Utility of alternate stable isotope calibration equations and informative priors when estimating the breeding origin of Arctic-breeding shorebirds
Understanding migratory connectivity is essential for monitoring and conserving Arctic-breeding shorebirds, particularly given divergent rates of decline across populations. Stable hydrogen isotope analysis of feathers offers a scalable, non-invasive method to assign breeding origin, but current applications are limited by the absence of shorebird-specific calibration equations, which associate feather isotope values with environmental isotope gradients ("isoscapes"), and by longitudinal isoscape bands in the Arctic that result in diffuse origin estimates across broad, heterogeneous regions. We evaluated the utility of alternate calibration equations and the incorporation of occupancy-based informative priors for improving breeding origin assignments. Specifically, we analyzed feather isotope data from hatch-year semipalmated sandpipers (Calidris pusilla) captured at staging sites in the Canadian Maritimes, as well as known-origin Arctic samples, to compare calibration equations from different reference datasets and test priors derived from eBird habitat occupancy predictions. We demonstrate that Arctic samples of known origin largely align with calibration equations based on broad passerine datasets, although incorporating Arctic samples into the calibration may correct for a marginal westward bias in origin estimates. Using occupancy-based priors that reflect habitat preferences increased precision by 18.8%, identifying hotspots within otherwise expansive isotopic bands. We highlight the validity of existing reference datasets with broad geographic coverage and demonstrate the value of informative priors to refine origin estimates. We recommend expanding Arctic sample collections across a wider longitudinal gradient to define a shorebird-specific calibration equation and further exploring potential biases in prior data sources to improve accuracy of Arctic-breeding shorebird origin assignments.