Experimental Evidence that Brighter Males Sire More Extra-pair Young in Swallows

Linda A. Whittingham and Peter O. Dunn

Department of Biological Sciences, UW-Milwaukee, whitting@uwm.edu, pdunn@uwm.edu

Across taxa, extra-pair mating is widespread among socially monogamous species, but few studies have identified male ornamental traits associated with extra-pair mating success, and even fewer studies have experimentally manipulated male traits to determine if they are related directly to paternity. As a consequence, there is little evidence to support the widespread hypothesis that females choose more ornamented males as extra-pair mates. Here, we conducted an experimental study of the relationship between male plumage color and fertilization success in tree swallows (Tachycineta bicolor), which have one of the highest levels of extra-pair mating in birds. In this study we experimentally dulled the bright blue plumage on the back of males (with non-toxic ink markers) early in the breeding season prior to most mating. Compared with control males, dulled males sired fewer extra-pair young, and, as a result, fewer young overall. Among untreated males, brighter blue males also sired more extra-pair young, and in paired comparisons, extra-pair sires had brighter blue plumage than the within-pair male they cuckolded. These results, together with previous work on tree swallows, suggest that extra-pair mating behavior is driven by benefits to both males and females. This research was supported by funds from the College of Letters and Science, UWM.