NatureScot Research Report 1375 - The predator-prey relationship between pine marten and capercaillie: could predator management offer a practical solution

Published: 2026

Authors: Jenny MacPherson, Samantha Bremner-Harrison (Vincent Wildlife Trust)

Cite as: Jenny MacPherson, Samantha Bremner-Harrison (Vincent Wildlife Trust)The predator-prey relationship between pine marten and capercaillie: could predator management offer a practical solution. NatureScot Research Report 1375.

Keywords

Western Capercaillie, Tetrao urogallus, nest predation, pine marten, Martes martes, predator management

Background

Numbers of Western capercaillie (Tetrao urogallus) in Britain have decreased by 51% in the last five years with the latest national survey (2021/22) estimating that there are only 532 capercaillie left in Scotland (Wilkinson et al., 2024). This is the lowest recorded level since the start of the national survey in 1992 - 1994. A subgroup of the NatureScot Scientific Advisory Committee advised that capercaillie could be lost within two to three decades without further action, and that renewed intensive measures are needed if the population is to be conserved. Poor breeding success is thought to be the main demographic driver limiting capercaillie population growth in Scotland (Metcalfe et al., 2022). To maximise population impact, measures to improve breeding success and survival should be targeted in the core of the range which is in the Cairngorms National Park where 85% of the UK capercaillie population now lives. The Cairngorms National Park Authority and NatureScot (formerly Scottish Natural Heritage) were therefore asked by the Scottish Government to lead on the development and delivery of a Capercaillie Emergency Plan (CEP) in collaboration with relevant stakeholders to achieve the long-term vision of a thriving capercaillie population in Scotland (CNPA & NatureScot, 2024). 

In 2005 Scottish Natural Heritage commissioned a review of the research on predation of capercaillie by pine marten (Martes martes) in response to growing concerns over the conservation status of the former and the available evidence of nest predation by the latter species. The influence of predation by pine martens on the distribution, abundance, survival and productivity of capercaillie was reviewed. The review concluded that, while pine martens do prey on capercaillie, the evidence at that time suggested that this predation did not significantly reduce breeding success or negatively impact the density of breeding adults (Reynolds, 2006). 

Since Reynolds’ (2006) literature review was undertaken, the pine marten population has continued to recover and recolonise more of its former distributional range (Croose et al., 2013, Croose et al., 2014, MacPherson and Wright, 2021, Sainsbury et al., 2019). In response to the pine marten recovery and continued decline in capercaillie numbers, there have been calls from some sectors for direct intervention in the pine marten population. In recognition of this, NatureScot commissioned the present study to update and expand upon the 2006 literature review, but with an additional focus on predator manipulation studies involving pine martens and their close relatives.

Executive Summary

• Capercaillie have undergone long-term declines in reproductive success across Europe, with stabilisation at low levels in many regions and ongoing declines in Scotland.
• Predation of eggs, chicks, and adults is a proximate cause of mortality, but the underlying causes (e.g., habitat degradation and climate interactions) remain complex and interlinked. Intensified land use and forest fragmentation have degraded preferred habitats and contributed to habitat loss, further amplifying edge effects and predator search efficiency creating ecological traps that may increase predation risk. Predation rates on nests and chicks are opportunistic but can be influenced by human-altered landscapes that increase predator efficiency.
• Capercaillie populations in Scotland have contracted to a core range in Strathspey, with smaller sub-populations at the periphery. Breeding success has declined significantly since the 1970s, with chick survival dropping from 3.2 to 0.8 chicks per hen between 1975 and 1995.
• Shifts in spring timing and warmer summers reduce capercaillie food availability, disrupt nesting patterns, and increase chick mortality through hypothermia or heightened predation risk. High rainfall during spring decreases insect availability, vital for chick survival, while potentially increasing predation risks through scent detection and other mechanisms.
• The decline in capercaillie populations is driven by a combination of habitat degradation, climate change, and predation pressure, with these factors interacting in complex ways. Conservation strategies must address habitat restoration, predator-prey dynamics and mitigate climate impacts.
• Mammalian predators limit population growth of grouse species by reducing reproductive success, with nest and chick predation identified as primary causes of failure in recent decades. Grouse species face predation losses from a wide range of avian and mammalian species, but red foxes (Vulpes vulpes) and pine martens are predominantly implicated as nest and chick predators, with regional variation in their impacts. However, pine marten predation is opportunistic, localised and unlikely to independently drive declines. Capercaillie populations coexist with predators in other regions (e.g., Scandinavia), indicating predation is a proximate, not ultimate, cause of decline. Historical evidence from Scandinavia during a sarcoptic mange outbreak in red foxes revealed capercaillie population increases as fox numbers declined, despite a concurrent increase in the number of pine martens.
• Predation on capercaillie nests aligns with vole population cycles; nest losses increase during vole crashes as predators switch to alternative prey. Addressing the interactions between predation, vole cycles, and habitat changes is critical for long-term capercaillie recovery.
• Heavy persecution in the 19th and early 20th centuries reduced pine martens to near extinction in Scotland, with survival limited to the northwest Highlands. Full legal protection in 1988 initiated the slow recovery and recolonisation of former ranges, but pine martens were largely absent from Strathspey until the 1990s.
• One study showed a significant increase in pine marten populations in the region from 1995 to 2009, based on scat survey indices, suggesting recolonisation by pine martens after long-term absence. However, scat density does not linearly correlate with population density, as variables like diet, vegetation, weather, and decay rates influence detectability.
• Non-invasive genotyping and spatially explicit capture-recapture modelling subsequently provided more accurate pine marten density estimates in Cairngorms forests with estimates of between 0.07 (95% CI 0.03–0.16) to 0.38 martens per km² (95% CI 0.11–1.07) between 2011-2012, consistent with densities observed in other Scottish forests. A 2023 study found that pine marten densities in the Cairngorms had remained stable since 2011-2012 but were still lower than expected, given the time elapsed for recolonisation. Continued monitoring using robust methods is essential to track population recovery accurately.
• Predator removal has been suggested in Strathspey to address capercaillie declines, including licensed removal of protected pine martens. However, evidence on the effectiveness of predator control for the conservation of grouse species remains limited and ambiguous. The success of predator control programmes depends on rigorous design and implementation, which may not be met by many existing programmes.
• Predator control is labour-intensive, costly, and ecologically complex. In addition, predator guilds are highly interconnected, and limited understanding of these interactions introduces uncertainties into predator control strategies. Targeted removal may benefit certain species but risks the development of unintended ecological consequences. Integrated approaches that balance predator-prey interactions and ecosystem health are critical for effective conservation.
• In Europe, reintroduction of apex predators like Eurasian lynx (Lynx lynx) has been shown to result in reduced mesopredator populations, such as red foxes, benefiting lower trophic levels, including capercaillie populations.
• Nest protection interventions like exclosures, fences, and guards can significantly improve nest success rates for ground nesting birds but may not be appropriate for capercaillie due to the difficulty in locating nests and the species’ sensitivity to disturbance. Conditioned Food Aversion has been shown to successfully reduce fox predation on nests but is less effective against pine martens.
• Providing alternative food sources can redirect predator behaviour, reducing predation without increasing predator populations. There is empirical evidence that diversionary feeding significantly enhances productivity in ground-nesting birds, such as capercaillie, offering a viable alternative to predator control. Careful monitoring and calibration are critical to avoid unintended consequences, such as increasing predator populations. Diversionary feeding can also be labour-intensive, however, seasonal feeding aligned with critical breeding periods can maximise the benefits relative to costs and reduce predation without increasing predator densities.
• Effective conservation strategies suggested include addressing the impacts of climate variability by enhancing habitat resilience and promoting favourable breeding conditions; restoring good-quality habitats with structural complexity, open glades, and robust food resources. Habitat modifications (e.g., larger, contiguous woodlands) may lower predation but require careful planning to avoid conflicting objectives and take time to become effective. The aim should be to reduce habitat fragmentation whilst balancing woodland connectivity to limit edge effects. We should continue to mitigate deer fence collisions and improve vegetation resilience through reduced browsing. Population reinforcement of capercaillie should also be considered as this has been successful in some regions but is unproven in areas with high predation pressure.
• Further research should include long-term studies on pine marten predation across capercaillie life stages and how martens interact with other predators. The effects of diversionary feeding on predator densities and behaviour should be monitored in the long-term. Modelling can be used to assess future climate scenarios and their long-term effects on chick survival and predation dynamics. The feasibility and ecological benefits of reintroducing apex predators to regulate mesopredators should be explored along with comprehensive Cost-Benefit Analyses to evaluate the financial and ecological trade-offs of various conservation measures.
• Capercaillie conservation requires a multifaceted approach, prioritising habitat restoration, climate adaptation, and targeted predator management. Diversionary feeding is currently the most effective strategy to reduce predation, but addressing ultimate drivers like habitat degradation and climate change is critical for long-term population recovery. Research into broader ecosystem dynamics and sustainable management practices will be essential to balance the recovery of predators like pine martens with the survival of vulnerable prey species.

Acknowledgements

We thank the following experts for their valuable input and comments: Cristian Navarro Waggershauser, UHI Inverness Institute for Biodiversity and Freshwater Conservation (IBFC); Tim Hofmeester, Swedish University of Agricultural Sciences; Jack Bamber, University of Aberdeen; an anonymous contributor from the Biodiversity Research Institute (IMIB), University of Oviedo, Spain; Dr Catherine McNicol, British Association for Shooting & Conservation; Xavier Lambin, University of Aberdeen and Dr Johnny Birks, Swift Ecology Ltd. We would also like to thank Rob Raynor, Dave Parish and Dan Haydon for commenting on an earlier draft of this report.

1. Introduction

The Western capercaillie (Tetrao urogallus) is a large grouse species found in boreal and montane forests of Europe and Asia. Its status and population trends vary regionally but show an overall decline attributed to habitat loss, human disturbance, climate change, and predation. The species is listed as ‘Least Concern’ globally by the IUCN due to its broad geographical range. Populations in Scandinavia and Russia are relatively stable or slightly fluctuating due to the availability of extensive boreal forests and conservation measures. However, in southern and western Europe there have been significant declines in remaining fragmented populations (Gil et al., 2020, Jiménez et al., 2022, Sachot et al., 2006, Storch, 2007b). 

On the Iberian Peninsula isolated populations are critically endangered due to forest fragmentation and climate pressures (Pollo et al., 2005, Vázquez et al., 2012). In Germany and the Baltic States, declines are linked to habitat degradation or loss as well as recreational disturbance (Coppes et al., 2019). In Scotland, fewer than 1,000 individuals remain, and the population is highly fragmented (Wilkinson et al., 2024). Conservation efforts are crucial to safeguard fragmented populations and ensure long-term survival.

Key threats that have been identified include habitat loss and deterioration (Storch, 2007a, Sirkiä et al., 2010), climate change (Selås et al., 2011, Braunisch et al., 2014), human disturbance through increased recreational activities and infrastructure development in forest areas (Thiel et al., 2008, Coppes et al., 2017). Predation, particularly on nests and chicks in fragmented forests or where there may be an increased abundance of predators can be a significant factor (Baines et al., 2016, Kämmerle et al., 2017), but most grouse populations considered to be threatened by predation occupy landscapes intensively used by humans (Kämmerle and Storch, 2019).

In 2005 Scottish Natural Heritage commissioned a review of the research on predation of capercaillie by pine marten in response to growing concerns over the conservation status of the former and the available evidence of nest predation by the latter species. The influence of predation by pine martens on the distribution, abundance, survival and productivity of capercaillie was reviewed. The review concluded that, while pine martens do prey on capercaillie, the evidence at that time suggested that this predation did not significantly reduce breeding success or negatively impact the density of breeding adults (Reynolds, 2006). 

The relatively recent recovery of the pine marten (Martes martes) in Britain, as a result of increased afforestation, legal protection in 1988 and successful conservation efforts, has sparked concerns about the potential negative impact of pine marten predation on forest grouse, particularly the vulnerable capercaillie. However, research on the direct relationship between pine marten predation and grouse populations is limited, and the extent of the threat remains unclear. Understanding this dynamic is crucial for managing conservation efforts effectively, as pine martens are, themselves, a protected species.

The aim of the present study was to assess the impacts of pine marten predation on capercaillie and other forest grouse species through a combination of literature review and consulting with experts, including those working on capercaillie conservation as well as those involved in the management of predators and mitigation of conflicts with pine martens. 

1.1 Objectives

To carry out a comprehensive up-to-date review of the available literature, including both the grey literature and PhD theses pertinent to the following research questions and to identify knowledge gaps and potential areas of future research.

1. What additional evidence concerning the predator-prey relationship between pine marten and capercaillie has become available since the previous review was carried out?
2. What is the current evidence that manipulation of the predator guild, of which pine marten is a component, can have benefits to vulnerable prey species such as capercaillie at the population level?
3. Is there any evidence that management of the pine marten population within the current capercaillie range could be expected to result in increased capercaillie productivity? Are there circumstances where time and/or area-limited management of martens might be useful, e.g. when vole numbers are low and prey-switching may occur?

Interpretation of information extracted in the course of the review will take account of any differences between the situation in Scotland and that which exists elsewhere when drawing conclusions from studies undertaken abroad. 

2. Methods

This systematic review was conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines (Page et al., 2021). The review aimed to comprehensively identify and evaluate existing literature on:

1. The relationship between pine martens and capercaillie and the role played by predation in capercaillie declines.
2. The effects of predator management (including predator removal as well as alternatives such as diversionary feeding) on vulnerable prey populations.

To ensure a thorough and systematic search, five major academic databases and platforms were used: Web of Science, Google Scholar, ResearchGate, the British Library’s interim online catalogue and the ProQuest database.

The literature search was conducted from 28^th October 2024 to 15^th November 2024 to include literature published between January 2006 and November 2024. The search terms were based on keywords and Boolean operators relevant to the research questions. The keywords included “pine marten”, “Martes martes”, “mustelid”, “capercaillie”, “Tetrao urogallus”, “forest grouse” and “predator management”. A combination of Boolean operators (AND, OR) was used to expand the search to cover a wide range of relevant studies. For example, search strings included the following: (“pine marten” OR “Martes martes”) AND (“capercaillie” OR “Tetrao urogallus”) AND “predation”

Searches were conducted within the Web of Science Core Collection (All editions). Only peer-reviewed articles were included, and results were filtered by publication date to ensure relevance. Google Scholar was used to capture a broader range of publications, including grey literature. The search was limited to the first 200 results per search term, as the relevance of results typically decreases after the initial pages. Only results published in English were included. ResearchGate was searched for relevant articles, preprints, and white papers. Priority was given to publications with open access and those from reputable authors or institutions. When available, ResearchGate’s recommendations and linked references were reviewed to identify additional relevant studies.

The British Library’s interim online catalogue was used to access some doctoral theses and dissertations. Only full-text theses were included, and they were filtered based on publication date, discipline, and relevance to the topic. ProQuest was also used to access academic theses and dissertations, as well as scholarly articles. The search was limited to English-language dissertations and articles. 

To ensure that the included studies met the scope of the review, predefined inclusion and exclusion criteria were applied. We included peer-reviewed journal articles, theses, dissertations, conference proceedings, and grey literature written in English and published since 2006, where they were directly relevant to pine martens, capercaillie, predation, and predator management. Exclusion Criteria were publications not in English, studies not directly related to the review topic, non-scholarly articles, opinion pieces, and editorials or those studies with insufficient methodological rigour as determined by initial quality screening.

All identified records were imported into a reference management software (EndNote) to organize and manage citations. After removing duplicates, titles and abstracts were screened for eligibility based on the inclusion and exclusion criteria. Full-text articles were obtained for studies that appeared relevant based on the title and abstract screening. We also included some of the literature cited in these studies and in existing reviews where applicable. 

Extracted information included the study design, sample characteristics, methodology, key findings, and limitations. Extracted data were then organized into themes relevant to the review questions. A qualitative synthesis of the findings was performed, with a focus on identifying patterns, strengths, limitations, and gaps in the literature. This comprehensive methodology allowed for a systematic and rigorous approach to capturing the relevant literature on the predator prey relationship between pine marten and capercaillie and the potential management options, ensuring that the findings and synthesis are based on a wide range of high-quality and relevant sources.

To supplement the literature review, we also made direct contact with relevant experts in the field. These were 19 representative individuals from a number of stakeholder groups and organisations who have authored relevant peer-reviewed publications and/or have a proven track record in the subject. Organisations included Forestry and Land Scotland, Cairngorms Connect Predator Project, IUCN Grouse Specialist Group, Cairngorms Capercaillie Project, Cairngorms National Park Authority, Game and Wildlife Conservation Trust, Royal Society for the Protection of Birds, British Association for Shooting and Conservation, the Mammal Society, the Martes Working Group and the Universities of Aberdeen, Highlands and Islands, Inland Norway University of Applied Sciences, Swedish University of Agricultural Sciences and University of Oviedo. When interviewing experts on the topic of pine marten predation on capercaillie, it was essential to frame questions in a neutral way to avoid leading the responses or implying any bias. All experts were asked the same five unbiased questions to help gather balanced insights. We include the questions and a summary of the responses in section 3.2 on page 20.

3. Results

3.1 Literature search

The database searches identified a total of 505 research papers, 9 theses and 3 reports. After removing duplicates and screening titles for direct relevance to the research questions, the remaining 168 papers, 7 theses and 3 reports were downloaded and screened through abstracts or more detailed reading. The review was carried out on the final set of 59 papers, 5 theses and 3 reports

3.1.1 Capercaillie population trends and contributing factors

A recent review by Jahren et al. (2016) looked at long-term changes in the reproductive success of capercaillie and black grouse populations from 16 countries across Europe from 1930 to 2012. They highlighted a consistent decline in reproductive output, with stabilisation at low levels in many regions, and an ongoing decline in Scotland.  Woodland grouse produce numerous offspring and exhibit high potential growth rates. As a result, losses in net production can be significant. However, when reproductive output consistently declines and fluctuating patterns of reproductive output decrease, it indicates mortality rates to which the population cannot adapt. A persistent lack of consecutive years with positive growth rates has prevented some capercaillie populations from reversing their current downward trends. 

The Jahren et al (2016) study attributed these trends to changes in habitat due to intensified land use, climate change, as well as predation pressure. They found that, although predation is the proximate cause of mortality of eggs, chicks and adults throughout the capercaillie range, the underlying (ultimate) causes are less obvious. It is difficult to separate between causal and correlative relationships, therefore the precise link between predation and other factors including climate change and habitat remains unclear. It is acknowledged to be likely that there is not one single factor causing declines in woodland grouse reproduction but several acting in unison. Nonetheless, a key recommendation is to address the issue of low productivity. 

Climate warming is predicted to reduce the range of the cold-adapted capercaillie in central Europe (Braunisch et al., 2013, Braunisch et al., 2014, Huntley et al., 2007).  Links between climate change and capercaillie reproductive success have already been identified (Moss et al., 2001). Shifts in the onset of spring caused by climate change may result in a mismatch between egg production and the nutritional development of forage plants, potentially having adverse effects on female body condition, reducing endurance during incubation and affecting the production of high-quality offspring (Coppes et al., 2021). This has been observed in vast areas of Fennoscandia where the start of the growing season now is not only increasingly early but also increasingly irregular (Karlsen et al., 2009). This mismatch could also lower re-nesting rates as these are closely tied to body weight. Selås et al. (2011) suggest that increasingly warm summers may indirectly have a negative effect on capercaillie reproduction because bilberry, its main food plant, will be less resource depleted after warm summers and able to rebuild chemical defences against herbivory more quickly. In support of this, they found that population indices of capercaillie in southern Norway were negatively related to summer temperatures in the previous one or two years. Two multi-species studies carried out in the UK reported significant loss in capercaillie habitat under climate change (Walmsley et al., 2007, Fuller et al., 2005). A study in Germany found that the number of days of in which snow cover was greater than 10cm was positively correlated with capercaillie presence (Braunisch and Suchant, 2007). Records show that since the 1960s, Scotland’s average climate has proved to be wetter and warmer (James Hutton Institute, 2023) with snow season shortened since 1961, especially in north and west Scotland (Werritty and Sugden, 2012, Rivington et al, 2019).

It is likely that climate plays a larger role in chick survival than nest success as chicks depend on insect-rich diets during their first weeks after hatching (Atlegrim and Sjöberg, 1995, Wegge and Kastdalen, 2008). High rainfall in spring can mean that insect food is less available or may lead to higher chick mortality from hypothermia. Apart from incidental mortality directly caused by adverse weather or lack of food, chicks are more susceptible to predation under these conditions (Wegge and Kastdalen, 2007). If chicks become chilled, they may call more to be brooded, alerting predators to their location (Wegge and Kastdalen, 2007, Selås et al., 2011, Summers et al., 2017). Evaporation of scent molecules increases under wet conditions, assisting olfactory predators in their search (Conover, 2007). It has been shown that predation rates on grouse chicks and eggs can increase immediately after precipitation (Herman‐Brunson et al., 2009).

Over the past 80 years, forestry has significantly transformed coniferous forests throughout the capercaillie’s range as a result of clear-cutting methods, increased fragmentation and shorter rotation periods for forest stands (Storch, 2007a). Capercaillie broods prefer specific habitat types (Wegge and Kastdalen, 2008) and a decline in the availability of these habitats can create ecological traps by constraining nesting birds to small areas, increasing the search efficiency of predators through edge-related effects. Jarhren et al (2016) argue, however, that the observed decrease in Fennoscandian capercaillie nest success cannot be explained by increased predator efficiency alone. Forest grouse eggs occur at low densities and are difficult to find so they are not a reliable source of food for predators. It is therefore likely that nest predation occurs opportunistically when found while searching for other prey. Human-driven habitat changes may indirectly increase predation by altering the distribution and composition of predators in the landscape.

3.1.2 Predation on capercaillie and other grouse species

Both experimental and correlative studies have shown that mammalian predators can affect grouse population dynamics in the short-term, by limiting reproduction and therefore population growth rates. During the past 80 years, the proximate causes of nest failure have changed. In early studies, direct and indirect anthropogenic causes were more prominent than predation whereas in later studies the proximate causes have been nest and chick predation (Jahren et al., 2016).

Grouse are predated upon by a wide range of both mammal and bird species, including stoats (Mustela erminea), weasels (Mustela nivalis), goshawks (Accipiter gentilis), eagles and corvids. Dominant predator species vary dependent on habitat characteristics and community composition (Summers et al., 2009, Coates and Delehanty, 2010) but studies assigning individual depredation events to a specific predator species are rare. Nonetheless there is a broad consensus in the literature that mammals, particularly red fox and pine marten, are the most important nest and chick predators (Wegge and Kastdalen, 2007, Wegge and Rolstad, 2011, Jahren, 2012, Pass et al., 2019) although the relative impacts of fox and pine marten vary between studies.

Kämmerle and Storch (2019) synthesised evidence from published studies to evaluate how predation impacts grouse populations. From a total of 15 peer reviewed studies investigating the effects of differences in predator abundance on grouse population development and/or reproductive success using data from Fennoscandia (5), the US (3), the UK (4), Germany (1), Canada (1) and the eastern Alps (1), they found 8 studies which concluded that reproductive success of grouse was negatively related to the abundance of mammalian or avian predators. Of these, only one (Baines et al., 2016) was focused on capercaillie. 

In that study, Baines et al. (2016) analysed capercaillie count data from 26 forests in Scotland and found that adult densities of grouse were negatively correlated to red fox abundance. Similarly, Kämmerle et al. (2017) linked capercaillie range contractions in Germany to variation in red fox abundance. Evidence of the impact of foxes on capercaillie was also seen during an outbreak of sarcoptic mange in the Scandinavian red fox population during the late 1970s and 1980s. Capercaillie densities increased and declined at the same time as fox numbers crashed and then recovered (Smedshaug et al., 1999).  It is notable that pine marten populations also increased during the time when foxes were scarce, suggesting that martens were not limiting grouse populations, but that foxes were suppressing pine marten numbers (Lindström et al., 1994)

In Abernethy, Scotland, where foxes were controlled and pine martens are protected, Summers et al. (2009) found that pine martens were the only predators recorded predating artificial capercaillie nests. More recently, Palencia and Barroso (2024) also used artificial nests to examine the factors influencing nest predation rates in habitats with capercaillie present in the Alt Pirineu Natural Park, Spain. The researchers monitored 82 artificial nests with camera traps. Martens (Martes spp.) were identified as the primary predators, responsible for 23% of predation events, despite their lower population density compared to other predators like foxes. This included stone martens (Martes foina), which were also identified as the main predators of 50 artificial black grouse (Tetrao tetrix) nests monitored with camera traps by Cukor et al. (2021) in the Ore Mountains of the Czech Republic. The study by Palencia and Barroso (2024) highlighted the impact of vegetation structure and predator densities on nest vulnerability. Holopainen et al. (2024) also found that vegetation structure was a significant factor in predation risk for artificial ground nests, with nest survival higher in landscapes with more continuous forest habitat and less edge habitat. 

Artificial nests have been widely used to estimate egg predation of ground nesting birds including grouse, because collecting data on active nests is often difficult. However, because eggs in artificial nests are not incubated by a cryptically coloured female, such nests are more exposed than natural nests. A brooding female may divert or deter some predators, although capercaillie hens flush relatively early, prioritising their own survival over that of the nest (Angoh, 2024). Artificial nests also lack the scent cues which may attract predators (Conover, 2007). This can result in biased information about the rate of loss and the relative role of predators. Acknowledging these limitations, artificial nests are still a useful tool for uncovering the relative abundance and composition of nest predators.

Using camera traps to monitor 194 known capercaillie nests in Norway between 2009 and 2014, Jahren (2017) found that red fox and pine marten were the most frequent nest predators. Of the identified predators, red foxes predated 42.4 % whereas pine martens predated 41.0 % of capercaillie nests. Jahren investigated the mechanisms driving predator-specific predation patterns and overall predation by modelling the relationship between nest predation, populations of red foxes and pine martens, abundance of voles and the vole cycle. Populations of voles, the principal prey of both foxes and pine martens, undergo population fluctuations which typically follow a multi (often 3-5) year cycle characterised by increase, peak, decline and low-density phases. The alternative prey hypothesis suggests that when the abundance of the primary prey declines, predators may switch to consuming alternative prey. In one example, it was shown that the impact of predation by pine martens on the ground nesting willow ptarmigan Lagopus lagopus was minimal during the high abundance phase of cyclically fluctuating vole populations, their primary prey. However, predation did have a negative impact on the growth rate and breeding success of ptarmigan when vole abundance was low (Breisjøberget et al., 2018). In Britain, field vole (Microtus agrestis) populations also show wide amplitude fluctuations which can have a large impact on predators and other prey species (Slade et al., 2022).

Jahren (2017) found that patterns of pooled predation on capercaillie nests followed the vole cycle with high losses to predators in the crash phase of the cycle. The best fit model showed that patterns of red fox predation on capercaillie nests agreed with those described by the alternative prey hypothesis, with high nest losses during the crash phase of the vole cycle followed by reduced predation as voles increased and peaked. However, this pattern was only true for pine martens at low abundance. At higher pine marten abundance, probabilities of predation on capercaillie nests during the peak of the vole cycle exceeded those of the increase phase. Numerical fluctuations are a characteristic of woodland grouse population dynamics, but under this scenario, capercaillie hatching success was not able to compensate in years with high vole numbers.

The reduction in variation in the number of capercaillie chicks per hen seen in Fennoscandia aligns with a simultaneous dampening of vole population cycles and observed increases in pine marten and red fox populations. Previously vole cycles followed a regular 4- to 6-year pattern. However, these cycles have shifted to diminished or irregular patterns (Cornulier et al., 2013, Ims et al., 2008). Up until the early 1970s, voles exhibited strong population cycles and served as an unpredictable food source for predators that relied on them. Since then, Jahren (2017) suggests that the widespread dampening of vole cycles may have turned voles into a more stable and predictable food resource, enabling vole-dependent predators to reproduce consistently rather than only during vole population peaks. 

Capercaillie distribution in Britain has contracted into a core range in Scotland centred on Strathspey in the eastern Scottish Highlands, with smaller, isolated sub-populations at the periphery (Wilkinson et al., 2024). “Broods per capercaillie hen in Scotland decreased from 0.59 to 0.46 between 1983 and 1991. The capercaillie chicks per hen in Scotland declined from 3.2 to 0.8 chicks over the 20 years between 1975 and 1995” (Jahren et al., 2016)

Baines et al (2016) examined factors contributing to the decline of capercaillie populations in Scotland. Researchers analysed data from 26 Scottish forests between 1991 and 2009, finding that breeding success averaged only 0.6 chicks per female, the lowest rate recorded in 16 previous studies. The study found that reduced breeding success was due to fewer females rearing chicks, rather than a reduction in brood size. Capercaillie bred less successfully in Perthshire at the southern edge of their range, where declines in female and male densities were highest. In contrast, female densities remained stable in Strathspey, the core of their range, where breeding success was higher.  This decline was attributed to climate change factors, including rising April and June temperatures, combined with increased predation by mammals such as pine martens and red foxes, whose indices of activity rose 3.9-fold and 2.2-fold respectively since 1995. 

3.1.3 Pine marten recovery in Scotland

Heavy persecution in the 19^th and early 20^th centuries led to the near extinction of pine martens everywhere but the far northwest of Scotland (Langley and Yalden, 1977). Since full legal protection in 1988, the species has been slowly recovering and recolonising its former range (Croose et al., 2013, Croose et al., 2014, MacPherson and Wright, 2021, Sainsbury et al., 2019). However, pine martens were largely absent from Strathspey until the 1990s, with only occasional records from the late 1980s (Balharry et al., 1996).

In 2013, Baines et al. (2013) examined the population trends of pine martens and red foxes in Scottish forests by comparing scat surveys conducted in 1995 and 2009. The findings suggested a significant increase in pine marten populations and a concurrent decline in red fox numbers over this period. However, this was based solely on an observed increase in marten sign indices (scats per 10 km surveyed per day). The authors acknowledged that this apparent increase in martens was likely to reflect recolonisation after their long absence due to historic persecution. They also recognised that the precise relationship between marten numbers and scat density is unlikely to be linear. This and subsequent studies highlight the uncertain relationship between faecal abundance and population density.  Variables quite unrelated to animal abundance may influence variation in defecation rate or the number of scats detected on a survey transect, such as diet, the physical characteristics of the transect route, the nature and extent of vegetation cover, and differential scat decay rates depending on weather and other factors (see review in Kubasiewicz 2014).

A subsequent study used a robust combination of non-invasive genotyping of hair samples and spatially explicit capture recapture modelling to derive population density estimates of pine martens in three forests in the Cairngorms (Kubasiewicz, 2014). Population density estimates of 0.07 (95% CI 0.03 - 0.16) to 0.38 per km² (95% CI 0.11 - 1.07) were obtained, which are within the range of previous estimates for forests elsewhere in Scotland (Kubasiewicz, 2014). A more recent study using similar methods showed that the density of pine martens in the Cairngorms remained very similar to density estimates from the same forests in 2011 and 2012 and concluded that this was lower than might be expected, considering the length of time that the region has been re-colonised (Hobson, 2023). 

3.1.4 The effects and effectiveness of predator removal

There have been calls from some for increased predator control and licensed removal of protected pine martens in Strathspey to halt the decline of capercaillie. A review of predation, predator control and grouse populations carried out by Kämmerle and Storch (2019) highlights the complexity of managing grouse populations in multi-use landscapes. They found considerable evidence in the literature that predation and predator abundance may limit adult abundance and reproductive success across a range of grouse species. However, although they found that predator control resulted in a net increase of adult abundance, it did not have a net positive effect on nest success or brood size, but it had a significantly greater impact on the ratio of juveniles to adults than on brood size. This suggests that predator control may reduce brood losses, leading to a higher proportion of juvenile birds in the population. While many studies targeted potential nest predators, and predation is often cited as a leading cause of nest failure, nest success showed confidence intervals that overlapped zero. However, the average effect on nest success was positive, albeit with the widest confidence interval among all parameters, indicating significant variability in the effectiveness of predator control across studies. The authors suggested that this variation may be due to differences in study environments and the specific predators removed, which were not accounted for within their analysis.

Furthermore, Kammerle and Storch (2019) report that reviews of other taxa have found that while predator control can improve breeding success, it often does not affect post-breeding adult populations. Evidence from before-and-after studies suggests that any increases in grouse population size due to predator control are temporary, as the benefits diminish once predator removal ceases and predator populations recover. However, well-executed predator removal programs can significantly reduce predator numbers and may provide short-term benefits for grouse. While their review found strong evidence that predators can limit grouse populations and that predator control can be beneficial, they acknowledge that understanding of the mechanisms behind these effects remains limited, particularly regarding the influence of environmental conditions, habitat suitability, and the role of specific predator species. They discuss how little research has focused on competitive interactions among predators, such as resource and interference competition, and how these dynamics shape predation risk for grouse. They suggest that even less is known about how selective predator control, which targets specific predator species, affects these interactions. For instance, some predators, like pine martens, may benefit from the decline of competitors such as red foxes, but the implications for grouse populations remain unclear. The authors also highlight that it is uncertain whether these cascading effects depend on other environmental factors. Some evidence suggests that higher-order predators, although they prey on grouse, may indirectly reduce mesopredator pressure on grouse populations, though findings are inconclusive.

Kammerle and Storch (2019) conclude that grouse populations most threatened by predation are typically found in heavily fragmented and degraded habitats, particularly in Europe. Declines in these populations have been linked to high mesopredator numbers in areas where top-down predator control is absent or weak. They recommend that addressing these issues requires further research, as the findings would be highly relevant for conservation and management strategies.

Treves et al. (2019) argue that predator control measures require robust scientific evaluation through unbiased randomised experiments using a cross-over design, thus reducing biases and accounting for confounding variables like habitat and prey density. The authors emphasize that such rigorous methodology is essential to accurately assess the effectiveness of predator control in achieving conservation goals, ensuring that interventions are evidence based and ecologically sound. Without certainty about the functional effectiveness of interventions to prevent future threats followed by reasoned discrimination between alternatives, most human decisions about how to intervene rely on assumptions and beliefs (i.e., perceived effectiveness) rather than evidence. 

Kämmerle et al (2019) examined the impact of localised red fox culling on predation risk in fragmented montane forests with capercaillie present in Germany (Kämmerle et al., 2019a). Cameras were used to monitor 273 artificial nests to assess whether targeted fox culling reduced predation risk and fox occurrence probability. The findings indicated that restricted-area culling did not lead to local reductions in predation risk or lower chances of fox sightings. Instead, predation risk appeared to be influenced by the overall abundance of red foxes in the surrounding landscape. The study concluded that uncoordinated recreational fox culling is ineffective as a conservation measure and does not support its use for reducing predation risk.

Another study examined the effects of restricted area culling on red fox populations in southwestern Germany (Kämmerle et al., 2019b). Researchers found that while local fox abundance decreased temporarily in spring following winter culls, populations rebounded by autumn. This suggests that such culling methods are insufficient for sustained population control during critical periods for conservation of prey species such as capercaillie. The study concluded that effective predator management requires coordinated efforts that align with the biological cycles of both predators and their prey.

Moreno-Opo et al (2015) carried out a study to assess the impact of removing mesocarnivores, such as martens (Martes spp.) and red foxes, on the demographic parameters of the endangered Pyrenean capercaillie (Tetrao urogallus aquitanicus) (Moreno-Opo et al., 2015). Over six years (2008–2013), researchers conducted a mesocarnivore removal experiment and compared the results with seven years of prior demographic data (1999–2007) using a before-after control-impact (BACI) design. The findings revealed that reducing mesocarnivore populations, particularly both stone martens and pine martens, during key reproductive months led to increased breeding success and adult survival rates in capercaillie. Breeding success was inversely related to mesocarnivore occupancy levels, significantly so for martens. The study suggests that targeted mesocarnivore management can benefit endangered prey species like the Pyrenean capercaillie.

In other grouse species, a study published in Wildlife Biology examined the causes of mortality in willow grouse (Lagopus l. lagopus) chicks and assessed the impact of intensive, localized predator control on their chick production (Steen and Haugvold, 2009). Over four years (1997–2000), researchers radio-tagged 253 chicks in Dalsbygda, Norway, finding an average survival rate of 33% from hatching to two weeks of age. Predation accounted for up to 73% of chick deaths, with no instances of disease or food scarcity as direct causes. Additionally, from 1998 to 2004, intensive predator control targeting species such as red foxes, martens, mink (Neovison vison), ravens (Corvus corax), and hooded crows (Corvus cornix) was implemented in Numedal, Norway. Despite these efforts, there was no significant difference in chick production between areas with and without predator control. The study concluded that such predator control measures did not effectively enhance chick survival or production in willow ptarmigan populations.

When looking at the impacts of predator control on red grouse Lagopus lagopus scotica, Fletcher et al. (2013) found that, while reducing populations of predators such as red foxes, carrion crows, stoats and weasels initially led to an increase in red grouse numbers, grouse populations subsequently declined, despite ongoing predator control. They suggested that this may have been due to the presence of avian predators.

Smith et al. (2010) carried out a comprehensive meta-analysis to examine the impact of predator removal on bird populations. Their analysis included 83 studies on predator removal, covering 128 bird species with relevant outcome measures. Of these, 51 studies assessed breeding population size, while only 19 examined post-breeding population numbers. Reproductive success was measured in 36 studies as hatching success and in 26 studies as fledging success. Only three studies used a before-after control-impact (BACI) design. Most studies (63%) compared areas where predators were removed with control areas where no removal occurred, and in 10 of these 49 studies, the removal and control sites were switched during the study. The remaining 31 studies focused on bird populations before and after predator removal. There was considerable variation in effect sizes between studies for all of the measured population parameters, related to prey species type and intervention methods. Some of the heterogeneity in effect size for breeding populations was linked to whether all predator species or only a subset were removed. In both cases, breeding populations increased significantly, but eliminating all predator species resulted in more significant improvements in breeding populations compared to targeting only specific predators. They conclude that on mainland areas, increased post breeding bird populations following predator removal confirms that it is an effective strategy for gamekeepers, who want to achieve a surplus of young birds for shooting in the autumn, whether or not any of them survive to breed. However, its benefits are short-lived unless removal efforts are sustained, as predators quickly recolonize the area. Thus, long-term management is necessary. Additionally, predator removal must be effective; most of the included studies did not report predator densities before and after removal, preventing a thorough assessment of its success. The information that was provided indicated that the predator reduction or elimination that was aimed for was not always achieved. Nonetheless, Smith et al (2010) suggest that their synthesis of findings confirms the effectiveness of predator removal in bird conservation while highlighting critical considerations for future removal programs. They note that ethical and practical challenges may lead managers to explore alternative, non-lethal solutions.

Predator control as a management tool is both labour intensive, costly and usually, at least to some degree, controversial. It may also result in increased populations of non-target predators, which can counteract the effort. Curveira-Santos et al. (2019) looked at the ecological consequences of predator control programmes, particularly their effects on mesocarnivore communities. Their findings highlight that non-specific and spatially variable removal of predators can lead to unintended changes in mesocarnivore community structure. These may include shifts in dominance, compensatory increases in non-target predator populations, and altered interspecies interactions, which can undermine conservation goals. The study by Curveira-Santos et al (2019) underscores the importance of considering ecological complexity and predator guild dynamics when designing and implementing predator control measures, advocating for integrated approaches that balance predator-prey interactions with broader ecosystem health. 

This was supported by the findings of Waggershauser et al. (2021), who conducted a systematic literature review to examine lethal interactions among forest-grouse predators in Europe. The researchers identified 44 pairwise lethal interactions among 12 predator species, highlighting a highly interconnected predator community. Of these interactions, three resulted in some degree of population suppression of the victim species, while another three did not; the majority (38) had not been evaluated for population suppression. The study found that these lethal interactions are primarily driven by food scarcity and, in mammals, by competition for carcasses. Interactions leading to demographic suppression were characterized by impacts on individuals with high survival elasticity—those whose survival significantly influences population growth rates. The authors emphasize that the complexity and limited understanding of these predator interactions introduce significant uncertainties into conservation strategies that involve managing predator populations. They recommend careful evaluation of such conservation actions to account for the intricate dynamics within predator communities.

3.1.5 Alternatives to predator removal

The presence or reintroduction of apex predators has been shown to decrease mesopredator populations in some instances, leading to positive effects on lower trophic levels (Jiménez et al., 2019, Pasanen‐Mortensen et al., 2013). For example, in capercaillie conservation, the resurgence of Eurasian lynx was associated with a decline in red fox numbers (Elmhagen et al., 2010). For other forest grouse species, researchers in Finland found that higher densities of golden eagle (Aquila chrysaetos) correlated with an increase in the proportion of juveniles in both black grouse (Tetrao tetrix) and hazel grouse (Tetrastes bonasia) populations. They suggest that golden eagles may indirectly benefit juvenile grouse by imposing predation risk on mesopredators such as red fox and pine marten, thereby reducing their predation pressure on grouse (Lyly et al., 2016). 

A 2024 systematic review and meta-analysis evaluated non-lethal methods for protecting wild bird nests from predators (Gautschi et al., 2024). The study found that interventions such as exclosures, fences, and guards significantly increased nest success rates, whilst other measures (deterrents, conditioned taste aversion, chemical camouflage and diversionary feeding) did not. However, direct nest protection may not be the most effective management strategy for safeguarding capercaillie. Capercaillie nests are notoriously hard to find, and the hens are highly sensitive to disturbance (Summers et al., 2009). In such situations, altering predator behaviour offers a more practical approach. 

Conditioned food aversion has been trialled to mitigate nest predation, showing success in reducing predation by foxes (Tobajas et al., 2020) but with lower effectiveness against martens (Tobajas et al., 2023). Diversionary feeding has also been proposed as an effective strategy to reduce predation pressure on capercaillie (Bamber et al., 2024b, Finne et al., 2019). Diversionary feeding involves providing food to redirect the behaviour of a target species away from conflict-causing actions, without intending to increase its population density.

In a review of the use of diversionary feeding, (Kubasiewicz, 2014, Kubasiewicz et al., 2016) analysed 30 trials from 21 studies. The authors found that while diversionary feeding led to a mean increase of 15% in success measures, only five trials achieved their ultimate conflict-reduction objectives. Additionally, cost-effectiveness analyses were seldom conducted, with only a third of studies reporting relevant information. The authors proposed a decision-making framework that incorporates ecological knowledge, financial costs, and evidence from previous studies to aid the planning and implementation of diversionary feeding in an adaptive format. The authors recommended that future studies clearly report objectives, results, costs, and effort to allow the return-on-investment to be calculated for different levels of management effort.

A later study conducted in southeast Norway found that providing alternative food sources to red foxes during spring increased the productivity of forest grouse species, such as capercaillie and black grouse (Finne et al., 2019). By diverting the foxes’ attention away from preying on grouse nests and chicks, diversionary feeding led to higher breeding success among the grouse populations. This approach suggests that targeted supplemental feeding of predators during critical breeding periods can be an effective conservation strategy to enhance the reproductive success of vulnerable prey species. However, careful calibration and regular monitoring are essential to prevent potential increases in predator populations or other unintended ecological consequences. 

A study published in 2024 evaluated diversionary feeding as a non-lethal method to mitigate predation on capercaillie nests in Scotland (Bamber et al., 2024b). Researchers conducted a large-scale experiment over two years, deploying artificial nests across 60 paired sites with and without diversionary feeding. The results showed that diversionary feeding significantly reduced nest predation, leading to an 82.5% increase in predicted nest survival over the 28-day incubation period. This effect was primarily due to decreased predation by pine martens, the main nest predators. The study concluded that diversionary feeding is an effective and easily implementable strategy for reducing predator impacts without resorting to lethal control measures, recommending its immediate application for capercaillie conservation in Scotland.

As previously discussed, experiments using artificial nests may have their drawbacks and there may be scepticism about whether results obtained in this way would translate to a similar reduction in predation on actual capercaillie nests. However, in addition to their findings above, a recent study by Bamber et al (2024a) provides empirical evidence that diversionary feeding can enhance productivity in ground-nesting birds, specifically capercaillie. Conducted over three years across 30x1km²  grid cells, the randomised control trial found that hens in areas with diversionary feeding were more than twice as likely to have broods compared to those in unfed areas. The probability of detecting a hen with a brood was 0.85 in fed locations, compared to 0.37 in unfed locations. Additionally, overall productivity, measured as the expected number of chicks per hen at the end of the sampling season, was more than double in fed sites compared to unfed sites, being 1.90 in fed sites compared with 0.82 chicks per hen in unfed sites (Bamber et al., 2024a). These results provide compelling empirical evidence that diversionary feeding can effectively enhance productivity in ground-nesting birds, offering a viable non-lethal strategy to mitigate conservation conflicts involving protected predator and prey species.

3.2 Expert elicitation questions and summary of responses

These questions were designed to encourage experts to discuss evidence, research methods, and potential solutions without steering them toward any particular viewpoint. Seven of those invited to respond were able to participate, due to the relatively short timeframe of the review. Responses are summarised below.

1. What, in your opinion, are the primary factors that influence capercaillie population dynamics, and where do you think predation by pine martens fits within these factors?

Experts agreed that the decline of capercaillie populations is a complex issue driven by habitat loss, climate change, and low productivity, with predation and human-related pressures exacerbating the problem. Habitat destruction, fragmentation, and degradation—such as overgrown heather and a degraded food base—are major contributors. Climate change further worsens habitat shifts and creates unfavourable weather conditions critical for chick survival and hen health during spring and summer. Poor reproductive output is central to the decline, with nest predation by opportunistic predators like pine martens, adverse weather during incubation and chick-rearing, and inadequate food availability all limiting recruitment.

Predation pressure, while significant, plays a secondary role in comparison to habitat and weather factors. Generalist predators like pine martens, foxes, and corvids, along with recovering raptors, contribute to nest predation, but their impact is unlikely to drive declines independently. Human-driven factors, such as mortality caused by poorly marked deer fences and recreational activities, further stress populations. Ultimate causes, including habitat loss and climate change, amplify the effects of these proximate pressures.

To conserve capercaillie, efforts must prioritize improving habitat quality and availability, mitigating predation impacts where possible, reducing fence-related mortality, and addressing weather and food-related challenges. While pine martens play a localized role in nest predation, their impact is minor compared to overarching issues, and conservation should focus on managing the broader drivers of decline.

2. To what extent do other predators or environmental pressures contribute to capercaillie population declines, and how do you think these compare with the impact of pine martens?

The opinions highlight a complex set of factors contributing to capercaillie declines in Scotland, emphasizing habitat loss, climate change, and human landscape management as ultimate drivers, with predation, especially by pine martens, playing a secondary but localised role. Habitat degradation, fragmentation, and loss are primary challenges, though some suitable habitat remains unoccupied, suggesting additional pressures such as predation and adverse weather are limiting recovery. Climate variability, including unfavourable weather during the critical breeding periods, reduces chick survival by affecting food availability and increasing mortality, while recreational activities, infrastructure (e.g., deer fences), and predator subsidies from game management exacerbate declines.

Pine marten predation, though opportunistic, can hinder population recovery in stressed areas but is not abnormal compared to elsewhere. Capercaillie populations coexist with a full guild of predators in other regions (e.g., Scandinavia) and remain stable, indicating predators are proximate, not ultimate, causes of decline. It was thought that pine martens are perceived as a threat partly because their recovery is relatively unfamiliar to land managers. Other predators, such as goshawks, are known to predate capercaillie heavily in other regions but are under-discussed in Scotland. Broader ecosystem changes, such as the absence of apex predators, may have allowed meso-predators like martens to thrive, increasing predation pressure. However, excessive focus on pine martens risks oversimplifying a multi-factorial issue. Effective conservation requires addressing ultimate drivers such as habitat restoration, sustainable land management, and climate adaptation, alongside targeted measures like diversionary feeding. Long-term solutions may also involve ecosystem-level interventions, including apex predator reintroductions, to restore balance and create more sustainable conditions for capercaillie recovery. In summary, pine marten predation is one of many pressures on capercaillie populations, but the primary threats are habitat loss, climate variability, and human activities. Addressing ultimate drivers through ecosystem restoration and sustainable land management is critical to reversing declines.

3. Please list any conservation or management strategies that you believe have been effective in balancing the presence of pine martens with capercaillie protection

The opinions focus on strategies to mitigate pine marten predation on capercaillie, particularly diversionary feeding, habitat management, and broader ecological interventions. Diversionary feeding has proven highly effective in Scotland, significantly increasing chick productivity to levels comparable with Scandinavia’s best years under favourable climates. It is currently the most practical and ethically acceptable method, though its long-term effectiveness requires further study. Seasonal variations in pine marten feeding behaviour suggest feeding can align with critical periods of reduced predation. Other strategies, such as lethal control or translocation, are controversial or untested in Scotland, while habitat modifications, like increasing woodland patch size or reducing edge habitats, may help reduce predation rates but conflict with reforestation goals that favour martens.

Beyond direct predation management, it is thought that restoring apex predators such as lynx could help regulate pine marten populations, as seen in Finland. Habitat restoration and predator deterrent methods, combined with integrated land-use planning, are also critical. However, the broader ecological context, including climate variability and habitat fragmentation, plays a significant role in capercaillie survival, often beyond direct management control. While pine marten predation is a natural ecological pressure, it is unlikely to drive long-term declines if other conditions are favourable. Population reinforcement has been proven to be successful for capercaillie but untested in the presence of pine martens.

In conclusion, while diversionary feeding is the most effective immediate solution, a sustainable approach requires integrated conservation efforts. This includes habitat restoration, exploring alternative predator management methods, and addressing broader drivers like climate change and land use to ensure capercaillie populations can recover.

4. How might changes in forest management or habitat restoration affect the interactions between pine martens and capercaillie?

The opinions emphasize that habitat management plays a critical role in the conservation of both capercaillie and pine martens, with an emphasis on improving habitat quality and addressing fragmentation. While habitat availability is not a limiting factor for capercaillie, poor-quality and fragmented landscapes expose nests and chicks to greater predation risks. Restoring good-quality habitats with structural complexity, open glades, and food resources for chicks is essential. Increasing woodland connectivity through replanting could have mixed effects, potentially increasing predator-prey interactions in patchy habitats but reducing predation in larger, contiguous woodlands with less edge habitat. Strategic restoration must balance the needs of both species to promote coexistence.

Habitat management also influences predator-prey dynamics. Expanding mature woodlands and reducing clear-felled areas could lower pine marten densities by limiting alternative prey like field voles. However, this trade-off might increase capercaillie predation if martens shift their focus to nests. Complementary measures, such as reducing deer fence collisions and managing deer browsing to enhance vegetation resilience, could improve capercaillie survival. While predator-prey interactions remain complex, creating diverse habitats rich in alternative prey could mitigate marten impacts, though this approach remains unproven.

Despite the potential benefits of habitat restoration, climate change poses a significant long-term challenge, affecting chick-rearing conditions and threatening forest ecosystems. While habitat improvements can enhance resilience and reduce predation pressures, they cannot fully offset climate-driven risks. A holistic, adaptive approach addressing habitat quality, predator management, and external threats like climate change is essential for the sustainable conservation of capercaillie populations.

5. What gaps remain in the current research on pine marten predation on capercaillie and what future studies would you recommend?

The relationship between pine martens and capercaillie is complex, requiring further research and targeted conservation strategies. Diversionary feeding has shown potential in reducing nest predation but does not seem to affect chick predation. Long-term studies are needed to assess its effectiveness across various conditions and its impact on predator abundance, behaviour, and densities. Key questions include whether it reduces predation across all capercaillie life stages and how it interacts with natural food availability for martens. Research must also address whether diversionary feeding could unintentionally increase pine marten densities, exacerbating predation pressure.

Predation dynamics are influenced by multiple factors, with martens being key nest predators and other predators (e.g., foxes, corvids) likely responsible for chick predation. Understanding marten abundance and its relationship to predation rates, as well as identifying which predators target capercaillie at different life stages, is critical for designing effective interventions. Habitat quality and structure also play a pivotal role, as poor-quality habitats may increase predator-prey proximity. Improving habitat could reduce interactions, though restoration must consider broader ecological factors like climate change.

Predator management, such as fox or corvid removal, may inadvertently increase pine marten predation by reducing interspecies competition. The sustainability and feasibility of pine marten removal are debated, with concerns about recolonisation, unintended ecological consequences (e.g., spread of grey squirrels), and long-term benefits for capercaillie. Research into predator guild dynamics and climate change modelling is vital to address how future conditions, such as wetter springs, may exacerbate predation impacts and reduce chick survival.

To conserve capercaillie, a multifaceted approach is essential, combining habitat restoration, predator management, and innovative techniques like diversionary feeding. Conservation strategies must be underpinned by rigorous research into predator-prey interactions, climate resilience, and socio-ecological considerations. Long-term solutions may also involve capercaillie population reinforcement and exploring apex predator reintroductions, such as lynx, to regulate meso-predators like pine martens. There is a need to investigate long-term predator-prey interactions under climate change and human management scenarios, including apex predator reintroductions.

The opinions present a range of perspectives on the relationship between pine martens and capercaillie, emphasizing the need for further research, targeted conservation strategies, and an understanding of broader ecological dynamics. The capercaillie-pine marten dynamic highlights broader challenges in balancing the recovery of one native species with the conservation of another.

4. Discussion

Some additional evidence concerning the predator prey relationship between pine marten and capercaillie has become available since the previous (Reynolds, 2006) review was carried out. In Abernethy, Scotland, where foxes were controlled, Summers et al (2009) found pine martens to be the sole predator of artificial capercaillie nests. More recently Palencia and Barroso (2024) showed that martens (Martes spp) were the primary predators on artificial nests in an area with capercaillie, accounting for 23% of predation events, despite their lower population densities compared to other predators. Red fox was the second most frequent nest predator. Whilst acknowledging the limitations of artificial nest experiments, the findings underscore the role of martens in nest predation dynamics, although there was no distinction between stone and pine martens. 

In a PhD study by Jahren (2017), camera traps were used to provide empirical data on predation at 335 capercaillie and black grouse nests in the boreal forest. Nest predation patterns were analysed in relation to predator and vole populations.  Red foxes and pine martens were responsible for 84% of documented predation events, while other predators were rare, and hens successfully defended their nests against corvids. For capercaillie, red fox predation patterns supported the alternative prey hypothesis, suggesting that foxes turned to grouse nests when their primary prey was scarce. In contrast, pine marten predation was unaffected by vole abundance and increased with pine marten density, indicating a more specialised nest predation strategy. The study concluded that artificial nests are inadequate substitutes for real nests when aiming to identify and understand the mechanisms behind nest predation. Additionally, findings suggest that woodland grouse nests serve as alternative prey for red foxes, whereas pine martens are more specialised and efficient nest predators.

In another PhD study, Angoh (2024) used data from camera traps at capercaillie nests to see how different landscape factors affected the risk of nest predation by pine martens and red foxes. Results showed similar daily predation rates by both species, but pine marten predation declined with increasing density of agricultural land and increased from clearcuts to forest interiors, whereas fox predation rates were unaffected by these factors. 

There is a range of current evidence discussed here about whether manipulation of the predator guild, which includes pine marten, can benefit vulnerable prey species such as capercaillie. The report to NatureScot’s Scientific Advisory Committee (2022) concluded that predator removal would likely improve the breeding success of capercaillie but to be effective, this would need to be intensive and sustained over large areas. Furthermore, culling or removal of a legally protected species such as pine marten would be likely to produce an adverse public reaction and it is not currently clear whether capercaillie would benefit from the reductions in numbers of just one species from this guild of interacting mesopredators. Where the predator guild is incomplete, as it is in Britain, reintroducing apex predators, such as lynx, has shown promise in reducing mesopredator populations (e.g., foxes), benefiting lower trophic levels, including capercaillie. Some studies support the hypothesis that the absence of apex predators leads to increased mesopredator populations, known as the Mesopredator Release Hypothesis (MRH), However, in a 2024 review, Van Schaik et al (2024) analysed 47 European studies and 38 apex predator-mesopredator pairings to assess mesopredator responses to both decreases and increases in apex predator numbers. Findings showed that only 10 pairings predominantly supported the MRH, while 23 instances across 17 pairings contradicted it. They found that the MRH was more predictive in scenarios where there were moderate to large body size differences between predators and where mesopredators were primarily hunters. The study concluded that mesopredator responses to apex predator population changes are complex and not uniformly explained by the MRH. Nonetheless, exploring this option in Scotland could provide a natural regulation mechanism for predator populations, but feasibility and ecological impacts require further study. 

We found some evidence to suggest that management of the pine marten population within the current capercaillie range might result in increased capercaillie productivity.  The results of the study by Jahren (2017) could indicate that, time or area-limited management of martens might be useful, for example when or where vole numbers are low and prey-switching is likely. This could be when management interventions such as diversionary feeding would be most effective. However, depending on marten density, this may be needed regardless of vole numbers as Jahren (2017) also found that when pine martens reached very high abundances, the alternative prey hypothesis broke down and nest predation rates were unaffected by the vole cycle. 

The synthesis of findings presented here reveals that capercaillie population declines are driven by a multifaceted and interlinked set of factors, requiring an integrated approach to conservation. This concurs with the recommendations of the Capercaillie Emergency Plan produced by the Cairngorms National Park Authority and partners (CNPA, 2024).  Long-term reproductive success has declined across Europe, with Scotland experiencing ongoing reductions, driven by habitat degradation, climate change, and predation pressures. Habitat fragmentation and degradation have resulted in the loss of preferred environments, increased edge effects, and enhanced predator efficiency, creating ecological traps that expose capercaillie nests and chicks to elevated predation risks. These human-altered landscapes exacerbate predation rates by making prey more accessible, particularly to opportunistic predators like pine martens and red foxes, which adapt their foraging behaviours in fragmented habitats. Climate change further complicates the situation by disrupting capercaillie nesting patterns, reducing food availability during critical breeding periods, and increasing chick mortality due to hypothermia or predation exacerbated by extreme weather events.

Predation, while significant, is a proximate rather than ultimate cause of decline, as demonstrated by the coexistence of capercaillie populations with full predator guilds in other regions, such as Scandinavia. Opportunistic predators, including pine martens and red foxes, predominantly target nests and chicks, but regional variation in their impact underscores the need for localised conservation strategies. For example, studies from Scandinavia indicate that capercaillie populations benefited from reduced red fox numbers during a sarcoptic mange outbreak, despite a simultaneous increase in pine marten densities. This highlights the complexity of predator-prey dynamics and the need to address interactions among predators, habitat conditions, and prey species. The experts consulted expressed a range of perspectives on the relationship between pine martens and capercaillie, emphasising the need for further research, targeted conservation strategies and an understanding of broader ecological dynamics. Pine marten predation is just one of many pressures on capercaillie populations, but the primary threats are habitat loss, climate variability and human activities. Addressing ultimate drivers through holistic ecosystem restoration and sustainable land management is critical to their long-term survival. 

Climate change poses an overarching challenge to capercaillie conservation, influencing habitat suitability, food availability, and chick survival. Wetter springs and warmer summers disrupt breeding cycles, reduce insect availability, and increase predation risks. Conservation strategies must address these impacts by promoting habitat resilience, improving vegetation quality, and ensuring capercaillie populations can adapt to changing conditions. Population reinforcement has been suggested (CNPA, 2015) and methods have advanced in recent years to increase survival and site fidelity (Merta et al., 2015b, Merta et al., 2016, Merta et al., 2015a). This could enhance population resilience and boost numbers in areas where local populations or genetic diversity are critically low. It could help offset low reproductive success and mitigate the impacts of habitat fragmentation and predation pressure in the short term. Additionally, reinforcement could serve as a complement to ongoing long-term conservation measures such as habitat restoration. 

Strathspey experiences high levels of anthropogenic disturbance from recreational activities, including visitors using new forest tracks and remote off-track areas. This disturbance reduces the amount of suitable forest habitat available to capercaillie. Establishing refuge areas where recreation is discouraged has the potential to mitigate these effects. However, results of modelling simulations testing population growth under scenarios with varying numbers of refuges, suggest that a network of refuges could support a growing capercaillie population but that refuges alone will not prevent overall population decline driven by other factors (Geary et al., 2023). The species long term survival will depend on improving its performance across a broader range of conditions. 

Predator control has been widely used to manage grouse populations, particularly in Europe where grouse are often managed for hunting purposes. Some studies show that predator control can improve grouse breeding success, particularly when generalist predators like foxes and corvids are targeted. However, the effectiveness of predator control depends on factors such as predator density, the intensity and consistency of control measures and habitat quality. In their Review of Capercaillie Conservation and Management (NatureScot, 2022), NatureScot’s Scientific Advisory Committee concluded that management initiatives aimed at removing predators can produce immediate effects, but it presents practical and societal challenges. Predator control remains controversial, labour-intensive, and ecologically complex. Evidence on its effectiveness is mixed, with outcomes often dependent on perceived effectiveness rather than robust scientific evidence. Predator guilds are highly interconnected, and the removal of one predator can lead to compensatory increases in others, introducing unintended ecological consequences. For example, removing red foxes may inadvertently increase pine marten predation due to reduced interspecific competition. Similarly, removing pine martens (either by licensed culling or translocation) is unlikely to address long-term declines as there are other protected predators in the region including badgers (Meles meles), goshawks and European wildcats (Felis silvestris), which may also predate capercaillie (Moreno-Opo et al., 2015). Furthermore, emptied territories will most likely act as sinks, admitting new individuals from nearby areas in the short term, with the removed animals being replaced by younger, dispersing ones over time. 

Removing pine martens for translocation to support reintroduction projects elsewhere in Britain has been proposed, however the timing for this is not compatible with the optimal timing of predator removal for protecting capercaillie nests. Removals with this objective would need to be carried out in March/April, immediately prior to the period when capercaillie lay their eggs. Unfortunately, this also coincides with the period when female pine martens have dependent young, and so trapping of any females would be both unethical and pose significant welfare issues. The same would also be true of any control carried out during the marten breeding season (late February-end of May). Pine marten translocations of adult, breeding-age animals should be carried out in early autumn, by which time the young are independent and the adult females have already mated (Macpherson, 2017). This is when young of the year are dispersing and seeking their own territories so if adults were translocated in the autumn for reintroductions, but with the dual objective of protecting capercaillie nests in the following spring their territories would immediately be occupied by dispersing juveniles, negating any effect. 

Recommendations and priorities for further research

The most promising immediate conservation strategy is targeted diversionary feeding, which has shown success in reducing predation on capercaillie nests and chicks by redirecting predator behaviour. However, this approach requires careful monitoring to avoid unintended consequences, such as increasing spatial aggregation of predators or altering ecosystem dynamics. Current research suggests that vole cycles affect the levels of predation on alternative prey by foxes but have less effect on pine marten predation at high marten abundance. More robust information is needed on marten densities in priority capercaillie regions.  Further research should also focus on how vole cycles affect the uptake of diversionary feeding by different predator species, as well as the interactions between them at feeding sites. 

It is recommended that long-term studies are undertaken on pine marten predation across capercaillie life stages and how marten abundance is affected by that of other predators. Modelling can also be used to assess future climate scenarios and their long-term effects on chick survival and predation dynamics. Models should also incorporate the effects of climate change on habitat availability with and without additional forest creation and expansion schemes. The feasibility and potential ecological benefits of reintroducing apex predators to regulate mesopredators should be explored along with comprehensive Cost-Benefit Analyses to evaluate the financial and ecological trade-offs of various conservation measures.

Habitat restoration is critical for reducing predation pressures and improving capercaillie reproductive success. The importance of habitat quality may be secondary to habitat quantity (i.e. extent) (Sirkiä et al., 2011, Lande et al., 2014, Wegge and Rolstad, 2011), therefore longer-term conservation strategies should include creating larger, contiguous woodlands with structural complexity, open glades, and robust food resources, while minimizing edge effects and addressing deer fence collisions.