Ecology of the Hazel Dormouse Contents of presentation Britain’s small Mammals

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Ecology of the Hazel Dormouse

  1. Ecology of the Hazel Dormouse

Contents of presentation

  1. Britain’s small Mammals

How do you define a small mammal –either by size (but when does small mammal stop being a small mammal and become a medium mammal?) or by likelihood of being trapped in a Longworth small mammal trap.

Wood mouse )

Yellow neck mouse ) All readily trapped during Longworth Field vole ) trapping

Bank vole )

Common shrew )

Water shrew )

House mouse non-native but now rarely seen

Harvest mouse smallest mouse, Britain’s only mammal with a prehensile tail

Pygmy shrew worlds 2nd smallest mammal; difficult to trap as very light weight may not trip trap

Hazel dormouse not known to use Longworths

People want to see dormice which can lead to false recording – this picture appeared in the Wisbech Standard in November 2009 identified as a dormouse rather than the more common woodmouse

  1. Class: Mammalia, Order: Rodentia

Family Muridae and Family Gliridae
Mice is a derivation from the Greek humus and means ‘of the earth’

Generally people think of ‘mice’ as from Family Muridae - Dormice are not mice!

General features of the Families Muridae and Family Gliridae
Mice, rats and gerbils are a large family within rodents, r strategists (short-lived, high fecundity, low parental care, habitat generalists)
Dormice are a small family within the rodents, ecologically K strategist (long-lived, low fecundity, high parental care, habitat specialists)
R or k strategist each have their each own advantages. R strategist relies on number of individuals, while a K strategist relies on the quality of individuals. R strategist has a great flexibility at the system level of the population. This means that the species can survive major catastrophes, even if only at the expense of very large number of individuals. On the other hand, K strategists have great flexibility at the level of individual. They depend on the survival ability of a complex and resilient individual which is able to cope with all but the worst of natural catastrophes.
r-strategists, named after the statistical parameter “r”; the symbol for the growth rate coefficient
K-strategists, named after the symbol for flattened portion of a population growth curve

  1. European Species

For details of European species and in the Appendices

  1. What’s in a name?

Origin of the names of our two species of dormice.

  1. Fat dormouse

The edible dormouse in Britain (Pat Morris) PDF

  1. Hazel dormouse

Identification features of the hazel dormouse

  1. The Hazel dormouse year

Graphic of the dormouse year – summer at top

  1. The Hazel dormouse year

Graphic of the dormouse year – Jan at top

  1. The Hazel dormouse year

Graph of 2010 dormouse weight distribution from NDMP sites

  1. Torpor and hibernation

  1. Hibernation

During the winter, when little food is available, dormice save energy by going into hibernation. Having spent all the summer in trees and shrubs, they now descend to the ground and stay there all winter. A small tightly woven nest is made and the animals usually spend the winter there alone. They hibernate under logs, under moss and leaves or among the dead leaves at the base of coppice stools and thick hedges. Dormice choose a moist place to hibernate, where the temperature will remain cool and stable and the humidity high. Cool conditions are vital as metabolic processes are slowed at lower temperatures and fat reserves will then last longer. Damp conditions are also necessary because water vapour is lost during the animal’s breathing. Hibernating in a moist place will ensure the animals do not desiccate during the winter, as they do not wake up and drink regularly.

Hibernation begins when the nights become cool in the autumn and there is little food left in the trees, generally around the time of the first frosts. Larger dormice (weighing up to 40 g) appear to enter hibernation earlier, while smaller animals continue to feed until later in the autumn. These may remain active until December in mild years, especially in the south of England. Dormice do not normally hibernate successfully in nest boxes because the temperature inside is too variable.

  1. Torpor

During cool or wet periods in summer, dormice may spend several hours a day in a state of torpor. In early summer, more than nine hours per day may be spent in torpor, although by autumn the average time is less than half an hour. The animals become moribund and inactive as the body cools, with their normal temperature (similar to our own) falling to little above that of their surroundings. The torpid animal is tightly rolled up with the tail curled over its belly and face. Torpor economises on energy expenditure, but at the cost of delaying breeding, often until long after other woodland mammals have already raised young. Early in the season, dormice may go torpid in flimsy nests, from which they are easily dislodged by the wind. It is not uncommon to find a torpid animal lying on the ground in the open, on a footpath for example. In such cases the animal is best moved to nearby shelter and allowed to wake up and look after itself.
Dormice are highly sensitive to the weather and on cold nights will abandon their nocturnal feeding early. A 5 °C fall in air temperature may result in feeding activity being curtailed by an hour or more. In addition, dormouse fur is very fine and poorly suited to throwing off water droplets, so rain and drizzle are a danger to these animals, whereas mice and voles, with a different fur structure, are better

adapted to cope. In the autumn, when nights are cold (below 9 °C at midnight), dormice will often compensate for reduced nocturnal activity by coming out during the day.

  1. Hazel dormouse diet

Dormice feed on a wide variety of arboreal foods including flowers (nectar and pollen), fruits (berries and nuts) and some insects (especially aphids and caterpillars). They will also eat buds and young leaves, but cannot efficiently digest large amounts of mature leaves as they lack a caecum in their digestive system. In the autumn there is abundant food in the form of berries and nuts, but these are not generally ripe before about August. So, in the early summer, the dormouse must move from one tree species to another as the different flowers become available. When these are over, but fruits are not ready, there is a period of potential food shortage. At this time, insects may become important in the diet, but insects (particularly aphids and caterpillars) are also consumed at other times. It follows that a high degree of diversity among tree and shrub species is desirable in order to ensure that an unbroken sequence of foods is available throughout the summer.


  1. Dormouse food requirements

Certain tree species are particularly valuable as providers of food at different times of year. Hazel appears to be an important provider of insects, and its nuts form the main food used to fatten up for hibernation. Where hazel is scarce or absent smaller fruit seeds, such as those from hornbeam or blackthorn sloes may suffice, but offer less food in exchange for the gnawing needed to open them. In addition to hazel, oak, bramble and honeysuckle are especially valuable food sources, but dormice can survive without at least one of them if an appropriate substitute is available. Most good dormouse sites will have most of these species, but the absence of any, or even all, of them is not evidence that dormice are also absent.

  1. Nests and breeding

They may weave their own nests (up to the size of a grapefruit)

in bushes and shrubs, dormice prefer to use more robust resting

places such as hollow tree branches, squirrel dreys and old bird nests. Nest boxes are a particularly attractive substitute for natural tree holes and, where boxes are provided, a high proportion of the dormouse population may use them. Provision of nest boxes can increase the local population density, suggesting that the availability of nest holes may be one of the factors that limit dormouse numbers.

  1. Dormouse breeding

Male dormice are territorial in the breeding period (May to September) and may attack each other, so the population tends to be spread thinly. Even the best habitats may not support more than about four adult males per hectare. Females give birth to (usually) four or five young, from early June until September (but mainly in July or August). The young remain with their mother for up to two months, delaying her production of a second litter.

  1. The ages of Hazel dormice

  1. Dormouse home range

Dormice do not normally travel far from their nest (usually less than 70 m), so the different trees and shrubs necessary to maintain a sequence of foods through the seasons must be present within a small area. This implies that a very mixed habitat is desirable.

  1. Dormouse population density

Dormice live at low numbers, even in the best habitats. In early summer there are typically only 3 to 5 (but sometimes up to 10) adults per ha in deciduous and conifer habitats. The National Dormouse Monitoring Programme suggests an average of between 1.75 and 2.5 adults per ha based on 83 sites in various habitats, with the lowest densities in the north of England (1993 to 2000 inclusive). Across the country, including sub-optimal habitats, the average population density is only about 2.2 per ha.

  1. Dormouse predators

  1. Dormouse activity

  1. Dormouse distribution

  1. Dormouse habitats

  1. Dormouse habitats

Photo: Canopy connection across a minor road in Hampshire

  1. Dormouse Ecology Summary

  1. Why are Dormice good?

Benefits from diverse, well structured woodlands linked in the landscape with well managed hedgerows and/or scrub patches

What’s good for dormice is good for many other species

  1. FINAL PTES slide

Conservation of the Hazel Dormouse

  1. Dormouse Conservation

Contents of presentation

  1. Dormouse survey methods

List of dormouse survey methods used

  1. Hazel nuts (picture)

Dormouse opened hazelnuts found under a single hazel stool in Litchfield Copse, Hampshire Sept. 2009

  1. Nut hunt methodology

The best way to establish dormouse presence at a site is to look for gnawed hazel nuts. Although this is obviously impractical where hazel is absent, it is worth searching any adjacent areas with hazel to see if dormice are nearby and thus likely also to be present on the site under investigation. Several species of rodents open hazel nuts, but dormice leave a smooth round opening. Dormouse tooth marks will be found around the rim of the hole, smoothing it out, with a few tooth marks on the nut surface. There are no transverse tooth marks across the rim of the nut shell. Dormouse tooth marks may be visible to the naked eye, but use of a magnifying glass is recommended.
Recommendations for dormouse nut searches (Vaughan 2009)

  1. Focus survey effort on trees which have a stool diameter at breast height of at least 150 cm and preferably 170 cm;

  2. Focus survey effort within 100 cm of the trunk of such hazel trees;

  3. Search a maximum of ten such trees and do not spend excessive time searching for hazelnuts under each tree, and if no dormouse gnawed hazelnut shell is found within 5 minutes then it is likely to be preferable to move on to another suitable tree.

To conduct a systematic search select an area of heavily fruiting hazel and search a square of ground measuring 10 m x 10 m for 20 minutes. If no dormouse nuts are found, repeat the process in another part of the site. There is an 80 per cent probability that, if dormice are present, the characteristically gnawed nuts will be found by the time three such squares have been searched (Bright, Mitchell & Morris 1994). If five squares fail to yield dormouse nuts, it is about 90 per cent certain that dormice are not present, although this is still not proof of absence from the site.

An alternative way of achieving an adequate sampling intensity is to collect 100 hazel nuts that have been opened by small rodents (voles and mice, but avoiding caches made by these species and also ignoring nuts opened by squirrels). If this sample contains no nuts that have been opened by dormice it is highly probably that dormice are not present.

  1. Nest tubes(1)

Nest tubes are an inexpensive means of detecting dormice in habitats where nut searches are unlikely to be effective. Nest tubes should be considered as an excellent tool for surveys, but not for long-term population monitoring.

The tubes are made from stiff double walled black plastic sheet, 5 x 5 cm in cross section and 25 cm long. A small plywood tray is placed inside, projecting 5 cm beyond the tube’s entrance to allow the animals' easy access. The opposite end of the tube is sealed with a wooden block mounted on the tray. Tubes can be made easily, or fruit juice cartons can be adapted but they are also available for purchase from The Mammal Society. They can be suspended by wire or tape, fixed firmly underneath horizontal limbs, where they resemble a hollow branch.
For survey work, small numbers of tubes are likely to miss dormice, even where they are known to be present. It is recommended that at least 50 tubes be used to sample a site, spaced at about 20 m intervals (Chanin & Woods 2003). They should also be left in place for several months. Nest tubes are most frequently occupied in May and August/September/October. Timing their deployment is therefore important. Setting them out in April may get early results, while setting them out in June may be less immediately successful. It is best to leave them out for the entire season, from March onwards, for checking in November.
Using 50 nest tubes as a standard and the table below as an index of the ‘value’ of different months for surveying, a score can be devised as an indicator of the thoroughness of a survey.


Index of probability

















Thus, 50 tubes left out for the whole season scores 25 (the sum of the indices for all 8 months), but 25 tubes left out in April and May scores only 2.5 (1 + 4, divided by 2 because only half as many tubes are used) (Chanin & Woods 2003).
This methodology is based on a single study and it has recently been suggested that the Index of probability score for October may be too low. It is therefore best to leave them out for the entire season, from March onwards, for checking in November.
Dormice are much more likely to use nest boxes than nest tubes Chanin and Gubert (20011)

  1. Nest tubes(2)

Nest tube shown in place (diagrammatic) (from Dormouse conservation handbook)

  1. Nest tube3 (picture)

Nest tube shown in use (picture from German study)

  1. Nest boxes

Wooden nest boxes, similar to bird boxes, but with the entrance

hole facing the tree, are readily used by dormice and offer a means of detecting the animals in the absence of gnawed hazel nuts. They are also an important conservation tool and evidently boost the local dormouse population density (Morris and others 1990). Those put up near honeysuckle are more likely to be occupied.

Although nest boxes will reveal the presence of dormice, they are often not used immediately. Sometimes they remain empty for several years. Often the first signs will be nests rather than the animals themselves. Nest boxes need to be used in large batches to be an effective survey method. Fifty or more boxes are recommended, and they should be put up in a grid, spaced about 20 m apart. A single line along the edge of a wood may achieve a higher occupancy rate (due to better quality edge habitat), but cannot give a reliable population density estimate. Similarly, putting clusters of boxes in ‘good places’ will enhance occupancy rates and increase the probability of detecting the presence of dormice, but reduce the potential for scientifically valid comparisons between sites. Since they are expensive, nest boxes should be considered mainly as a means of population monitoring rather than as a survey tool.

  1. Other survey methods

Picture: Natural dormouse nest in hedge in Devon (Rob Wolton Nov.2007)

Where hazel is absent, other signs of dormice must be sought, such as natural nests. Dormice nests are typically grapefruit-size and often found in brambles or other low-growing shrubs and are most likely to be found in the autumn. Dormouse nests are woven from strips of honeysuckle bark, or similar material, and frequently have whole leaves incorporated into the outer layers. These are often collected fresh and are either green or faded to grey. The nests are spherical and lack an obvious entrance hole. Searching for nests is time consuming and often unsuccessful – even where dormice are known to be present – as they mainly use other places to rest (for example, tree holes) and do not often construct nests of their own. Thus, failure to find woven nests should not be used as evidence of absence.

Hair tubes are simple to use and cheaper than nest boxes They can be made from plastic sink-outlet pipe approximately 3 to 4 cm diameter. Two 25 mm square openings must be cut in the 'roof' with a saw and chisel, then adhesive tape stretched across each opening with the sticky surface facing inwards. The tubes, baited with jam or peanut butter, are then fixed to horizontal branches using string, cable ties or sticky tape. As the dormice (or other small mammals) squeeze through the tube to get at the bait, they leave hairs stuck to the sticky tape. After a few days, the tapes can be collected. Hair samples may be acquired from mice, voles and shrews, not just dormice, and a microscope is needed to tell them apart. This is not difficult, with practice, and will be made easier by having comparative samples mounted on glass microscope slides.
The success rate of hair tubes is low. Fewer than 10 per cent of tubes may catch dormouse hairs, even where the animal is abundant. Their advantage is that they may yield results within a week, whereas nest boxes need to be put up and left, often for months, before they are found and used by dormice. Hair tubes are also easier to set up among the branches of dense shrubs and tightly trimmed hedges. They are also cheap, so large numbers can be put out, increasing the probability that dormice will be detected. Because of their low success rate, hair tubes should be used in large numbers. A density of at least twice that for nest boxes is recommended. Their relative inefficiency suggests that this is not a good survey method and absence of hairs in the tubes cannot be accepted as evidence that dormice are not present

  1. Dormouse surveys – Good practice

Although it is virtually impossible to prove that dormice are absent from any area of appropriate habitat within their natural range, an adequate survey will give confidence that any significant populations have been detected. For environmental assessment or development sites, survey proposals should be based on the following table:





Search for gnawed hazel nuts

Most efficient method. Gives quick results, but only where hazel is present..


Nest tubes

Good method, especially where hazel absent. Only useful March–November. Likely to take one survey season


Nest boxes

As for nest tubes, but much more expensive and better suited to long-term monitoring.


Hair tubes

Cheap; limited to summer months; requires hair identification skills. Low ‘hit rate’. Very difficult to quantify search effort. Not recommended.


Nest searches

Not recommended as a survey method, but nests may well be found when clearance is in progress.



Labour intensive. Needs a licence. Low ‘hit rate’. Not recommended.

The following is a recommended approach to a survey:

  1. Check whether the site falls within or close to the known range of the dormouse

  2. Check for the existence of dormouse records with the local biological records centre or on the National Biodiversity Network (NBN)

  3. Check with the site owner to see if they know whether dormice are present.

  4. If the presence of dormice is possible, carry out a survey using a recommended method at an appropriate intensity.

  5. If dormice are found, submit data to the local biological records centre.

  1. Dormouse surveys – legislation

No licence required for non-invasive surveys; nut hunts, hair tubes, looking for, but not disturbing, natural nests

If boxes or tubes are put out speculatively to detect presence, this in itself does not require a licence, but a disturbance licence is essential once the first dormouse has been found.

  1. Where to look for dormice

Dormice may be present in any wood or scrub habitat within their current range

  1. Important elements for dormouse conservation

Dormice are arboreal, sequential feeders that require autumn food resource to build up weight for hibernation. They occupy small ranges so habitat requirements are needed within relatively small area (approx. 1ha )

  1. Dormouse population density

Estimated dormouse population in different habitats in Britain

  1. Problems for dormice

  • Inappropriate woodland management – no/poor shrub regeneration

  • Deer and rabbits – impact upon shrub regeneration

  • Poor hedgerow management – low diversity, excesses cutting, poor structure, lack of continuity

  • Habitat fragmentation – small isolated woodlands/roads/railways

  • Development – dormice inhabit clos to human populations but impacted by woodland fragmentation/poor management and predation i.e. cats

  • Climate change

    • Warm winters – greatest mortality probably over winter due to energy loss when temperature awakens them from hibernation

    • Wet summers – generally don’t feed in the wet

  1. Mitigation for dormice

  1. Dormouse reintroductions

Picture: Dormice being put in nest boxes inside a soft release cage at Freeholder Wood, Yorkshire June 2007

Over the past 20 years there has been an effort to return the dormouse to some of the counties from which it has been lost or where their numbers are low. PTES, in partnership with various other organisations, has successfully re-introduced dormice into ten counties since the first re-introduction to Cambridgeshire in1993. A further 16 reintroductions have taken place in 11 counties. Unfortunately the releases at three of these sites have not been successful, probably due to inappropriate or insufficient woodland management. Dormice have survived at five of the sites and have dispersed throughout the woodland in which they were released. At seven sites however the dormice have not only dispersed throughout the wood but they are now starting to move out of the woodland into the wider countryside.

  1. Why are Dormice good?

Benefits from diverse, well structured woodlands linked in the landscape with well managed hedgerows and/or scrub patches

What’s good for dormice is good for many other species

  1. Why are dormice good?

20. Final PTES slide

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