Supplementary Food and the Winter Dynamics of Bank Vole Populations *

The food supply of two populations of Clethrionomys glareolus ( S e h r e b e r, 1780): one living in forest on a 4 ha lake island, and the other inhabiting an open forest, was exprimentally increased by sup-plying oats ad libitum for a period of two years. The food was supplied in wooden boxes, which on the island were arranged in a grid of squares, each 15X15 m. In the open forest the boxes were placed on two plots each of 2.7 ha: (1) intensive supplementation with the grid of boxes 7.5X15 m, and (2) extensive supplementation with the grid of boxes 30X30 m. Population parameters were estimated by the CMR method each individual being marked. Food supplementation caused intensive winter reproduction to such extent that 50 % of individuals found in the spring were born during the winter. On the other hand, food supplementation did not affect winter survival, although a few individuals (females) survived two winters. During the winter, sexually active males were heavier than females. The individuals born during the winter promptly become sexually active. A result of the increased food supply was that the population numbers in the spring were 2 to 4 times higher than in control years. A similar situation was observed in the autumn. Hence the food carrying capacity of the habitat is a significant factor affecting the numbers of small forest rodents despite the fact that, as indicated by energy balance studies, they utilize only a small part of food available in the ecosystem. 1