The ecology of wood mice (Apodemus sylvaticus) in fields of oilseed rape (Brassica napus oleifera)
The ecology of wood mice (Apodemus sylvaticus) found in fields of oilseed rape (Brassica napus oleifera) around Newburgh, Aberdeenshire was studied from October 1990 to December 1992. The animals' population dynamics were studied by live trapping with mark and recapture, their home range sizes and habitat utilisation were determined by radio-tracking, their diets were analysed by microscopic examination of stomach contents, and their preferences for oilseed rape were assessed by feeding trials. Population densities of wood mice in oilseed rape fields, at 1.6 to 12.5 per ha, were generally lower than those of woodland mice, but were comparable to those of wood mice on arable land found in other studies. The seasonal fluctuations in wood mouse densities in oilseed rape fields differed slightly from those described for woodland mice, with peak densities found in spring or summer. Both agricultural practices and the type of adjacent habitats affected the mean densities and seasonal fluctuations in densities of wood mice in oilseed rape fields. The mean home range size of radio-tracked wood mice in this study (0.34-0.88 ha) was larger than those reported for woodland mice, but smaller than those reported for wood mice on sand dunes. Although there were no statistically significant differences between seasons, the mice tended to make less use of oilseed rape fields during the early stage of the crop, but increased their use of oilseed rape fields as the oilseed rape plants grew older. The diet of wood mice caught in oilseed rape fields was similar to those of wood mice in agricultural fields in other studies, in that seed formed the major part of their diet, while vegetative parts of plants and animal food were taken seasonally as supplementary foods. Oilseed rape was eaten by wood mice in a noticable amount only in April, when flowering buds and flowers were always eaten, and in June, when young seeds of oilseed rape were frequently eaten. Only young seeds of oilseed rape, not old ones, were found to be as preferred by wood mice as young seed of barley or wheat.