An island bird species discovered in Japan looks just like a similar species from 1,000 km away, but has been genetically isolated for millions of years. Takema Saitoh, Per Alström, and colleagues report the existence of the Tokara Leaf Warbler, a small insectivorous songbird with an olive-green back and a silvery gray breast from the Tokara Islands in Japan. The Tokara Leaf Warbler is a cryptic species that looks identical to Ijima’s Leaf Warbler ( Phylloscopus ijimae ) from the Izu Islands. However, genetic analysis reveals that the lineages diverged around 2.8–3.2 million years ago, and there’s no evidence of any contemporary gene flow between the island groups. Genetic divergence between the birds is twice as large as the divergence between the Collared Flycatcher ( Ficedula albicollis ) and the European Pied Flycatcher ( F. hypoleuca ), which live in overlapping ranges without any genetic exchange. In addition, the two leaf warblers sing different songs. Ijima’s Leaf Warblers were already listed as vulnerable by the International Union for Conservation of Nature (IUCN). The understanding that the group comprises two separate species suggests the need for a fresh evaluation of each species’ conservation status. The Tokara Leaf Warbler faces threats from introduced weasels ( Mustela itatsi ) and environmental modification by domestic goats, as well as potential threats from volcanic eruptions, as the Tokara Islands are volcanic and show signs of activity. The authors propose the name Phylloscopus tokaraensis for the species.
PNAS Nexus
Discovering and protecting cryptic biodiversity: A case study of a previously undescribed, vulnerable bird species in Japan
17-Mar-2026