Epaulet Color and Sexual Selection in the Red-Winged Blackbird: A Field Experiment

The epaulets of male Red-winged Blackbirds (Agelaius phoeniceus) function in both intersexual and intrasexual contexts, but there is little evidence that they covary with reproductive success in this well-studied species. We used path analysis of male/territory traits, mating success, and reproductive success of unmanipulated males to estimate current directional selection. Territory size had a positive effect on number of within-pair mates. Number of extra-pair mates had a positive effect on number of extra-pair fledglings, and number of within-pair and extra-pair fledglings had positive effects on total number of fledglings. We also reddened epaulets of free-living territorial males to determine whether manipulated and control males differ in territorial behavior, mating success, or reproductive success. Compared with control males, males with reddened epaulets incurred elevated rates of trespassing and territorial challenges, lost their territories more often, and were unable to produce extra-pair offspring. Despite these differences, however, the realized reproductive success of experimental and control males did not differ significantly, perhaps because males with reddened epaulets devoted more time to anti-predator vigilance and were more aggressive toward a simulated predator than were control males. An apparent lack of current sexual selection on epaulet color may be a form of counter-balancing sexual selection in which male aggression against redder epaulets opposes female preference for redder epaulets, or it might be the result of males compensating for lost extra-pair fertilizations by increasing their parental care.