Habitat Selection and Heterospecific Flocking Associations of a Single Male Scaly-Sided Merganser (Mergus squamatus) over Three Winter Seasons in Northern China

The Scaly-sided Merganser (Mergus squamatus) is an endangered duck endemic to East Asia. It usually gathers in single-species flocks through winter. Existing field records focus almost entirely on southern winter flocks. No prior work has described lone individuals spending winter in northern China. We conducted standardized three-year winter surveys (2019–2022) across five isolated ice-free river patches along the Taizi River, Liaoning Province, northern China, to document habitat use and flocking behavior of one solitary male. Despite the availability of five discrete overwintering sites, the focal bird only occupied two patches characterized by dense riparian concealment and low human disturbance, avoiding open, highly disturbed sites. Resource Selection Function (RSF) modeling indicated habitat attributes (concealment and human disturbance) strongly predicted site use (AICc weight = 0.79), whereas the abundance of heterospecific flocking partners (Common Mergansers Mergus merganser) exerted negligible influence (AICc weight < 0.01). This solitary male associated only with Common Mergansers, the species with the closest phylogenetic relatedness (mitochondrial genetic distance = 0.04365), and never aggregated with other sympatric waterbirds. Model comparison confirmed phylogenetic relatedness outperformed dietary similarity as a predictor of heterospecific flocking (AICc weight = 0.76). Our three-year observations of this single individual demonstrate that, when conspecifics are absent and suitable winter habitat is limited, this male prioritized high-quality, low-disturbance ice-free patches over social aggregation opportunities, and selectively formed mixed flocks with its closest phylogenetic congener. This multi-year case study provides rare empirical baseline data for understanding winter behavioral trade-offs of endangered cavity-nesting waterfowl under severe frozen river conditions, though all conclusions herein apply only to this focal male and cannot be generalized to the entire species.