Contrasting assignment of migratory organisms to geographic origins using long‐term versus year‐specific precipitation isotope maps

Summary As a result of predictable large‐scale continental gradients in the isotopic composition of precipitation, stable isotopes of hydrogen (δ2H) are useful endogenous markers for delineating long‐distance movements of animals. Models to predict patterns of δ2H in precipitation (δ2Hp), and consequently determine likely geographic origin of migratory animals, have traditionally used static, amount‐weighted long‐term average values of δ2Hp over the growing season. However, animal tissues reflect H incorporated from food webs that integrate precipitation over a single year's growing season or portions thereof. Inter‐annual variation in precipitation and other climatic variables may lead to deviations from predictions derived from long‐term mean precipitation isotopic values and could therefore lead to assignment errors for specific years and locations that are atypical. We examined whether using biologically relevant short‐term δ2Hp isoscapes can improve estimates of geographic origin in comparison with long‐term isoscapes. Using δ2H data from known‐origin tissues of two migratory organisms in North America and Europe, we compared the accuracy, precision and similarity of assigned origins using both short‐ and long‐term δ2Hp isoscapes. Relative to long‐term δ2Hp isoscapes, using short‐term isoscapes for assignment often resulted in dissimilar regions of likely origin but did not significantly improve accuracy or precision. This was likely due to reduced spatial coverage in the data used to generate the short‐term δ2Hp isoscapes. We suggest that continued efforts to collect precipitation isotope data with a large spatiotemporal range will benefit future research on incorporating temporal variation in the amount and isotopic composition of precipitation into geospatial assignment models.