Stable hydrogen isotope (δD) values in songbird nestlings: effects of diet, temperature, and body size

Stable hydrogen isotopes (delta D) can be a powerful tool for estimating the large-scale movements of animals, but the cause and extent of isotopic variation within animal tissues remain poorly understood. Here, we simultaneously examined three hypotheses to explain 8D variation in the blood (delta D-B) of nestling Tree Swallows (Tachycineta bicolor (Vieillot, 1808)) from a single nest-box population in southern Ontario, Canada: (1) microgeographic and (or) temporal variation in delta D of local diet, (2) potential evaporation of protium resulting from variation in nest-box temperature, and (3) differences in body size resulting in higher rates of water loss and isotope fractionation in larger birds. delta D-B ranged from -128 parts per thousand to -94 parts per thousand, with a mean +/- SD of -113 parts per thousand +/- 7 parts per thousand. Nest-box temperatures ranged from 21.5 to 38 degrees C immediately prior to blood collection. A general linear mixed-effects model explained 80% of the variation in delta D-B and provided evidence that delta D values in prey, maximum temperature I day prior to blood collection, and body size were all significant predictors of delta D-B. Our results suggest that isotopic variation in individuals and local populations arises from a combination of physiological factors and local environmental variation. To gain a complete understanding of how 8D values can be used to estimate animal movements, additional work is needed to determine how these factors influence other tissues, such as metabolically inert feathers, and animals of different age classes.