Don't be alarmed: this business with the bags is standard practice for weighing mice during monthly monitoring checks, all done by qualified and experienced licence holders who carefully return the animals to their nest box unharmed a few minutes later.
Today we were checking boxes in hazel coppice stools along a woodland edge in southern England. We found seven dormice in total, scattered among 50 boxes, including a female with "pinkies" (newborn young, which we left undisturbed), a couple preparing to breed and a pregnant female. Dormice are clearly thriving here, despite their national decline, and small wonder when you look at the food sources around them: hazel trees bursting with ripening nuts, sprawling bramble covered with pink berries and a plentiful supply of insects.
Dormice are exceptionally lively at this time of year, even though they normally sleep during the day. As we tried to transfer them from nest box to weighing bag, they scampered and leapt around a giant plastic sack and tried to escape by running up our arms. Inside the box, they weave cosy nests with strips of honeysuckle bark and moss, on a bed of fresh green leaves. In its heart is a small cavity lined with soft dry grass for the breeding den.
They seem to favour remnants of ancient woodland on the site which offers a greater diversity of plants and insects. The wood borders a field of wheat where we spotted a roe deer among the corn, a silage heap popular with basking adders, and shelters a number of badger setts.
Of course dormice are quite capable of nesting without our wooden boxes. We put them up so that we can monitor population numbers and breeding activity on different sites. If you see one, please leave it in peace. Dormice are strictly protected by law, as a fast declining endangered species, and you need a licence to disturb or handle one.