A polygynous population of northern harriers was studied for 4 years in New Brunswick to establish the costs and benefits to females within harems. Polygynous harems of up to five females were observed, and clear hierarchies in clutch size and reproductive success were apparent in each year. Alpha females raised more fledglings than any other harem female. Seasonal declines could explain the tendency of polygynous females to lay smaller clutches but not their reduced reproductive success relative to monogamous females. Later-settling polygynous females suffered higher rates of predation than either monogamous or alpha females. This could not be explained by differences in nest site quality; nor could female inexperience be cited, since older females predominated in harems. Reduced success of polygynous females was best explained by male provisioning rates. Alpha females received significantly more prey items per day than any other harem female. We surmise that the increased predation of later (beta, gamma, and delta) harem nests was also a result of low male provisioning rates; later-settling females were forced to forage by themselves and to leave their nests unattended. We contend that a skewed sex ratio is not the prime reason for the evolution of polygyny, as suspected in other harrier populations, but that differences in male quality or controlled resources contribute to the crossing of the polygyny threshold.