Investigating non-invasive survey methods and building a predictive occupancy model for Harlequin Ducks (Histrionicus histrionicus) in Western Montana and Northern Idaho

Holmes, Holli

Harlequin Ducks (Histrionicus histrionicus; HADU) are an imperiled sea duck that breeds along white-water mountain streams, but challenges in surveying them have made it difficult to understand population trends in their Northern Rocky Mountain breeding population. In this study we had two objectives: (1) evaluate non-invasive survey methods for detecting HADU on their breeding grounds in Western Montana and Northern Idaho, and (2) develop an occupancy model to predict potential breeding stream use for HADU across the region. First, we assessed the efficacy of ground-based foot surveys (GBS), environmental DNA (eDNA), and camera traps to detect breeding HADU during time periods corresponding to the incubation and brood rearing phases of their annual cycle. GBS (0.51, SE: 0.108) and eDNA (0.49, SE: 0.146) had the highest mean detection probabilities during incubation season and time-lapse detection camera traps (0.16, SE: 0.067) were more effective than motion detection camera traps. We found that combining methods could improve our cumulative detection probability in a single visit to a stream; taking five eDNA samples in tandem with one GBS achieved a cumulative detection probability of 0.97 (SE: 0.037), emphasizing the power of combining methods. Second, we built an integrated species distribution model for breeding HADU using detection / non-detection data from GBS and eDNA samples collected from 2009 to 2024 across 1,025 one-mile stream segments. Our results indicated that stream geomorphology such as stream features like pools and braiding, human disturbance from changes to the landscape, and mean annual flow are important for predicting HADU site-use. Using posterior estimates from the occupancy model, we predicted site-use to streams within our sampling frame across the region. Predicted site use indicates that the northwest portion of the study area around Glacier National Park has the highest probability of site-use (0.29, SD: 0.07). To our knowledge this is the first large-scale statistical model for HADU site-use in Western Montana and Northern Idaho. By integrating findings from both chapters, we offer actionable strategies and recommendations for agencies to more efficiently detect and protect HADU.