Exposure to electromagnetic fields (EMFs) can affect a wide range of biological processes, including reproduction, growth and development. Experiments aimed at investigating the biological effects of EMFs, focused on potential harmful effects on humans, have been mostly carried out in vitro or with animal models in laboratory conditions. By contrast, studies performed on wild animals are scarce. The effects of EMFs created by an electric power line on reproductive traits of a wild great tit (Parus major) population were explored by analysing data gathered during nine breeding seasons. EMF exposure significantly increased clutch size (7%) and egg volume (3%), implying a 10% increase in clutch volume. This indicates an increase in reproductive investment from parent birds exposed to EMFs as compared to the adjacent reference area. These results cannot be attributed to habitat or adult quality differences between the exposed and reference group. Nevertheless, no differences in hatching success or final productivity (fledging and reproductive success or nestling body mass) could be detected. Our study clearly shows that EMFs created by power lines can have biological consequences in wild organisms that live intimately with them. To our knowledge, this is the first study showing an increase in clutch size, and one of the few reporting an increase in egg size, associated with EMF exposure. The possible mechanisms by which great tits invest more under EMF exposure are discussed, and future research directions to evaluate the effect of EMFs on avian reproduction in the wild are suggested.