First record of brown-headed cowbird parasitism on Cassin’s Sparrow in Oklahoma

Abstract

Cassin's Sparrow (Aimophila cassinii), an emberizid with a distribution limited to the shrub-grasslands of the southwestern U.S. and northern Mexico, has been reported to be an infrequent host of the brood parasite, Brown-headed Cowbird (Molothrus ater). We present the first documented records of parasitism on Cassin's Sparrow by cowbirds for Oklahoma. Eleven of 45 (24.4%) Cassin's Sparrow nests found during 2006-2008 were parasitized by cowbirds. This may indicate that Cassin's Sparrows are more frequently parasitized than previously thought or that this species experiences a higher rate of brood parasitism in the northeastern part of its range in Oklahoma. Reasons for locally increased parasitism rates may include small-scale geographic variation in host preference by cowbirds or variation in host nest availability due to the highly fluctuating nature of Cassin's Sparrow abundance.