Overwinter population change of small mammals in southern Finland

Patterns and variations in the overwinter population change of small mammals (shrews, voles, and mice) were studied on the basis of long-term data collected using snap trapping in autumn and spring 1981-2005 in southern Finland. In particular, attempts were made to reveal possible negative effects of mild winters on the wintering success of populations. The winter decline in density varied considerably both within and between species and groups. It was not significantly steeper in the coastal than in inland vole populations, but there were significant differences between the local populations of the southern coast. Small mammals seemed to overwinter more successfully in forests than in fields, and the negative effects of mild winters seemed to impact the populations of field habitats. The direct contribution of high ambient temperatures on the overwinter population change seemed to be largely restraining, but especially so in the depth of winter. There seemed to be indirect inverse effects involved. Population density exhibited only minor effects.