Elusive winter engineers: hazel dormouse hibernation nests

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Many animals exhibit similar patterns of behaviour throughout the year, whilst others dramatically change theirs with the changing seasons. Hazel dormice, which are usually reluctant to come to the ground during their active season, make a beeline for the forest floor when it’s time to hibernate.

Searching for nests

Those of us lucky to monitor dormice are familiar with their characteristic double-layered, beautifully woven breeding nests. Bright green hazel leavers carefully wrapped around an inner core of shredded honeysuckle bark is a sure sign a dormouse is nearby. We know much less, however, about their behaviour over winter, and what factors influence where and how they build hibernation nests.

Winter nests have been found at ground level, under moss, nestled in leaf litter, and at the base of coppice stools or thick hedgerows. But compared with summer nests, very little is known about these nests that dormice make to hibernate it. So, PTES provided funds to help support Leo Gubert’s investigations in Devon and Cornwall. Leo has been busy over the past few years determining whether winter nests vary in their construction, as summer nests do, how far dormice are willing to travel to gather the material they need and whether ambient temperatures impact the nest types too.

Travelling across the two counties, Leo looked at broad-leaved and conifer woodlands, hedges and roadside habitats. He found most hibernation nests by radio-tracking individual animals, the rest he found doing finger-tip searches and incidental finds during habitat management. Six dormice moved and build new nests whilst Leo was radio-tracking them. Four animals moved once, one moved twice and another three times, building a total of four nests over its hibernation period.

What material makes up a nest?

Leo’s findings confirmed that winter nests can be classified into the same types as summer nests: layered nests, which were the most common, woven grassy nests (made of honeysuckle strips, bluebell stems or fern bark), foliar and mixed nests. The dormice made their nests from between one and five different materials, though on average only two were used. Moss was the most common material that was available within 3m of each nest but was only used in one quarter of them. Ferns, bracken and oak leaves were the most commonly used material. When they were used to build a nest, grasses, ferns, bracken and honeysuckle made up the majority of the material in the nests they were found in, and dormice were willing to travel slightly further to get them (1m-3m away, whilst most other material was less than a metre away from where the nest was located, and mostly under 50cm.)

For grass, ferns and beech, the proportion of the material used in the nest was greater when it was found closer to the nest site. Dormouse showed a preference for grass in areas where conifer needles, hazel and honeysuckle were least abundant; honeysuckle was mostly used where grass was not common, bracken was preferred in areas where oak was present but with little grass, ferns and willow, whereas moss usage was associated with abundant willow where bracken, conifers and oak were scarce.

Clockwise from top left: layered nest with an outer layer made of moss with a distinct core of stripped fern stems and grass, typical woven structure of grassy nests or core of layered nests (this example is built mostly with honeysuckle bark and the dormouse has been made visible), a foliar nest utilising ferns and broad-leaved tree leaves and an in situ mixed nest made of leaves and grass stems. Credit: Leo Gubert.

Sourced locally

In general, the material dormice used to make their nests reflected general availability locally rather than any selective preference. Bracken was one of the most extensively used materials and very versatile. Leo was interested to find that moss was present at all but one of his study sites. Mosses are good indicators of ground moisture and humidity which are both thought to be important factors for dormice when they choose where to hibernate, preventing dehydration over winter. Previous studies have also highlighted that dormice don’t seem to travel very far to collect hibernation nest material. This may a deliberate behaviour to minimise time on the ground building the nest in order to avoid predators. And using material close at hand helps it blend into the surroundings, camouflaging it well.

Leo’s work confirms that dormice are versatile and adaptable in terms of where and how they build their nests. If a site is suitable for a dormouse’s needs during the active months, then there should be plenty of suitable material to fulfil their needs in winter too. Bracken, grass and honeysuckle are all important plants which should be considered during woodland management in order to provide dormice with what they need during hibernation.

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