The Clutch Size and Numbers of Eggs of Brown-Headed Cowbirds: Effects of Latitude and Breeding Season

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Abstract
A comparative study of populations of parasitic cowbirds [Molothrus ater] in central California, S Oklahoma and N Michigan [USA] showed a mean clutch size of 3.91-4.05 in all 3 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 N Michigan. Laying rates (mean number of eggs ovulated during the previous 10 days) were the same in all 3 populations. The smaller number of eggs laid by females in N 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.

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