Acquisition of plumage polymorphism in white-throated sparrows

Abstract

The White-throated Sparrow (Zonotrichia albicollis) was thought to be polymorphic in breeding plumage, having distinct white and tan phases that are bimodal in distribution and independent of sex. Vardy (1971), however, submitted evidence that plumage type is determined primarily by age, sex, and molt cycle. We reexamined this question by measuring plumage characteristics on a quantitative scale with the Munsell system of color notation. Using techniques of numerical taxonomy, we examined immature and adult birds before and after prenuptial molt. Winter plumages were basically unimodal and continuous in distribution, whereas spring plumages were bimodal, as the birds separated into two morphs independent of sex. Winter plumage was significantly more variable than spring plumage. We hypothesize that winter plumage variability could function to maintain stable social relationships in winter flocks.

Citation

Atkinson, Carter T.; Ralph, C. John. 1980. Acquisition of plumage polymorphism in white-throated sparrows. The Auk 97(2): 245-252