• Dufour, Kevin W.
  • Weatherhead, Patrick J.


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), withinpair 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.