A comparative study of populations of parasitic cowbirds in central California, southern Oklahoma, and northern Michigan showed a mean clutch size of 3.91 to 4.05 in all three areas. Estimates of the mean number of eggs laid in a season were 25.0 for Oklahoma, 24.1 for California, and 11.3 for northern Michigan. Laying rates (mean number of eggs ovulated during the previous ten days) were the same in all three poulations. The smaller number of eggs laid by females in northern Michigan is due to the shorter breeding season of the local passerine hosts. Recoveries of banded cowbirds in the northeast show that most return to their hatching site in later breeding seasons, and genetically effective dispersal is limited to a few kilometers. No differences in clutch size were found that could be attributed to latitude, to total number of clutches laid in a season, or to the history of the populations.