Abstract
BackgroundIsotopes can provide unique solutions to fundamental problems related to the ecology and evolution of migration and dispersal because prior movements of individuals can theoretically be tracked from tissues collected from a single capture. However, there is still remarkably little information available about how and why isotopes vary in wild animal tissues, especially over large spatial scales.Methodology/Principal FindingsHere, we describe variation in both stable-hydrogen (δDF) and strontium (87Sr/86SrF) isotopic compositions in the feathers of a migratory songbird, the Tree Swallow (Tachycineta bicolor), across 18 sampling sites in North America and then examine potential mechanisms driving this variation. We found that δDF was correlated with latitude of the sampling site, whereas 87Sr/86SrF was correlated with longitude. δDF was related to δD of meteoric waters where molting occurred and 87Sr/86SrF was influenced primarily by the geology in the area where feathers were grown. Using simulation models, we then assessed the utility of combining both markers to estimate the origin of individuals. Using 13 geographic regions, we found that the number of individuals correctly assigned to their site of origin increased from less than 40% using either δD or 87Sr/86Sr alone to 74% using both isotopes.Conclusions/SignificanceOur results suggest that these isotopes have the potential to provide predictable and complementary markers for estimating long-distance animal movements. Combining isotopes influenced by different global-scale processes may allow researchers to link the population dynamics of animals across large geographic ranges.
Highlights
Understanding the ecology, evolution, and life-history strategies of animals requires detailed knowledge of individual movements throughout the year [1,2]
Mechanisms of geographic variation in dD in feathers (dDF) and 87Sr/86SrF As predicted, we found that Tree swallows with more positive dDF values tended to be from breeding sites with more positive dDGS, and that individuals with higher 87Sr/86SrF ratios tended to be from breeding sites with older underlying bedrock
Our results provide evidence that 87Sr/86Sr and dD can be used as complementary geographic markers to estimate the origins of animals in North America
Summary
Understanding the ecology, evolution, and life-history strategies of animals requires detailed knowledge of individual movements throughout the year [1,2]. Animals incorporate isotopic signatures into their tissues through local diet sources and, depending on the turnover rates within tissues (days to weeks in blood and liver: [10,11]; up to a year in bone tissue: [10]), samples from individuals in one period of their life cycle can be used to infer their origin from the period in which the tissue was formed. Successful application of this technique relies partly on predictable geographic variation of the isotopic composition of a give element [7,12]. There is still remarkably little information available about how and why isotopes vary in wild animal tissues, especially over large spatial scales
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