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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.

The ecological mechanisms driving an observed decline in the mean size-at-age of adult Pacific halibut (Hippoglossus stenolepis) in the eastern North Pacific Ocean have yet to be defined. Here, we present the results of a study designed to investigate the relationship between one potential mechanism — diet — and size-at-age using carbon and nitrogen stable isotope analysis. Our results indicate significant differences in the skeletal muscle δ₁₃C and δ₁₅N values between size-at-age categories for each sex, with larger-size-at-age fish consuming diets with higher δ₁₅N values, indicating higher trophic level feeding. Analysis of Bayesian standard ellipse areas showed that for females, intermediate size-at-age categories have the largest dietary range. For males, the largest dietary range was observed in the largest size-at-age category. Our results suggest a size-based stratification in dietary strategy for Pacific halibut with implications for observed declines in size-at-age.