This report was commissioned by NatureScot, following a meeting of the Scientific Advisory Committee (SAC) in September 2021. A sub-group of the SAC, chaired by Professor Neil Metcalfe, produced this report, published on 25 February 2022.
The Board of NatureScot requested its Scientific Advisory Committee (SAC) in 2021 to form a sub-group to review the evidence and advise on further measures for the conservation and management of Capercaillie in Scotland, with emphasis on the core area of the species in the Cairngorms. This was motivated by concerns over apparent declines in the Scottish Capercaillie population in recent years, and differing opinions on how the scientific evidence concerning threats to the population should be interpreted, and consequently what conservation management options should be prioritised. The sub-group was asked to address the following Objectives:
a) Critically assess key constraints on Capercaillie population extent, numbers and productivity, based on the available evidence;
b) Explore the relative merits of predator control; reintroduction/reinforcement and establishment of refuges;
c) Advise on the potential impacts of climate change on habitat, and hence on Capercaillie population viability. This would include assessment of the importance of the Scottish population in UK and European contexts;
d) Advise on any geographical differences in management measures across the Capercaillie range in Scotland; and
e) Propose a range of options for further work aimed at conserving the Scottish population, supported by a consensus on the evidence base.
The sub-group focussed primarily on the scientific evidence of changes in the size, geographical extent and breeding success of the Capercaillie population in Scotland, the underlying drivers of those changes, and the likely effectiveness of different management options. The evidence considered comprised both published scientific papers and unpublished reports and survey data. The sub-group met virtually three times and invited the Director of Nature & Climate Change for the Cairngorms National Park Authority, and Chair of the Scottish Capercaillie Group, to attend the latter two meetings so as to be able to present an overview of the perspectives of members of the Scottish Capercaillie Group and the management initiatives that had been taken to date.
With respect to the five Objectives, the sub-group came to the following conclusions:
a) Population size: There has been a marked reduction in the geographical range and size of the Scottish Capercaillie population since the 1970’s, which appears to have continued since the last national survey in 2015-16. If this recent trend continues it is likely that Capercaillie will become extinct within 2-3 decades. Initial declines were associated with high adult mortality, while more recent declines appear to be driven by poor breeding success.
Mortality rates: The only factor demonstrated to have caused changes in juvenile and adult mortality rates in recent decades is the presence of deer fences, causing injury or death to birds in flight. This is thought to have been an important driver of the initial decline in population size in the 1980s – early 2000s. While many fences were removed or made more visible once this factor was identified, there are still unmarked fences that will cause mortality; the most vulnerable birds are those dispersing in their first year of life.
Breeding success: This has declined over the period of population decline; some forests initially maintained a high breeding success but in the last decade it has become more uniformly low. Success is reduced in years of high rainfall at the time of chick hatching and possibly delayed April warming, both of which may have slightly increased in frequency due to climate change. There is no clear evidence that these effects have been greater over the last 10 years of poor reproductive success. A more likely factor is predation on eggs and young chicks. The main predators affecting the Scottish Capercaillie population are crows, foxes and pine martens, and increases in predator numbers (notably the pine marten) are contributing to the decline in breeding success, along with increased human disturbance due to significant increases in visitor numbers and recreational activities. There is no compelling evidence that changes in habitat quality or availability (independent of effects of disturbance), parasites or inbreeding can explain the recent reduction in breeding success.
b) It is likely that reductions in the numbers of predators (foxes, pine martens, crows) would rapidly improve the breeding success of Capercaillie. A potentially effective (but less tested) alternative would be diversionary feeding of predators (i.e. provision of alternative food so as to minimise their predation on Capercaillie eggs and chicks). In order to prevent this causing an increase in predator populations or concentrations, the duration of feeding should be limited to the most critical period of the breeding season and should be accompanied by monitoring of the predator population so that the level of feeding can be carefully calibrated. Either of these options poses practical and societal challenges, since they would require intensive and sustained work over large areas. Creation of refuges from human disturbance would likely improve breeding success. This could be achieved by closure or removal of paths in sensitive areas. Current evidence suggests that the only period when disturbance is critical is during the breeding season, and so path closures could be seasonal. An alternative to path closure is better vegetative screening of the path, but this takes longer to achieve any beneficial effects. With current recruitment and survival rates there is no obvious case for reinforcement or reintroduction of Capercaillie as an urgent measure. However, reinforcement could prove valuable in future, once ecological constraints on breeding success and survival are resolved.
c) There is some evidence for a long-term slight increase in the likelihood of warm April temperatures and high June rainfall. Both of these correlate with lower breeding success and so may have contributed to the long-term gradual decline in breeding success, but not the most recent decade of poor success. As yet there is no evidence that climate change is having an adverse effect on Capercaillie habitat. The Scottish population is small relative to the total European population, but there is evidence that populations elsewhere in Europe are also declining. The species is considered an icon of the ancient Caledonian pine forests and has considerable cultural significance in Scotland, and so its loss would be likely to have societal repercussions beyond its ecological significance.
d) The major investment of resources and greatest intensity of management measures for Capercaillie should take place in the core region of Badenoch and Strathspey. However, consideration of the needs of Capercaillie should still be taken into account when considering the management of coniferous forests in other areas that currently or have recently contained Capercaillie, for instance in the placing of any new paths or fences.
e) Refreshed intensive measures are needed if the population is to be conserved. These should focus on options that will improve breeding success (primarily survival of eggs and young chicks), but there may also be scope for increasing survival of dispersing young birds. Given the current rate of decline there is a need for action that will achieve immediate results. It is also clear that any interventions will need to be carried out on as wide a scale as possible within the core area of Badenoch and Strathspey. The options that are likely to have the greatest immediate positive impact on the population are:
- Predator control.
- Diversionary feeding of predators.
- Creation of refuges through permanent or seasonal closure of paths and tracks.
- Fence marking/removal.
It is unlikely that employing a single option of those listed will be sufficient to prevent further population declines. There is currently limited or only circumstantial evidence as to whether diversionary feeding, path closure or path screening will have a positive impact on the Capercaillie population. However, any delay in enacting these might result in the population declining to a point where extinction becomes inevitable. Instead, it would be preferable to adopt an adaptive management approach, in which the effectiveness of the intervention measures is continually monitored and the management regime amended accordingly. Modelling should prove a useful, and possibly essential accompaniment to any adaptive management programme.
The current monitoring of leks and breeding success should also continue alongside the 6 yearly population survey, since these data have proved to be essential tools for Capercaillie conservation.
All of the above measures have the potential to provoke an adverse reaction (to a greater or lesser extent) from stakeholders, recreational users and/or the general public. It is, therefore, essential that an assessment of likely impacts is made, and deployment of each option is combined with dialogue with the relevant groups so that the need for the measures can be properly explained and any adverse impacts mitigated. The cultural impact of the potential extinction of this iconic species should also be explored and taken into consideration.
Finally, the efforts made to conserve Capercaillie in Scotland to date have involved impressive collaborations between a large group of organisations, communities, landowners and individuals, and it will be essential for these collaborations to continue (with support from NatureScot, Cairngorms National Park Authority and the Scottish Government).
In Europe the conservation status of the Western Capercaillie Tetrao urogallus L., hereafter Capercaillie, is classified as ‘least concern’ (Birdlife International, 2021), but in Scotland it is ‘endangered’ and red-listed (Stanbury et al., 2021). The main pressures on the European population of 360-550 thousand pairs are habitat deterioration, hunting and climate change (heavier spring rainfall affecting productivity, and habitat change due to climate warming) (Lõhmus et al., 2017, Coppes et al., 2021). As hunting of Capercaillie has been prohibited in the UK since 2001 (and was preceded by a voluntary ban), a range of other factors have been suggested to underlie the documented decline in the UK Capercaillie population since the 1970’s. The last full winter survey in Scotland (in 2015-16) reported only approximately 1,100 individuals (Wilkinson et al., 2018), and subsequent annual lek and brood counts gave rise to concerns that the Scottish population had further declined.
Considerable sums of money have been spent on Capercaillie conservation over the last 4 decades. However, in recent years there have been different opinions on how the scientific evidence concerning threats to Scottish Capercaillie populations should be interpreted, and consequently what conservation management options should be prioritised. Following discussions with the Cairngorms National Park Authority and some key partners concerned with Capercaillie conservation and management, NatureScot formed a SAC sub-group to review the evidence and advise on further measures for the conservation and management of Capercaillie in Scotland, with emphasis on the Cairngorms.
The Objectives of the Sub-group were to:
- Critically assess key constraints on Capercaillie population extent, numbers and productivity, based on the available evidence;
- Explore the relative merits of predator control; reintroduction/reinforcement and establishment of refuges;
- Advise on the potential impacts of climate change on habitat, and hence on Capercaillie population viability. This would include assessment of the importance of the Scottish population in UK and European contexts;
- Advise on any geographical differences in management measures across the Capercaillie range in Scotland; and
- Propose a range of options for further work aimed at conserving the Scottish population, supported by a consensus on the evidence base.
The sub-group was asked to report to the Science Advisory Committee (SAC) of NatureScot, which in turn would report to the NatureScot Board.
The sub-group was composed of the following members: Professor Neil Metcalfe (SAC member; chair), Professor Dan Haydon (SAC member), Professor Rob Marrs (NatureScot Expert Panel member) and Professor Jane Reid (SAC member). Observers: Judith Webb (Board member of the Cairngorms National Park Authority, chair of its Upland Advisory Group and member of its Nature Strategy Group) and Sarah Fenn (NatureScot CASE PhD student working on avian conservation). Specialist Adviser: Dr Sue Haysom (NatureScot ornithologist).
Virtual meetings were held on 13/9/21, 14/10/21 and 18/11/21. Dr Peter Mayhew (Director of Nature & Climate Change for the Cairngorms National Park Authority, and Chair of the Scottish Capercaillie Group) was invited to attend the latter two meetings so as to be able to present an overview of the perspectives of members of the Scottish Capercaillie Group and the management initiatives that had been taken to date.
1.4 Forms of Evidence Examined
The sub-group focussed primarily on the scientific evidence of changes in the size, geographical extent and breeding success of the Capercaillie population in Scotland, the underlying drivers of those changes, and the effectiveness of different management options. This involved careful reading and evaluation of the relevant published scientific literature and unpublished reports, focussing primarily on studies of the Scottish Capercaillie population but also drawing on relevant studies of Capercaillie elsewhere in Europe and on studies of the effectiveness of alternative management approaches applied to other species.
This information was supplemented by unpublished summaries of recent surveys of Scottish Capercaillie (counts of males at leks - the traditional display sites used by males at the start of the breeding season - and measures of breeding success of females). J. Reid and S. Fenn also developed a simple modelling tool that allowed prediction of changes in population size under different levels of breeding success and age-specific mortality, so as to better evaluate the likely population consequences of management interventions acting at different life stages.
While relying primarily on the above scientific evidence, the sub-group was further informed through questioning of P. Mayhew, J. Webb and S. Haysom, in particular in relation to previous and ongoing conservation initiatives for Capercaillie in Scotland, and received further written information from R. Moss (Centre for Ecology and Hydrology), X. Lambin, C. Navarro Waggershauser and J. Bamber (University of Aberdeen), J. Matthiopoulos and H. Niven (University of Glasgow).
Capercaillie are difficult to survey and study: they are secretive (except when males are lekking), live at low densities and their nests are extremely difficult to locate. Given these constraints, the sub-group was very impressed by the extent and quality of the data available on Scottish Capercaillie. They also wish to pay tribute to the dedication of those individuals, landowners and organisations involved in documenting the size and breeding success of populations in many different locations across the breeding range over long time periods. It will be essential for this level of monitoring to continue so that the effectiveness of any conservation measures can be assessed; this will require the continuation of support from NatureScot and the Scottish Government for the activities of the Scottish Capercaillie Group and its members.
2. Objective (a): Critically assess key constraints on Capercaillie population extent, numbers and productivity
2.1. Population extent and numbers (Objective a.1)
The Capercaillie became extinct in the UK in the late 18th century but was reintroduced successfully to Scotland in the middle of the 19th century and appears to have reached its widest geographical distribution (from Sutherland to Argyll) in the early 20th century. Its distribution subsequently contracted, but in the 1970’s (when the population was thought to be around 20,000 birds) it still had healthy populations in Easter Ross, Moray, Strathspey, Perthshire, the Trossachs and on the islands of Loch Lomond, with small populations in Argyll and Fife. While the accuracy of the figure of 20,000 birds is questionable, it is nonetheless clear that the population has declined significantly since that time (Baines et al., 2004, Catt et al., 1998, Moss, 2001). A national survey conducted in the winters of 1992-94 estimated the population to be approximately 2200 birds (Catt et al., 1998). This survey has been repeated approximately every 6 years, and the most recent survey (2015-16) produced an estimate of 1114 birds, with 95% confidence intervals of 805-1505 (Wilkinson et al., 2018). The confidence intervals are relatively large due to the difficulties in surveying this species, but this estimate is in line with the previous survey (2009-2010; 1285 birds; 95% CI: 822-1882). There has been a retraction of the range within Scotland along with the reduction in population size: by the time of the 2015-16 survey there were no longer populations in Fife or Argyll, possibly no longer in Loch Lomond and the Trossachs, and were much reduced in Perthshire. An estimated 83% of the birds were in the core area of Badenoch and Strathspey – a higher percentage than in previous surveys due to the decline in populations in other areas (Wilkinson et al., 2018). The sub-group considered the survey methodology and method of analysis to be appropriate, and the conclusions of a contraction in range sound.
The wide confidence intervals prevented any strong conclusion as to whether the population had reduced in size between 2009-10 and 2015-16, and there has not been a national survey since 2015-16 (fieldwork for the next one is currently taking place over the 2021-22 winter). However there is other evidence of a further decline in population size and range. Over recent years (excepting 2020 due to covid restrictions) counts have been made each year of the number of displaying males at almost all known active lek sites in Scotland (unpublished data provided by members of the Scottish Capercaillie Group, collated by the Capercaillie Advisory Officer). The sub-group concluded that there was compelling evidence of a continuous downward trend from 2016-2021 in both the number of occupied leks (74 in 2016 down to 45 in 2021) and the total number of displaying males (228 in 2016 down to 149 in 2021). This decline was occurring across the range, resulting in very small subpopulations remaining in the areas outside the core area (i.e. Moray & Nairn – 4 males, Easter Ross - 10, Deeside & Donside - 5, and Perthshire – 1; note that Easter Ross and Perthshire showed no decline over this time period but were already very small). Moreover, for the first time there was evidence of a decline within the core area of Badenoch and Strathspey (from 194 males in 2016 to 127 in 2021). The proportion of Scottish Capercaillie lekking males found in Badenoch and Strathspey thus remained very stable (approximately 85% of the total) over this period. The downward trends in the number of males were steady and consistent, implying little statistical noise in the counts, and occurred even where survey effort was constant. While these counts have not yet been subjected to rigorous statistical analysis, the trends are so clear that it is highly likely that they are real.
The sub-group concluded that;
- There has been a marked reduction in the geographical range of the Capercaillie in Scotland since the 1970’s, which has continued since the last national survey in 2015-16. If current trends continue it is likely that Capercaillie will be lost from Deeside & Donside, Moray & Nairn and Perthshire within the next 10-15 years.
- The size of the Scottish population at the time of the last national survey (around 1100 birds) was much reduced from its peak in the 20th century. Furthermore, recent lek counts very strongly suggest that there has been a marked decline in the number of displaying males since 2016.
- It is reasonable to assume that lek counts reflect the size of the breeding population. Taken together, these outputs indicate that there has been a continual decline in the breeding population since 2016, including within the core area of Badenoch and Strathspey, where there had been a stable or increasing population up to 2015.
- If this trend were to continue then the Scottish Capercaillie population would likely become extinct within 2-3 decades.
2.2. Productivity (Objective a.2)
It is challenging to measure productivity in Capercaillie due to the difficulties in locating nests and following broods once chicks hatch, and the need to avoid disturbance. The usual method is to use trained dogs to locate hens during the late chick stage and then count the number of chicks per hen. While this may underestimate the success of each breeding attempt, since not all of the located hens (especially 1-year-old birds) will have attempted to breed, it provides an index of breeding success that can be compared across sites and over time. It is a measure that is subject to statistical noise due to small sample sizes (especially when the population is declining), but there is nonetheless good evidence from published studies that overall breeding success (chicks/hen) has declined in Scotland since the 1980’s and is now low relative to sites elsewhere in the species’ range (Baines et al., 2004, Summers et al., 2004, Summers et al., 2009, Summers et al., 2015, Baines et al., 2016). Studies conducted 10-30 years ago indicate spatial variation in breeding success, with birds in some forests being more successful than those elsewhere. However, more recent information suggests that there are now no forests in which breeding success is consistently high. Current estimates of an average of approximately 0.5 chick per hen indicate a breeding success, even in the core area of Badenoch and Strathspey, that is unlikely to be sufficient to prevent population declines (see below).
The sub-group concluded that:
- It is not easy to measure the breeding success of Capercaillie, so estimates of productivity have wide confidence intervals.
- However, there is clear evidence of declines in breeding success in Scottish Capercaillie in the last 40 years. During the initial phase of this decline the breeding success remained high in some areas, but it is now low in all parts of its range in Scotland.
- Current breeding success appears to be too low to allow recovery of the population.
2.3. Factors affecting breeding success (Objective a.3)
A number of factors have been proposed to explain the decrease in breeding success of Capercaillie in recent decades. The sub-group, therefore, considered the evidence that breeding success was influenced by predation, weather/climate, habitat loss, disturbance, parasites and inbreeding. There have been a number of independent studies examining spatial and temporal variation in the breeding success of Scottish Capercaillie. These have generally been constrained by the difficulties of obtaining large enough sample sizes per year or per location, and the majority involved correlative rather than experimental approaches. Nonetheless, the impressive duration and spatial scale of several of the studies have resulted in statistically significant and consistent patterns.
There is clear evidence from several independent studies that predators have a major impact on breeding success, primarily through consumption of eggs and young chicks (Baines et al., 2004, Summers et al., 2004, Summers et al., 2009, Summers et al., 2015, Baines et al., 2016, Kämmerle and Storch, 2019). Up until the early 2000’s the major predators were foxes and corvids (mainly crows); both routine and experimental control of the numbers of these predators led to higher breeding success (chicks per hen) (Baines et al., 2004, Summers et al., 2004). More recently the population increase and recovery in range of the pine marten has led to it becoming relatively common in forests that are important for Capercaillie (Baines et al., 2013, Croose et al., 2014). It has been documented to predate Capercaillie clutches (Summers et al., 2009, Fletcher and Baines, 2020), and in recent years Capercaillie breeding success has been negatively correlated with levels of pine marten activity (Summers et al., 2015, Baines et al., 2016). However, it should be noted that pine martens are not the only cause of recent Capercaillie declines, since the reduction in the Capercaillie population in Deeside occurred before pine martens were present. Recently badgers have increased in abundance in some areas, and may also cause some predation on eggs, but the relative importance of the different predators is yet to be established. It should be noted that, while Capercaillie do not form a major part of the diet of any of these predators, which appear to encounter and feed upon the eggs opportunistically, even a low level of predation on eggs/chicks can still have a significant impact on Capercaillie populations due to their current low numbers.
The sub-group concluded that:
- Predation is a significant contributor to variation in breeding success.
- Increases in some nest predators (notably the Pine Marten) in recent years are likely to be contributing to the decline in Capercaillie breeding success and hence population size. The main predators affecting the Scottish Capercaillie population are foxes, pine martens and crows; badger populations are increasing in Capercaillie breeding areas and must be considered a threat for the future.
- It is not currently possible to place the predators in rank order of impact, and it is more their cumulative effect that is important, but the recent addition of the pine marten to the suit of mesopredators in Capercaillie core breeding areas is likely to have accelerated the decline in Capercaillie population size.
A consistent pattern found across several independent studies is that high rainfall in early summer (i.e. June, around the time of hatching of eggs) causes a reduction in breeding success (Moss, 1986, Summers et al., 2004, Summers et al., 2015, Baines et al., 2016, Coppes et al., 2021). The mechanisms are not known but could be due to young chicks becoming chilled and/or their invertebrate prey becoming less available (or the chicks having less foraging time). There is also evidence of a correlation between April (i.e. pre-breeding) temperatures and breeding success, but the pattern is not clear: in an early study breeding success was greatest when temperatures rose quickly in April (Moss et al., 2001) but a later one suggests that more hens had chicks when Aprils were cooler (Baines et al., 2016). While weather is likely to be an important factor affecting breeding success, this need not be related to climate change. There was a slight trend for increasingly wet weather in June in the Cairngorms areas over the 20th century (Summers et al., 2010), but there was no significant temporal trend over the period from 1975-1999 when breeding success was first noted to be declining (Moss et al., 2001), nor between 1991 and 2009, the period analysed by Baines et al. (Baines et al., 2016). Mean April temperature increased over that period (Baines et al., 2016), but there was scant evidence of climate change being the driver for the recent consistently low breeding success.
The sub-group concluded that:
- Breeding success was reduced in years of high rainfall in early summer, and was possibly also affected by April temperatures, but there is little compelling evidence that climate change may have contributed to the long-term decline in breeding success. There is also no clear evidence that climate change was the main driver of the decline in breeding success in the last 10 years.
2.3.3. Habitat loss
Capercaillie breed in old-growth conifer forests and plantations. While these are often dominated by Scots pine, the structure of the woodland is more important than the tree species, since the birds require a mix of open canopy areas (to allow growth of the understory, especially blaeberry, needed to provide the insect diet of the chicks) and dense areas that provide cover. Several detailed studies have examined whether habitat structure or vegetation type influences breeding success, but results are equivocal and inconsistent (Baines et al., 2004, Summers et al., 2015, Baines et al., 2016). It is noteworthy that breeding success can be higher in plantations (provided that they have some clearings) than in ancient Caledonian pine forest, suggesting that other drivers (such as predation) are more important than habitat features in determining breeding success provided that a minimum quality of habitat is surpassed. Moreover, there is little evidence that the extent or quality of the available habitat has decreased over the period that the breeding success has declined once any effect of disturbance on availability is taken into account (see section 2.3.4).
The sub-group concluded that:
- Habitat quality was likely to only have a small impact on breeding success provided that the minimum requirements were met, comprising a patchwork of open forest canopy mixed with dense vegetation to provide cover.
- There was no evidence that the decline in breeding success of Capercaillie over the last 30 years was due to changes in habitat quality or availability (independent of effects of disturbance).
There is clear evidence from a number of independent studies that Capercaillie avoid areas frequented by people, both during the breeding season and at other times of year (Summers et al., 2007, Moss et al., 2014, Coppes et al., 2017). The density of hens is significantly reduced in otherwise suitable vegetation that lies close to roads, tracks or footpaths. A useful rule of thumb is that any area within 100 m of a path is unlikely to be used by breeding birds. The core area of Badenoch and Strathspey is a popular tourist destination and so many of the potentially most important areas of woodland for Capercaillie also have a high density of paths and tracks that are well used by walkers, runners and cyclists. If Capercaillie avoid all woodland that is close to paths and tracks then it reduces the available habitat (Coppes et al., 2017). This in itself would not necessarily result in lower breeding success (unless birds are forced into undisturbed, but marginal, habitats). However, incubating hens will leave the nest if there is any disturbance from either humans or dogs (and do so at a greater distance than related species such as the Black grouse), and in doing so leave their eggs exposed to predators and/or chilling.
The marked increase over recent years (exacerbated by the Covid pandemic) in the number of people visiting woodland areas has led many users to seek out new and less frequented areas (which are then flagged by apps such as Strava). This results in a greater risk that any incubating hen will be disturbed either by humans or their dogs. As yet there is no concrete evidence that this has directly led to a reduction in mean breeding success, but it seems highly likely that the significant increase in visitor numbers to breeding areas (in particular those in the Cairngorms National Park) will have resulted in increased disturbance, which will contribute to a decline in the likelihood of successful reproduction.
The sub-group concluded that:
- Disturbance to breeding birds is likely to have increased markedly, especially in the core areas of the range, due to significant increases in visitor numbers and recreational activities.
- While quantitative evidence is as yet lacking, it is likely that this has contributed to the decline in breeding success over recent years, through increasing the risk of egg predation or egg chilling.
There is evidence that ticks may have increased in abundance in woodland/upland areas of Scotland in recent decades (Millins et al., 2017), but there is no direct evidence that tick abundance has an impact on Capercaillie. It seems unlikely that tick abundance would be a major driver of declines in breeding success but it may contribute if a higher tick burden causes health problems or weakening of chicks, making them more susceptible to predation or bad weather.
The sub-group concluded that:
- At present there is no evidence that ticks or other parasites were a significant contributor to the recent decrease in Capercaillie breeding success, but the increase in tick burden may provide an additional pressure interacting with other factors – this may become significant in the future but needs further investigation.
In general, inbreeding can cause reductions in breeding success. The rate of increase of inbreeding is greatest in fragmented populations with small effective population sizes, when the risk of mating with relatives increases. There is evidence of juvenile Capercaillie dispersing between forests (Moss et al., 2006, Fletcher and Baines, 2020), suggesting that there may be sufficient panmixia to avoid sub-populations becoming entirely genetically isolated. There is currently little evidence that inbreeding, or other genetic problems stemming from lack of genetic variation, are likely to be over-riding constraints on breeding success. This is because the current Scottish population was founded with a mixture of individuals from different source populations (possibly with some subsequent reinforcement), and effective population sizes have not yet been critically small for many generations. Inbreeding is therefore unlikely to have been the primary cause of rapid declines in breeding success as have apparently occurred in recent years. However, it remains possible that inbreeding could have some negative effects, especially in small and isolated subpopulations, which would be expected to increase gradually across several generations rather than suddenly.
The sub-group concluded that:
- There was currently no evidence that inbreeding or other genetic effects had contributed to the recent declines in breeding success. However, inbreeding effects could gradually become more important if the population continues to decline.
2.4 Factors affecting juvenile and adult mortality rates (Objective a.4)
The sub-group considered the evidence underlying suggested causes of changes in juvenile (i.e. post-fledging) and adult mortality rates in recent years. These included collisions with deer fences, predation, parasites, inbreeding, disturbance and weather/climate. The only one of these for which there is conclusive evidence of an increased mortality risk to post-fledging juveniles and adults is collisions with deer fences. There is good documentation that these fences cause injury and mortality to Capercaillie, especially during the first year of life where birds are dispersing and so are less likely to be aware of the locations of fences (Moss, 2001, Moss et al., 2006). There has been much progress in removing/marking deer fences over the last 20 years, especially in prime Capercaillie areas, but many unmarked fences remain and so fence mortality has not been eliminated. As regards the other potential causes of an increased juvenile or adult mortality rate, predation rates on fully grown birds are low (except possibly on incubating hens), there is no direct evidence of any effect of parasites, disturbance, weather/climate or inbreeding or other genetic effects, although inbreeding could become an issue if the population continues to decline in size.
The sub-group concluded that:
- The only factor demonstrated to have caused changes in juvenile and adult mortality rates in recent decades is the presence of deer fences, causing injury or death to birds in flight.
- While many fences were removed or made more visible once this factor was identified, so reducing the mortality rate, there are still unmarked fences that will cause mortality. The most vulnerable birds are those dispersing in their first year of life.
- However, given that fence mortality is likely to have decreased rather than increased in the last 20 years, it is highly unlikely that such mortality in post-fledging juveniles and adults have caused the apparent declines in population size over more recent years.
2.5. Relative importance of breeding success and mortality rates (Objective a.5)
The relative importance of changes in breeding success and mortality are shown in a simple matrix population model developed by sub-group members J. Reid and S. Fenn.
The model considers a basic Capercaillie life-history. The current implementation is simplistic in that it considers constant vital rate values with no among-year or spatial variation. This means that population growth rate will likely be slightly over-estimated, and hence that population projections will likely be on the optimistic side. Projected values of population growth of 1 imply a stable population, while values greater or less than one imply increasing and decreasing population sizes respectively.
This model can be parameterised with plausible values for adult annual survival (0.72), chick survival over first 4 weeks (0.89), juvenile survival over rest of first year (0.59), probability of breeding aged 1, 2 and 3+ (0.50, 0.95 and 1.00 respectively). Even if hatched chicks per breeding attempt is set to 1.05 the model predicts a declining size of population. Further explorations, achieved by adjusting parameter values, imply that the population would remain stable if each breeding hen hatched 1.25 rather than 1.05 chicks, or if post-fledging juvenile survival increased from 0.59 to 0.70, or if adult annual survival increased from 0.72 to 0.76. More modest adjustments of combinations of these parameter values would achieve the same effect. Further increases in these parameter values would be predicted to result in an increasing population size. Parameters such as the probability of breeding at age are likely to be less flexible or open to manipulation by management interventions, since they are more fixed biological traits.
While this model is simple and undoubtedly does not capture many nuances, it does illustrate the need to increase egg and chick survival, post-fledging juvenile survival (e.g. through further fence removal) and/or adult survival rates if the decline in the population is to be reversed. Given that the inputted baseline value for adult annual survival rate (0.72) is similar to those reported for populations elsewhere in Europe, it is perhaps unlikely that this could be increased markedly by any management interventions.
The sub-group concluded that:
- The model, albeit simple, suggests that short-term conservation interventions should be targeted at increasing clutch and young chick survival;
- It is possible that post-fledging juvenile survival could also be increased, e.g. through further fence removal.
3. Objective (b): Explore the relative merits of predator control, reintroduction/ reinforcement and establishment of refuges
3.1. Predator control (Objective b.1)
There have been a number of studies designed to evaluate the effectiveness of controlling predators as a conservation measure for Capercaillie and/or Black grouse (which occupy similar habitats, also nest on the ground and have similar predators), mostly focussed on monitoring the impact on breeding success. These studies have either compared areas (forests) with/without a history of predator control or adopted an experimental approach in which predators were culled for limited periods (Marcström et al., 1988, Baines et al., 2004, Summers et al., 2004, Summers et al., 2015, Baines et al., 2016, Kämmerle and Storch, 2019). The most commonly/easily controlled predators were crows; removal of foxes when attempted was usually incomplete. One Swedish study had culled pine martens as well as foxes (Marcström et al., 1988). As yet no studies have examined the impact of removing badgers, and so their importance as predators is not yet clear. In general, reductions in the numbers of predators immediately resulted in greater breeding success of Capercaillie and/or Black grouse, probably as a result of a greater proportion of eggs and clutches surviving through to hatching. Predator control is thus a potentially effective means of increasing the breeding success of Capercaillie. However, given the current low density of breeding Capercaillie it would need to be conducted over a wide area for it to have a measurable impact on the Scottish Capercaillie population. Moreover, two of the predators (pine marten and badger) are protected species. It is currently not possible to say what the impact would be of removing only one predator species, since this has not been experimentally tested and the predators form a guild of interacting species (e.g. foxes are predators on pine martens).
The sub-group concluded that:
- It is likely that reductions in the numbers of predators (crows, foxes, pine martens and possibly badgers) would significantly improve the breeding success of Capercaillie.
- Management initiatives aimed at removing predators can produce immediate results, but they pose practical and societal challenges.
- To be effective they require intensive and sustained work over large areas; furthermore, any culling or removal of wild animals can produce an adverse public reaction, and this would be heightened in the case of a species that is legally protected.
- It is, therefore, necessary to inform and engage with stakeholders and the public prior to any management initiative aimed at removing predators.
- It is not currently clear whether Capercaillie would benefit from the reductions in numbers of just one species from this guild of interacting meso-predators.
3.1.1. Diversionary feeding of predators
An alternative to removing predators is to provide substitute food for them (usually in the form of carcasses that are then scavenged) at times/locations where they might otherwise prey upon the species of conservation concern. The provided food thus acts as an alternative food source, reducing the predator’s impact. A large-scale study in Norway showed that provision of alternative food for foxes (and potentially pine martens) during the breeding season immediately led to higher breeding success of Black grouse and greater number of Capercaillie hens with chicks, presumed to be due to a reduction in predation on clutches (Finne et al., 2019). A similar protocol has shown significant reductions in impacts of predators on ground-nesting birds in other contexts (Redpath et al., 2001, Mason et al., 2021). However, provision of additional food to predatory species runs the risk of producing an increase in predator populations, so potentially making the situation worse. While this did not occur in any of these previous studies, possibly because provision of additional food was limited to the brief time of year when predation on nests was a problem, it is an approach that requires careful calibration and regular monitoring of its impact on the predators as well as on their prey.
A study led by Aberdeen University is currently underway in one area of the Cairngorms to see if this approach could work in reducing predation on Capercaillie nests. The results are not yet fully analysed, but show promise, with extensive scavenging by badgers, pine martens and foxes (and occasionally crows) of the additional food (deer carcasses) provided during the Capercaillie breeding season. This was associated with a reduced predation rate on artificial nests of hens eggs in the vicinity, compared to artificial nests in other locations. Fifty percent of false nests were predated and camera traps showed that pine martens were responsible for approximately 75% and badgers 10% of recorded nest predation events. There remains uncertainty as to the contribution of foxes as they showed evidence of camera trap shyness.
Diversionary feeding is an approach that is likely to be more acceptable to the public than the removal of predators, but it still requires good liaison and communication with both stakeholders and the general public so that (a) the reasons for providing food to a problematic species are clear and (b) the feeding can be done in a way that minimises any other potential adverse effects.
The sub-group concluded that:
- Diversionary feeding of predators (i.e. provision of alternative food so as to minimise their predation on Capercaillie eggs/chicks) could be an effective conservation measure that has the potential to create an immediate improvement in Capercaillie breeding success.
- However, as with predator removal, it requires an investment of time over large areas each breeding season for it to have an impact on the Scottish Capercaillie population. In order to prevent it causing an increase in predator populations or concentrations it needs to be carefully designed and the duration of feeding should be limited to the most critical period of the breeding season.
- Use of this approach must also involve monitoring of its impact on predator populations and should be preceded by dialogue with stakeholders and the general public to avoid adverse reactions.
- Given the recent rate of decline in the population there is a need for urgent action to reduce the impact of predators, which cannot wait until the outcome of further research is known – an adaptive management approach is needed.
3.2. Reinforcement/reintroduction (Objective b.2)
If populations are very small and fragmented then reinforcement (i.e. supplementation of existing populations with additional individuals from elsewhere) may be beneficial. However, while there have been a number of Capercaillie reintroduction or reinforcement attempts in Scotland, and elsewhere in Europe, very few have been successful (one being the original reintroduction to Scotland in the 1830s, but subsequent reinforcement attempts did not achieve their aims). Releasing captive-bred birds is rarely successful since they are prey- and predator-naïve; the practice of ‘head starting’ (collection of eggs from the wild and rearing of chicks for later release) is not really feasible given the difficulties and disturbance involved in finding nests. A more plausible approach is to capture wild adults from a source population and then release these. However, in order for reintroduction or reinforcement to be considered, all other factors affecting the failing population should be addressed. Given that the current decline is unlikely to be due to inbreeding (see section 2.3.6 above) and that there is as yet no evidence of particularly low genetic diversity, there is no current justification for an immediate reinforcement programme. It should only be considered at some point in the future if the ecological causes of low breeding success and juvenile survival have been addressed but in the meantime the population has declined in size to a point where inbreeding or low genetic diversity are thought to be limiting the potential for recovery.
The sub-group concluded that:
- There is currently no case for reinforcement or reintroduction of Capercaillie as an immediate objective since this has rarely been found to be successful and would not address the causes of the current decline.
- It should only be considered in future if the causes of the decline have been addressed but the population by then is so small and fragmented that recovery may be prevented by inbreeding depression or a lack of genetic diversity.
3.3. Establishment of refuges (Objective b.3)
There is no direct evidence that the creation of refuges (i.e. areas of suitable forest habitat that have a low incidence of human disturbance) leads to an increased breeding success. However, given that Capercaillie hens leave the nest unprotected if approached by humans or dogs, and predation of eggs and chicks (including by dogs) is one of the main causes of breeding failure, it is highly likely that the significant increase in human/dog disturbance levels in recent years has contributed to the decline in breeding success. The increasing penetration of recreational users into the forests means that there are now few areas within the breeding range that are not at risk of disturbance. The creation of refuges from disturbance within areas of suitable forest therefore has the potential to reduce one cause of breeding failure. In many breeding areas the current density of paths and number of visitors is such that simply asking walkers to keep dogs under control or on a lead is unlikely to be sufficient to prevent disturbance to many breeding birds.
Temporarily closing paths that cross key areas for the duration of the breeding season is likely to be more effective. In some areas this could be done without significant loss of amenity value, especially if alternative routes are provided. Advantages of this conservation measure are that the investment in time and resources is minimal (and even the permanent removal of a path only requires an initial rather than an ongoing investment) and the impacts felt immediately. However, any path closure/removal may elicit an adverse reaction from users unless the action is proportionate, the reasons clearly explained and alternative options are provided. Dialogue with users and user groups (e.g. orienteers, mountain bikers etc) is essential. Police Scotland-badged signs that indicate users would be breaking the law if they continue down a path that would take them close to the breeding site of a protected species have recently been used in the area surrounding a sensitive lek; this approach may be necessary (and could be extended to include potential nesting areas) to ensure that at least some parts of the core breeding range are as little disturbed as possible.
An alternative to path closure/removal is to promote the growth of dense vegetation alongside paths so that the birds are better screened (and humans/dogs less likely to leave the path) (Coppes et al., 2017). This approach is less likely to provoke any adverse response from users, but a drawback is that any beneficial effects may not become apparent for some years (since it may take c.10 years for the vegetation to become sufficiently dense). There is currently no evidence that disturbance outside of the breeding season is having a measurable impact on the viability of the Capercaillie population.
The sub-group concluded that:
- Creation of refuges from human disturbance would be likely to reduce the most important cause of breeding failure (egg and chick mortality).
- This could be achieved by closure or removal of paths in sensitive areas.
- Signage that highlights use of a path would risk breaking the law due to disturbance of a protected species should be considered in key areas.
- Current evidence suggests that the only period when disturbance is critical is during the breeding season, and so path closures could be seasonal.
- All cases of path closure/re-routing should be accompanied by dialogue with users so that adverse impacts on recreational activities can be minimised.
- An alternative to path closure is better vegetative screening of the path, but this takes longer to achieve any beneficial effects.
4. Objective (c): Advise on the potential impacts of climate change on habitat, and hence on Capercaillie population viability. Assessment of the importance of the Scottish population in UK and European contexts
The breeding success of Capercaillie has been shown to be lower in years with high rainfall at the time of chick hatching in June (Moss, 1986, Summers et al., 2004, Summers et al., 2015, Baines et al., 2016, Coppes et al., 2021). A slight increase in June rainfall was documented over the 20th century (Summers et al., 2010), but this trend is not significant over the period when breeding success has declined (Moss et al., 2001, Baines et al., 2016). April temperatures have also been found to be associated with variation in breeding success and the mean April temperature has increased in recent decades (Baines et al., 2016), but the link between April temperature and reproductive success is not straightforward (Moss et al., 2001, Baines et al., 2016). Climate change may therefore have played a small part in the long-term decline of the Capercaillie population, but there is no evidence that the steeper population decline over the last 5 years is due to changes in the timing of spring warming or the likelihood of June rainfall. It is possible that climate change is causing alterations to the habitat of the birds, but as yet there is no evidence of this having an impact on their population; changes in habitat quality are not currently considered to be a factor underlying the population decline in the core area of the Capercaillie over the last 5 years.
There are no Capercaillie in the rest of the UK; while the Scottish population is small relative to the total global population, there is evidence of declines in populations elsewhere across its range (northern Europe), despite these generally being much less intensively monitored than in Scotland (Lõhmus et al., 2017). There is growing concern that habitat fragmentation and increasing recreational activity in European woodlands is leading to greater disturbance levels to breeding Capercaillie (Coppes et al., 2017). The trends seen in Scotland may therefore be indicative of what is happening on a broader scale across the range of the species.
While the Scottish population has always been relatively small, the Capercaillie is considered of great cultural significance in Scotland, being an iconic and instantly recognisable species that is particularly associated with ancient Caledonian pine forests. There are therefore societal as well as ecological reasons for attempting to conserve it in Scotland; it is unclear how its loss might be perceived by the general public.
The sub-group concluded that:
- There is little evidence as yet that climate change is having an adverse effect on the forest habitat of Capercaillie.
- The Scottish population is small relative to the European total, but there is evidence that populations elsewhere in Europe are also declining, possibly for similar reasons as in Scotland.
- The Capercaillie is an iconic species of the ancient Caledonian pine forests that has considerable cultural significance; loss of the species from Scotland would have more than simply ecological significance.
5. Objective (d): Advise on any geographical differences in management measures across the Capercaillie range in Scotland
While Capercaillie used to be widespread across northern and eastern Scotland, their range has contracted in recent decades and now around 85% of the population is thought to be found in the core area of Badenoch and Strathspey (Wilkinson et al., 2018; also recent unpublished lek counts (Scottish Capercaillie Group)). The sub-populations found in other areas (Moray & Nairn, Easter Ross, Deeside & Donside, and Perthshire) are all very small, declining and vulnerable to local extinction. The Badenoch and Strathspey sub-population appeared to be stable during the initial phase of the current decline in the Scottish Capercaillie population, but it too is now showing reductions in breeding success and lek counts. It can no longer be considered a potential source population from which other regions could be re-populated, and while it is currently still of sufficient size to be considered viable, a continued decline would put it at risk of extinction. Therefore a cost-benefit analysis would suggest that the most intensive, urgent and resource-hungry conservation measures should be focussed on this core area of Badenoch and Strathspey, since it is still possible that this sub-population can be rescued if action is taken fast (whereas those in other areas are unlikely to persist if the one in the core area continues to decline).
The sub-group concluded that:
- The major investment of resources and greatest intensity of urgent management measures for Capercaillie should take place in the core region of Badenoch and Strathspey.
- The needs of Capercaillie should be taken into account when considering the management of forests in other areas that currently, or have recently, contained Capercaillie, for instance restricting the placing of any new paths or fences.
6. Objective (e): Propose a range of options for further work aimed at conserving the Scottish population, supported by a consensus on the evidence base
There is clear evidence that the Scottish Capercaillie population is currently small and is continuing to decline. Its range is contracting and the decline is now evident even in its core area of Badenoch and Strathspey. Removal/marking of deer fences appears to have been successful in reducing adult mortality. The evidence suggests that the recent decline is largely driven by low breeding success, mostly due to poor survival of clutches prior to hatching. Breeding success is now poor throughout the Scottish range of the species. There may also be reduced post-fledging juvenile survival, although comparative evidence for this is currently lacking. If current trends continue then the Scottish population could become extinct within 20-30 years.
The sub-group concluded that:
- Renewed intensive measures are needed if the population is to be conserved. These should focus on options that will improve breeding success (primarily egg and young chick survival), but there may also be scope for increasing juvenile survival and continuing to minimise known threats to adult survival.
- Given the current rate of decline there is a need for action that will achieve immediate results.
- It is also clear that any interventions will need to be carried out on as wide a scale as possible within the core area of Badenoch and Strathspey.
- The options that are likely to have the greatest immediate positive impact on the population are:
- Predator control. The evidence suggests that removal of crows, foxes and pine martens would likely lead to an improvement in breeding success. However, any control of pine martens would be contentious, given its protected status.
- Diversionary feeding of predators. An alternative approach to removing predators is to provide them with alternative food during the breeding season, which has been shown to improve the breeding success of other ground nesting birds. Preliminary results of an ongoing test of this approach in Badenoch and Strathspey are encouraging; it therefore holds the potential to be effective provided that it is designed carefully and accompanied by monitoring of the predator population so that the level of feeding can be carefully calibrated.
- Path closure. Greater consideration could be made of creating more/larger refuges from human disturbance through the closure of paths and tracks either temporarily during the breeding season or by permanent removal. Temporary closure of paths needs to be made as effective as possible, for instance through greater use of signage that indicates that continuing along a path leading to a lek site or breeding area will constitute breaking the law. An option in some situations (that will only yield benefits after some years) is to increase the visual screening of path users by allowing greater vegetative growth alongside the path. This would be most effective if the most suitable sites for this approach were identified pro-actively.
- Fence marking/removal. While there has been much progress in marking or removing deer fences, many still remain and so can be contributing to juvenile and adult mortality.
- It is unlikely that employing a single option of those listed will be sufficient to prevent further population declines. Instead action should be taken on all key areas, i.e. reducing the impact of predators, reduced disturbance and fence removal/marking.
- Any delay in enacting these might result in the population declining to a point where extinction becomes inevitable. Instead, it would be preferable to adopt an adaptive management approach, in which the effectiveness of the intervention measures is continually monitored and the management regime adjusted accordingly. Modelling should prove a useful, and possibly essential accompaniment to any adaptive management programme.
- All of the above measures have the potential to provoke an adverse reaction (to a greater or lesser extent) from stakeholders, recreational users and/or the general public. It is, therefore, essential that an assessment of likely impacts is made, and deployment of each option is combined with dialogue with the relevant groups so that the need for the measures can be properly explained and any adverse impacts mitigated.
- The existing monitoring (annual lek counts and measures of breeding success, backed up by 6-yearly population surveys) should continue, since the data generated is essential for quantification of the effectiveness of conservation measures.
- It would be valuable to assess the cultural value of Capercaillie and the impact on people of its potential extinction, given its iconic status, association with ancient Caledonian pinewoods and previous history of being successfully reintroduced to Scotland.
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