The Laying Cycle of Brown-Headed Cowbirds: Passerine Chickens?

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Abstract
We collected 270 female Brown-headed Cowbirds (Molothrus ater) in southern Ontario in 1976 to determine ovarian and oviducal growth during the breeding season and 188 female cowbirds in 1977 to estimate clutch size and the interval between clutches. Ovaries and oviducts grew rapidly in April, reached breeding size in early May, and remained at this size, without regression between clutches, until the end of the breeding season in early July. Ovaries and oviducts then regressed rapidly and by the end of July weighed about the same as those in early April. The clutch size averaged about 4.0-4.6 eggs, similar to clutch sizes of nonparasitic icterids. It varied greatly from 1 egg, in about 10% of the birds, to about 7 eggs. Two days without laying usually separated consecutive clutches, but some birds appeared to miss laying for only 1 day. Although atresia of large yolky follicles was common in birds between clutches, it did not always precede the end of a clutch. The long reproductive period without regression and the short interval between clutches are atypical of passerine reproduction. Rather, the laying cycle of cowbirds is similar to that of domestic chickens (Gallus gallus). The laying cycle of cowbirds seems adapted to a continuous supply of host nests; atresia may be related to a shortage of nests.