brood.

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P40.0001), and study area (F3,70=8.59, P40.0001). In paired annual comparisons, breeding success at S exceeded that at N (w21=15.33, P40.0001), and that at E (w21=17.24, P40.0001). Between E and N it did not differ significantly (w21=1.69, P=0.19). The analysis excluded A because it had no hens in many summers and few in most others, but mean breeding success there resembled that on E. Analysis of the proportion of hens with broods explained 72% of the total deviance (387.0). Again, this showed effects of year (F29,70=4.89, P40.0001), and area (F3,70=23.0, P40.0001). The proportion of hens with broods at S resembled that in unperturbed populations (Table 4, and Watson et al., 1998), and exceeded that at N (w21=39.29, P40.0001), while at N it exceeded that at E (w21=5.98, P=0.015). The analysis excluded A because it had small samples, but the mean proportion with broods resembled that on E. Analysis of brood size showed an effect of year (w21=2.80, P=0.0007), but not of area (F3,50=0.93, P=0.92). Hence birds that successfully passed the nest and early-chick stages reared similar-sized broods on E, N, and S, and differences in breeding success among these areas arose largely from differences in the proportions of hens with broods. 4.5. Crow numbers and ptarmigan breeding success after development Watson (1996 and unpublished data) saw no crows and very few gulls on the plateau in 1943–1966, but saw crows each summer in 1967–1995, and gulls each summer in 1969–1994. He saw more crows and gulls on land visited by many tourists (A > E > N > S), and very few on DS. Crows took all six ptarmigan nests that he found on A and all 10 found on E. On N they took four out of seven nests found, and two out of six on S. The proportion taken on the undeveloped N+S exceeded that on the developed A+E (Fisher exact P=0.0036). Each summer in 1967–1996 he saw shells from eggs eaten by crows, except in 1996 when very few crows visited high land following increased killing of breeding crows in the valley from 1992 onwards. In 1967–1995 he found shells on A in each summer with ptarmigan hens present, and in all summers on E. With similar observer effort per km2 during the nesting season, he found shells more frequently on N (29/30 summers) than on S (2/30, Fisher exact P < 0.0001). The annual number of crows seen per day on the plateau in 1967–1996 was negatively related to breeding success at E and at N (Table 5), but not at S. Breeding success at N was correlated with that at E (r=0.87, P 40.0001), but at S not with that at N or E (r=0.15 and 0.12). An analysis of covariance (R2=0.38) examined the relationships of crows to breeding success at E, N, and S. Breeding success was related to study area

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Table 5 Pearson correlations between crow numbers on Cairn Gorm plateau and ptarmigan breeding success in 1967–1996

Young per hen Brood hensa Brood size

Area E

Area N

Area S

0.63*** 0.70**** 0.01

0.44* 0.49** 0.07

0.13 0.00 0.16

*P <0.05. **P <0.01. ***P <0.001. ****P 40.0001. All measures are loge(measure+0.1) except brood size [loge(measure)], and sample size was 30 summers, except for brood size at E (22) and N (25). a Proportion with a brood.

(F2,84=7.63, P=0.0009), crow numbers (F1,84=12.8, P=0.0006), and the latter’s interaction with area (F2,84=5.87, P=0.004). A similar analysis for the proportion of hens with broods (R2=0.51) also showed effects of area (F2,84=16.7, P 40.0001), crow numbers (F1,84=23.3, P 40.0001), and the latter’s interaction with area (F1,84=6.21, P=0.003). For brood size, however, neither area nor crow numbers had significant explanatory value. In short, breeding success was determined largely by the proportion of hens with broods (foregoing section), which at S resembled that of unperturbed populations (Table 4, and Watson et al., 1998). The proportion of hens with broods on E and on N was inversely related to crow numbers (Fig. 5). Annual crow and gull numbers were correlated (r=0.45, P=0.012). Analyses of breeding success as above, but with gulls instead of crows as explanatory variables, showed significant effects for gulls, though weaker than for crows. Stepwise analyses with crow and gull numbers as potential explanatory variables selected crows but not gulls. Hence crows were more closely associated than gulls with poor breeding. 4.6. Population dynamics on the different areas Population growth rates on E, N, and S were unlikely to be independent, because young from one area very probably recruited to another. Furthermore, population growth on each area was not significantly correlated with breeding success there (Table 6), which does not fit the model for a closed population [Eq. (1)]. Therefore we used principal component analysis to study population growth and breeding success on E, N, and S simultaneously (Table 6). Weightings or eigenvectors for the first principal component (PC1) showed densities on E, N, and S rising together, as shown by positive eigenvectors for population growth. Unexpectedly, the increases on E and on N followed lower breeding success there, shown by negative eigenvectors for breeding success. This differed from the positive correlation between breeding success and change in numbers at the unperturbed D

Fig. 5. The annual proportion of hens with broods at E and N against the mean number of crows seen per day on the plateau. The abscissa is the residual from a regression of the proportion of hens with broods at E (or N) upon the proportion at S, with each expressed as loge(proportion+0.1). This controls for annual variations in the proportion of hens with broods, due to factors other than crows. Table 6 (a) Correlations (Pearson—top right, Spearman—bottom left) in 1973–1996 between population growth (Rt1) at areas E, N, and S, and between breeding success (number of young reared per adult [loge(1+ Bt1)]) there, and between population growth and breeding success there, and (b) principal component analysis of the same variables, with the second row showing the proportion of the variance accounted fora a)

Growth at S

Growth at N

Growth Young Young per at E per adult adult at N at S

Growth at S 0.79**** 0.25 Growth at N 0.72**** 0.17 Growth at E 0.21 0.19 Young/adult at S 0.39 0.40 0.19 Young/adult at N 0.30 0.31 0.41 Young/adult at E 0.22 0.26 0.15 (b)

PC1

Eigenvalue 2.20 Variance a/c for 0.35 Eigenvectors: Growth at E 0.37 Growth at N 0.46 Growth at S 0.45 Young/adult at E 0.44 Young/adult at N 0.49 Young/adult at S 0.15

0.28 0.29 0.11 0.01 0.02

0.12 0.16 0.31 0.18

Young per adult at E 0.03 0.09 0.18 0.01 0.90****

0.94****

PC2 1.85 0.31 0.02 0.42 0.46 0.48 0.48 0.39

****P 40.0001. a Densities on E and N fell in 1967–1972 and transient effects from these transitional years might have biased the results, so Table 6 covers 1973–1996, though data for 1967–1996 (not shown) led to similar conclusions.

(Watson et al., 1998). A generalised linear model (total deviance 38.9, residual deviance 23.4) showed that PC1 scores were related to crow numbers (w21=5.33, P=0.02), and to population growth on DS (w21=3.93, P=0.048). Obviously, higher crow numbers could not cause higher population growth (Fig. 6). Possibly, more crows visited the plateau in years of rising ptarmigan

A. Watson, R. Moss / Biological Conservation 116 (2004) 267–275

numbers there, so eating higher proportions of eggs and chicks on E and on N. This may explain why annual breeding success at E was correlated with that at N, but at neither E nor N with that at S (Table 6). The association between PC1 scores and population growth at DS (Fig. 6) fits the idea that recruits from DS partly fuelled population growth on E, N, and S. Eigenvectors for PC2 showed densities on N and S rising together when adults on E, N, and S bred well. This resembled the unperturbed population on D (Watson et al., 1998). However, E differed in that density there did not rise after good breeding on E, N, and S. A generalised linear model gave no evidence of PC2 being influenced by crow numbers, or by population growth at DS. So, PC2 seemed to reflect a largely unperturbed population, whereas PC1 showed perturbations associated with crow predation. The PC2 eigenvector for population growth at E was close to zero, showing that young reared locally on E, N, or S probably did not

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cause E’s population growth. The low PC1 eigenvector for breeding success on S presumably occurred because crows took few eggs or chicks there. Eigenvectors for population growth and breeding success at N contributed substantially to PC1 and PC2, so dynamics there showed characteristics of both perturbed and unperturbed populations. 4.7. Deaths and movement after development In summer 1967–1996, 40 winter deaths (mid October to mid April, the season of death being told from the plumage) were found on N and 26 on S, all by predators and 89% by foxes. Summer adult deaths found on N and S were few, five and three respectively (six by predators and two dead on the nest after blizzards), on A three by wires and one by a predator, and on E one by a wire. Area A had more ski wires and snow fences than the much larger E (Figs. 1 and 2). In spring–summer 1971– 1996, when both areas had wires, 137 deaths by wires in winter were found on A and seven on E. No deaths by predators were found on A, but three on E. A generalized linear model explained 75% of the total deviance (210.5) in the number of winter wire deaths recorded on A and E, in terms of wire length (type 3 w21=14.5, P=0.0001) and study area (type 3 w21=8.8, P=0.003). Neither fieldwork method (count of live birds with dogs or count with dogs plus extra searches under wires) nor year had significant explanatory value. A mean of 1.10 (95% CL 0.94–1.29) deaths was found per km of wire per year on A, but only 0.19 (95% CL 0.09–0.40) on E. We multiplied these by mean wire lengths over the 30 study years (4.6 km on A and 1.2 km on E). This showed that wires on A killed about 20-fold more birds than on the larger E. Despite no live adults being seen on A in spring or summer 1981–1995, birds must have moved onto it in winter. In 1981–1995, dusk watches in each of nine winters revealed 2–3 crowing cocks, and in all winters small flocks occurred and birds died on wires. 4.8. Direct disturbance on ptarmigan

Fig. 6. Scores for principal component 1 (PC1 in Table 6) against crow numbers (top) and population growth on DS (bottom). Score values are residuals from regressions of PC1 on Rt1(DS) and loge[(crow numbers+0.1) in year t1] respectively. In the year with the highest score, 1981, numbers peaked at N and S, but at DS had just declined from a 1980 peak.

In summer, many people visited A (Watson, 1979, 1991, 1994, and unpublished), where breeding was very poor. Despite the south-east part of E (Table 1, notes) being rarely visited, breeding was also very poor (no young reared in 18 of the 27 summers with hens). Summer disturbance on A declined greatly after 1970 as new roads and paths channelled walkers, and a major fall in summer visitors to Cairn Gorm that began between 1972 and 1973 continued into the 90s (Watson, 1994), yet ptarmigan numbers did not recover (Fig. 3). It seems that direct disturbance was not the main factor affecting numbers or breeding success.

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5. Discussion 5.1. Changes in breeding success and numbers after ski-development After the development, ptarmigan bred poorly on and near the development. We infer that crows attracted by the development reduced breeding success up to 4 km from the car park. Other generalist predators, such as gulls, might have reduced breeding success, but the evidence points strongly to crows. Despite local extirpation on A for 17 summers, ptarmigan died on its ski-lift wires in every winter. It had much heath-dominated ground (Table 2), good ptarmigan habitat, and held high density in the development’s early years (Appendix). We suggest that it became a population sink because of many deaths on wires and very poor breeding due to crows. Population dynamics at E and N were intermediate between local extirpation at A and apparently unperturbed cycles at S and DS. Wires on E killed few adults, but birds nonetheless bred very poorly (Table 4). A constant population on E, given such poor breeding, would have required a mean adult survival rate of 0.85 (95% C.L. 0.81–0.91), far higher than recorded elsewhere. Data from a nearby massif (S. Rae, pers. comm.) showed a hen survival rate of 0.48 per year (95% CL 0.29–0.66), consistent with other studies (Holder and Montgomerie, 1993). Many young of Lagopus species emigrate and breed away from where they hatched (e.g. Holder and Montgomerie, 1993; Martin et al., 2000). We suggest that immigration sustained numbers on E, and that, like A, it became a population sink. The 20-fold more winter deaths recorded on A than on E suggest that A became a deeper sink. 5.2. Prey cycles, cycle damping and conservation Inferences from unreplicated large-scale field experiments are convincing if effects are big and at odds with a well documented reference situation (Raffaelli and Moller, 2000). An increase in generalist predators, crows and less importantly gulls, was followed by a strongly damped ptarmigan cycle on E and weaker damping on N. After showing the widespread 1971 peak, the trajectory at E (Fig. 3) differed so greatly from that of ptarmigan far from the development that it met the criteria of Raffaelli and Moller. We regard this as an unreplicated natural experiment (on a scale impracticably large for designed experiments) that resulted in cycle damping. Similarly, an increase in generalist raptors damped a cycle in red grouse in south-west Scotland (Thirgood et al., 2000). Their natural experiment and the Cairn Gorm one fit the idea that prey cycles are damped (and at Cairn Gorm also local extirpation) when increased

generalist predators kill a higher proportion of prey. Similarly, forest grouse and rodents show cycles in north Fennoscandia, but not in the more agricultural south where increased generalist predators are thought to suppress them (Hanski and Henttonen, 2002). An interesting aspect of conservation arises. Much conservation work tries to increase numbers or distribution of a species. The Cairn Gorm case raises the question whether man should also conserve demographic patterns such as cycles of full natural amplitude. Cycles remain a focal problem in ecology (Moss and Watson, 2001) and their loss would diminish the richness of nature and thereby mankind. 5.3. Spatial scale Heavier egg predation occurred on N, at 2–4 km from the main car park, than on S at 3–6.5 km. Crows often travelled 2–3 km in one flight. At dawn they flew up from woodland roosts in the valley and down at dusk (Watson, 1996), the distance to A being 2.5 km and 5 km to S. Conservation of unperturbed ptarmigan populations may therefore require big areas more than 5 km from any development that attracts increased generalist predators such as crows.

Acknowledgements D. Gowans, D. Holland, and J. and M. Porter helped with searches of wires in 1981 and J. Porter gave other information on deaths at wires and fences. Z. Bhatia, D. Morris, L. Rankin, K. Scott and A.C. Watson helped with dawn or dusk watches. P. Rothery advised on statistics, P. J. Bacon, R. Boonstra and N. Aebischer commented on the manuscript, J. Watson helped with Fig. 1, and anonymous referees gave valuable comment.

Appendix. Adult counts and deaths Total counts of adults on A, E and D showed generally good agreement between spring and summer (Table A1). Numbers sometimes changed between spring and summer, but numbers in summer closely resembled either those in the previous spring or those in the next spring. Hence trajectories of spring or summer counts were very similar. Many more birds died than were recorded in our searches. Jo Porter (pers. comm.), who worked at Cairn Gorm in 1967–1979 bar 1970, saw about one killed on wires per winter week (about five times as many as we recorded) and one per summer month. Snow fences were not searched for the present study and the dogs avoided them because they restricted movement. However, Jo Porter saw several deaths at fences on A, and during

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weekly visits in the 11 winters 1990–2001 he found at least six killed per winter on the same 2 km of fencing, indicating substantial loss. Table A1 Number of adults (cocks, hens) in spring and summer

Area A

Area Da

Area E

Year Spring Summer Spring Summer Year Spring Summer 1967 23,19 1968 10,7 1969 10,6 1970 3,2 1971 8,3 1972 2,1 1973 2,2 1974 2,2 1975 3,2 1976 2,1 1977 2,1 1978 1,1 1979 1,0 1980 1,0 1981 0,0 1982 0,0 1983 0,0 1984 0,0 1985 0,0 1986 0,0 1987 0,0 1988 0,0 1989 0,0 1990 0,0 1991 0,0 1992 0,0 1993 0,0 1994 0,0 1995 0,0 1996 1,1

9,6 8,6 2,2 2,3 1,1 3,1 2,2 2,2 3,2 2,1 1,1 1,1 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,1

– – – – – – – – – – – – – – 11,11 11,10 10,9 10,10 9,9 8,8 9,10 9,9 8,8 11,11 11,10 11,10 9,9 9,9 10,11 6,6

28,22 20,18 24,14 12,11 25,22 15,12 10,10 10,10 8,8 8,7 9,8 9,9 9,9 10,10 10,11 10,10 9,9 9,10 9,9 8,8 9,10 9,9 9,8 10,11 9,10 8,10 9,9 9,9 11,11 6,6

1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1968 1971 1973 1974 1975 1976 1977

29,20 23,13 22,12 21,11 18,9 14,8 11,5 7,4 7,8 14,11 26,26 24,26 24,24 26,24 47,36 29,18 25,19 13,8 5,5 7,8

28,15 21,11 19,11 20,10 17,10 12,5 10,6 7,4 7,7 14,11 21,21 22,24 23,23 25,23 42,32 27,16 13,9 10,6 5,4 7,8

a In missing years in 1963–72, summer counts were samples, not covering the whole area.

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