Introduction
Over the past 50 years, the UK has seen a severe decline amongst many of its native mammal species, including hazel dormice (Muscardinus avellanarius ), harvest mice (Micromys minutus ) and hedgehogs (Erinaceus europaeus ). Even populations thought to have been stable, and widespread, such as those of stoats (Mustela erminea ) and weasels (Mustela nivalis ), are being shown, through new research, to be decreasing at alarming rates (Coomber et al. 2021). Habitat loss through urban expansion and changes in farming practices are cited as being key drivers of population decline, as well as changes in forestry management. These changes decrease structurally complex and spatially heterogeneous woodlands (Hopkins & Kirby 2007) affecting vulnerable mammal species such as red squirrels (Sciurus vulgaris ) (de Raad et al. 2021) and pine martens (Martes martes ) (Caryl 2021). Many of our UK mammal species are vitally important contributors to biodiversity directly through interactions with various plant and invertebrate species, and also indirectly as prey for other species such as birds of prey and larger mammals (Occhiuto et al. 2021). It is therefore important that we understand the specific requirements of these vulnerable species to provide suitable mitigation and enhancement, and inform new protective legislation as required.
Hazel dormice have severely declined with a reported 50% reduction in population size since 2000 across the UK (Wembridge et al. 2019). This decline is primarily attributed to a reduction of traditional woodland management techniques, specifically coppicing (Bright & Morris 1995) and habitat fragmentation (Bright & Morris 1994, Capizzi et al. 2002). Hazel dormice are habitat specialists and thus, particularly sensitive to environmental changes and habitat fragmentation (Capizzi et al. 2002). In addition, hazel dormice have slow reproduction rates and live at low densities (Bright & Morris 2008) which makes the species especially vulnerable to local declines and extirpation. The rarity of other dormice species, such as the garden dormouse, forest dormouse and mouse-tailed dormouse led to the creation of EU legislation which affords enhanced protection of individuals, prohibiting their capture, injury, killing, or deliberate disturbance, and their breeding sites and nests (UK Statutory Instruments: The Conservation of Habitats and Species Regulations 2017. No 1012).
The alarming decline of hazel dormice has prompted several conservation initiatives designed to protect the species and enhance or create suitable habitats (Bright & Morris 1994, 1995, Ramakers et al. 2014). For example, active woodland management has led to local recoveries (Goodwin et al. 2018). Management includes coppicing hazel to maintain a successional status, which slows the progression from an unshaded and productive shrub layer to a high forest with an overshaded understorey (Bright & Morris 1990). This provides dormice with habitat rich in foraging material that can support healthy populations of invertebrate species, which are a vital part of the diet of hazel dormice over the summer (Bright et al. 2006). Conservation initiatives have also involved reintroductions and in 2021 Natural England celebrated the reintroduction of the 1000th individual in Lancashire (People’s Trust for Endangered Species 2021). Success has been linked to adequate habitat management (Bright & Morris 2002) and improved connectivity to allow population expansion (Mitchell-Jones & White 2009).
The installation of nestboxes is also widely employed in the UK to help reverse dormice declines (Morris et al. 1990). Nextboxes can improve local densities (Morris et al. 1990), potentially due to enhanced survival of young in consistently dry and secure boxes, or because boxes offer greater nesting opportunities than might be naturally available in some habitats. With entrances holes approximately 20mm in diameter, dormice nestboxes are less likely to be used by other woodland species, such as squirrels and woodpeckers, which reduces competition (Madikiza et al. 2010). While nestboxes can be important, their uptake by hazel dormice depends on the surrounding environment which needs to be carefully considered when placing boxes (Juškaitis et al. 2013, Mortensen et al. 2022). Placement near food and nesting material sources is likely beneficial. Hazel dormice feed on a variety of resources that vary seasonally. After coming out of hibernation in Spring, dormice feed on the nectaries and anthers of understorey tree species, including hawthorn (Crataegus monogyna ) and rowan (Sorbus aucuparia ), as well as herbaceous field layer species such as wild garlic (Allium ursinum ) (Richards & Hurrell 1984, Eden 2009). Dormice can also consume pollen from certain canopy tree flowers, including oak (Quercus spp.) and sycamore (Acer pseudoplatanus )(Richards & Hurrell 1984). Spores of bracken(Pteridium aquilinum), bryophytes and fungi have also been found in faecal samples (Richards & Hurrell 1984). In the Summer dormice have an invertebrate-based diet, mainly feeding on aphids and lepidopteran larvae (Richards & Hurrell 1984, Eden 2009, Chanin et al. 2015). Finally, in Autumn, in preparation for hibernation, hazel dormice feed on high-calorie food such as hazelnuts (Grodziński & Sawicka-Kapusta 1970). Regarding nesting material, honeysuckle (Lolium periclymenum ) is the preferred material, with graminoids providing an alternative (Bright & Morris 2005). Proximity to these resources is likely important, because while dormice can travel up to 50 m to collect materials, when resources are available regular travel is generally limited to within 10 m of the nest site (Bracewell & Downs 2017). In fact, a study found that over 70% of nests in nestboxes were made from the plant on which the nestbox was attached (Bracewell & Downs 2017). A shorter journey when encumbered by heavy nest materials reduces the risk of predation, and conserves energy, especially for lactating females (Prentice & Prentice 1988, Juškaitis 2014).
Proximity to key resources is important, but their accessibility is also key. As an arboreal species, hazel dormice require a well-developed tree canopy, and/or understorey layer with plenty of horizontal branches (Bright & Morris 2009). However, an extensive tree canopy and/or understorey layer could limit the amount of sunlight reaching field layer plants affecting flowering and fruiting and encouraging vertical growth which is less useful for travel (Bright & Morris 1990), so probably intermediate to high tree canopy and/or understorey layer cover is optimal (Juškaitis & Augutė 2008). An extensive tree canopy and/or understorey layer could also shade the area beneath, leading to lower temperatures in nestboxes. At temperatures below 14-15℃, dormice often enter a state of torpor (Juškaitis 2005) that could be particularly detrimental during Autumn when foraging is key in preparing for hibernation. Temperature and sunshine hours have also been found to influence breeding success (Goodwin et al. 2018).
Previous research has described general habitat preferences of hazel dormice and explore how overall local conditions affect nestbox occupancy (Bright & Morris 1990, Panchetti et al. 2007). However, limited information is available on how microhabitat within a suitable local site influences nestbox use (Mortensen et al. 2022). After an area is identified as suitable for installing nestboxes it is still important to determine the optimal locations for nestboxes within the site. Here we address this knowledge gap, linking data from 76 microhabitat variables to dormice occupancy for 45 nestboxes installed in a UK National Trust woodland. Microhabitat variables were defined to test our hypotheses that the probability of nestbox occupancy increases with relatively high tree canopy and/or understorey coverage that facilitates movement but does not fully block sunlight, with greater abundance and diversity of key plant species for nest building and foraging resources in close proximity to the nest, and with increased distance from sources of anthropogenic disturbance. Our results offer recommendations for where to best place nestboxes and guide microsite management to aid in the protection and conservation of the hazel dormouse.