Last autumn we announced the provisional results of the sixth national Capercaillie survey. This week saw the publication of the scientific paper for the survey in the journal Bird Study. Here, Nick Wilkinson, Conservation Scientist, explains more about the survey findings.
Capercaillie - Ben Andrew (rspb-images.com)
The latest national survey showed that the Capercaillie population in Scotland had fallen to an estimated 532 (95% confidence interval, 227-810) birds in winter 2021/2022. This is a fall of 52% from the previous estimate of 1,114 birds (95% confidence interval, 805-1505) just six years earlier in winter 2015/2016. While numbers have fluctuated considerably over the last nearly 30 years, these periodic snapshots of the population every 5-6 years reveal a consistent downward trend since 2003-04. Sadly, the population is now at its lowest level since the start of national monitoring in 1992-94 when there were approximately 2,200 birds (see Figure 1).
Figure 1. Capercaillie population size in Scotland estimated from periodic national surveys. Vertical lines show the 95% confidence intervals.
Declines in population stronghold
Alongside the declines in abundance, the species has become increasing concentrated in the east Highlands of Scotland, centred around Strathspey. This one region now supports 80% of the population and Capercaillie are now very scarce in the rest of their range. Unlike previous surveys however, the current survey recorded marked declines in abundance in this core region as well as the peripheral regions such as Deeside, Easter Ross, Moray and Nairn.
Poor breeding success
In contrast to the earlier phase of Capercaillie population decline during the 1980s to 2000s, when high mortality of birds from fence collisions was the main causal factor, poor breeding success appears to be primary driver of recent population declines. The factors affecting breeding success are complex and include cold, wet spring weather impacting the fitness of female birds before the breeding season, high summer rainfall affecting chick survival, predation of eggs and chicks and habitat quality, with productivity higher in forests with a rich understorey of Bilberry supporting abundant invertebrate food for chicks. On top of this is the likely growing impact of recreational disturbance which causes Capercaillie to avoid areas with high human activity and results in a loss of useable forest habitat. While habitat is continually being extended and enhanced, it remains fragmented in places and this work takes much longer to make a difference.
These latest survey results highlight the need to step up action to tackle these threats, focusing on interventions to improve breeding success, minimise human disturbance at sensitive times of year (such as the creation of refuges around Capercaillie hotspots) and expansion of high-quality connected areas of habitat. Together with other partners, we are working with Scottish Government, landowners, and local communities to develop an Emergency Plan to halt the decline and secure the future of Capercaillie, with a particular focus on the Cairngorms National Park area which is estimated to hold 84 percent of the total population.
The sixth national Capercaillie survey was conducted by the RSPB Centre for Conservation Science and funded by NatureScot, Cairngorms National Park Authority, Forestry and Land Scotland, RSPB Scotland, and Scottish Forestry.
Population estimates (footnote)
The population estimate of 532 birds (95% confidence interval, 227-810) reported in the published paper is slightly lower than the provisional estimate reported in the press release of results in autumn 2022 (542 birds, 95% confidence interval: 255-832). This difference is due to the discovery of two minor errors in the survey dataset and subsequent re-analysis of the corrected dataset. While these corrections led to a slight change in the final figure, it falls well within the 95% confidence interval of the previous estimate.