Quantifying the Information Gap Between Standard and Extended Bird Survey Windows Along the East Asian–Australasian Flyway: A Five-Winter Study at the Janghang Coast, Republic of Korea

Coastal wetlands can serve different ecological functions across the non-breeding season, so a fixed midwinter survey period may not fully represent seasonal bird use. We evaluated how much information from an extended October–March window was captured by the December–January standard window at the Janghang coast, Republic of Korea, using monthly surveys across nine fixed sections over five wintering seasons (30 surveys). We compared species richness, community composition, species-level non-detection, threatened-species representation, and peak observed counts. The standard window recorded 67–77 species per winter versus 84–95 in the extended window, with a pooled observed representativeness of 78.33%. At a common sample coverage of 0.9850, estimated species richness was 94.00 versus 111.41 species (descriptive coverage-standardized representativeness, 84.37%; no formal test). PERMANOVA indicated an association between survey period and community composition (R2 = 0.145, p = 0.004), although dispersion differed and ANOSIM showed little separation between periods. Twenty-six species were never detected during December–January, including several threatened species, and the standard-window maximum matched the extended-window maximum in 30.18% of species–wintering-season combinations. These results suggest that standardized midwinter surveys remain useful for long-term comparison, while surveys outside the core midwinter period may provide complementary information on seasonal occurrence, community variation, and conservation-priority species.