Up in the trees – research into dormouse survey methods – The Applied Ecologist


Author Debbie Barlett shares her latest study that assesses whether current dormice survey methods are fit to produce accurate results.

Where did the idea come from?

I’ve always felt my role as programme leader for the MSc in Environmental Conservation at the University of Greenwich was to encourage students to question what they are being taught. This is particularly important during fieldwork, from the preparatory risk assessment, the techniques used on site and to querying data collected to determine what it reveals and how this converts into useful information.

Having mixed groups of students from a wide diversity of backgrounds always made for lively discussions, particularly during our residential weekends at Sandwich Bird Observatory, as well as out in the woods combining dormouse license training with botanising.

Torpid dormice in a disused bird nest – these were not disturbed © Debbie Bartlett

One question which came up year after year was if dormice are arboreal, spending their summers active in the tree canopy, then why do we think artificial nest boxes on trees at a convenient height for us to survey would give valid results?

What did we do?

The 2011 intake included Sam Bower, a fully qualified and proficient tree climber, who decided this question would be the focus of his MSc research project, so we set up paired artificial nest boxes, one high in the canopy, the other at the standard 1.4m from ground level on our long-term survey site.

This was Sam’s project, but it was still an excuse for group activities not least as for safety reasons there always had to be at least one other climber qualified in aerial rescue present.

What did we find?

The first summer demonstrated that of the 23 pairs of boxes, five of the high and just three of the standard height boxes were used by dormice. Interestingly, had we been carrying out a population estimate, we could have missed 20% using only the lower boxes.

This study was continued, with dormice found in 20 of the high boxes and 21 of the low ones over three years. In eight cases, both the high and low boxes were used in the same year but in 11, only the high one was occupied, clearly showing that the standard survey method alone would not have given accurate results.

installing high box_screenshot
Installing a high box © Debbie Bartlett

A question that  puzzled us was the composition of the dormouse nests. Received wisdom is that these are usually made by plucking green leaves from the tree, with shredded honeysuckle bark used for the breeding chamber, and that dormice generally only come to ground level to hibernate in winter. But we were finding nests composed entirely of brown leaves – so where had these come from?

We picked some fresh green leaves, put them in an artificial nest box, and photographed them weekly. They didn’t ever go brown suggesting dormice were collecting material from the woodland floor so might be travelling regularly to ground level. It was agreed that this should be tested in the next research project.

A group of former students, led by Sam, set up a further research project on a site that had never been surveyed for dormice, part of the National Trust’s Sissinghurst estate. Assisted by Peter Dear, National Trust ranger and another qualified climber, 25 survey stations, with three artificial nest boxes at each, were set up.

In addition to the high and standard height boxes, one was on a post so could only be accessed by dormice travelling across the woodland floor.

resarch team_screenshot
MSc student and graduate survey team © Debbie Bartlett

Over three survey seasons (2017-19) more high boxes were occupied by dormice than standard ones, and 19 of the 25 post boxes had evidence of use clearly showing dormice regularly come to the woodland floor and move around, despite the good aerial connectivity above the other boxes.  We continued surveying – when COVID-19 restrictions permitted – for a total of six years.

We’ve been carrying out dormouse surveys for many years and, as anecdotally reported by others, we’ve found that occupancy of nest boxes falls off after the first few years. This is widely interpreted as population decline.

But before boxes were put up dormice must have been existing and breeding without them so could there be another explanation? Are dormice curious so explore new features but then move on, perhaps because of parasite build up? This is an area that needs more investigation.

What does this mean?

This has confirmed our suspicion that surveys using artificial nest boxes at around 1.4m from ground level may not result in robust ‘likely absence’ conclusions and this is potentially serious as dormice are protected species

What next?

Surveys carried out using the standard method are likely to be missing dormice but routine surveys in the tree canopy are not a practical option. The logical conclusion is that, as we know dormice hibernate at ground level, we should consider how to survey them then.  

We now moving into the fourth year of a research project aiming to do just that – it will probably raise yet more questions – but we hope to publish results in 2025.

Read the full article: “Investigating the use of artificial nest boxes positioned at different heights on trees and in isolated positions by dormice (Muscardinus avellanarius): Implications for current survey guidelines” in Ecological Solutions and Evidence.

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