Long-term population dynamics of Calidris canutus rufa (Red Knot) and Limosa haemastica (Hudsonian Godwit): Insights from 26 years of aerial surveys

Abstract Long-term population monitoring at key nonbreeding sites is critical for assessing the conservation status of long-distance migratory shorebirds. We used a joint-likelihood Bayesian state-space model to estimate 26-year population trends (2000–2025) for Calidris canutus rufa (Red Knot) and Limosa haemastica (Hudsonian Godwit) at Bahía Lomas, Tierra del Fuego, Chile—the most important nonbreeding site for rufa Red Knots and one of the most important for L. haemastica. Aerial surveys transitioned from visual estimates (2000–2022) to photograph-based estimation analyzed post-survey (2020–2025), with paired counts in overlap years. The joint-likelihood specification allowed both observation methods to inform a single underlying population trajectory and provided a direct test for systematic differences between methods; the estimated method offset was indistinguishable from zero for both species. Calidris c. rufa declined by ∼63% between 2000 and 2025 (from ∼38,600 to ∼13,600 birds), with most of the decline occurring during 2000–2007 and the population stabilizing thereafter at ∼11,000–14,000 birds; the mean annual growth rate was –4.2% yr−1 (95% credible interval [95% CI]: –6.9% to –1.7%). Limosa haemastica showed no decisive directional trend across the same period (mean annual growth rate +1.5% yr−1, 95% CI: –1.4% to + 3.7%) but high interannual variability. The contrasting trajectories—a clear long-term decline in C. c. rufa and substantial unstructured variability in L. haemastica —have distinct conservation implications: for C. c. rufa, maintaining a consistent local monitoring program at Bahía Lomas is instrumental for evaluating the long-term effectiveness of full-flyway management, whereas L. haemastica require a coordinated, regional monitoring framework spanning multiple South American nonbreeding sites to capture their unstructured variability.