Generating population estimates for migratory shorebird species in the world’s largest flyway

Population estimates are widely used to underpin conservation decisions. However, determining accurate population estimates for migratory species is especially challenging, as they are often widespread and it is rarely possible to survey them throughout their full distribution. In the East Asian–Australasian Flyway (EAAF), this problem is compounded by its size (85 million square kilometres) and the number of migratory species it supports (nearly 500). Here, we provide analytical approaches for addressing this problem, presenting a revision of the EAAF population estimates for 37 migratory shorebird species protected under Australian national environmental legislation. Population estimates were generated by (1) summarizing existing count data in the non-breeding range, (2) spatially extrapolating across uncounted areas, and (3) modelling abundance on the basis of estimates of breeding range and density. Expert review was used to adjust modelled estimates, particularly in under-counted areas. There were many gaps in shorebird monitoring data, necessitating substantial use of extrapolation and expert review, the extent of which varied among species. Spatial extrapolation to under-counted areas often produced estimates that were much higher than the observed data, and expert review was used to cross-check and adjust these where necessary. Estimates of population size obtained through analyses of breeding ranges and density indicated that 18 species were poorly represented by counts in the non-breeding season. It was difficult to determine independently the robustness of these estimates, but these breeding ground estimates were considered the best available data for 10 species that mostly use poorly surveyed freshwater or pelagic habitats in the non-breeding season. We discuss the rationale and limitations of these approaches to population estimation, and how they could be modified for other applications. Data available for population estimates will vary in quality and extent among species, regions and migration stage, and approaches need to be flexible enough to provide useful information for conservation policy and planning. © 2021 British Ornithologists' Union