According to optimal foraging theory, a predator should select the most beneficial prey in terms of energy gain relative to handling time. For the pine marten (Martes martes), rodents such as bank voles are the primary prey; however, the pine marten can switch to a variety of secondary prey items (e.g., birds, fruit and frogs) when the rodent abundance is low. We measured frog ilia found in pine marten scats during several consecutive years to assess whether the body mass of consumed frogs varied with changes in rodent abundance. Rodent density was inversely related to the frequency of occurrence of frogs in the marten's diet. Moreover, during the years with low rodent abundance, pine martens ate smaller frogs than in the years with high rodent availability. The average frog weight (18 g) during the years with high rodent density was similar to average vole weight, suggesting that prey of this size has an optimal ratio of foraging effort to energetic gain. During the lean years, martens preyed on frogs smaller than the average vole weight, and during spring this frog size-group is usually the most abundant in the environment. Our results suggest that functional responses of predators to low abundance of primary prey involve not only a prey-switch but also a decrease in the size of secondary prey consumed. Fluctuations in the proportion of frogs eaten in relation to yearly rodent density can affect both marten and frog population demography.