Thermal imagers for monitoring pine marten den boxes

Enabling quick checks of den boxes from a distance, thermal imagers help pine marten conservation projects to build a picture of den box use while not disturbing the inhabitants.

In this blog we share our experiences from using thermal scopes to monitor pine marten den boxes on the North York Moors and in Galloway Forest, including some recommendations of thermal scopes that we think are great for the job.

Thermal imagers are increasingly being used in wildlife conservation as a unique way to observe wildlife from a distance. As technology is improving and the prices of thermal equipment are decreasing, we’re seeing more uptake in the use of thermal scopes for a diverse range of uses, including bat surveys, mammal and bird watching and den/nest box monitoring.

Pulsar Helion 2 XP50 thermal imaging scope

Earlier this year I spent some time with John Martin and Johnny Birks in Galloway Forest, where we put some Pulsar thermal scopes to the test for detecting heat signals in pine marten den boxes. The pine marten population has been growing in Galloway Forest over the past 30+ years. The installation of Galloway's den boxes, led by John and Johnny, has provided Galloway's pine martens with a network of resting and breeding dens. There's been interest in the use of thermal imagers for monitoring den boxes for several years, so this was a great opportunity to see how this equipment performs at lots of different den box locations. Over two days we visited 86 den boxes and our thermal scopes provided us with clear indications of whether or not den boxes contained a resident.

Side view of a Galloway Lite den box showing a strong thermal signal through a Pulsar Helion 2 XP50 Pro on the 'hot white' setting.

Front view of a VWT style den box showing a thermal signal through a Pulsar Axion XM30S on the 'hot white' setting.

Nobody home! Two empty Galloway Lite den boxes blending into the environment when no animals are present. Image taken with a Pulsar Helion 2 XP50 Pro.

Forestry and Land Scotland are increasingly using thermal imagers to monitor pine marten den boxes, with a trial over the past year in Galloway Forest showing positive results that may now lead to extending this approach to more sites across Scotland.

Why monitor pine marten den boxes with thermal imagers?

Without the use of technology like thermal scopes and camera traps, pine martens can be challenging to monitor. They are highly territorial, with home ranges that can cover up to 30km². Couple this with much of their waking activity taking place during the night, and the chances of seeing a pine marten in person become slim! Monitoring den boxes helps with focussing survey efforts towards specific areas and at key points during a pine marten’s life cycle, such as during springtime when kits are reared by their mothers in den boxes, or in winter when pine martens are seeking warm, safe dens.

Thermal imagers can be used to confirm if a den box is currently in use without having to open the box to check inside. This is beneficial as it means that animals are not disturbed, and for those carrying out the monitoring, the use of a thermal imager is a much quicker and easier process. If a den box is confirmed to be occupied with a thermal imager, this may then lead follow up monitoring depending on project aims. This may include the use of camera traps to monitor activity, or for those with pine marten disturbance licenses to open the box and see if kits are present in areas where pine marten populations are being closely monitored.

Front and base view of a VWT style den box on the 'red hot' setting through a Pulsar Axion XM30S.

In collaboration with Forestry England, NatureSpy has installed 46 den boxes as part of the Yorkshire Pine Marten Project. The use of thermal scopes means that quick checks of den boxes are now possible. Whenever passing a den box while out on other survey work, we can simply check the box from a distance. This means we’re able to keep closer tabs on den box activity year-round, as well as carry out more structured annual surveys.

Camera trap footage of a pine marten exploring several den boxes on the North York Moors.

How effective are thermal scopes for monitoring den boxes?

Our experience to date shows that thermal imagers are an effective tool for finding out if a pine marten den box is occupied.

Although pine marten den boxes have been designed with a particular species in mind, there are other species that may use them too, such as squirrels. It seems that red squirrels are less interested in den boxes than grey squirrels, but this is currently based on anecdotal observations from Galloway (where reds are present) and the North York Moors (where greys are present). The image below is of a den box containing a grey squirrel, producing a clear thermal signal, so camera traps are a useful follow up monitoring approach for confirming the species while still keeping a distance from the den box. We’re currently working on a guide for monitoring pine marten den boxes with camera traps - watch this space!

A VWT style den box with a squirrel inside showing a clear thermal signal through a Pulsar Helion 2 XP50 (2020 model).

Thermal imagers work year-round to detect heat signals in den boxes. For example, the Pulsar Axion XM30F operates in temperature ranges from -25c to +40c. Thermal images are clearest in cooler temperatures, so this technology is well suited for use at dawn and through spring, winter and autumn. They work effectively in summer too, although the image clarity may be reduced.

Den boxes that are exposed to direct sunlight will hold on to heat during the day. The image below shows an extreme example of this, with two Galloway Lite den boxes side by side in the middle of a clear fell site, therefore completely lacking shade from trees.

Two Galloway Lite den boxes that have absorbed a lot of midday sunlight in an unshaded, open area. Image captured with a Pulsar Helion 2 XP50 Pro.

Which thermal scopes are recommended for monitoring pine marten den boxes?

There are a range of thermal imagers on the market and the specifications can be confusing! In broad terms, the main parts of the performance that change in higher specification models are more image clarity and greater performance at longer ranges.

When monitoring pine marten den boxes, it is best to keep as great a distance as possible between yourself and the box to avoid disturbance to any animals present. Pine martens are a protected species, so close approaches to den boxes should only be carried out by those with a pine marten disturbance licence. In Galloway Forest earlier this year, we found we were able to detect heat signals out to 60m, but this was often constrained by obtaining a clear line of site. In most scenarios, we found a range of 25-45m proved to be most effective.

Thermal scopes such as the Pulsar Axion XM30F work great for monitoring pine marten den boxes, as this scope can produce clear images at the 25-60m range. The XM30F can also take photographs and record videos, proving useful for records and later comparisons of images.

If you’re planning on using a thermal scope for a variety of wildlife work, we also suggest considering the Pulsar Axion 2 XG35 and the Pulsar Telos XP50. These scopes produce clearer images out to longer ranges and can record higher resolution videos, making them more flexible for a variety of wildlife conservation work.

Learn more about our pine marten conservation work over on our project pages.

Article by Ed Snell