Reproductive consequences of bilateral asymmetry for individual male red-winged blackbirds

We used morphological and breeding data from a 2-year field study of red-winged blackbirds (Agelaius phoeniceus) to test the hypothesis that males characterized by low levels of bilateral asymmetry (i.e., high developmental competence) realize a reproductive advantage. Specifically, we evaluated each of several distinct components of male reproductive success relative to asymmetry measures made on five bilaterally paired characters. Results of a male removal experiment generally failed to support the prediction that symmetry would be associated with success in competition for access to breeding territories: established territory owners and nonterritorial replacement males were effectively indistinguishable in this regard. Similarly, there was no indication that symmetrical males were more likely to establish territories in high-quality marsh habitat than in marginal upland field habitat. Finally, monitoring of breeding activity in high-quality habitat revealed that male symmetry was generally unrelated to recruitment of social mates (i.e., harem size), the productivity of those mates (average female reproductive success), within-pair paternity (assessed using DNA-based analysis of parentage), or extrapair mating success. Collectively, these results indicate that symmetry is not an important determinant of reproductive success among individual male red-winged blackbirds. This observation, in combination with the results of several other recent investigations, suggests that the fitness consequences of subtle departures from perfect symmetry may be less significant and/or less ubiquitous than initially suggested.