Japan's First New Bird Species in 45 Years Was Hiding in Its Song
Ijima's Leaf Warbler is one of the rarest songbirds in Japan: a small greenish bird that breeds only on the Izu Islands south of Tokyo and on the Tokara Islands, roughly 1,000 kilometres to the south-west, and is protected as a Japanese Natural Monument. This spring it stopped being one species. The Tokara population is now the Tokara Leaf Warbler (Phylloscopus tokaraensis) — Japan's first newly described bird species since the Okinawa Rail in 1982.
The discovery was published in PNAS Nexus by a team from Uppsala University, the University of Gothenburg and two Japanese institutions. What makes it unusual is that nobody could see the difference. The two populations are, to the eye, the same bird.
Two kinds of evidence
The first clue was genetic. Ten years ago, DNA sequences showed that the birds on the two island groups were clearly distinct; that led to years of fieldwork on the islands, work in museum collections and full genome analysis. The whole-genome comparison showed the Tokara birds are very unlike the Izu birds.
The second line of evidence is the one this site cares about: their songs. Careful comparison of recordings corroborated what the genomes said. Where appearance failed, sound and DNA agreed.
“The new species is a little cryptic and tricky to define. In terms of appearance, it doesn't differ from the Ijima's Leaf Warbler. It is DNA analyses and differences in song that show that this is a separate species,” says Per Alström of Uppsala University.
This is the quiet, ongoing role of field recording in biology. Leaf warblers — the genus Phylloscopus — are a famously difficult group: small, similar, often greenish, and prone to being split and merged as evidence accumulates. For birders, song is usually the only practical identification tool in the field. For taxonomists, it has repeatedly been the tie-breaker that reveals two species where there appeared to be one. A birder humming a phrase into a recorder may, unknowingly, be collecting the evidence that ends an argument.
Why “cryptic” species matter
Cryptic species — species that look alike but are reproductively separate — are one of the clearest arguments for the tools that reveal them. Every year a few new bird species are described worldwide, and a growing share of them are found this way: through genetics, through voice, or through both.
The conservation consequence is immediate. The two island groups where these warblers live are tiny. The Tokara archipelago has a combined land area of just over 100 square kilometres spread over twelve islands. Both species have very low genetic diversity, which leaves them vulnerable to environmental change, habitat shifts and disease — though both also show signs of having recovered somewhat from earlier declines.
Ijima's Leaf Warbler is already listed as Vulnerable by the IUCN and protected as a Natural Monument in Japan. Because the Tokara Leaf Warbler is at least as rare, the researchers recommend that both be classified as Vulnerable, and that both be monitored for future population change.
“This shows how important it is to use genetic methods to reveal hidden biodiversity at a time of global biodiversity crisis. These methods can help provide more complete knowledge on which to base future nature conservation efforts,” Alström says.
What a leaf warbler sounds like
Leaf warblers are not the loudest birds in the forest, but in spring their songs carry a long way precisely because the birds sit high and repeat. The two recordings below are relatives from the same genus rather than the new species itself — the Tokara Leaf Warbler's own song is what split it from its neighbour, and it is best heard on the islands. Listen for the pattern: a short, high, rising series repeated with small variations. If you ever hear two leaf warblers singing side by side and they seem to answer each other but never overlap in phrasing, that is the sort of difference the taxonomists were measuring.