Intraclutch variation in egg appearance of Brown-headed Cowbird hosts

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 eggs). We predicted that rejecters with white maculate eggs should have lower intraclutch variation than accepters, whereas accepters and rejecters with blue eggs should have similar amounts of intraclutch variation. The intraclutch variation between accepters and rejecters with blue eggs did not differ, which matched our predictions. However, rejecters with white maculate eggs did not consistently have lower intraclutch variation than accepters; thus, our hypothesis was not supported for this group. A more comprehensive study, focused on cowbird hosts nesting in grassland and edge habitats, is warranted to determine whether a pattern between intraclutch variation and egg rejection exists among hosts with white maculate eggs.