Dormouse Survey

Image by saguari from Pixabay

Yesterday we were really lucky to be invited along to watch a group of trained and licensed volunteers undertaking a Hazel Dormouse survey in some woodland on Exmoor. This all came about because I fancy painting a Dormouse and for a while now we have been on the look out for a good photo!  Anyway, I didn’t get a photo good enough to paint from, but we did have a really interesting time.  Dormouse have beautiful golden coloured fur, very large eyes and long furry tails.  Unfortunately with the loss of ancient woodlands their numbers are thought to have declined by a third since 2000.  Once their habitat is destroyed they do not disperse to new areas across open land, so their populations simply die out or lose their genetic diversity.  The south of England tends to be one of the areas where they can still be found, with small populations scattered elsewhere in the UK.  Careful management of habitats including the planting and coppicing of hazel alongside the provision of nesting boxes is helping, but there is still a long way to go.  The work of volunteers to monitor and hopefully increase populations is vital if this delightful little animal is to remain in the UK.

A Dormouse box has many similarities to the wooden bird boxes that we monitor and indeed they are sometimes used by birds.  However, unlike bird boxes which have an entrance on the front or side, the Dormouse box has it’s entrance tucked away around the back.  The extra wooden batons attached to the back keep the box away from the tree allowing mice to enter easily.  Monitoring these boxes is also different as the aim is to weigh, sex and age as many mice as possible.  This can only be done if they can be caught!  They are very sensitive to sound and may make a hasty exit so a quiet approach is needed.  The entrance hole is quickly blocked and the detachable lid is carefully lifted to show whether a nest has been created inside.  Should there be a nest inside the whole box is taken off the tree.

The box is placed gently into a large plastic bag and the contents very carefully examined to discover if mice are present.  At this point they have quite often run out into the bag, but there is still a chance of further mice being inside the box.  From their entrance hole, the mice will have formed a nest chamber in the centre of the material, gently feeling into this chamber will reveal whether there are any further mice inside.  In the above photo this is illustrated outside the bag, but in reality this searching takes place within the safety of the plastic bag in case of any escapees.  Once the number of mice has been established and they are all accounted for in the bag the box is carefully removed.

Their comparatively large eyes and golden fur are clearly shown above and what incredibly long whiskers too!  On average they are 10 – 17cm long but very rarely seen as they can spend up to 7 months of the year asleep.  They tend to sleep during the day and are active at night.
The next tricky job is to catch the extremely quick and agile mammals.  As we observed, this isn’t always easy!  

Once in the hand, they are sexed and then transferred to a smaller bag to be weighed.   They weigh no more than 30/35g, this little one was under 20g – so at this time of year probably a juvenile.  Dormice will usually have 4 to 5 young, born in the summer inside a nest made from strips of bark and grass and usually constructed amongst branches or in a hedgerow.  The mother tends to her young for up to 8 weeks, at about 3 weeks old they leave the nest to forage with her – distinguishable by their greyer fur.  They keep this colouration for a year until they become sexually mature themselves when it changes to the golden colour associated with Hazel Dormice.
The information on the mouse’s age, weight and sex is carefully recorded and the data submitted to the National Dormouse Monitoring Programme.
The whole process is now reversed, a few leaves are collected from nearby trees, the mouse is carefully ‘posted’ back into the box hole and when the surveyor is sure that their furry tail is well out of the way the hole is plugged with the leaves.  The mouse is perfectly capable of chewing through these to get out again but they ensure that the box can be re-fastened to the tree with the mouse safely tucked up inside. 

Dormice diets vary depending on food availability throughout the year.  Flowers, insects, caterpillars, nuts, seeds and berries all form part of the food they eat.  One of the best indicators of their presence in a woodland are indeed neatly nibbled hazel nut shells.

©People's Trust for Endangered Species Guidelines - Click for more information.
©People's Trust for Endangered Species Guidelines - Click for more information.

Not all Dormouse boxes will have animals inside, many will be completely empty and some will contain bird nests or old disused mouse nests.  Of the 15 boxes we observed yesterday, there were only 4 with animals inside – 2 were dormice, but we also had a Pygmy Shrew and a Wood Mouse.  Very interestingly the Dormice will not over-winter in their boxes, they make a nest on the ground in amongst some dense cover instead.  One of the winter jobs for the volunteers, the same as with bird nest recorders, will be to empty all the boxes ready for the new season next year.  
This was a brief but fascinating insight into a different type of survey and data collection and we extend our very grateful thanks to all those involved in this yesterday.  And whilst I didn’t get any photos, Jo at least brought home a handy reminder of just how sharp rodent teeth are!!  😂

The following links may be useful for more information on Dormice.
The Woodland Trust
National Dormouse Monitoring Programme
People’s Trust for Endangered Species Nut Hunt

Thank you for following my blog, and until the next time, keep enjoying the wildlife near you. Paul