The UK’s hazel dormice are under threat, but breeding and reintroduction initiatives mean that there’s still hope for these endearing little animals, says David Tomlinson

David Tomlinson takes a look at the state of hazel dormice in the UK, and finds out what is being done to protect them.

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My first encounter with a dormouse was my most memorable. I was out pigeon shooting with a friend, wildlife artist Tim Greenwood, in a hazel coppice in the Weald of Kent. Suddenly, Tim dropped his gun and, as quick as a flash, caught a dormouse that he had spotted climbing in a nearby tree. That dormouse was immortalised in a copper-plate etching that was exhibited at the Royal Academy Summer Exhibition, where the edition sold out.

That incident took place more than 50 years ago. These days, catching or even handling a dormouse is strictly illegal unless you have a licence, for these attractive rodents are fully protected by law. Schedule 6 of the Wildlife and Countryside Act 1981 gave the dormouse partial protection and this was followed by full protection as a Schedule 5 species in 1986. Sadly, protection doesn’t mean a great deal. True, trapping, killing and trade in the species are unlawful, but the law offers no defence against habitat loss, the greatest threat to the survival of the species in the UK today.

Small, chiefly nocturnal and almost exclusively arboreal, the dormouse is a mammal that few people have seen in the wild. I lived for many years in prime dormouse country, but the only reminders I had of their presence were nuts I picked up that they had nibbled. Unlike mice and voles, which leave clear teeth marks on hazelnuts they have eaten, a dormouse leaves a nut with a smooth and round hole, readily identifiable if you know what to look for. Finding nuts they have eaten is much the best way to establish the presence of this elusive and rarely seen mammal.

It was the Great Nut Hunt of 1993, organised by the People’s Trust for Endangered Species (PTES), that established the state of dormice in England and Wales (they are absent from Ireland and Scotland). Approximately 11,000 survey packs were distributed to anyone interested in taking part, and almost 250,000 nuts were inspected. No fewer than 13,171 nuts were submitted for confirmation that they had been opened by a dormouse. Each one was inspected by dormouse expert Dr Pat Morris, who established that there were 1,352 dormouse nuts. As a result, some 334 new dormouse sites were identified in England and Wales, allowing a new and accurate distribution map to be drawn. In 2001, a second Great Nut Hunt took place, finding a further 136 sites with signs of dormice. This brought the total number of known dormouse sites throughout the UK to a little over 800.

What the Great Nut Hunts achieved was confirmation that our dormouse population had contracted and declined. There was no evidence of dormice
from Cheshire, Derbyshire, Lincolnshire, Norfolk, Staffordshire, Warwickshire and Yorkshire, all counties where dormice were known to occur in the 19th century. The loss from Yorkshire was particularly significant, for it once had at least 20 dormice woods. Some 83 sites in Yorkshire were checked, but no signs of dormice were discovered.

Overall, it is reckoned that dormice have disappeared from around half their former range, making this one of the most threatened of all our native animals. On the Continent, dormice occur from southern Sweden south to Italy and eastward to Russia, but they are absent from the Iberian peninsula and all the Mediterranean islands except for Sicily and Corfu. It is generally thought that numbers have fallen in Europe just as they have here, but there is little in the way of data to prove this: enthusiasm for dormice is a peculiarly English and Welsh passion.

It’s not hard to find reasons for the dormouse’s disappearance from so much of England and Wales in the past 150 years. Most significant is the felling of ancient semi-natural woodland, although the increasing fragmentation of surviving woods, with the loss of linking hedgerows, has also made survival more difficult. Where woods are highly fragmented, dormice are most likely to be found in only those of more than 100 acres, but in regions of high connectivity, very small woods may be occupied. The decline in traditionally managed hazel coppices is also a problem. Dormice love hazels, as their official name reminds us, and when coppices become overgrown, they become less attractive. Dormice can thrive in hedgerows, but the latter must be species rich and overgrown, not subject to annual flaying.
Another important factor in the decline seems to be climate change. Dormice like sunshine, and a map of their distribution in the UK shows they much prefer sunny areas. Sunshine encourages insects and flowers, as well as helping to ripen the nuts and fruit on which they feed, so though dormice may rarely be seen out in sunshine, they depend on its warmth for survival. A warming climate, one would think, should suit them, but its increasing unpredictability is the problem. They don’t like rain, so summer downpours provide a challenge. Hibernation allows them to cope with cold winters, but unexpected mid-winter warm spells that wake them unnecessarily reduce survival rates.

There are other factors that may be connected to the decline but for which, at the moment, there is little evidence. The European Squirrel Initiative (an anti-grey squirrel group) believes that grey squirrels may well pose a major threat to dormice, especially in poor hazelnut years, when grey squirrels will outcompete dormice in their quest for nuts. It is worth remembering that grey squirrel populations are generally three to six times higher than those of the red squirrels they have replaced. Dormice may have evolved to live in the same woodland habitat as red squirrels, but greys are a different proposition.

Another pressing concern is increasing numbers of deer altering and degrading vital woodland habitat. The fallow deer population explosion in southern England
is a relatively recent phenomenon, with herds of hundreds occurring today where, only 30 years ago, there were few to be found. The huge increase in numbers of muntjac in southern and eastern England may also be a factor. Muntjac have been implicated in the disappearance of nightingales from many woods where these birds once bred, eating the undergrowth needed for nesting. It seems likely that they have had a similar impact on dormice, too.

In recent years, reintroductions of lost species have become increasingly popular, and with an iconic species such as the dormouse there is considerable enthusiasm for bringing it back. A reintroduction scheme started as part of the English Nature Species Recovery Programme in 1993, and a reintroduction has taken place approximately every year since. The project’s aims are simple: to restore dormice to counties and areas where they have become extinct. However, this is far more complicated than it might appear, for the first essential step is to establish why the dormice became extinct in the first place. There is no point in going to the trouble and expense of releasing animals if they, too, will either fail to establish themselves or die out within a few years.

Translocating animals from healthy existing populations is a method that is commonly used in reintroductions, but it hasn’t been employed with dormice because there are few English populations thought to be robust enough for animals to be trapped and removed. Instead, captive-bred animals are used, supplied by the Common Dormouse Captive Breeders Group (CDCBG). Dormice are challenging to keep in captivity, but techniques for breeding have been refined by the CDCBG, producing sufficient young animals every year for the releases to take place. A key member of the CDCBG is Wildwood in Kent: it not only breeds a significant number of dormice for the programme, but also maintains the Dormouse Stud Book. The latter is important, as it avoids inbreeding and ensures that animals released are of a strong genetic mix. The Zoological Society of London is also much involved, quarantining animals due to be released.

In 2020, a landmark was reached when the 1,000th captive-bred dormouse was released into the wild. The site was woodland in the Arnside and Silverdale Area of Outstanding Natural Beauty in Lancashire. The release was part of the Back On Our Map Project, led by the University of Cumbria and Morecambe Bay Partnership and supported by The National Lottery Heritage Fund.

Introducing captive-bred dormice to their new environment requires careful preparation, followed by detailed monitoring. The animals are first liberated into large soft-release cages where they are fed a mixture of nuts and seeds, fruit and berries. After 10 days, doors are opened at the top of the cages, so the animals can explore their new surroundings. Experience shows that animals released in June may well breed during the summer, increasing the population before hibernation in October. All introduced populations are monitored by the National Dormouse Monitoring Programme (NDMP).

The NDMP oversees the long-term population trends. Each site that it monitors contains a minimum of 50 boxes, which are checked at least twice a year, in May or June and September or October. The number, sex, weight and age of individuals found are recorded, giving an indication of the health of the population. The NDMP is the world’s longest-running annual, terrestrial small-mammal monitoring project in the world. Around 600 sites have contributed data and, since 2011, over 300 have submitted records annually.

If you have dormice in a wood on your land and would like to set it up as an NDMP site, then contact the PTES for details of how to register it, along with advice on installing nest boxes. Dormice readily adopt nest boxes, though in a bid to avoid competition with birds, dedicated dormouse boxes are recommended, hung no more than chest high above the ground. The entrance hole should be facing the tree trunk to make it easier for the dormouse to enter. Studies have shown that where sufficient boxes are erected, almost the entire dormouse population will use them.

Once one of our least-known native mammals, the dormouse is now one of the most intensely studied. We know what it likes, where it lives and what it eats. We know how to breed it in captivity, and how to establish new populations. Whether this will be sufficient to maintain the population at its current level remains to be seen. Few of us have ever seen, or are likely to see, a wild dormouse, but fortunately an increasing number of people are doing their best to ensure that these charming animals remain an essential member of our woodland fauna.