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Individual study: Reintroduction of hazel dormice Muscardinus avellanarius to Brampton Wood, Cambridgeshire, England

Published source details

Bright P. & Morris P. (2002) Putting dormice (Muscardinus avellanarius) back on the map. British Wildlife, 14, 91-100


The hazel dormouse Muscardinus avellanarius is an uncommon British species that has disappeared from at least six counties where it was present in the late 19th century; it now occurs almost entirely south of a line between Suffolk and Shropshire. Research has revealed reasons for this decline (Bright & Morris 1993; Bright & Morris 1996) and established a scientific basis for English Nature's dormouse Species Recovery Programme. Part of this conservation strategy envisaged reintroducing dormice to some counties where they had become locally extinct. Dormouse ecology is complex and simply letting them go is likely to leave many susceptible to starvation. Dormice require a sequence of suitable foods to be available in the tree and shrub canopy, throughout the summer and in an unfamiliar place, they need time to explore, establish territories and know where to find the food. Different methods of release were tested using various sources of dormice. Some were captive bred animals, others were translocated from a wild population nearby. Animals were released at different times and in different ways, then radio tracked for at least a week to compare their behaviour and performance (Bright & Morris 1994, for a summary please see: In the light of findings from this trial, a full-scale release was then undertaken in a woodland in Cambridgeshire, eastern England.

Study site: Brampton Wood, a 140 ha woodland with a vegetation structure and woody species diversity considered appropriate to support a population of dormice, was selected as the reintroduction site. This woodland probably historically had dormice although there is no direct evidence for this.

Dormouse releases: Captive bred dormice were held in 'pre-release cages' (about 50 x 50 cm and 1 m high) made of weldmesh and fixed to a tree or shrub about 60 cm off the ground. They were kept in the cages for 10 days before being released. After release, food was continually replaced in the open cages, providing a supplement if the dormice were unable to find sufficient natural food after release. Several wild-caught dormice were held in cages, sometimes for several weeks, until the release period. In June and early July, 1993, 11 wild caught and eight captive bred dormice were liberated from the pre-release cages.

Others were later taken to the site but died quickly in persistent heavy rain and cold weather. These weather conditions proved very unsuitable for dormice survival and the captive bred animals had probably never experienced rain before.

Radio-tracking: Several dormice were fitted with transmitters for post-release radio tracking studies.

Radio tracking confirmed that the dormice swiftly learned their way about, visiting neighbouring release cages and using nearby nest boxes. Some dormice would undertake lengthy excursions, then return to their cage by the shortest direct route, passing through areas not previously visited.

After release, captive bred dormice lost weight, shedding excess put on in captivity. however, by autumn they had similar weights to translocated wild animals. Wild caught dormice travelled further at first but by autumn there was no difference between the two groups. Both groups increased the distances they travelled each night, although only two male dormice dispersed more than 150 metres. They all made heavy use of supplementary food, until August when natural fruits became available, confirming the need for artificial support during the establishment phase.

At least 36 young were born in the first summer, in seven litters (three to captive bred and four to wild caught females), probably the first born in Cambridgeshire for nearly 100 years. Two females, one wild caught the other captive bred, produced second litters that summer, their first litters having died, apparently due to the inclement wet weather. By October, at least 45% of the wild caught animals and 22% of the captive bred group were still alive. Equal proportions of young born to wild and captive bred mothers survived until October, the onset of the hibernation period.

Higher mortality among captive bred animals seems likely to be due to predators. Two captive bred dormice were seen frequently sitting on branches in the open, three others often crossed woodland tracks on the ground (something that wild dormice normally avoid doing). Such naive behaviour would make the dormice vulnerable to predators, e.g. tawny owls Strix aluco, two pairs of which were resident in the release area.

Winter began early, curtailing the period for fattening up before hibernation. Persistent rain followed, a further threat to dormice as they hibernate on the ground. Nevertheless, at least ten animals were alive in May 1994, three being already pregnant. Over 16% of the original released animals survived into 1994 and at least five (11%) of those born in the wood in 1993 survived the winter. One of these produced second generation, site native young in 1994. Many animals that had not been found earlier in the nest boxes reappeared later, so actual survival rates are higher than appears from monthly monitoring of nest boxes.

Conclusion: It appears that the reintroduction was successful. By 1999 the Brampton Wood population had spread throughout the woodland. That year, over 50 animals were found in the nest boxes, a substantial increase on the number released, and representing only a small proportion of the new population as only part of the wood was actually monitored. In autumn 2000 similar numbers were present.

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