We studied mating patterns in Saltmarsh Sparrows (Ammodramus caudacutus), a species with no territorial defense, no paternal care, and no pair bonds. Fifty-seven of 60 broods with at least 2 sampled chicks (and all broods that were completely sampled) resulted from multiple mating by females. About one-third of the broods had a different father for each chick, but the most common pattern, found in 36 of 60 nests, was multiple sires, with at least 1 male having sired 2 or more chicks. The level of multiple paternity in Saltmarsh Sparrows may be the highest documented in any bird. Levels of multiple mating increased with population density but not with nesting synchrony. We were able to assign paternity to particular males for about half the 206 chicks that were genotyped. Males sired chicks as far as 1.4 km from their original capture site, and some males sired chicks in multiple nests, separated by up to 0.5 km. We also document a case of 2 females laying eggs in the same nest.