Geographic clines of age ratios of Black-throated Blue Warblers (Dendroica caerulescens)

Abstract

I investigated the age structure of breeding populations of Black-throated Blue Warblers (Dendroica caerulescens) in eastern North America. The proportion of yearling males increases as relative abundance decreases toward the margins of the breeding range. Geographic dines in age structure are hypothesized to correlate with gradients in habitat quality radiating from the Appalachian Mountains, the probable axis of source populations. Data from the southern Appalachians suggest that despotism of older males may impose a ceiling on the proportion of yearlings in saturated high-quality habitats, regardless of local productivity. Populations that exhibit low variance in yearling recruitment and high population densities may fall toward the ''source'' end of the ''source-sink'' continuum, and vice versa. It is hypothesized that the regional distribution of source and sink populations of the Black-throated Blue Warbler might closely mirror the prevalence of successful multiple brooding.