Intraclutch variation in egg appearance of Brown-headed Cowbird hosts

Abstract

ABSTRACT Several species of avian brood parasites have evolved egg mimicry, which can interfere with host egg rejection. Parasitic egg mimicry may select for decreased intraclutch variation in host egg appearance to facilitate the recognition and rejection of parasitic eggs. This hypothesis has received scant attention in hosts of the Brown-headed Cowbird (Molothrus ater) because the cowbird apparently has not evolved mimetic eggs. Nonetheless, hosts with eggs similar in appearance to cowbird eggs should minimize intraclutch variation to increase the likelihood of detecting parasitism. By contrast, there may be minimal selection pressure to reduce intraclutch variation in hosts with eggs that are divergent from cowbird eggs. Using reflectance spectrometry, we compared the intraclutch variation between accepters and rejecters of 2 groups of host species: those with eggs similar in appearance to cowbird eggs (white maculate eggs) and those with eggs that clearly diverge in appearance from cowbird eggs (blue...