Plumage Variability in Redpolls from Churchill, Manitoba

Abstract

Redpolls (Carduelis flammea and C. hornemanni) are well known for their extensive phenotypic variability. Various explanations have been proposed for the phenomenon, ranging from extreme variability within a single inclusive taxon, to more modest variation within frequently hybridizing or strictly isolated taxa. The predominant view favors the recognition of two largely sympatric and only slightly differentiated species: the Common Redpoll (C. flammea), comprised of the four subspecies flammea, cabaret, islandica, and rostrata; and the Hoary Redpoll (C. hornemanni) comprised of the two subspecies hornemanniand exilipes. We investigated the possible existence of sympatric redpoll forms in a large sample (n = 209) of breeding individuals from Churchill, Manitoba, where the putative forms flammea and exilipes were reported to co-occur. Groups of individuals of specific age and sex were analyzed independently. The sample was not divided a priori into flammea and exilipes subgroups. The pattern of variation within each age-and-sex group is described, and the differences between these groups are analyzed. We show that several pigmentation characters in specific age-and-sex groups are not distributed normally, and that pale and dark subgroups of second-year (SY) and after-second-year (ASY) males can be identified in a principal-components analysis (PCA) of seven characters reflecting the extent of melanin and carotenoid pigmentation. The pink- and red-breasted subgroups of ASY males match almost perfectly the pale and dark groups found in the PCA analysis. Further, when ASY males of different breast color (pink vs. red) are analyzed independently, all plumage characters are distributed normally. The analyses of SY and ASY females failed to provide good evidence for a plumage polymorphism. Our results support the idea that two relatively distinct redpoll forms breed at Churchill, which differ in several plumage characteristics involving both melanin and carotenoid pigmentations. These forms are more clearly differentiated in males than in females. They may be specifically distinct (C. f. flammea and C. hornemanni exilipes), as has frequently been suggested, but they also could be the product of different types of intraspecific genetic or ecophenotypic polymorphisms. Preliminary observations are presented that support the idea that plumage variants are largely genetically determined, but experimental work is needed to confirm this suggestion.