A Phenetic Analysis of the Redpolls Carduelis flammea flammea and C. hornemanni exilipes

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Abstract
A review of the historical treatment of common and hoary redpolls (C. f. flammea and C. h. exilipes) revealed no diagnostic means of distinguishing the 2 taxa. Interpretations of species limits have varied, resulting in conflicting treatment of intermediate forms. Variability in plumage and skeletal characters of redpolls from 9 breeding locations in Alaska [USA] and Canada were examined to determine if a means of discriminating between the 2 taxa could be found, and to determine if individuals with intermediate plumage characteristics (potential hybrids) were also morphologically intermediate. Because sexes differed in most skeletal dimensions, all analyses were duplicated for the 2 sexes. A character index was developed based on 3 plumage characteristics: rump streaking, undertail covert streaking, and lateral (side) streaking. This index characterized redpolls on a darkness gradient from extreme C. f. flammea (dark, heavy streaking) to extreme C. h. exilipes (light, no streaking). The observed range of scores was divided into thirds to designate C. f. flammea, unknown (intermediate) and C. h. exilipes classes. Females showed a much narrower range of plumage variability than males. No females were white enough to be considered C. h. exilipes on the scale used for males. Most birds from the C. f. flammea and C. h. exilipes extremes of plumage variability could be distinguished by skeletal criteria (using discriminant function analysis); however, no discontinuity existed, as other birds were intermediate for both plumage and skeletal characteristics. Intermediates were found at all sampled localities where both plumage morphs occurred. Results of analyses based on plumage (character index) and skeletal measurements were very similar and showed that samples containing intermediate plumage birds were also intermediate skeletally. Considering the continuous variability of both plumage and skeletal characters and the lack of any diagnostic criterion for identifying the taxa, the 2 redpolls should be merged into a single taxon, C. flammea.