Fantastic beasts and where to find them

Pine Marten Terry Whittaker/2020VISION

Josie, pine marten project officer, tells us about pine marten residential preferences and how we can make habitats more attractive for pine martens to den in…

Lockdown life

As the country enters another lockdown and we all become reacquainted with staying indoors for the foreseeable future, it seemed apt to use this blog to discuss the pine marten’s ‘working from home’ set up. And if you’re going to have to stay at home, why not do it in style like a pine marten, with a penthouse view from a rustic wood panelled pad.

Pine martens are well adapted to a lockdown life. They are solitary by nature; only forming social bubbles with their kits, who they’ll tolerate in the den and at feeding sites for several months after weaning. They aggressively remove other martens from their patch, they don’t meet up with friends, and only leave the den for their ‘daily exercise’ and excursions for food from the forest. They were born ready for lockdown.

What isn’t so pandemic-friendly though is the way they cache food in their houses (although not toilet paper we assume) and their habit of moving den sites each night would probably be frowned upon, especially as we’re pretty sure a mask wouldn’t fit over their pointy noses (1). Martens have huge territories for their body size and it is thought they move den sites often to patrol their patch, but also to avoid parasite burdens (i.e. build up of fleas) and depleting the prey source too much in any one area (2). They are very clever little creatures, with good memories, and we find that they will circle back and reuse den sites again and again especially if it’s in good foraging ground.

Denning habitat

Pine martens have long been considered ‘old forest specialists’ meaning they need trees with large hollow cavities to thrive (3). Despite their name, pine martens are not dependant on pine forests for their survival…but they do like to have trees in some form. They much prefer mixed habitat, especially when in comes to denning, with the rich variety of cavities, nooks and undergrowth for foraging that broadleaf trees of various ages provide (4).

Pine martens rarely excavate their own dens, those claws are for climbing, not digging! Instead, they prefer to use a range of woodland structures. Not only will they den in tree cavities, but they will use squirrel dreys, windthrow, root bowls from upturned trees, and scrubby crags and crevices. They will also occasionally be found on the ground in rabbit warrens, badger sets and piles of brash (1).

Safe from predators

Predation is an important element influencing selection of resting and denning sites. In England, red fox Vulpes vulpes, is their biggest predator but in other countries lynx, eagle owls and golden eagle have preyed upon them. Whilst the adults are occasionally predated by foxes directly, it is the kits that are particularly vulnerable. Kits are slow to develop and womble about for several weeks helplessly. As a result, dry and roomy cavities high up off the ground are crucial to protected them from predators. Studies have shown female martens prefer arboreal cavities all year round as well as secure shelter for birth and rearing of kits, whereas males are happier to ‘rough’ it in ground level dens as they are bigger and less susceptible to predation (5). Anecdotally, we can say that this has been mirrored in the Forest of Dean, with all of the kits being born and raised in tree cavities.

Some studies have shown that martens at more Northern latitudes tend towards subnivean denning sites (dens underneath the snow) to stay warmer and that these sites seemed particularly important for resting in winter (6). If only it snowed enough in Gloucestershire!

Naturally occurring den sites

Natural tree denning opportunities can be rare in the managed conifer forests that make up much of the UK’s commercial forestry, where trees are grown on a short rotation for specific markets. In more recent years, forest managers have made efforts to preserve living and dead trees with cavities to the benefit of martens (and other tree cavity dwellers). But as pine marten-suitable cavities in Europe are often initially excavated by black woodpeckers (a species which is not present in the UK) there is still a lack of suitable denning sites for these animals in Britain (7).

Artificial den sites

In the absence of natural structures, they will often occupy owl boxes and old abandoned buildings, occasionally venturing into loft spaces in still occupied houses. But even early feasibly work from the 90’s highlighted the probable need for artificial den sites before any reintroduction in the British Isles. Den boxes were first introduced to Britain by VWT in 2003 and their use is now widespread across much of Scotland and Wales. These wooden boxes, with their three chambers and two entrances, mimic the warm dry tree cavities martens need to rest, cache food and raise their kits in safely.

Whilst we are so fortunate in the Forest of Dean to have over 50% of the woodland comprised of broadleaf trees, we still install den boxes for pine martens, particularly in areas where the habitat lacks premium pine marten real estate. This not only improves the chances of martens taking up residence in the area, but they provide us with a great monitoring tool. As you’ll have heard by now from our blogs, martens are notoriously elusive, so having artificial boxes that are at a reasonable height and easy to find are much simpler for the pine marten team to monitor than cryptic cavities often 4+ meters up a tree and covered in ivy. 

Monitoring martens

We will use annual den box checks to monitor the martens, and their breeding, into the future. These checks are undertaken in late spring and, as pine martens are a protected species, are only carried out by people who have Natural England licences to do the work. However, we will be training volunteers how to monitor these boxes from a safe distance and let us know if they look to be in use. And the tell-tale sign? You guessed it….scat! (for all scat related curiosities check out this blog).

References

1 - Croose, E. et al. 2016. Den boxes as a tool for pine marten Martes martes conservation and population monitoring in a commercial forest in Scotland. Conservation evidence 13, pp.57-61.

2 - Birks, J., Messenger, J., Halliwell, E., 2005. Diversity of den sites used by pine martens Martes Martes: a response to the scarcity of arboreal cavities? Mammal Review 35(3&4), pp.313-320.

3 – Clevenger, A. P., 1994. Habitat characteristics of Eurasian pine marten martes martes in an insular Mediterranean Environment. Ecography 17(3), p.p 257-263.

4 – Brainerd et al, 1995. Pine marten (martes martes) selection of resting and denning sites in Scandinavian managed forests. Annales Zoologici Fennici 32, p.p. 151-157.

5 - Ruette, S. et al. 2015. Quantifying the age- and sex-dependent morphological variation in two syntopic mustelids: Martes martes and Martes foina. Mamm Biol 80, 414–423.

6 – Pullianinen, E, 1981. Winter habitat selection, home range and movements of the pine marten in a Finnish Lapland forest.

7 – Johnsson, K., 1993. The black woodpecker (Dryocopus martius) as a keystone species in a forest. Ibis 135(4), p.p. 410-416