A receiver bias for red predates the convergent evolution of red color in widowbirds and bishops

Preexisting receiver biases are well known and empirically supported alternatives or complements to signal evolution through coevolving mate choice, but largely neglected as drivers of sexually or socially selected agonistic signal evolution. In further pursuit of a recently revealed receiver bias for red agonistic signaling in Euplectes (17 species of widowbirds and bishops), we investigate here its presence in the yellow-crowned bishop (Euplectes afer), a representative of the earliest phylogenetic branch of the genus. In a captive population in southern Spain, where the yellow-crowned bishop is invasive, we staged and filmed 10-min dyadic contests over access to a feeder, between males with experimentally yellow- (control-) and red-painted crown plumage, respectively. Red males secured significantly more time at the feeder, and tended to win more of the limited number of supplant attempts observed. This suggests that the previously demonstrated agonistic signal function of red carotenoid coloration in widowbirds also applies to the bishop birds, and may derive from a receiver bias (aversion) that is substantially older than the convergent gains of red plumage pigmentation in Euplectes, and perhaps also predating the evolution of red in a few other weaverbird (Ploceidae) lineages. Given the similarities in ecology and behavior across Euplectes, the color diversity appears to primarily be a consequence of evolutionary limitations on mechanisms for achieving red coloration.