Are pine martens contributing to capercaillie declines?

Where the claim came from

Recent discussions over the struggles facing Scottish capercaillie have intensified, with conversations centring on the threats facing the birds. The role of predation in capercaillie declines is often controversial, with some supporting that predation, and specifically predation from pine martens, is the main issue facing capercaillie^17,18 while others contest this^19,20.

What the science says – the fuller picture

Capercaillie are an iconic species found in old pine forests in Scotland, notably within the Strathspey area of the Cairngorms National Park, where 85% of remaining birds occur^6,8,15,21,22. The birds went extinct in Scotland in the 1700s following deforestation and over-harvesting and were reintroduced in 1837, with recovery possible because of afforestation and intensive predator control efforts throughout the 19th and into the 20th century^5,23. The species is listed as ‘least concern’ across its range in central, northern, and eastern Europe^24,25 but is listed as endangered in the UK^26,27. Hunting of capercaillie has been banned in Scotland since 2001²⁸ following a voluntary ban throughout the 1990s.

There were thought to be 20,000 capercaillie in Scotland in 1970, although the origin of this estimate is not clear; since then, there have been notable declines in numbers^6,29. National surveys in the mid-1990s found that there were around 2,200 birds remaining³⁰. This suggests there was a steep rate of decline between the 1970s and 1990s, with this seemingly slowing in the 2000s and 2010s, with population estimates fluctuating between 1,000-2,000 birds¹⁵. However, by 2016 capercaillie were thought to be extinct in areas such as Fife, Argyll, and Loch Lomond and the Trossachs – with the remaining birds concentrated in Badenoch and Strathspey^10,15. The declines observed between 1994 and 2018 are the largest long-term decrease for any bird species in Scotland³¹.

The most recent calculations estimate that just 304 birds remained in 2020¹⁰, suggesting that the rate of decline has steepened since the 2010s. The yet unpublished sixth national survey estimated that 542 individual birds remained in winter 2021/22^6,14, with the number of lekking males declining across their Scottish range and worryingly, in the core Badenoch and Strathspey area¹⁰. As a result, scientists have calculated that without remedial action there is a 23% chance the species will be extinct within 25 years, with a 95% chance it will be extinct within 50 years¹⁰.

Why are capercaillie populations declining?

Many studies, reports, and conservation organisations agree that low breeding success is the biggest issue for capercaillie, being the main driver of declines and what is preventing population recovery in Scotland^4,6,32.

Changes in annual weather patterns in Scotland can influence capercaillie breeding success, with cool, wet June weather negatively affecting chick survival. This could be the result of chicks becoming damp and cold, or because less food is available in these conditions^5,16,33-36. This may become a bigger issue with climate change, with shifts in temperatures and rainfall patterns potentially leading to a mismatch in the timing of plant growth, invertebrate emergence, and capercaillie egg laying and hatching^5,37-39. These weather changes may also improve conditions for parasites such as sheep ticks, making them more prevalent and potentially increasing their impact on chicks. Tick numbers have increased in Scotland over recent decades⁴⁰ and they are known to impact breeding success in and spread louping ill virus to red grouse^41,42. Less is known about the impact of ticks on capercaillie, but ticks can parasitise and weaken chicks, leaving them unable to forage or avoid predators^2,43.

Disturbance from human activity is another growing threat to capercaillie – with the number of visitors to the Cairngorms National Park increasing by some 32% between 2009 and 2019⁴⁴. Outdoor recreation is a long-recognised problem and source of disturbance for capercaillie and other wildlife in Europe^6,13. Generally, any area within 100 metres of a woodland track is likely to be avoided by capercaillie⁷. A recent study found that capercaillie in the Black Forest stay at least 145 metres away from mountain bike trails and up to 320 metres away from other recreational infrastructure¹³. Studies from Scotland found that capercaillie droppings – a measure for capercaillie activity – were less common within 70-235 metres of woodland tracks⁴⁵, with another study finding that capercaillie avoided woodland tracks by some 61-108 metres, equating to avoidance of one hectare of woodland for every 46-82 metres of track⁴⁶. These studies suggest that human disturbance could be limiting capercaillie distribution in Scotland, but at present there is no evidence that disturbance has an impact on breeding success⁴⁷. In central Europe, the increased presence of recreational users in suitable forests is thought to have created a ‘landscape of fear’ for capercaillie^12,13. Given that higher densities of roads and woodland tracks are associated with fewer signs of capercaillie, the use of tracks and roads in Scottish woodlands may need to be modified⁴⁷.

In Scotland, the old pinewood forests rich in bilberry that capercaillie prefer ^{5,16,22,48-50} often exist as fragmented patches^5,47,48,51. These forest fragments have greater ‘edge effects’ with other habitats such as farmland, which can support higher predator numbers⁴⁷ and are more susceptible to disturbance where habitats are fragmented by pathways and roads⁴⁶. Where habitats are more fragmented, populations can become smaller and more isolated, and so the risk of birds breeding with their relatives can increase⁵². However, the fact that Scottish capercaillie populations are the product of multiple reintroductions⁵² and that juvenile females disperse and move between forests mitigates this threat^2,53,54.

Boundary fences, or those designed to keep deer out of regenerating areas of woodland, both injure and kill capercaillie that fly into and collide with them, for example when birds are disturbed or being pursued by predators. This is thought to be a particular problem for juvenile birds, with collisions occurring when they disperse into new forests and so may be unfamiliar with existing fencelines^10,29,53. This was thought to increase mortality rates and quicken the rate of decline between 1980 and 2000, with both the removal and marking of fences over the last 30 years thought to have had a positive impact on adult survival rates^6,7,10. Fence marking may be a useful way of reducing fence collision rates, particularly where fences cannot be removed, with a research trial able to reduce capercaillie collisions by 64% by marking fences with orange netting⁵⁵. Recent modelling suggests that fences may still impact capercaillie numbers, and in the absence of deer fences, numbers could be 16% higher and the risk of extinction within 50 years might fall from 95% to just 3%¹⁰.

What about predation?

Numbers of some predators have increased across the UK in recent decades, including foxes⁵⁶, pine martens⁵⁷, and crows⁵⁸. Within capercaillie range in Scottish forests, fox and pine marten indices increased 2.2 and 3.9-fold between 1995 and 2009⁵. Whether the numbers of these species have changed since is not known, with uncertainty surrounding if other species such as badgers may have on capercaillie⁷.

A wide range of independent studies agree that predators have a major impact on capercaillie breeding success^{1,5,16,22,35,59}. Predators such as foxes, crows, and pine marten are likely to influence capercaillie numbers through the consumption of egg clutches and chicks, with foxes and pine martens also known to predate adult birds^1,5,8. In cases where foxes and crows were controlled capercaillie breeding success improved, possibly to higher rates of egg and chick survival^22,35. Additionally, the abundance of crows and pine martens has been linked with poor capercaillie breeding success⁵.

So, are pine marten to blame?

Pine marten activity has been negatively related to capercaillie breeding success – meaning that where there is more pine marten activity, there is poorer capercaillie breeding success^5,16. Pine martens have undergone their own population recovery following habitat loss and intensive culling through the 19th and into the 20th century^4,57,60,61, gaining full legal protection in 1988 and recolonising much of their previous range since then^4,62,63. This recovery has made pine marten more common in woodlands containing capercaillie^3,4, with pine marten indices 3.9 times higher in 2009 than in 1995^4,5.

The increased presence of pine martens is likely to be intensifying the pressure on capercaillie populations, particularly by reducing breeding success, with a recent review by NatureScot echoing this⁷. Research investigating the impact of predators on capercaillie supports this, with one study finding that pine marten were the main cause of nest losses, predating 33% of observed nests¹; another in Norway found that around 90% of observed nests were lost due to predation, with pine marten being the main culprit¹¹.

While it is highly likely that increased numbers of pine marten have helped to accelerate capercaillie population declines, it is not doing so alone. It is much more probable that increasing pressure from pine martens is part of a cumulative impact on capercaillie breeding success. The combination of warmer April and wetter June weather, and increased predation pressure is proving to be particularly significant^4,5,8-11. At present, there is a lack of evidence that changes in habitat quality or availability, parasites, or inbreeding can explain the observed reductions in breeding success^7,10. These factors require more investigation, but given that around 500 birds are thought to remain in Scotland^6,10,14 there may no longer be time to study these factors without first taking decisive, remedial conservation action to prevent a second extinction of capercaillie in Scotland.

Conclusion

Many studies agree that predation, deer fencing, and changes to spring and summer weather are the biggest threats facing capercaillie. These combined with other contributing factors such as habitat change, parasite burden, and disturbance from human activity create a steep uphill battle for the remaining capercaillie populations^5,7,10,12,13. Given how small capercaillie populations are now estimated to be^6,10 every threat they face will push them closer to extinction, with predation – particularly from growing pine marten populations – adding additional pressure on the birds^5,7,16.

It cannot be overstated that these birds face perils from all directions, with deer fencing remaining a very significant threat to adults and dispersing juveniles¹⁰; increases in human activity potentially leading to habitat avoidance^45-47; and worsening weather conditions making successful breeding attempts even more challenging^16,33-36.

Conservation actions are best directed at boosting breeding success to effectively improve population recovery and prevent a second extinction of Scottish capercaillie^6,7. Continued removal and marking of deer fences should also be a management priority to help minimise mortality of adults and dispersing juveniles. This is essential, as maintenance of high adult survival rates means that reduced breeding success can be tolerated for longer, giving conservation management more time to work¹⁰.

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