Stable isotope signature of philopatry and dispersal in a migratory songbird

Stable isotope analysis is widely promoted as a practical method for tracing the geographic origins of migratory birds. However, the extent to which geospatial patterns of isotope ratios in avian tissues are influenced by age-specific, altitudinal, and temporal factors remains largely unexplored. We measured carbon ((13)C/(12)C) and nitrogen ((15)N/(14)N) isotope ratios in feathers of black-throated blue warblers (Dendroica caerulescens) breeding along a relatively steep altitudinal gradient in the Appalachian Mountains to evaluate the effects of altitude and year on the isotopic signatures of yearling (first breeding season) and older males (>2 years). Breeding males (n = 302) collected during 7 consecutive years exhibited significant age-specific and altitudinal effects in δ(13)C values and age-specific and temporal effects in δ(15)N values. The δ(13)C values of older males increased with altitude at the rate of ≈1.3‰ per 1,000 m, suggesting a high degree of year-to-year philopatry to narrow altitudinal zones, if not to breeding territories. In contrast, absence of altitudinal patterns in yearlings most likely reflects natal dispersal. Carbon isotope variation (δ(13)C = −26.07 to −20.86‰) observed along a single altitudinal transect (755 m) nearly brackets the range of δ(13)C values recorded in feathers across the North American breeding range of the warbler from Georgia to New Brunswick (11° of latitude) and from New Brunswick to Michigan (22° of longitude). These data indicate that age-specific and altitudinal effects must be considered when using δ(13)C values to delineate the geographic origin of avian species with large altitudinal and latitudinal ranges.