Summary
Colored epaulets in female red-winged blackbirds (Agelaius phoeniceus) could be due either to direct sexual selection favoring the maintenance of this trait in females due to intrasexual competition for breeding opportunities, or to sexual selection favoring bright epaulets in males indirectly causing expression of the trait in females by genetic correlation. Older females tend to be brighter than younger females. Also, brighter females tend to breed earlier in the season than dull females. These patterns are consistent with both of the hypotheses. In a series of experiments in which males and females were presented with taxidermic mounts of dull and bright females, the plumage of these mounts had no influence on the response of either males or females. Also, the response of females was independent of their own plumage. The results of all of the experiments consistently supported the interpretation that male plumage characteristics expressed in female redwinged blackbirds have no functional value and are a consequence of genetic correlation with males. Since recent studies have also indicated that female aggression has no functional value, we speculate that this too could be due to genetic correlation with a trait favored in males.
Methodology
Research was conducted along paved highways within 30 km of the Queen's University Biological Station north of Kingston, Ontario in the spring and summer of 1986 and 1987. On a daily basis beginning 1 April, we monitored female settlement on the territories centered around cattails (Typha latifolia) growing in the ditches. All observations were made using binoculars from a truck parked along the road. Females were not individually marked until after they were established on territories so we did not influence settlement patterns. Early in the spring females were sometimes observed on particular territories and sometimes not. We considered a female to have settled on a territory once we saw a female on three consecutive visits to that territory (Teather et al. 1988). When females had settled we caught them either by decoy trapping (Smith 1972; Picman 1979) or by mist net. All birds were banded with a unique combination of three colored aluminum bands and a numbered U.S. Fish and Wildlife Service band. Most females remained on the territories on which we banded them, confirming our assumption regarding settlement. We measured body mass of each female to the nearest 0.5 g, wing length to the nearest mm, and quantified plumage brightness (see below).