Long-Term Population Trends and Spatial Redistribution of Shorebirds at a Critical Migratory Stopover

Understanding population change in migratory shorebirds is critical for conservation, particularly at key stopover sites where individuals concentrate during migration. Using nearly four decades of standardized peak aerial count data (1986-2025), we examined long-term population dynamics of four abundant shorebird species at the Delaware Bay spring stopover: Red Knots (Calidris canutus rufa), Ruddy Turnstones (Arenaria interpres), Sanderlings (Calidris alba), and Semipalmated Sandpipers (Calidris pusilla). First, we evaluated species-specific trends in peak abundance to distinguish interannual variability from sustained population declines. Log-linear regression analyses for each species showed a statistically significant decline in Red Knot and Ruddy Turnstone populations from 1986 to 2025 (p < 0.001), whereas trends for Sanderlings and Semipalmated Sandpipers showed declines from historic numbers, but the declines were not statistically significant (p > 0.5). Focusing on the two significantly declining species, we assessed patterns of shoreline use within Delaware Bay. Red Knots showed a substantial shift in shoreline use within the bay over time, concentrating on the New Jersey (eastern) shoreline over time, with more than 85% utilizing the New Jersey shoreline from 2019 to 2025, while Ruddy Turnstones continued to use both shorelines, with values fluctuating between 35% and 65% across the time series. By integrating long-term abundance trends with patterns of shoreline use, we demonstrate how long-term monitoring is essential for interpreting population trajectories and informing conservation management at critical migratory stopover sites.