Hydrogen isotopic variation in migratory bird tissues of known origin: implications for geographic assignment

Continent-wide variation in hydrogen isotopic composition of precipitation is incorporated into animal diets, providing an intrinsic marker of geographic location at the time of tissue growth. Feathers from migratory birds are now frequently analyzed for stable-hydrogen isotopes (delta D) to estimate the location of individuals during a preceding molt. Using known-origin birds, we tested several assumptions associated with this emerging technique. We examined hydrogen isotopic variation as a function of age, sex, feather type and the timing of molt in a marked population of American redstarts (Setophaga ruticilla) breeding in southeastern Ontario. We measured delta D in feathers and blood from individuals that bred or hatched at our study site during the year in which those tissues were grown. Juvenile tissues from 5- to 10-day-old birds had more negative delta D values than those from adults, which most likely reflected age-related differences in diet. Within adults, primary feathers had more negative delta D values than contour feathers. The mean delta D value in adult primary feathers was relatively consistent among years and with the value expected for our study population. However, among-individual variation in delta D corresponded to an estimated latitudinal range of 6-8 degrees (650-900 km). We conclude that feathers sampled from recently hatched juveniles may not provide a reliable estimate of expected local isotopic signatures for comparison with adult feathers of unknown origin. Furthermore, we urge researchers to use caution when using delta D values in feathers to infer geographic origin, and suggest that the best approach is to assign individuals to broad geographic zones within a species' potential molting range.