We determined if extra-pair paternity existed in an eastern population of Song Sparrows (Melospiza melodia melodia), and compared the frequency of extra-pair paternity in first and second broods. Over both broods, a total of 10.5% young were of extra-pair origin, but this frequency is considered conservative as it was based on only three loci. Forty percent of females had extra-pair young in at least one of their two broods. One of five females changed from genetic monogamy to genetic promiscuity between broods, despite remaining paired with the same male. Studies of double-brooded species are ideal for examining factors other than mate quality that might influence rates of extra-pair paternity.