Bilateral symmetry as an indicator of male quality in red-winged blackbirds: associations with measures of health, viability, and parental effort

Abstract

Recent evidence suggests that in many animal taxa, males characterized by low levels of bilateral asymmetry realize a mating advantage. The mechanisms responsible are poorly understood, but there is much interest in the possibility that symmetry reflects aspects of underlying individual quality and that females use symmetry as a criterion for mate selection. We used data assembled from a 5-year field study of red-winged blackbirds (Agelaixu phoeniceus) to explore the possibility that symmetry provides females with information regarding the quality of potential mates. Asymmetry measures made on a variety of paired characters, whether considered alone or in combination, were generally unrelated to the nutritional status (i.e., body condition) of individual males. Similarly, we found no differences in asymmetry levels exhibited by males infected and those uninfected with avian hematozoa. Composite (i.e., multicharacter) asymmetry was related to infestation by two broad classes of ectoparasites (hematophagous mites and ischnoceran lice), but these relationships were weak and opposite to those expected under the male quality hypothesis. Direct assessment of male viability based on annual return to the breeding grounds generally did not support the prediction that symmetrical males have higher survival prospects, nor was symmetry predictive of male nest defense, the primary form of paternal care in our study population. Collectively, these results are inconsistent with the proposal that symmetry reflects high mate quality, which in turn suggests that, in red-winged blackbirds, the potential for female choice based on this aspect of morphology is probably weak or nonexistent.