Conspicuous sexual signals in a socially monogamous passerine: the case of neotropical Blue-black Grassquits

Abstract

Sexual selection has shaped many traits involving communication, behavior, morphology, and reproductive strategies. Sexual communication in the Blue-black Grassquit Volatinia jacarina involves an iridescent, nuptial plumage and the exhibition of a complex motor and acoustic mating display. Blue-black Grassquits breed in small, aggregated territories, are socially monogamous, but have high rates of extrapair fertilization and intensive paternal care. Here, we discuss the evolution of sexually selected ornaments in this species and review some studies that have evaluated their costs and benefits. Our data suggest that females have an active role in obtaining extrapair fertilizations, but appear to be indiscriminate in their choice of extrapair males. However, it remains necessary to evaluate female choice in natural circumstances, and the effects of such on male reproductive success. This is particularly important because, in the field, we found that the male motor display may provide a useful cue for nest predators, and therefore evolution of female choice may be constrained by the probability of higher nest predation associated with choosing males exhibiting high display rates. Our studies also indicate that small males have an advantage in aggressive interactions with larger males. We hypothesize that small male size may be driven by female choice for male motor performance, including aspects of display agility and acrobatic capacity, leading to better performance by small males in intrasexual interactions involving flight within confined spaces. Alternatively, the food resource may be more valuable to smaller males, resulting in higher aggression to access the food.