Plumage coloration and morphology in Chiroxiphia manakins: interacting effects of natural and sexual selection

Abstract

I examined how natural and sexual selection may have influenced the morphology and coloration of Chiroxiphia manakins (Aves: Pipridae). In the first chapter, I investigated age and sexrelated patterns of plumage coloration and molt timing in longtailed manakins, C. linearis. I examined how plumage coloration changed with age in males and females based on 1315 mist net captures in northwestern Costa Rica and examined molt patterns in an additional 585 museum specimens. Males followed a remarkably agespecific pattern of plumage maturation before attaining definitive adult plumage in their fifth year. Some females developed malelike plumage characteristics as they aged, but not reliably so. Although moltbreeding overlap occurred at the population level, few individuals molted while breeding. In the second chapter, I examined the explanatory power of natural and sexual selection hypotheses in explaining patterns of sexual dimorphism in Chiroxphia manakins. I measured six morphological traits in 361 wild C. linearis and a subset of these traits in 872 museum specimens of Chiroxphia manakins. My findings were consistent with both sexual selection and natural selection hypotheses, suggesting that both mechanisms have strongly influenced the evolution of sexual dimorphism in this group. In the third chapter, I investigated how the perceptual environment influences the conspicuousness of plumage displays in longtailed manakins. I measured the reflectance of 62 males and 59 females, the reflectance of the background vegetation, and the irradiance of light at display perches. Male manakins were highly conspicuous against the visual background, whereas females were relatively cryptic. Males did not appear to adjust the location or timing of their displays to take advantage of particular light conditions but rather displayed in the most common light environment of forest shade. Displays performed in forest shade may optimize the shortdistance conspicuousness of male plumage ornaments while minimizing the longdistance conspicuousness of male ornaments and the short and longdistance conspicuousness of female plumage patterns. In the fourth chapter, I evaluated the degree to which the color of study skins is representative of coloration in wild birds. I measured the plumage reflectance of 58 wild and 55 museum specimens of longtailed manakins. I found significant differences in color between museum specimens and wild birds, and the degree of difference depended on the coloration mechanism. Potential sources of these differences include the specimen preparation process, the age of specimens, and geographic variation. Although caution is warranted for some types of studies, most differences were relatively subtle, justifying the use of museum specimens to assess color in many instances.