Pine martens predate capercaillie eggs and chicks and contribute to reduced breeding success. Poor breeding success is the main reason for ongoing capercaillie declines.
What the science says
Pine marten predate capercaillie eggs and chicks1,2, and increases in their numbers and distribution in Scotland means there is now more interaction between the two species compared to 30 years ago3-5. Several studies, including a recent NatureScot review, suggest that increases in predator numbers are contributing to reduced capercaillie breeding success, which is driving their decline in Scotland6,7.
Whilst pine martens are contributing to the decline, several factors combine to have a cumulative negative impact on capercaillie breeding success. The combination of warmer April and wetter June weather, and increased predation pressure from foxes, crows, and pine marten alike all appear to be driving declines and may interact, for example by adverse weather making chicks more susceptible to predation4,5,8-11. Additionally, the impact of remaining deer fencing on adult birds cannot be overlooked. Recently modelled data suggest that in the absence of deer fencing, capercaillie numbers would be 16% higher and the risk of extinction within the next 50 years would fall from 95% to just 3%10. These factors, alongside possible threats from habitat change, parasitic sheep ticks, and disturbance from increasing human activities create a steep uphill battle to prevent a second extinction of Scottish capercaillie5,7,10,12,13.
Around 500 birds now remain in Scotland6,10,14 with 85% of those found in one small area of Strathspey in the Cairngorms National Park15. Given such small numbers, every threat, including that from pine martens and other predators, has the potential to significantly impact the remaining birds5,7,16.
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 capercaillie17,18 while others contest this19,20.
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 occur6,8,15,21,22. 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 century5,23. The species is listed as ‘least concern’ across its range in central, northern, and eastern Europe24,25 but is listed as endangered in the UK26,27. 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 numbers6,29. 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 Strathspey10,15. 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 2021/226,14, 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 Scotland4,6,32.
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 conditions5,16,33-36. 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 hatching5,37-39. 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 grouse41,42. Less is known about the impact of ticks on capercaillie, but ticks can parasitise and weaken chicks, leaving them unable to forage or avoid predators2,43.
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 Europe6,13. 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 capercaillie12,13. 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 patches5,47,48,51. 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 threat2,53,54.
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 fencelines10,29,53. 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 rates6,7,10. 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 success1,5,16,22,35,59. 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 birds1,5,8. In cases where foxes and crows were controlled capercaillie breeding success improved, possibly to higher rates of egg and chick survival22,35. 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 success5,16. Pine martens have undergone their own population recovery following habitat loss and intensive culling through the 19th and into the 20th century4,57,60,61, gaining full legal protection in 1988 and recolonising much of their previous range since then4,62,63. This recovery has made pine marten more common in woodlands containing capercaillie3,4, with pine marten indices 3.9 times higher in 2009 than in 19954,5.
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 significant4,5,8-11. 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 success7,10. These factors require more investigation, but given that around 500 birds are thought to remain in Scotland6,10,14 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.
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 populations5,7,10,12,13. Given how small capercaillie populations are now estimated to be6,10 every threat they face will push them closer to extinction, with predation – particularly from growing pine marten populations – adding additional pressure on the birds5,7,16.
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 avoidance45-47; and worsening weather conditions making successful breeding attempts even more challenging16,33-36.
Conservation actions are best directed at boosting breeding success to effectively improve population recovery and prevent a second extinction of Scottish capercaillie6,7. 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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