Epaulet Brightness and Condition in Female Red-Winged Blackbirds

Abstract

Epaulets of female Red-winged Blackbirds (Agelaius phoeniceus) vary in bright- ness from brown to bright red-orange. We test predictions of the hypothesis that condition at the time of molt determines female epaulet brightness and that females in superior con- dition produce brighter epaulets. We compared each female's epaulet with a series of color photographs ranked from 1 (dull brown) to 12 (bright red-orange) and considered females to have increased in brightness between years if their color increased by at least two ranks. More first-year females than older females increased in brightness, more older females in- creased in brightness after low reproductive effort than after high effort, and more first-year females increased in brightness after a year of superabundant food (an emergence of periodical cicadas, Magicicada spp.) than after other years. In each case, we expected an improvement in condition. We estimated female condition and regressed condition index on day of the breeding season. During the breeding season, females that later increased in brightness improved in condition relative to females that did not increase in brightness. We confirm a previously reported correlation between age and epaulet brightness, but age does not explain the relationship between change in brightness and either reproductive effort or the emergence of cicadas. We conclude that the most likely explanation for our results is that condition, at least in part, determines epaulet brightness in female Red-winged Blackbirds and that superior condition is reflected by brighter epaulets. Our results support assumptions of the status- signaling hypothesis and mate-choice hypotheses for the evolution of variable plumage. The best explanation for the evolution of variable plumage among female Red-winged Blackbirds is that brightness signals status in female-female aggressive encounters. Received 21 November