Integrating telemetry data and stable isotope ( δ 2 H) signatures to refine estimates of Greater Scaup breeding and molt origins

Determining the breeding period origins of migratory waterfowl that occupy high-latitude, remote areas is challenging, yet key given that annual breeding population surveys are the predominant source of waterfowl population monitoring data to guide effective management. We integrated telemetry and stable-isotope (δ²H) datasets to estimate breeding period origins of Greater Scaup (<em>Aythya marila</em>), an Arctic- and subarctic-breeding diving duck and declining species for which limited information is available on breeding areas and phenology. Specifically, we deployed satellite transmitters on non-breeding Greater Scaup in Rhode Island, USA (<em>n</em> = 10) and the lower Laurentian Great Lakes of North America (<em>n</em> = 6) and collected feathers from these same individuals plus others captured or harvested at both locations (<em>n</em> = 29 and 31, respectively). Using these data, we (1) compared breeding period origins and phenology of telemetry-marked individuals from the two non-breeding sites; (2) demonstrated that telemetry-derived breeding period locales can be used to refine likely breeding and molt origin assignments derived from feather stable isotopes; (3) identified differences in likely breeding origins for the two non-breeding sites; and (4) compared likely breeding versus molting origins for the Rhode Island non-breeding site. The breeding period phenology and distributions of Greater Scaup that we document do not align well with the timing or stratification of the Waterfowl Breeding Population and Habitat Survey, specifically in northern Quebec, Nunavut, and Alaska. Further, for waterfowl species that breed in remote habitats that are logistically challenging to access, the integration of telemetry and feather isotope information as outlined here provides a promising approach for delineating breeding and molting origins for distinct non-breeding subpopulations. Thus, our approach, when applied to larger samples, could effectively inform waterfowl monitoring programs and address conservation challenges for high-latitude breeding migratory birds.