Egg pecking and discrimination by female and male Brown-headed Cowbirds

Abstract

Brood parasitic Brown-headed Cowbirds (Molothrus ater) remove host eggs in conjunction with parasitism, and also peck and destroy host eggs and nestlings in nests they have apparently not parasitized. Egg and nestling destruction by Brown-headed Cowbirds has been hypothesized to force hosts to re-nest, providing cowbirds with a future opportunity to parasitize a nest. We examined the egg-pecking behavior of captive male and female Brown-headed Cowbirds by presenting them with nests containing plaster eggs resembling a parasitized host nest to determine how often this behavior occurs. There was no significant difference in the number of female (67 %) and male (42 %) cowbirds that pecked both egg types, but females pecked more eggs than males, and both were more likely to peck host eggs than cowbird eggs. Our results demonstrate that both females and males peck eggs, and suggest that parasites may have an even greater effect on hosts than previously realized.