Abstract:
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 (13C/12C) and nitrogen (15N/14N) 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]13C values and age-specific and temporal effects in [delta]15N values. The [delta]13C values of older males increased with altitude at the rate of [apprxeq]1.3[permill] 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]13C=-26.07 to -20.86[permill]) observed along a single altitudinal transect (755 m) nearly brackets the range of [delta]13C values recorded in feathers across the North American breeding range of the warbler from Georgia to New Brunswick (11[degree] of latitude) and from New Brunswick to Michigan (22[degree] of longitude). These data indicate that age-specific and altitudinal effects must be considered when using [delta]13C values to delineate the geographic origin of avian species with large altitudinal and latitudinal ranges
