Assessing trends and density of bird species in bottomland hardwood forests and riparian forests using simulation and sample size optimization for surveys.

The decline of neotropical migratory birds in North America is closely tied to habitat loss, including the degradation of bottomland hardwood and riparian forests, which provide essential habitats for numerous species. To address this, the U.S. Fish and Wildlife Service conducts bird surveys to monitor restoration efforts and evaluate conservation outcomes. This study assessed avian surveys from three National Wildlife Refuges in Texas and Oklahoma, using simulations, field data, and literature to evaluate current sampling protocols. Our findings revealed that achieving acceptable precision in bird density estimates (coefficient of variation: 0.15, 0.25) often requires more than 200 bird point counts, depending on the species and study area. While data aggregation across sites and years improved precision, it masked local trends critical for refuge-specific management. Imprecise results, particularly for rare species, underscored the need for improved protocols, such as repeat visits within a year, targeted sampling for priority species, and adaptive designs incorporating forest composition and structure data. These adjustments would enhance the precision of multispecies surveys, making them more effective for detecting changes in habitat quality. This study provides actionable recommendations to support service-wide efforts in strategic, data-driven monitoring and long-term conservation planning for neotropical migratory birds.
Competing Interests: Competing interests: The authors declare no competing interests.
(© 2025. This is a U.S. Government work and not under copyright protection in the US; foreign copyright protection may apply.)