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The aim of this protocol is to establish evidence of Harvest Mice Micromys minutus breeding attempts, based on nest searches, and to assess the extent and suitability of habitat for Harvest Mice.
Estimating population size
Obtaining accurate population estimates for harvest mice is notoriously difficult and live trapping produces unreliable results compared to other small mammals, possibly due to poor detection. (MacDonald & Tattersall, 2001 cited in Sibbald et al., 2006; Haberl & Krystufek 2003).
Using trapping to obtain reliable population estimates, or even a robust minimum would require time and resources that are not currently available to Sheffield and Rotherham Wildlife Trust. Owl pellet analysis and bait tubing have shown some success in detecting presence/absence (Poulton & Turner, 2009; Meek, 2011) but the costs of recruiting identification experts, and the inability of these methods to provide abundance indices preclude their use here.
Survey techniques will therefore be focused on obtaining nest density estimates, and on the correlation between nest density and selected habitat variables.
Harvest mice weave 5-10cm spherical grass nests, 0.3-1m above ground in the stalk zone of vegetation (Mammal Society Species Factsheet) providing a useful signal of their presence in a particular habitat patch.
Common consensus agrees that nest number does not provide an index of population size (Kettel et al., 2016). Nest density (number per unit area) number can, however, provide an indirect proxy of activity level and evidence of habitat patch occupancy. Over time, changes in nest frequency counts may indicate underlying population changes (Poulton & Stone, 2008).
Harvest Mice are thought to range around 20m, and so a buffer of that size around each nest can provide a proxy for the range covered by the species across the site.
Habitat preference
Given that nest density indicates harvest mouse presence and can be considered a proxy of activity level, their location can indicate which habitat types are favoured by harvest mice.
Vegetation height, dominant plant species, and whether or not a habitat is wet/dry and connected/unconnected are all thought to affect habitat suitability for harvest mice (Haberl & Krystufek, 2003; Sibbald et al., 2006; Poulton & Turner, 2009; Meek, 2011; Kettel et al., 2016).
Previous studies indicate that preferred habitat features for nesting include >0.5m high, dense vegetation, including grasslands left to grow for two-three years, field margins, hedgerows, scrub, ditches and lowland wetland habitats with associated vegetation such as rushes and reed stands.
>0.3m tall, dense grass or scrub margins including ditches, river banks, field margins and hedgerows provide connectivity from one area of activity to another.
Key attributes
- Expansion of range, based on 2018.
- Within areas of recorded harvest mice activity (2017 onwards), vegetation consisting of rank grasses, rushes and/or brambles should exceed >50cm in height
- Within areas of recorded harvest mice activity (2017 onwards), territories should have access to connectivity such as vegetation consisting of rank grasses, rushes and/or bramble >30cm tall, ditches, field margins, hedgerows and/or scrub
Materials/equipment
- GPS
Timing
Surveys should take place in late autumn, early winter. During this period there is less chance of disturbing breeding mice. Harvest mice become more ground dwelling in the winter so risk of encountering mice in nests is low. Vegetation die-back facilitates nest location, while any further into winter risks destruction of nests due to harsh weather conditions.
Method
Suitable areas of habitat, including along field boundaries, have been split into 10m grid squares, which should be searched for nests.
Thoroughly and systematically search vegetation in each grid square, as far as vegetation allows. Note that occasional nests may be constructed on the ground (Ishikada et al., 2010), or aerial nests dislodged.
Within each quadrat measure:
- Sward height: record whether >50% vegetation >0.5m.
- Dominant species: may be more than one species.
- Whether suitable areas of connectivity are available from each known nest site (>0.3m tall, dense grass or scrub margins including ditches, river banks, field margins and hedgerows).
It may be more efficient to split the two tasks above, allowing an individual to work quickly across the site to carry out vegetation assessment ahead of those searching for nests.
Other information
References
Haberl, W. and Krystufek, B. (2003) Spatial distribution and population density of the harvest mouse Micromys, minutus in a habitat mosaic at Lake Neusiedl, Austria. Mammalia 67: 355-365.
Ishiwaka, R., Kinoshita, Y., Satou, H., Kakihara, H. and Masuda, Y. (2010) Overwindertering in nests on the ground in the harvest mouse. Landscape and Ecological Engineering 6: 335-342.
Kettel, E. F., Perrow, M. R. and Reader, T. (2016) Live-trapping in the stalk zone of tall grasses as an effective way of monitoring harvest mice (Micromys minutus). European Journal of Wildlife Research 62: 241-245.
Meek, M. (2011) Suffolk’s harvest mice in focus. Report to the People’s Trust for Endangered Species and The Chadacre Trust. Suffolk Wildlife Trust 2009-2011.
Poulton, S. and Stone, E. (2008) Pilot study for a national monitoring scheme for small mammals in the United Kingdom and the Republic of Ireland. The Mammal Society Research Report No. 8.
Poulton, S. and Turner, P. (2009) A comparison of nest searches, bait tubes and live trapping for monitoring harvest mice (Micromys minutus) and other small mammals. The Mammal Society Research Report No. 9.
Sibbald, S., Carter, P. and Poulton, S. (2006) Proposal for a national monitoring scheme for small mammals in the United Kingdom and the Republic of Eire. The Mammal Society Research Report No. 6.
Species Factsheet: Harvest Mouse (Micromys minutus). The Mammal Society.
http://www.mammal.org.uk/discover-mammals/species-harvest-mouse/. Accessed 18th February 2018.