A team of ornithologists from the National University of Singapore, Birdtour Asia Ltd., and the University of Queensland has described a new and cryptic species of the hawk-cuckoo genus Hierococcyx from Borneo, a large island shared by the Malaysian states of Sabah and Sarawak, the Indonesian state of Kalimantan, and the sultanate Brunei.
Breeding range map of Hierococcyx bocki from Sumatra and Peninsular Malaysia, Hierococcyx sp. from Borneo, and Hierococcyx sparveriodes. Image credit: Ayuwat Jearwattanakanok / Robert O. Hutchinson / James A. Eaton.
Hierococcyx is a small genus of birds within the cuckoo family Cuculidae.
First established in 1845, the genus currently includes nine scientifically recognized species.
Commonly known as hawk-cuckoos, they can be found in South, Southeast, and East Asia.
“Within the hawk-cuckoos of the genus Hierococcyx, two tropical and subtropical Asian species form a close-knit pair,” said lead author Dr. Frank Rheindt from the National University of Singapore and his colleagues.
“They are variably merged into one species or recognized as two monotypic species: the large hawk-cuckoo (Hierococcyx sparverioides) and the Bock’s hawk-cuckoo or dark hawk-cuckoo (Hierococcyx bocki).”
“These hawk-cuckoos are found in forests from the Himalaya and China to South-East Asia.”
“Hierococcyx sparverioides is the more widespread taxon, breeding widely across montane tropical, subtropical and even temperate mainland Asia and wintering to the south.”
“Hierococcyx bocki ranges in the montane forests of Sundaland.”
“Historically, species delimitation in Hierococcyx cuckoos, as in most other birds, has been guided by plumage differences,” they noted.
“Yet there is only subtle plumage variation across these two species.”
“Since the 1990s, the taxonomy of many tropical and subtropical bird species complexes has undergone a revolution with increasing insights into the importance of bioacoustic characters for species delimitation.”
“Species limits within numerous tropical and subtropical Asian bird species complexes have been re-drawn on the basis of bioacoustic evidence.”
In their new study, Dr. Rheindt and co-authors aimed to investigate the taxonomy of the Hierococcyx sparverioides–Hierococcyx bocki pair and to uncover evidence for cryptic species.
To do this, they used an integrative approach based on bioacoustic, plumage and morphometric data.
They analyzed a total of 107 sound recordings of these two species obtained from online sound libraries.
They also assessed plumage differences and morphometric traits measured on the basis of a series of museum specimens.
Their analysis unearthed a distinct, hitherto unnamed population of Hierococcyx bocki residing on Borneo that can be separated from other populations chiefly by its unique three-syllabled main song.
Given the importance of vocalizations in cuckoo taxonomy, they described this population as a new species to science: Hierococcyx tiganada.
“The new species from Borneo closely resembles its sister species Hierococcyx bocki from Sumatra and Peninsular Malaysia,” the researchers said.
“The two are almost identical visually, but adults of the new species have a slightly grayer mantle, especially the upper mantle towards the crown, leading to a distinct lack of contrast between a gray crown and more brownish back that is apparent in Hierococcyx bocki.”
“The best distinguishing feature between the two species is the structure of their brainfever song — always three-syllabled in the new species and two-syllabled in Hierococcyx bocki.”
“Hierococcyx sparverioides differs from the new species in the same characters that set it apart from Hierococcyx bocki,” they added.
“Hierococcyx sparverioides also has a two-syllabled brainfever song.”
“In addition, adult Hierococcyx sparverioides has a larger body size, is less dark-gray in front of the eye, has a browner mantle less contrasting with the crown, and exhibits distinct dark-rufous streaking on a whitish background on the upper breast (solid rufous with no streaking in the new species).”
According to the scientists, Hierococcyx tiganada inhabits montane rainforests above 1,000 m elevation.
“Despite the alarming conservation status of many South-East Asian birds which rely on rainforest habitat, Hierococcyx tiganada is probably not threatened,” they said.
“While Borneo’s lowland rainforests have suffered considerable loss over the last three decades, much of the montane forest in the heart of Borneo survives thanks to its remoteness and inaccessibility to logging companies and other extractive industries.”
“Hierococcyx tiganada is relatively well known from three general locations in Sabah and one in Sarawak.”
“There are extremely few records from Kalimantan, and the species appears to be genuinely absent from the Meratus Mountains in South Kalimantan.”
“However, the species is probably ubiquitous over vast areas of montane northern Borneo, often near the border between Kalimantan, Sarawak and Sabah, and the lack of widespread records likely reflects poor observer coverage away from well-established birdwatching areas.”
Hierococcyx tiganada is the first Hierococcyx species to be described in the 20th/21st centuries.
“This discovery highlights the continuing importance of bioacoustic research in biodiversity discovery and further elevates the biogeographic importance of Borneo,” the authors concluded.
Their paper will be published in the Journal of Asian Ornithology.
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Frank E. Rheindt et al. 2025. A new and cryptic species of hawk-cuckoo (genus Hierococcyx) from Borneo. Journal of Asian Ornithology 41: 34-43
