Serial sampling provides chronological evidence that endogenous protein is used for primary growth in a molt-migrant goose

This is a proof of concept paper based on chronological samples of growing feathers from geese thought to be molt-migrants. When molt-migrant birds initiate molt shortly after migrating to a new isoscape, isotope values measured along the length of their feathers should change continuously. To assess long-term changes and daily cycling in δ¹⁵N and δ¹³C values, we serially sampled growing primaries of three presumed molt-migrant geese. Two showed changing δ¹⁵N signatures along the length of their growing primaries, indicating they were molt-migrants, while the third, presumably a resident, showed no change. We then resampled these feathers at closer intervals for evidence of the predicted diel cycle in the use of exogenous and endogenous protein for feather growth, generated by the diel feeding cycle of these geese. As predicted, the two geese that were equilibrating to a new isoscape showed oscillations of approximately 24-hour periodicity in δ¹⁵N values, measured along the length of their primaries. In contrast, the goose that was not equilibrating to a new isoscape showed no 24-hour periodicity in its δ¹⁵N values. Our results demonstrate that chronological sampling along the length of individual primaries holds great potential for identifying individuals that are molt-migrants.