| Summary: | Abstract Determining the processes that shape the relative timing of energetically-costly events in the annual cycle of migrating birds is important to our understanding of avian phenology and ecology. We paired satellite tracking and hydrogen stable isotope analysis (δ²H) to examine the relative timing of two such events — migration and feather molting — in tundra swans from four breeding areas in Alaska, USA. Our results show a trend of increasing intra-individual variability in breast feather δ²H values with increasing migration distance, suggesting the overlap of breast feather molting and migration. However, when individual samples were pooled by breeding area, the δ²H values of breast and head feathers showed no trend with migration distance, presumably resulting from high levels of inter-individual variability in δ²H values within each breeding area. We explore potential reasons for this variability, propose potential mechanisms influencing feather δ²H values of tundra swans, and recommend further research into methods for exploring the temporal configuration of events in the annual cycle of migrating birds.
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