Auk’s Top Cited, #4: Variation in Cowbird Host Egg Appearance

FIGURE 1. Fresh eggs representing the host eggs measured from the Chicago Field Museum’s egg collection, with the average spectral reflectance of eggs from each species measured.

This week we’re looking back once again and featuring a highly cited paper from 2014: Intraclutch variation in egg appearance of Brown-headed Cowbird hosts by V.E. Abernathy and B.D. Peer, from the October 2014 issue of The Auk.

Unlike some nest parasites, the Brown-headed Cowbird has not evolved to lay eggs that mimic those of its host. Still, some host species’ eggs look more like cowbird eggs (white and spotted) than others, and these birds should face more selection pressure to produce clutches of very similar-looking eggs so that cowbird eggs will stand out and be easier to detect.

Using reflectance spectrometry, Abernathy and Peer compared egg clutches from four different categories of cowbird host species: species that accept cowbird eggs (“accepters”) and lay white, spotted eggs; species that reject cowbird eggs (“rejecters”) and lay white, spotted eggs; accepters that lay blue eggs; and rejecters that lay blue eggs. Intraclutch variation did not differ between accepters and rejecters with blue eggs, which supported their hypothesis, but the pattern for hosts with white eggs was less clear – intraclutch variation did vary between species, but it didn’t follow the accepter/rejecter categories. The results suggest that the degree of intraclutch variation in a Brown-headed Cowbird host may not be sufficient to predict rejection frequency; other factors, such as length of exposure to cowbird parasitism, the costs associated with parasitism, and geographic location, may affect a species’ response to cowbird eggs.

Read the full paper at