Discussion
Microhabitat variables describing vegetation within 10 metres of a
nestbox and local human disturbance effects influence occupancy by hazel
dormice. In particular, we found that nestboxes were more likely to be
occupied by dormice in sites with higher abundance of key vegetation
resources (hazel, hawthorn and honeysuckle), with tree abundance that we
hypothesized offered multiple movement routes but without creating fully
closed canopies that would limit understorey growth and solar radiation,
and when located further from human disturbances (footpaths and woodland
margins). These are features that should be considered when selecting
where to place nestboxes in mitigation and conservation actions and also
to guide specialised woodland management promoting favourable features
to encourage use of nestboxes by hazel dormice.
Hazel dormice can adapt their diet to their surroundings (Eden 2009),
but proximity to preferred and suitable resources is likely beneficial
and previous research found a positive effect of tree diversity locally
on nest box occupancy (Mortensen et al. 2022). In our study area,
increased probability of nestbox occupancy was linked to abundance of
three plant species (hazel, hawthorn and honeysuckle). Hazel and
hawthorn were also associated with higher occupancy in a managed forest
in Denmark (Mortensen et al. 2022). High abundance and cover of hazel
trees was a key predictor of occupancy, perhaps not too surprising for a
species named after the plant. Hazel is a source of high-calorie food
between July and October when dormice need to build up fat reserves to
overwinter (Tooke & Battey 2010, Bracewell & Downs 2017). Hazel leaves
are also a favoured nest material, thus, close range availability should
reduce energy costs and predation risk (Prentice & Prentice 1988).
Nestboxes located in areas with higher abundance of hawthorn were also
more likely to be occupied. Previous research found dormice seek out
hawthorns when emerging from hibernation (Juškaitis 2013). This tree
flowers in April/May providing valuable resources at a critical time.
The third key plant was honeysuckle, one of the few plant with leaves
eaten by dormice (Richards & Hurrell 1984). Honeysuckle leaves can
represent half of dormice’s diet during May (Richards & Hurrell 1984)
and are also a commonly used nesting material (Bracewell & Downs 2017).
Honeysuckle bark can be peeled away from the stem in small strips
offering a light, easy to transport, material that is readily available
as dormice come out of hibernation. Dormice can use leaves of other
plants for nesting (eg. beech and oaks) but often these leaves are not
available until later in spring (Lechowicz 1984, Roberts et al. 2015).
Our analyses also showed that proximity to some plants, in particular
dog’s mercury M. perennis , could reduce occupancy. Whilst there
are many ground flora plants which are not utilised by dormice for food
or nest material, dog’s mercury is a poisonous plant toxic to many
mammals and, dormice might specifically avoid it because of this (Rugman
et al. 1983). Further research into its toxicity would be valuable. In
addition, dog’s mercury is very prolific and can completely cover large
areas of the field layer, competing with other species which are
suitable for dormice.
Our results also reveal the importance of tree abundance and tree canopy
cover for hazel dormice, predicting that the probability of occupancy
should double when the relative abundance of trees goes from 10 to 30
within 10 metres of a nestbox. Dormice are arboreal and can travel up to
152 metres from their nestbox in search of food, this movement is
facilitated by an abundance of trees with suitable branching structures
(Bright & Morris 2009). Indeed, previous research indicated dormice
prefer nestboxes in forest stands with higher cover or denser (Bright &
Morris 1990, Juškaitis & Augutė 2008, Mortensen et al. 2022). However,
we show here that very high tree canopy and understorey cover
(>85%) is likely not ideal, with an apparent optimal
around 80-85%. An effect that may not be detectable when using coarser
density indices (Mortensen et al. 2022). Very closed canopies can
prevent solar radiation leading to lower temperatures and may also limit
sunlight reducing plant growth below the tree canopy. On the other hand,
open canopies allow too much radiation and occur under limited tree
cover that limit arboreal movement.
Higher occupancy was also associated with lower human disturbance.
Occupied nestboxes were more likely to be located further from woodland
edges and footpaths. Hazel dormice do not completely avoid disturbed
sites, and have been reported at roadside habitats in Germany (Schulz et
al. 2012). However, our study site is a well-visited National Trust
property with high footfall of people, especially at weekends and in the
Spring and Summer when dormice are active. Nestboxes located closer to
the footpaths and woodland margins are likely exposed to higher noise
levels and potentially people could disrupt dormice (trying to look
inside nestboxes) if these are visible from paths.
Collectively, these results lead to management recommendations for the
placement of nestboxes and site management that build on previous
research that focused on wider habitat and nestbox design (Morris et al.
1990, Juškaitis 1997, Madikiza et al. 2010). In particular, given
footpath effects, larger and less visited woodlands should be preferable
sites for nestbox placement. Within those, nestboxes should be placed
preferentially in core woodland areas with high abundance of hazel,
hawthorn and honeysuckle, good tree abundance and a late-spring tree
canopy and understorey cover around 80-85%. If these conditions are not
present, management to promote them should be implemented through
felling or coppicing. Coppicing is often employed as a management
strategy but the planting of honeysuckle, a fast-growing species, is not
generally considered and based on our results could improve dormice
occupancy. Management of trees can also be important to avoid fully
closed canopies. Finally, the spacing between nestboxes should be
considered. Our results show higher occupancy for nestboxes within 5
metres of each other and when located around 50 m away. The first result
may reveal individual dormice moving among nestboxes located in very
closed proximity, which is an optimal outcome if the aim is to increase
population size (i.e., maximize the number of distinct dormice using
nestboxes). While additional
research is necessary and may not be practical in smaller settings, we
tentatively suggest placing nestboxes around 50 metres from each other
if possible and within the optimal microhabitat conditions described
above.
In conclusion, our study addressed a knowledge gap to understand the
role of microhabitat on nestbox occupancy by hazel dormice. However,
additional information is still needed to facilitate the recovery of the
hazel dormouse. For example, despite collecting data on dozens of plant
species during our vegetation surveys, dormice occupancy seems to be
influenced by just a handful of key plants. Surveys required working
closely to nestboxes, and thus, to minimize disturbance we completed
these during the scheduled monthly monitoring by a Natural England
dormouse class licensee. More frequent surveys may identify rarer but
potentially important plants or seasonal changes we were unable to
monitor. In addition, our occupancy time-series did not allow analysis
of temporal patterns, but it would be interesting to consider how past
occupancy influence future use. Research on variation among individual
dormice in their preferences will also be valuable. Marking dormice
using pit-tags and camera traps could be used to understand temporal and
individual patterns of nestbox use. While we wait for this additional
understanding, our results reveal microhabitat variables that influence
hazel dormice occupancy of nestboxes offering advice to placement and
local scale management to promote conservation of this little mammal.