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 delta(13)C values and age-specific and temporal effects in delta(15)N values. The delta(13)C values of older males increased with altitude at the rate of approximately 1.3 per thousand 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 (delta(13)C = -26.07 to -20.86 per thousand) observed along a single altitudinal transect (755 m) nearly brackets the range of delta(13)C values recorded in feathers across the North American breeding range of the warbler from Georgia to New Brunswick (11 degrees of latitude) and from New Brunswick to Michigan (22 degrees of longitude). These data indicate that age-specific and altitudinal effects must be considered when using delta(13)C values to delineate the geographic origin of avian species with large altitudinal and latitudinal ranges.