Home Among the Trees: How Pine Martens Choose and Use Dens
Pine martens are a denning species, relying on safe, elevated, and insulated dens to survive and thrive in the wild. In this article, we explore the essential role of dens in a pine marten’s life, the key features of an ideal den, and how artificial den boxes can support their survival where natural denning options are limited.
A pine marten exploring the forest.
Why Dens Matter: Safety and Shelter for Pine Martens
Pine martens rely on dens year-round for resting and as safe spaces for mothers to raise their kits in spring. When choosing a den, they primarily seek protection from predators and shelter from harsh weather. With their excellent climbing abilities - thanks to adaptations like rotating hind paws and semi-retractable claws - pine martens are well-suited to off-ground dens, which provide the best opportunities for safety and shelter.
Protection from Predators
In the UK, pine martens face predation risks from foxes and larger birds of prey, such as goshawks and golden eagles. To minimise this risk, they seek elevated dens, which are especially important for mothers with kits. Kits are particularly vulnerable in their early months when they are small and reliant on their mother to learn essential survival skills, including climbing and hunting. By choosing off-ground dens, mothers provide a safer environment for their young as they develop these abilities.
Shelter from the Elements
As a mostly nocturnal species, pine martens spend around two-thirds of their active time outside their dens at night. While dens provide daytime protection from predators, they also serve as shelters against the elements. Due to their slender bodies and limited fat reserves, pine martens tend to remain in their dens for longer periods during chillier months. Concealed dens protect them from snow, rain and wind, helping them stay warm and dry in winter.
Cavity in a veteran oak.
Home Sweet Home: What Makes a Perfect Den?
Pine martens use a variety of resting sites throughout the year. A study in Poland found that males, on average, utilise nearly 200 different resting locations annually, while females use closer to 150. The type of den a pine marten chooses depends greatly on its surroundings and the available resources, which are often shaped by human influence. In Poland’s old-growth Białowieża Forest, arboreal resting sites comprise over 95% of their chosen dens, suggesting that these elevated locations are their preference. However, due to deforestation and other land management practices, ideal denning options may not always be accessible, resulting in the use of suboptimal denning places like rocky crevices and buildings.
Pine martens adapt their den choice according to the climate and season. For example, birds’ nests in spruce trees are popular resting places during warmer months, while tree cavities provide the insulation needed during colder months. Oak and lime trees are especially valued for denning in cavities. As oaks age, their natural decay process forms soft areas that can be excavated by woodpeckers, creating ideal denning spaces for martens.
Cavity in a veteran oak (image credit John Martin)
In regions with fewer tree cavities and harsher winters, such as Scandinavia’s boreal forests, pine martens may resort to ground-level dens, like burrows, to stay warmer; however, this choice increases their predation risk. As a result, ground-level dens are a last resort, used only when warmth is paramount.
When it comes to natal dens, where a mother gives birth and raises her young, space and protection are essential. With litters ranging from one to four kits, mothers need roomy, safe dens. In continental Europe, cavities made by black woodpeckers offer ample space and security for raising young. These dens are typically used for 45 to 70 days, so it is important for mothers to find the right den first time to avoid the risks associated with moving vulnerable kits to another den.
In Scotland, research found that pine martens used trees for 44% of their dens, rocks for 28%, and buildings for 14%. However, for natal dens specifically, 61% were located in buildings and other man-made structures, with only 23% in trees. This reliance on human-made structures reflects a scarcity of suitable arboreal dens, forcing martens to use riskier alternatives. The shortage of natural denning options highlights a need for conservation efforts to provide safer, more suitable choices, particularly in areas lacking tree cavities.
Thermal photograph of a cavity in a beech tree, likely containing a pine marten (Image credit John Martin)
Restoring Shelter: Artificial Den Boxes Support Pine Marten Recovery
Woodlands in Britain have been dramatically shaped by centuries of timber harvesting and land management practices. By the early 1900s, Britain’s woodland cover had dwindled to an all-time low of around 5%. Today, reforestation efforts have increased woodland cover to approximately 13%. However, a challenge remains for wildlife: most of this woodland is relatively young, with only 2.5% of Britain’s land classified as ancient woodland. Ancient woodlands provide essential resources for a wide variety of species, including mature trees with more abundant natural den cavities for pine martens.
Ancient Caledonian pinewood at Alladale Wilderness Reserve.
In areas where natural denning cavities are scarce, artificial pine marten den boxes have been introduced to provide safe, elevated, and insulated dens. These boxes are typically installed in forestry plantations, which, while offering foraging opportunities, often lack the suitable denning sites more commonly found in mixed and deciduous woodlands.
The original pine marten den box design, known as the VWT-style den box, was created to replicate the size of cavities made by black woodpeckers in mainland Europe. This size of den is suitable for a mother to raise her young, or for a solitary pine marten to rest in during colder days. A study in Galloway Forest found that 30–70% of the VWT-style den boxes installed there showed signs of pine marten use each year, highlighting the importance of these artificial dens within plantation forestry.
Installing a VWT-style pine marten den box in Yorkshire.
Pine marten resting inside a VWT-style den box.
Other designs of pine marten den boxes have also been developed, such as the Galloway Lite den box. This box features a slightly smaller wooden denning chamber, surrounded by a recycled plastic barrel, making it more suitable for installation on smaller trees compared to the VWT-style den box. Installing a variety of artificial den box styles in a forest provides a better range of denning options for pine martens.
The slightly more compact Galloway-Lite den box.
Pine marten prospecting a Galloway Lite den box for shelter.
Supporting Pine Martens Now and in the Future
NatureSpy has installed den boxes in the North York Moors National Park as part of the Yorkshire Pine Marten Pathways Project, increasing and improving denning opportunities for this elusive species. Drawing on our experience in Yorkshire and from pine marten initiatives in Scotland, we’ve collaborated with experts to develop guidance on monitoring den boxes using trail cameras.
By providing safe, insulated, and elevated dens where natural options are scarce, artificial den boxes play a vital role in supporting pine marten recovery. As woodlands evolve and conservation efforts continue, these carefully placed dens offer an important stepping stone in ensuring a future where pine martens can thrive.
