Survival conditions over the winter and determinants of variability in the onset of reproduction are major factors shaping the life history of most organisms in strongly seasonal environments. During the long non-breeding season availability and quality of food may be major constraints determining the onset of reproduction. We studied the effects of supplemental animal protein over winter on the onset of reproduction in the bank vole, Clethrionomys glareolus, an iteroparous seasonal breeder, granivorous-herbivorous, with small amounts of animal food in its diet. We predicted that animal protein would be especially important during spring, when energy demands are increasing due to the onset of reproduction and availability of new plant food is still limited. Field experiments were conducted in 6 open forest plots (2.25 ha each), and bank voles were fed supplemental whey protein during the winter and spring. Supplemented males were heavier at the onset of reproduction than non-supplemented males. Maturation and parturition of first litter occurred significantly earlier in females fed supplemental animal protein. No differences in litter size and litter mass were detected between treatments. We conclude that animal protein in the form of whey may play an important role in the timing of spring reproduction. Therefore, natural animal protein food sources like insects, pupae, and possibly carcasses of mammals and birds gain importance for the onset of breeding even in an granivorous-herbivorous species like the bank vole.