National Capercaillie Survey: FAQs

1st October 2022

Capercaillie – one of the UK’s most magnificent yet elusive bird species – is now found only in Scotland, within their preferred pine woodland habitat. Since the mid-1970s a significant decline in their numbers has been recorded. The national survey provides an updated estimate of the UK capercaillie population, which is a hugely important to help guide further conservation action for the birds.

Want to learn more about the national survey? Here are some frequently asked questions:

What is the survey?

This is the sixth national capercaillie winter survey, which aims to establish a robust population estimate for the species, using comparable methodology to the last five previous surveys. The survey takes place every five to six years with the fifth one happening over winter 2015/16.

Who is funding it and delivering it?

It’s funded by RSPB Scotland, NatureScot, Cairngorms National Park Authority, Cairngorms Capercaillie Project, Forestry and Land Scotland, and Scottish Forestry. RSPB Scotland have undertaken the fieldwork and follow up scientific analysis.

Why is it happening?

The national survey allows nature conservationists to see how the capercaillie population is faring and then target measures to help the species. Scotland’s capercaillie population is small and vulnerable. The species faces numerous threats, including lack of suitable habitat, predation, collisions with deer fences, human disturbance, and wetter summers (which result in fewer chicks surviving) likely becoming more frequent due to climate change.

This national survey provides an up-to-date estimate of the overall population which can be compared with the previous five surveys, and this is important for informing further conservation action.  

Data on capercaillie numbers collected by other methods in recent years, such as lek counts, suggests there has been a decline so it is critical to ascertain how the population has fared since the last national survey in 2015/16. All of the partners acknowledge the need for urgent action to help the species, and this survey will be vital to informing what steps to take next.

When is it happening?

Fieldwork started in November 2021 and was completed in March 2022 in Strathspey, Deeside, Moray, Nairnshire, Easter Ross and Perthshire. The scientific analysis was completed by the RSPB Centre for Conservation Science.

How was it undertaken?

The survey team walked 2km long triangular routes or transects through a random sample of woods throughout their range and counted all capercaillie seen, usually when flushed from trees (pine needles are their main food in winter) or the ground. From the numbers of birds counted at these sites, the numbers of birds in the areas not surveyed were estimated by extrapolation and so obtained an estimate of the total population.  

Statistics were used to estimate the number of birds that were missed in the surveyed woods because birds further away from the transect are more likely to be missed. The survey was carried out by six professional fieldworkers who collectively cover a total of c.750 transects.

How was disturbance minimised?

The minimum number of transects were walked, in order to calculate an accurate estimate whilst minimising disturbance. The survey takes place in the winter when the impacts of disturbance are lower compared to the sensitive breeding season. Surveys were not conducted, as far as possible, on particularly cold days when the need to conserve energy is essential for the birds and surveyors liaised with landowners to minimise disturbance, for example by planning surveys around other activities happening in or near the transects.

Why were the areas covered by the 2021/22 survey changed from previous surveys?

The areas surveyed change to some extent with every national survey due to updated information on alterations to the capercaillie’s range in Scotland, and the time window of records used to define what that range is.

To identify the areas of coverage for the 2021/22 survey, information spanning the past 10 years was used from a database compiled by the RSPB, collating records from annual lek monitoring, plus other sightings and signs of capercaillie presence that are assumed to give an accurate representation of the species’ current annual distribution. The 2003/04 and 2009/10 surveys included records as far back as 1968, from places where capercaillie have long since disappeared, and so for the 2015/16 survey this time window was reduced to include data from the past 15 years.

For the 2021-22 survey, the time window used to determine the current and recent historic range was reduced further to 10 years. This change removed large areas of former capercaillie range occupied before 2012 but for which there have been no records of capercaillie presence since, such as in Loch Lomond and Trossachs. Given the evidence for range contraction recorded by the 2007-11 Bird Atlas and the high profile of capercaillie, it was felt they were extremely unlikely to be persisting, undetected, in such areas. Some areas were removed from the survey where there have been no records of capercaillie in the last five years, and which are outside the current range including in the Black Isle and Donside. This is because it’s very unlikely that birds still occupy these areas.

Whilst the areas surveyed reduced, the number of landowners did not differ much to previous surveys as the land omitted this time round is generally part of large areas owned by the same landowners.

With different areas surveyed, how is it ensured that overall results are comparable between surveys?

These changes are accounted for in the scientific analysis stage of the survey by including data on the total length of transects walked and the size of survey area(s). 

How are the transects chosen?

The same process for selecting the transects has been used from the 2003/04 survey to now. For the 2021/22 survey, the survey area was split into two regions; one covering the core of the population in Strathspey and another covering the peripheral regions of the range in Deeside, Easter Ross, Moray, Nairnshire, and Perthshire. This is so that more transects can be allocated in the core of the range with more capercaillie than in the periphery where they exist in much lower numbers.

To sample the two regions, a grid of points is superimposed over the survey area and a defined proportion of points that fall within 200m of woodland are randomly selected. These points mark the starting locations of the triangular transects. The orientations of transects were selected at random from a subset of eight starting bearings, so long as a transect could fit within the available woodland on the chosen bearing. All suitable grid points in Strathspey were selected for survey, whereas for the peripheral regions, a proportion of grid points was randomly selected from the set of suitable points available.

Since 2009-10, a second grid of transects has been used to allow a greater level of sampling in Strathspey, which holds the bulk of the capercaillie population. This is done in order to fit in more transects in this region than was enabled by transects based on the initial grid alone. As before, all suitable grid points and the orientations of transects were randomly selected on the condition that it was possible to accommodate a transect within the woodland, even if they overlapped those on the existing grid.

In both cases, the same grids as used by the 2009/10 and 2015/16 surveys were used for the 2021/22 survey.  Many of the transects selected on the first grid are the same as those walked in the previous two surveys. Transects that differ from previous surveys arise because of changes to the set of woods included for survey and the defined proportions of points randomly selected.

Why aren’t the same transects used for every survey?

The changes in the capercaillie range mean that each survey has to have its design refined. This is so that the new population estimate is as reliable and precise as possible. Some transects will be covered from one survey to the next. The extent to which this happens varies between surveys, depending on changes to the areas selected for survey and numbers of transects chosen to sample these areas.

Why were some transects visited twice in the 2021/22 survey?

At least 60-80 sightings of capercaillie are needed to gain a reliable estimate of population size. However, given the indications of further declines in capercaillie numbers since the last national survey in 2015/16, there was a need to increase overall survey effort in an attempt to boost the number of sightings needed for a reliable estimate.

This additional survey effort was best placed in those areas holding most of the population – for 2021/22 this is Strathspey – but not sufficient enough numbers of new transects could be fitted into the available area, and so a random subset of the transects in Strathspey were visited twice to achieve the required increase in survey effort.

How will the survey results incorporate results from the same transect?

Repeat visits to the same transect are not independent of each other as counts of capercaillie on one visit are likely to be similar to those on a repeat visit to the same transect. This means they cannot be treated as if they are visits to different transects.

Transects visited more than once are accounted for in the scientific analysis by pooling the associated survey effort from both visits to a single transect, and recording the associated survey effort for that transect as the length of the transect line multiplied by the number of visits (in this case, two).