UPLANDS - CAPERCAILLIE DECLINES |
TABLE 1
Mean (SE) values of capercaillie breeding success and indices of female density and percentage annual female decline across six Scottish regions between 1991 and 2009
Region Strathspey
Aberdeenshire Perthshire Moray
Easter Ross Argyll
Forests Chicks per female Broods size 8 6 6 2 3 1
0.86 (0.14) 0.69 (0.18) 0.37 (0.09) 0.55 (0.21) 0.47 (0.18) 0.67 (0.36)
2.34 (0.13) 1.88 (0.17) 1.87 (0.18) 2.59 (0.42) 1.94 (0.28) 1.92 (0.44)
Broods per female 0.41 (0.06) 0.42 (0.09) 0.21 (0.06) 0.22 (0.11) 0.26 (0.12) 0.39 (0.20)
Females km-² % annual decline 1.92 (0.80) 1.62 (0.80) 0.94 (0.48) 0.77 (0.88) 0.66 (0.52) 3.19 (3.56)*
-1.3 (0.9) -13.0 (1.3) -16.4 (2.0) -16.2 (2.8) -8.8 (4.3) -23.0 (6.5)
*High density attributed to birds being confined to four small islands (combined area 1.9 km²) in the middle of a large lake (Loch Lomond).
Argyll towards the south, to Morangie near Nairn in Ross-shire to the north. We do this in July/August to estimate breeding success and simultaneously to give an index of adult densities. In 1995 and again in 2009, we estimated predator levels within these forests by counting crows and raptors and recording scats of fox and pine marten along transects. In 1995 and in 2011 we also measured forest ground vegetation composition and height within the same forests. We then analysed the bird, habitat and predator datasets, together with weather records from the nearest weather station to each forest for breeding seasons between 1991 and 2009. Annual breeding success from 26 Scottish forests surveyed between 1991 and
2009 averaged only 0.6 chicks per female, the lowest rate recorded in 16 previous studies within the species’ entire range. Reduced breeding success over the duration of the study (see Figure 1) was due to proportionally fewer females rearing chicks rather than a reduction in brood size. Birds bred less well in Perthshire towards the southern edge of the range, where declines in indices of female and male density were highest (see Table 1). Only at the core of the range in Strathspey, which now hosts more than 75% of Scotland’s capercaillie, did birds breed reasonably well and female densities were stable (see Table 1). Two weather variables, April temperature in the pre-breeding period, and
temperature at chick hatch in June, increased over the study period. Indices of pine marten increased 3.9-fold between 1995 and 2009, and those of fox 2.2-fold, whereas carrion crow and raptor numbers and forest floor vegetation showed no change. Neither forest type nor forest ground vegetation appeared to influence breeding success. Instead, females reared more chicks in years when hatch time in June was drier, and in forests with lower marten and crow indices. In addition, more females reared broods in years when Aprils were cooler. Brood size was unaffected by any of the measured variables. Densities of adult birds declined over time and were lower in forests with higher fox indices. Increased predation of clutches and chicks by martens and crows within these small, fragmented forests, as well as changes in climate, may explain reductions in breeding success and hence contribute to continued declines. To date, breeding success and adult numbers have remained similar over time
in Strathspey, now the last remaining stronghold for capercaillie in Scotland. This is despite increases in pine martens in this region too, but here the native forests are larger and less fragmented and hence generalist predators associated with the forest- agriculture interface such as crow and fox may have less impact. Under these condi- tions, it remains to be seen whether capercaillie breeding success can be maintained without further intervention or whether successful conservation of capercaillie may require adaptive predator management, including a licensed removal of martens to test the hypothesis that martens contribute to reduced breeding success. Such a short to medium-term approach would inform longer-term predator management policies and complement aspirations to increase the area and connectivity of forest habitat to benefit capercaillie.
www.gwct.org.uk GAME & WILDLIFE REVIEW 2016 | 53
KEY FINDINGS
Capercaillie range retraction is linked to declines in breeding success. Breeding success was lower in forests where there were more crows and pine martens and in years when June rainfall was higher. Three-quarters of the popula- tion is now confined to the larger forests in Strathspey, the remaining Scottish strong-hold.
David Baines Nicholas Aebischer
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