Bird-rich Indonesian island yields up new songbird species

  • Researchers have described a new species of songbird found only on the Indonesian island of Rote — the second new avian discovery there in less than a year.
  • The Rote leaf-warbler (Phylloscopus rotiensis) was initially assumed to be the same species as the Timor leaf-warbler from a neighboring island, but closer studies of its physical characteristics and genetic analyses have distinguished it as its own species.
  • Rote is home to a large number of species found only there or on neighboring islands, but lacks any major terrestrial protected area.

JAKARTA — Less than a decade after the world was introduced to the latest species of Asian leaf-warblers, a new songbird of that group has now been described by scientists. But the species, confined to a single island in eastern Indonesia, is already thought to be endangered.

The discovery of Phylloscopus rotiensis, or the Rote leaf-warbler, named after the island on which it’s found, involved a series of separate field observations between 2004 and 2015 by different groups of researchers, according to a paper published Oct. 23 in the journal Scientific Reports.

The description of this species came after scientists identified the limestone leaf-warbler (Phylloscopus calciatilis), in Vietnam’s karst mountains, in 2010. The announcement of this newly-described songbird from Rote also followed the news of researchers describing the Rote honeyeater (Myzomela irianawidodoae) in December 2017.

The Rote leaf-warbler. Image by Philippe Verbelen.

The first observation of a leaf-warbler of unknown identity on Rote in Indonesia’s East Nusa Tenggara province, part of a chain that makes up the Lesser Sunda Islands, was carried out by the Australian ornithologist Colin Trainor in 2004.

But Trainor didn’t make detailed observations and had no photographs as he assumed that the songbird was the same species as the Timor leaf-warbler (Phylloscopus presbytes), found on the nearby island of Timor, which is shared by Indonesia and Timor-Leste.

When a pair of Belgian ornithologists visited Rote in 2009 to carry out a survey on owls, they made very detailed observations and photographs of the unknown species. They returned in 2014 to conduct more fieldwork and compare the samples from Rote with the leaf-warblers in Timor. The following year, a team of researchers from the Indonesian Institute of Sciences (LIPI) and the National University of Singapore (NUS) went to Rote to study the bird in much greater detail, including carrying out a genetic analysis.

“Alarm bells went off when we realized how strikingly different the bill shape and the coloration of the Rote bird was compared to all other leaf-warblers,” Philippe Verbelen, one of the Belgian researchers, told Mongabay in an email.

The newly described bird’s bill, at 16 millimeters (about 5/8 of an inch), is “proportionately much longer” than other Phylloscopus leaf-warblers, the report noted.

“Bark-gleaning behaviour could be the reason behind the unusually long bill of this taxon: a longer bill could confer evolutionary benefits by permitting the bird to more easily extract prey from between fissures in tree bark,” the authors said in the report. However, they noted it was difficult to say whether such long bill could be a potential mechanism to widen the species’ ecological niche.

Beyond the bill shape, the Rote leaf-warbler can be distinguished from other Indo-Papuan leaf-warblers by the differences in plumage coloration and in the color and pattern of the lower jaw.

The researchers also used genetic analyses to show that the Rote leaf-warbler is genomically distinct from its closest known relative from the next island over, the Timor leaf-warbler.

Phylloscopus leaf-warblers are small, have a similar coloration and are species-rich,” study leader Frank Rheindt, who heads the Avian Evolution Lab at NUS’s Department of Biological Sciences, told Mongabay in an email.

“Hence, there is big potential in discovering new species, despite the fact that birds are generally so well-known, and despite the fact that there are only — on average — 3-4 new bird species being described to science every year,” he said.

The newly described leaf-warbler inhabits intact primary deciduous forest as well as secondary forest in Rote, but human population growth and expanding intensive agricultural activity have reduced the songbird’s habitat, the researchers said.

“These human population growth trends are expected to continue, bringing with them increased rates of road-building and land cover conversion and further decreasing the amount of habitat available to the Rote leaf-warbler,” the report said.

“The unique dry woodland habitats of Rote are under immense threat, along with the four species of avian endemics that are now known from this island, and an unknown number of other animal and plant endemics,” Rheindt said.

He added that despite being songbirds, Phylloscopus leaf-warblers were not a typical target of Indonesia’s notoriously expansive songbird trade.

“The reasons are difficult to know, but I suspect they die too easily in captivity, and although I personally find their songs pleasing, they don’t have the full-throated melodious songs typical of coveted pet songbirds,” Rheindt said.

Still, the researchers have proposed that the Rote leaf-warbler be given a categorization of “vulnerable” in the IUCN’s Red List of Endangered Species.

Only five to 10 new bird species are described each year, but the Rote leaf-warbler is already the second new species described from Rote in 2018, highlighting the island’s rich conservation value.

The number of bird species found only on Rote or nearby islands has long been underestimated; the island hosts many species-level endemics as well as an additional number of threatened, range-restricted species, such as yellow-crested cockatoos (Cacatua sulphurea), Timor green pigeons (Treron psittaceus) and Jonquil parrots (Aprosmictus jonquillaceus).

Rote’s northern peninsula has been identified by BirdLife International as a “key biodiversity area,” but isn’t among the priority sites for investment by the Critical Ecosystems Partnership Fund. Rote also currently lacks any major terrestrial protected area. Conservationists have called for the creation of new protected areas covering the most intact forest patches in the island’s north and south-central coast to secure the long-term survival of the many endemic and threatened species on the island.

“It will be up to the Indonesian government and Indonesian society to exercise wise stewardship of this unique ecological heritage,” Rheindt said.

“I am confident that our discovery will help highlight the extraordinary biological richness of the island to the Indonesian people.”

The Rote leaf-warbler. Image by Philippe Verbelen.

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