Little Furry Slugs and Teenage Dirtbags

Pine martens have been in the Forest of Dean since September and have been doing extremely well, they've made the forest their new home. In today's blog Project Officer Josie shares the latest pine marten updates including news of the new arrivals.

Our champagne moment

If you’ve been following the pine marten project over the last nine months, you’ll know that much of our time has been taken up with nocturnal radiotracking, trying to keep up with our wee beasties as they explore their new home. Martens tend to move den-site most nights - to patrol their territory, avoid build-up of ticks and fleas, and to avoid depleting the prey source in one area. But if a marten gives birth she will stay in the same den-site for weeks at a time1. With many of our animals still radio-collared, we were able to tell when they settled down to give birth to their young (called kits!). It was with great delight that we discovered at least four of our females gave birth in March this year. These are potentially the first martens born in England in over 100 years.

Since the initial discovery, it has been a joy to take on a slower pace of ‘tracking’ as the females have settled into a den-site for longer than a single night. As a result, we have been able to deploy cameras at den sites (under Natural England licence) to capture these secretive little animals in the most exciting period of the year. We are licenced to check den-sites at the start of May (when the kits are around 6 weeks old) to confirm how many kits have been born and their condition. But, our little ones just couldn’t wait to make themselves known to the world and poked their heads out a few days before we were due to check.

Pine marten kits A+B

Breeding ecology

Pine martens are very slow breeders, it’s one of the reasons why they have struggled to bounce back in the natural way polecats have. They typically only have 2-3 kits in a litter and of these, often only 1-2 will survive until adulthood2. In addition, they are slow to start breeding, having an extended ‘sub-adult’ phase and typically waiting to breed until they are around 3 years old. Compare that to a female stoat or a weasel that will have its first litter at around 3 months old3!

Pine martens mate in the summer months (July-August), and due to the overlapping nature of most marten’s territories both males and females will mate with several different partners4. Female martens then go through ‘diapause’ where the fertilised egg doesn’t implant or begin to grow until early the following year. The cue for this process to begin is thought to be caused by changing day length. This is when the females officially become pregnant and the hormonal changes cause the marten to search for the perfect maternity den site.

Den sites can range from natural tree cavities, to old squirrel dreys, to artificial den boxes, and occasionally ground sites like rabbit warrens or brash piles5. But when seeking a maternity den site, martens tend to be a bit pickier, choosing sites that are safer and a bit roomier. All our females with kits in Gloucestershire used natural tree cavities.

Pine marten pic C

The natural densite for a pine marten is a cavity in a tree, just like this old Yew tree

The gestation period is approximately 30 days, with the martens normally giving birth around March3. A newly born kit will only weigh about 30g, be lightly furred with silvery grey hairs, and have its eyes and ears closed (affectionately known as ‘furry little slugs’ by the pine marten team).

By about three weeks the kits look more like the animal they will become; with soft brown fur and a white (which will darken to creamy-yellow) bib now visible. At this age, the mothers will only leave the den for a couple of hours to help keep the kits warm and well fed, with lactation lasting around 45 days2,6.

Once weaned the kits first start to explore outside of the den and they are very clumsy; ‘wombling’ about without much hint of the real talent they will develop for arboreal climbing. They are also insatiably curious, and it is fairly common for kits to fall out of the tree. Whilst the fall itself seems to do little damage, opportunistic passing predators can be a real danger7,8. Even at a young age they have a real instinct to grip, but at this point still have no chance of climbing back up to safety. So instead they sit and ‘squawk’ on the forest floor until mum comes to pick them. During these regular rescues by the harried female, mum also makes a delightful ‘chittering’ noise that is reminiscent to mothers everywhere of a good telling off.

Pine marten pic D

These sweet little faces are quite good at getting themselves into trouble!

Making their own way in the world

Martens tend to disperse from their mother later than many other mustelids, and it’s not uncommon to spot this usually solitary animal in small family groups throughout the autumn and into winter. Pine martens reach adult body size at around 6-8 months old, but they retain their youthful look, with pristine fur, sharp shiny white teeth and playful behaviour3. With a tolerant mother and good food availability, marten youngsters quite commonly don’t disperse until the February after they were born (around 11 months old) the timing coinciding with when the mother should be pregnant again9.

With our females’ radio-collars reaching the end of their lives and our kits slowly but surely becoming more independent, soon we won’t be able to radio track the animals at all. Instead, we monitor them remotely, using camera traps and our records of their unique ‘bib’ markings to keep track of who is where. We can’t wait to find out what happens to these first-generation English pine martens.


Until next time!


Team pine marten

Pine marten pic E

One of FD19's kits almost fully grown.


  1.  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.
  2.  Kleef, H., and Tydeman, P., 2009. Natal den activity patterns of female pine martens (martes martes) in the Netherlands. Lutra 52(1), pp.3-14.
  3.  Birks, J. 2017. Pine Martens. Whittet Books Ltd, Essex, UK.
  4.  Macdonald, D., 2001. The New Encyclopaedia of Mammals. Oxford University Press, Oxford.
  5.  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.
  6.  Kleef, H., Wijsman, H.J.W., 2015. Mast, mice and pine marten (Martes martes): the pine marten’s reproductive response to wood mouse (Apodemus sylvaticus) fluctuations in the Netherlands. Lutra 58(1), pp.23-33.
  7.  Lindstrom, E.R. et al. 1995. Pine marten-red fox interactions: a case of intraguild predation? Annales Zoologici Fennici 32 (1), pp. 123-130.
  8.  Brainerd, S.M. et al. 1995. Pine marten (Martes martes) selection of resting and denning sites in Scandinavian managed forests. Annales Zoologici Fennici 32 (1), pp. 151-157.
  9. Larroque, J. et al. 2017. Level- and scale-dependent habitat selection for resting sites by 2 syntopic Martes species. Journal of Mammalogy 98(6), pp.1709-1720.