Combining non-invasive survey methods increases cumulative detection probability for breeding harlequin ducks Histrionicus histrionicus

The effective implementation of new technologies for wildlife population monitoring is limited by knowledge of factors that impact their efficacy. Population monitoring of harlequin ducks Histrionicus histrionicus on their breeding streams in Montana and Idaho in the Northern Rocky Mountains, has historically relied on ground-based foot surveys (GBS), which have produced variable count data, making it hard to decipher trends in stream occupancy and population size. We quantified the detection probability of GBS and compared it to two additional non-invasive technologies, environmental DNA (eDNA), and camera traps (time-lapse and motion-detection) for detecting breeding harlequin ducks. We surveyed for harlequin duck presence on 10 occupied streams during incubation and brood rearing in 2022 and 2023 using GBS, eDNA, and camera traps. We found that a single 5-km GBS had a mean detection probability of 0.51 (95% CI: 0.31–0.71) and varied with the relative abundance of harlequin ducks, streamflow, and survey pace, but not between incubation and brood-rearing periods. Both eDNA and camera traps proved effective in detecting harlequin ducks, particularly when replicated across space (multiple samples of eDNA along a stream reach) or time (camera traps deployed over several weeks). It is important to note that eDNA and camera traps only provide presence/absence data, while GBS provides relative abundance. Combining methods proved particularly effective; we estimated a cumulative detection probability > 0.95 for a single-day survey effort by collecting eDNA samples spread throughout a stream reach in tandem with a GBS. Utilizing and combining these non-invasive survey techniques could also be used to effectively detect other low density, cryptic, waterbirds occupying stream habitats.