New ways to Monitor Our Harvest Mice

The Wildlife Trust of South and West Wales’ have embarked on a new and innovative project designed to learn more about the elusive harvest mouse (Micromys minitus) at Parc Slip nature reserve near Bridgend. The project, which is being undertaken by WTSWW’s student placement, aims to identify the most effective survey methodology for the species and relies on several different techniques, including: live trapping, nest searching and the use of bait stations filmed by motion-sensitive cameras.

Harvest Mouse by Amy Lewis

Harvest Mouse by Amy Lewis

The trapping involves two different types of trap; the traditional Longworth and the relatively new Tube traps. 50 traps of each model are being used in a comparison study to investigate which is best for the specific trapping of harvest mice. A harvest mouse has been successfully caught with a Longworth trap, whilst other species of small mammal including; wood mouse (Apodemus sylvaticus) and common shrew (Sorex araneus) have been caught using both types of traps.

The traps were originally elevated off the ground by bamboo canes; the idea behind this was to attract the mice in the stalk zone, which is where harvest mice create their summer nests, with the vegetation providing seed and stability for both feeding and breeding. In the autumn when the vegetation starts to die back the traps were moved to the ground, harvest mice move from the stalk zone down to the ground in winter for food and nesting opportunities, hence the descent of the traps. Over all the number of small mammal captures has been poor, with the wet weather possibly to blame.

The nest searches took place in October when the vegetation had died back and nests were easier to be seen.  In total 18 nests were found in just one rough grassland habitat on the reserve; all of these being summer nests. Summer (breeding) nests are approximately 10 cm in diameter and non breeding nests are normally smaller. Recently constructed nests will most probably be green; this is because they are built with living vegetation and the foliage is weaved onto the living stalk, it is for this reason that nest searching was postponed to the autumn months.

The nest searching is a relatively non-invasive method of surveying, however numbers of individuals will not be provided, it simply allows us to say whether harvest mice are present in a particular area or not.

The other part of the project was to design an innovative way of monitoring for the presence of harvest mice without the use of traps, which can invasive to the individual captured, or nest-searching which can be destructive to the habitat.

To combat potentially damaging and time consuming harvest mouse survey methods, a combination of bait stations and motion sensitive cameras in the stalk zone were used. The cameras could then film visiting harvest mice attracted by the food, undisturbed and in their natural habitat, thus allowing important insights into their (and other small mammals) ecology to be made.

Different sizes of mesh were used on the bait stations in order to exclude other species such as wood mice which have been known to visit them and exploit the supplementary food source. The initial design attracted wood mice, field voles, brown rats and the occasional bird, but no footage of harvest mice. It was clear from the amount of other species visiting the bait stations that another design was needed.

This new design has immediately borne fruit with harvest mice recorded visiting the bait station at night and during the day. We have also recorded wood mice visiting the new station but unable to get through the narrow gauge mesh.

This is the first time we have seen live harvest mice on the reserve without the assistance of trapping even though mice presence has been confirmed through nest searching. Now that the harvest mice footage has been captured, one of the bait stations will be moved to an area where the nest searching has not been carried out to see if the station can detect harvest mice without the aid of nest searching.

The project is ongoing and research on the species and the new methodology will take place over a number of years, providing new information and data all the time. If you require any further information please feel free to contact Eloise Neighbour on 01656 724100 or field.assistant@welshwildlife.org