Summary
We tested whether the reproductive success of male red-winged blackbirds (Agelaius phoeniceus) varied with male secondary sexual traits or with haematozoa prevalence, and whether these patterns were consistent with females preferring genetically superior males. We also determined whether the traits that correlated with male success on their own territories were also correlated with male success at siring young on other males' territories. Our analysis included data from a 6-year study involving 617 nestlings for which paternity was determined by DNA profiling. Larger males sired more young on their territories, principally because they obtained larger harems. The success of larger males at acquiring more mates did not appear to be a consequence of larger males holding larger or better-quality territories. Older and longer-lived males sired more young by extra-pair fertilizations. Larger males sired the most offspring overall (on territory + off territory). Variation in epaulet size and color, responses to male and female models, nest defence and parasitism was not correlated with male success either on or off their territories or overall. Male success in a given year was significantly correlated with success the previous year, as expected if females were selecting genetically superior males. The male that was by far the most successful individual in this study was highly consistent from year to year. Because male body size is positively correlated with survival in this population (although not within the sample of males included in this study), female preference for larger males may have reflected a general preference for males with superior survival ability. We propose that the direct advantage realized by older males in extra-pair matings might indicate that experience is important, such that experienced males are better at creating or exploiting the opportunities for extra-pair mating. This hypothesis is consistent with a pairwise analysis of cuckoldry that showed that cuckolders were most often older than the males they cuckolded.
Methodology
Our data were collected from 1986 through 1991 as part of a study of red-winged blackbirds initiated in 1984 at sites 10 km from the Queen's University Biological Station in eastern Ontario. Each of the three sites is a spring-fed beaver pond surrounded by habitat inappropriate for red-winged blackbird nesting. The closest unstudied marsh with breeding blackbirds was 0.25 km from the nearest study area, and the study areas were 0.5-1 km from each other. We were able to capture, band, and collect blood from > 95% of males resident on the three marshes over the 6 years of this study. Data from the largest of the marshes were used in all 6 years. Preliminary paternity analyses from this marsh for 1986 were presented in Gibbs et al. (1990). In the other two marshes we only analysed paternity in 1988 and 1989, although we did monitor nesting activity throughout, so the age of the males present in 1988 and 1989 could be estimated.