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

Abstract

Stable hydrogen isotopes (δD) 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 δD variation in the blood (δDB) 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 δD 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. δDB ranged from –128‰ to –94‰, with a mean ± SD of –113‰ ± 7‰. Nest-box temperatures ranged from 21.5 to 38 °C immediately prior to blood collection. A general linear mixed-effects model explained 80% of the variation in δDB and provided evidence that δD values in prey, maximum temperature 1 day prior to blood collection, and body size were all significant predictors of δDB. 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 δD 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.