Abstract

Reliable population estimates are fundamental for conservation, yet many migratory species remain poorly quantified due to uneven survey coverage across their annual cycle. This is particularly true for species associated with under-monitored habitats, where limited data can bias global population assessments. The Surfbird (Calidris virgata), a migratory shorebird specialized on rocky intertidal habitats, is widely distributed along the Pacific coast of the Americas, but its non-breeding population size remains uncertain. We conducted standardized surveys during February 2025 across 147 rocky shoreline transects spanning 2,853 km of the Chilean coast (north of 43° S) and used a Bayesian zero-inflated negative binomial model to estimate population size while accounting for sparse detections and overdispersion. We estimate that Chile supports approximately 45,000 Surfbirds (95% credible interval: 20,000–107,000), indicating that a substantial proportion of the global population occurs within a single country during the non-breeding season. Our results provide the first model-based population estimate for Surfbirds in Chile derived from surveys targeting their preferred rocky intertidal habitat. Our estimate is consistent with an earlier, beach-focused shorebird atlas for Chile, and together these independent assessments suggest that the global total—largely unchanged for two decades—may warrant upward revision. These findings highlight the need to expand monitoring across the species’ South American range and demonstrate the value of combining citizen-science data with hierarchical models to estimate populations of sparsely distributed species.