Blog by Nick Wilkinson, Conservation Scientist, RSPB Centre for Conservation Science

The capercaillie is a spectacular large woodland grouse of old world boreal and temperate forests. It is famous for its communal displays at ‘leks’ in spring, where males gather to attract a mate. Across much of its fragmented European range, however, populations are declining. In the UK, the capercaillie is found only in Scotland, where the population of this ‘red-listed’ species has decreased substantially in both numbers and range since the mid-1970s.

By the time of the first national survey, in 1992-94, the population had fallen to just 2200 birds (95% Confidence Limits: 1500-3200) and then to 1073 birds (95% CL: 549-2041) when re-surveyed in 1998-99. Research showed that poor breeding success (due to wet weather at hatch-time in June, predation, and habitat) and the elevated mortality of adult birds, owing to collisions with deer fences, contributed to the decline. The parlous population status of capercaillie at the end of the 20th century prompted an ongoing programme of research and conservation action by public bodies, conservation NGOs and private landowners.

Picture: Capercaillie male in roost tree, Caledonian pine forest, Scotland, by Ben Andrew (rspb-images.com)

 

The importance of monitoring

Regular monitoring of the population status of capercaillie in Scotland is an essential part of assessing the impact of this conservation action and helps to inform the targeting of further work. The population is assessed every six years by RSPB and Scottish Natural Heritage (SNH) as part of a rolling survey programme for a range of bird species of conservation concern requiring periodic national population surveys (e.g. hen harrier). The latest national survey (the fifth) was undertaken during winter 2015-16. Unlike most national single species surveys, capercaillie are surveyed in winter when the birds feed in the tree canopy and are more readily flushed and detected by observers. This timing also avoids disturbance during the breeding season. As highlighted by Simon’s blog  on the hen harrier survey, such surveys require considerable preparation and planning to set up once funding is confirmed. This includes collating the most-up-to-date data to define the species’ known range (and thus area for survey), assessing modifications to the survey sampling design (in order to improve the precision of the population size estimate whilst maintaining comparability with previous surveys), selecting the random sample of sites for survey, identifying and contacting landowners, and recruiting and training a team of fieldworkers to undertake the survey.

Fortunately for me, the Capercaillie Project Officer (a post funded jointly by RSPB, SNH and Forestry Commission Scotland) was able to take on the substantial task of identifying and contacting the many landowners to arrange survey access.

Photo: Male and female capercaillie artwork, by Mike Langman (rspb-images.com)

In Scotland, capercaillie are found primarily in Scots pine woodlands (both remnants of the old Caledonian forest, and mature plantations) with an under-storey of bilberry. As with all previous national capercaillie surveys, the survey itself involved walking a sample of 2-km long triangular line transects selected randomly in woodland throughout the species’ known range to record all capercaillie seen. In addition to the required ornithological knowledge and experience of surveyors, this calls for good navigation skills (aided by hand-held Global Positioning System units) and physical fitness, particularly in the dense forests typical of plantations, where weaving a route between the trees can be a struggle in itself, let alone the joy of finding your transect route crossing areas of clear-felled and/or wind-thrown trees.

Although capercaillie are large birds (c. 74 cm long from bill to tail, with a wingspan of over a metre; males weigh approx 4 kg, twice that of the much smaller, well-camouflaged, females), they can be surprisingly difficult to see until they have been flushed, and then it is often just a fleeting glimpse. Occasionally, however, you can be rewarded with magnificent views of this rare and elusive bird flying through open woodland that seem to go on for ages, but probably last all of 5-10 seconds. On seeing capercaillie, in addition to recording the number and sex, we also collect data on the distance of birds from the transect line. Using the method of distance sampling, this information allows us to account for the fact that some birds may remain undetected in the surveyed area (because the probability of detecting a bird decreases as its distance from the observer increases). Distance sampling uses data on the distances of detected birds from the transect line to estimate the density of birds (number per unit area) and, by extrapolation to the known range, total population size.


Photo: Capercaillie male displaying, at Abernethy, by Dave Braddock (rspb-images.com)

The 2015-16 survey 

Thanks to relatively benign winter conditions (unlike the remarkably snowy winter encountered by the previous survey in 2009-10), the team of six surveyors covered nearly 750 transects (~ 1500 km) between November 2015 and March 2016. Most birds were recorded in the core of the range around the Spey valley with sadly few found in the peripheral areas of the range. Analysis of the survey results indicate that the provisional total capercaillie population estimate is 1114 individuals (95% Confidence Limits: 805-1505). This is a slight decline (-13%) since the previous survey in 2009-10 when the population was estimated to be 1285 individuals (95% CL: 822-1882), although this change was not statistically significant. Other data support the suggestion of relative population stability. Over the longer term of national surveys (dating back to the early 1990s), the population appears to have fluctuated between 1000 and 2000 birds, but is now very much at the low end of this range. As in previous surveys, the area around Strathspey held the bulk of the population (83%) and capercaillie are now very scarce in Easter Ross, Moray, Aberdeenshire and Perthshire.
Conservation measures to reduce mortality from fence collisions are relatively simple (marking fences to make them more visible, reducing their height or, ideally, complete removal) and have largely been put in place.

By contrast, measures to improve breeding success have proved much more difficult, with multiple factors involved, including weather, climate, habitat and predators, and this remains the key factor underlying the lack of population recovery. Breeding success is adversely affected by weather (high rainfall in June when the chicks hatch) and predation, and is only (relatively) high when both June rainfall and numbers of predators are low. Furthermore, wetter Junes have become more frequent and appear to be part of a long-term trend, while the small size and fragmented nature of forests within much of their Scottish range provides easier access to predators such as crows and foxes and likely exacerbates the effects of predation (‘edge effects’). Additionally, there is increasing evidence that human disturbance (use of vehicle tracks and footpaths) causes capercaillie to avoid using large areas of otherwise suitable woodland, reducing the area of habitat available to the birds and, potentially, limiting population expansion. Whilst the concerted conservation action of the last two decades appears to have made a second extinction of capercaillie in Scotland less likely, these latest results highlight that the population remains at a critically low level.

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