Morphometric Variability in Redpolls from Churchill, Manitoba

Abstract

Two plumage forms were observed in redpolls from Churchill, Manitoba, which correspond to the putative taxa Carduelis fiammea fiammea (dark-plumaged birds) and C. hornemanni exilipes (pale-plumaged birds). in a sample of breeding adults (n = 277), we examined whether morphometric differentiation parallels this plumage polymorphism. Males and females were analyzed independently, and age, date of capture, and status at measurement (study skin vs. live bird) were considered as covariates in univariate analyses. We used a separate phenetic analysis of plumage characters, rather than our subjective field identifi- cations, to establish the groups to be compared statistically; this ensured that mensural characters (e.g. bill shape) were not utilized as classification criteria for the present investi- gation of metric traits. in both sexes, redpoll plumage forms differed significantly in three of seven external mensural characters. Discriminant-function analyses, based on the same characters, showed that the forms can be distinguished morphometrically with great confi- dence (jackknifed estimate of correctly classified individuals was 87% in both sexes). in males and females, the distribution of discriminant scores of typical individuals and of a set of unidentified birds is bimodal. in a bivariate-reduced space of plumage and morphometric variability, pale- and dark-plumaged ASY (after-second-year) males form distinct groups. in SY (second-year) males and in females, plumage forms are not strictly distinct, but in no case was there an abundance of intermediates as predicted under the hypothesis that the forms are distinct species that frequently interbreed. Redpoll types may be specifically distinct, as has frequently been suggested, but they also may be examples of intraspecific genetic or ecophenotypic polymorphism. Experimental breeding and an assessment of mating patterns in the field are required to test these possibilities. Received 3 October 1991, accepted 5 September 1992.