Seasonal variations of diet composition in farmland field mice Apodemus spp. and bank voles Clethrionomys glareolus

Feeding habits of field mice Apodemus flavicollis Melchior, 1834 and A. sylvaticus Linnaeus, 1758, and bank voles Clethrionomys glareolus Schreber, 1780 in a mixed farmland area in Northern Germany were investigated from March to December 1992. For semi-quantitative analysis of diet composition, faeces were sampled in the margins of an arable field surrounded by hedgerows. Diets of both Apodemus-species were similar, with A. flaviocollis tending to eat more animal food and less green plant matter. Except in the early summer, C. glareolus consumed less animal matter than both Apodemus-species, but not consistently more green plant matter than A. sylvaticus. Beech flowers were important food for all species in May, whereafter rye grain became the dominant food item until August. In the autumn and winter, field mice took beechmast and acorns, while bank voles ate berries and fungi, and in December, large amounts of grass leaves. Dietary overlap was, altogether, highest from May to August (about 0.8). Overlap between A. flavicollis and A. sylvaticus dropped moderately after the summer, while that between C. glareolus and either Apodemus-species decreased sharply to levels of 0.2 and lower. Increased probability of competitive interaction between the species in the autumn and winter, corresponding to a more patchy distribution of food resources than in the summer, is discussed as a possible reason for the observed divergence of food habits.