Geographic Variation in Clutch Size in the Northern Flicker (Colaptes auratus): Support for Ashmole's Hypothesis

Abstract

AbstractThe Northern Flicker (Colaptes auratus), a common hole-nesting North American woodpecker, exhibits a highly significant latitudinal increase in clutch size. Using preserved clutches and information on climatic conditions and breeding densities of confamilial species, I examined the hypothesis that this geographic trend is the result of seasonal fluctuations in resources ("Ashmole's hypothesis"; Ashmole 1961, 1963; Ricklefs 1980). Clutch size is significantly correlated both with the ratio of summer productivity to estimated breeding densities of all woodpecker species and with the ratio of summer to winter productivity, supporting Ashmole's hypothesis. In addition, a substantial portion of the latitudinal gradient can be directly explained by the seasonality of resources, primarily because of an inverse correlation between clutch size and winter productivity. These results provide the first corroboration of Ashmole's hypothesis from an examination of intraspecific clutch-size variation and suggest that geographic trends in fecundity in some cases may be largely attributable to trends in seasonal fluctuations of resources.