Analysis of Polymorphism in the Sooty-capped Bush Tanager

Abstract

Information from distribution, ecology, behavior, and morphology indicates that Zeledon's Chlorospingus (Chlorospingus zeledoni Ridgway) is a grayish-green morph of the Sooty-capped Bush Tanager (Chloro spingus pileatus Salvin)the latter in typical form represents the yellow-green phase. Combined reflectance spectrophotometry and pigment biochemistry demonstrate that the two morphs differ in but one feature, the concentration of lutein pigment in the feathers of the breast band, flanks, and dorsum. A direct positive relationship exists between the concentration of pigment and degree of colorimetric purity. Extraction of lutein from the breast plumage of an example of the yellow-green phase resulted in a bird visually identical with examples of the gray-green phase. Comments are offered on the relationship between the three attributes of color (dominant wave length, brightness, and purity) and the physical basis for colors in feathers (pigment type, pigment concentration and placement, and feather structure) as exemplified by certain tanagers. The local confinement of the gray-green morph to two high mountains in central Costa Rica, Volcanes Irazú and Turrialba, a region of historically active volcanism where the background of ash, vegetation, and fog provides an environment of pervasive grayness, suggests that the polymorphism is maintained tliere by the advantages of matching coloration, particularly after volcanic eruptions. Theories of heterozygote advantage and of apostatic selection do not seem appropriate in this instance to account for the polymorphism.