Recently, possible biological effects of electromagnetic fields (EMFs) produced by high-voltage transmission lines have come under intense scrutiny, mostly with respect to human health. Because little has been done to assess possible effects of EMFs on the biology of free-ranging animals living within such fields, this study monitored the breeding biology of birds using nest boxes placed under transmission lines and in reference areas. Tree swallows (Iridoprocne bicolor) had significantly lower reproductive success (number fledged/clutch size) under the powerlines than in the reference sites. No such effect was apparent for house wrens (Troglodytes aedon). These results suggest that habitats under high-voltage lines may be suboptimal for tree swallows.