Annual fluctuations in harvest mouse (Micromys minutus) nest densities in southern England.

Aerial nest surveys are the principal method of monitoring the distribution and habitat preferences of harvest mice (Micromys minutus), but monitoring has rarely been undertaken for more than three years at any site. Although nest surveys have inherent errors, they remain the most easily accessible and cheapest method. The aim of this study was to count the number of nests found in consecutive years at two sites in southern England. We found that the harvest mouse nest densities fluctuated over time. This supports other studies that indicate harvest mice populations undergo major fluctuations from year to year. Although the drivers of this annual variation in nest numbers have not yet been identified, surveying for harvest mice using aerial nest counts in just one year could underestimate their likely presence at that site.