The edible dormouse is a specialized seed predator which is highly adapted to the fluctuations of food availability caused by mast seeding of beech and oak trees. Dormice produce young just in time with maximum food availability, and can completely skip reproduction in years with a lack of seeding. Because their decision to reproduce or not in any particular year is made long before the ripe seeds are available, it seems that dormice can anticipate the upcoming mast situation. We tested the hypothesis that the presence of high caloric food in spring affects their reproductive decision. Therefore, we supplementary fed dormice in a field experiment from spring to early summer with sunflower seeds, which also contain a high amount of energy. Supplemental feeding caused significant increases in the proportion of reproducing females and reproductively active males. These results suggest that edible dormice may use the occurrence of an energy rich food resource to predict the autumnal mast situation. Further, our data indicate that the decision to reproduce was not the result of an increased body mass due to the consumption of surplus food, but that sufficient seed abundance acts as an environmental signal to which dormice adjust their reproduction.