The eggs of the parasitic Screaming Cowbird(Molothrus rufoaxillaris) and its host, the Baywinged Cowbird(M. badius): is there evidence for mimicry?

Abstract

Measurements and descriptions of 239 eggs of Baywinged Cowbirds(Molothrus badius) and of 263 eggs of Screaming Cowbirds(M. rufoaxillaris) are presented; egg identification is ultimately based on nestling identification. Statistical analyses of host and parasite eggs showed no significant differences in measurements, weight and index of size. An index of shape was more useful for egg identification; eggs ofM. rufoaxillaris were more spherical, but their has been found considerable overlap. Background color and presence/absence of thick dark lines (“scrawls”) were the most useful criteria for the identification of host and parasite eggs. In this host-parasite system there is no evidence of egg mimicry, and the behaviour ofM. badius is not selecting for egg mimicry.