Using sizes of unovulated follicles to estimate the laying rate of the brown-headed cowbird

Abstract

Most female brown-headed cowbirds (Molothrus ater) collected at the peak of the breeding season have three large ovarian follicles. The largest, about 8 mm in diameter, is designated as an a follicle. A bird with an a follicle would ovulate the next day, but one without an a follicle would not. Thus, one can identify birds that will terminate or begin a clutch on a particular day. The proportion of such birds is used to estimate the clutch size and the interval (number of days without laying) between clutches.The clutch size is the reciprocal of the proportion of laying birds (those with an oviducal egg) that lack an a follicle. The interval is the reciprocal of the proportion of nonlaying birds (those in breeding condition but without an oviducal egg) that have an a follicle. The average daily laying rate equals the clutch size divided by the sum of the clutch size and the interval. This rate, based on a heterogeneous sample, is about 0.66 eggs.I present a technique for testing the hypothesis that there is no differential vulnerability to collecting between laying and nonlaying birds.