Relationship between age and method of population estimation and the impact of changes in range-size maps on the IUCN Red List assessments of birds

Population size is one component of several criteria in the International Union for Conservation of Nature (IUCN) Red List of Threatened Species (Red List). For these criteria, the quality of the population estimation can therefore have significant impact on the assessed status. To evaluate population estimate quality, we selected 473 species of land birds from the Americas considered by the Red List to be “Critically Endangered”, “Endangered”, or “Vulnerable” at the end of 2021, of which 414 (88%) had a population size estimate. For these species, we determined the age of the estimate and how the population estimate was derived, grouped into eight categories. For 87 species (18%) the population estimate was derived by sampling a small area and extrapolating to the entire range of the species; for these, the population size estimate depends on the estimate of range size. For the subset of 22 of these with complete data, we compared range size estimates obtained from maps published by IUCN with maps produced using the methods of Huang et al. (2021) to see how range map differences could affect population size estimates and therefore Red List status. Potentially half of these species (11 of 22) could change status using the new maps. More than one-third of the population size estimates (38%, 161 species with a date of population estimate) were made in 2000 or earlier. A majority of the species, 63% (300 of 473 species), do not have population size estimates made using a scientific sampling method, although the majority since 2010 have been made using a sampling method, reflecting an effort by Red List assessors to include more scientific information. We encourage the ornithological community to work to obtain current, high quality population size and range estimates to improve the quality of Red List status assessments.