Spatially explicit assignment of harvested waterfowl using stable isotopes
Abstract
Naturally occurring stable isotopes within animal tissues can provide intrinsic markers for predictable assignment to origin of migratory animals, without additional tracking devices. The use of feather stable-hydrogen isotopes (δ2Hf) in waterfowl research has been limited until recently and the opportunity to use stable isotopes in general to inform waterfowl management, particularly when assessing source origins and connectivity, is unrealized. Many of the current waterfowl monitoring programs (e.g., preseason banding) are spatially limited due to accessibility, but intrinsic markers provide a complementary method to estimate harvest source areas and evaluate biases. In my Ph.D. dissertation, across four data chapters, I used δ2Hf measurements to inform direct connections between harvest and source areas for harvested waterfowl in eastern North America, assessing and improving on the current methods. First, I used δ2Hf and stable-carbon isotope (δ13C) measurements to evaluate differences in the origin of harvested American Black Duck (Anas rubripes) across its range (Chapter 2). I found evidence supporting the flyover hypothesis, where American Black Duck harvested in Atlantic Canada showed predominantly southern (local) origins, while those harvested elsewhere originated farther north in the boreal. Second, I critically evaluated the current methods used to predict origins based on stable isotopes in waterfowl feathers (Chapter 3). Here I aggregated known-origin calibration data (δ2Hf vs. δ2Hp) and informed the best practices for assignment methods moving forward. Lastly, focusing specifically on leg-band returns and how they can be directly integrated into likelihood-based assignment, I explored spatiotemporal patterns in the natal source areas of waterfowl in eastern North America (Chapter 4) and critically evaluated the use of band returns as a prior probability of origin, directly comparing source areas derived from band returns and δ2Hf measurements (Chapter 5). I found evidence of flyway-specific natal sources with northward shifts later in the harvest period. When used as a prior probability of origin, band returns greatly refined derived source areas, despite their spatial bias. Together, these contributions address key conservation questions for species of conservation concern, inform best practices when using stable isotopes, and demonstrate the value of stable isotopes as a tool for waterfowl management.