Brown-headed cowbirds in northern lower Michigan breed from late May to early July. Throughout the breeding season laying females flocked with other breeding cowbirds. Communal roosting of breeding birds, flocking, and the occurrence of eggs laid by several females in a single host nest indicate that cowbirds do not maintain exclusive territories. Serial sections of the ovaries of 60 adults were examined for post-ovulatory follicles. Numbers of eggs laid and the timing of laying were determined by comparing the size and histology of cowbird follicles to those of blackbirds of known breeding history. Female cowbirds lay an average of 10 to 12 eggs during the breeding season. Variation between birds is great, as some lay no eggs while others lay at least 15. Cowbirds lay in clutches of one to six eggs. Mean clutch size is 3.1 eggs. Mean clutch size late in the breeding season is significantly lower than early in the breeding season. Both follicular growth rate and atresia limit clutch size by regulating the numbers of oocytes that mature in the ovary. Variation in clutch size suggests a high degree of environmental control of clutch size. Most cowbirds lay several clutches in a season. Times between clutches range from a few days to a few weeks. Mean clutch size has not increased in the evolution of the cowbirds. However, the numbers of clutches and the numbers of eggs laid in a season are greater in Brown-headed Cowbirds than in nonparasitic icterids. The large number of eggs laid by this brood parasite is associated with an evolutionary removal of an upper limit of young to be fed by the parents, rather than with a high mortality of the young.