For decades, the research community thought that the small, olive-green songbirds found on two Japanese islands were identical. But a new study has revealed these birds are actually two distinct species, ones that have been evolutionarily isolated for millions of years and are now facing the risk of extinction.
Researchers discovered a population of the newly named Tokara leaf warbler (Phylloscopus tokaraensis) on the remote Tokara archipelago in 1988. Back then, it was considered to be Ijima’s leaf warbler (Phylloscopus ijimae), found in the Izu Islands, some 1,000 kilometers (620 miles) away.
An international team of researchers, led by Per Alström from Uppsala University in Sweden and Takema Saitoh of the Yamashina Institute of Ornithology in Japan, has now analyzed the genetic data and songs of the birds on the two islands.
Genetic analysis showed that a “deep split” between the two lineages occurred approximately 3.2 million years ago, the authors write.
The researchers also found that while the two bird populations are virtually indistinguishable in appearance, their songs say otherwise.
In an email to Mongabay, Saitoh said the Tokara species’ songs are lower in pitch and faster in pace than those of its Izu relatives. This acoustic divide is so distinct that the researchers were able to correctly classify 100% of Tokara recordings based solely on their vocal patterns.
The recognition of the Tokara and Ijima’s leaf warblers as separate species means they’re even rarer than previously realized.
The Tokara leaf warbler is known to breed only on the island of Nakanoshima in the Tokara archipelago. “Its range is confined to a very limited area on a global scale,” Saitoh told Mongabay.
Genomic data also showed that both species exhibit low genetic diversity and signs of past population declines. This is consistent with small, isolated populations, the authors say.
Both the Tokara and Ijima’s leaf warblers face threats typical for island species. These include natural disasters, invasive predators like introduced weasels, and habitat modification by feral goats, Saitoh noted.
He added the birds are also vulnerable to forest decline caused by pine wilt disease and the constant threat of volcanic eruptions in the geologically active region.
Ijima’s leaf warbler is listed as vulnerable on the IUCN Red List. By formally describing the Tokara leaf warbler as a separate species, the researchers say they hope to ensure it receives an independent assessment of its own conservation status and the targeted monitoring it needs.
“The discovery that species which appear identical at first glance actually encompass cryptic species suggests that this research plays an important role in identifying hidden biodiversity and ensuring that these species are monitored and conserved so that they do not go extinct unnoticed,” Saitoh said.
Banner image: A Tokara leaf warbler photographed in Nakanoshima in June 2017, when it was still thought to be a separate population of Ijima’s leaf warbler. Image courtesy of Per Alström.
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