REVIEW Conspicuous sexual signals in a socially monogamous passerine: the case of neotropical Blue-black Grassquits Regina H. Macedo • Lilian Manica • Raphael I. Dias Received: 6 August 2011 / Revised: 6 December 2011 / Accepted: 12 January 2012 / Published online: 1 February 2012 Ó Dt. Ornithologen-Gesellschaft e.V. 2012 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 ex- trapair fertilizations, but appear to be indiscriminate in their choice of extrapair males. However, it remains nec- essary 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. Keywords Display Á Mating system Á Neotropical birds Á Ornaments Á Plumage Á Sexual selection Introduction: the challenges of interpreting male sexual ornamentation One of the major challenges in evolutionary ecology and behavior is to understand the selective forces that have shaped the development of complex and costly morpho- logical traits and courtship behaviors in animals. The the- ory of sexual selection, originally crafted by Darwin (1871), suggests that the evolution of elaborate traits depends upon the existence of variance in male mating success (Arnold 1994), and is based upon the advantages to females of choosing males that differ in value. Female preferences may be based upon male attributes indicating a capacity to supply females or offspring with material benefits, such as food, and other paternal care, such as protection. In other words, elaborate male traits may serve as indicators of males of high parental quality (Heywood 1989; Hoelzer 1989). Another much debated explanation for females favoring elaborate sexual signals is that these potentially costly traits signal high general fitness, that is, they indicate good genes. According to this scenario, only genetically superior males are capable of developing robust and elaborate sexual traits. An explicit prediction of this idea is that offspring of males with the preferred phenotype should be genetically superior to offspring of the non- Communicated by John Wingfield. R. H. Macedo (&) Departamento de Zoologia, Universidade de Brası ´lia, Brası ´lia, DF, Brazil e-mail: rhfmacedo@unb.br L. Manica Á R. I. Dias Programa de Po ´s Graduac ¸a ˜o em Ecologia, Universidade de Brası ´lia, Brası ´lia, DF, Brazil 123 J Ornithol (2012) 153 (Suppl 1):S15–S22 DOI 10.1007/s10336-012-0815-5