Seasonal Variation in the Genus Leucosticte in North America

Abstract

Variation in several mensural and color characters was examined through the breeding season for the 8 recognized taxa of rosy finches [Leucosticte griseonucha umbrina, L. g. griseonucha, L. g. littoralis, L. tephrocotis tephrocotis, L. t. wallowa, L. t. dawsoni, L. atrata, L. australis] in North America. Bill length in each taxon was shortest in June but thereafter 2 patterns emerge. In the 3 gray-checked forms length was greatest in July and declined thereafter whereas in the remaining 5 taxa length increased through Aug. or Sept. Bill depth also showed monthly variation, with the bills being thickest either in the same month or the month after the greatest length was attained. Bill shape (ratio of depth to length) showed greatest monthly variation in the 3 slender-billed forms (umbrina, L. g. griseonucha and L. t. dawsoni). Body weight increased throughout the summer in males and was greatest in late June in females. Wear of flight feathers was small in winter but increased rapidly during the summer months with greatest wear occurring in females. The change in bill color from black to yellow following breeding occurred 4 wk earlier in females than in males in most taxa and 6 or more weeks earlier in the gray-cheeked forms. There was no apparent difference in timing of the sexes in L. australis. Fade of plumage color, as examined by reflectance colorimetry, was accompanied by a shift to a shorter dominant wavelength, an increase in brightness and possibly a decrease in purity. Differences in monthly sample means for most of the characters examined were significant during at least parts of the breeding season and were too large to be ignored when examining geographic variation in the genus. The effect of wear on feather length is especially important and a new method of dealing with this problem is discussed.