Museums, taxonomic uncertainty and ‘lost species’: a case study of the Critically Endangered Siau Scops Owl

Of the world's Critically Endangered (CR) and Data Deficient (DD) birds, ~27% are ‘lost species’ (lacking verified documentation for >10 years). Some may await rediscovery, and others are probably already extinct, but a few may prove never to have been valid species. Identifying these latter cases is an urgent conservation priority so as not to waste finite resources on costly and futile searches. We identify 11 ‘lost’ bird species that warrant urgent taxonomic scrutiny, including three currently assessed on the IUCN Red List as CR. For these species, the world's natural history collections—particularly their specimens, archives and digitized metadata—often house the only data available and are therefore essential to resolving such taxonomic uncertainty and helping direct conservation resources accordingly. We use as a case study the Siau Scops Owl Otus siaoensis , a CR species known from a single specimen ostensibly collected on the island of Siau, Indonesia, in 1865 (not 1866 as previously assumed), currently housed at Naturalis Biodiversity Center, Leiden (RMNH). Combining examination of the holotype, museum skins and their metadata, archived 19th‐century correspondence held at RMNH, published literature and field survey effort, we conclude that the provenance of siaoensis is doubtful and find no character diagnosing it from neighbouring scops owls. Having already been the subject of multiple independent and NGO‐funded searches, we recommend that siaoensis is no longer recognized as a species and no further searches take place for it. The other 10 taxonomic uncertainties we identify equally warrant urgent investigation, which natural history collections are uniquely placed to help address.