The Ghost of Sumatra: The Century-Long Mystery of Rueck’s Blue-Flycatcher (Cyornis ruckii)

 The Vanishing Legend

Cyornis ruckii, internationally known as Rueck’s Blue Flycatcher, is one of the most mysterious birds in the world, often described as a “ghost” of the Sumatran rainforest. This songbird has achieved legendary status because its existence has not been conclusively confirmed in the wild for more than a century, with the last official record dating back to February 1918 (BirdLife International, 2021). Since then, this endemic species of Sumatra appears to have vanished, leaving a long-standing enigma for researchers and birdwatchers worldwide.

Rueck’s Blue-Flycatcher (photo by IA)

Globally, the International Union for Conservation of Nature (IUCN) classifies the species as Critically Endangered, the final category before being declared extinct in the wild (BirdLife International, 2021). Its extreme rarity has also led to its inclusion in CITES Appendix II, while the Indonesian government has granted it legal protection since 1972 (Collar, 2020). Despite its high conservation status, the lack of reliable information on its population and current existence makes conservation efforts exceptionally challenging.

The disappearance of this species has created a significant gap in the biodiversity record of Sumatra and represents a major challenge for modern ornithology. One of the primary difficulties in rediscovering it lies in its nature as a cryptic species, as its appearance closely resembles other members of the genus Cyornis. This challenge is further compounded by the absence of verified vocal recordings that could assist field identification. The century-long mystery surrounding this vanishing legend is not merely about the loss of a single species but also serves as a warning signal regarding the critical condition of Sumatra’s lowland primary forests.

Historical Records

The historical record of Cyornis ruckii is entirely based on four preserved specimens collected between the late nineteenth and early twentieth centuries, which remain the only scientific reference for this species to date. The first two specimens, consisting of an adult male and female, were described by the French zoologist Émile Oustalet in 1881, based on material obtained from a trader named Monsieur Rück at the port of Malacca in 1880. However, the validity of Malacca as the original locality is questionable, as these specimens were likely trade skins originating from an undocumented location (Oustalet, 1881; Collar, 2020).

Definitive evidence of the species’ occurrence in the wild was only established several decades later through the collection of two additional specimens in North Sumatra by August van Heyst. The first, a juvenile female, was collected in April 1917 in Delitua, followed by an adult male obtained in February 1918 in Tuntungan, near Medan. Since these last collections, no verified records of the species have been documented in the wild, reinforcing its status as one of the most enigmatic birds in Southeast Asia (Robinson & Kloss, 1919; Collar, 2020).

The four historical specimens of Cyornis ruckii are currently housed in several major institutions, including the Muséum national d'Histoire naturelle in Paris, the Naturalis Biodiversity Center in Leiden, and the American Museum of Natural History in the United States. These specimens constitute the sole basis for the species’ taxonomic assessment (Collar, 2020). Initially, the later specimens were described as a new species, Cyornis vanheysti, by Herbert Robinson and Cecil Kloss in 1919. However, subsequent taxonomic revision concluded that this taxon is synonymous with Cyornis ruckii.

Morphological comparisons reveal notable differences between the specimens described by Oustalet and those collected in Sumatra by van Heyst. The male from the earlier collection exhibited a blue belly, whereas the Sumatran specimens showed a greyish-white underpart and a slightly larger bill. These variations are thought to represent individual differences or possible taxonomic variation, such as subspecific divergence. However, this hypothesis remains unconfirmed due to the absence of data from living populations (Oustalet, 1881; Robinson & Kloss, 1919; Collar, 2020).

Portrait of a Ghost: Description and Ecology

Cyornis ruckii is a small passerine measuring approximately 17 cm in length, with a black bill, brown iris, and dark legs. The species exhibits clear sexual dimorphism: males display a glossy deep blue plumage, while females are reddish-brown with paler underparts (Oustalet, 1881; Collar, 2020). Identification is particularly challenging due to its resemblance to closely related species such as Cyornis hainana and Cyornis unicolor, as well as morphological variation among known specimens. Ecologically, the species is presumed to inhabit lowland primary forests and to be insectivorous. However, nearly all aspects of its biology, including breeding behavior and vocalization, remain unknown due to the absence of modern observational records (Robinson & Kloss, 1919; Collar, 2020).

Why Did They Disappear?

The absence of Cyornis ruckii from scientific records for over a century is closely linked to intense pressures on lowland forest ecosystems in Sumatra, particularly in Aceh and North Sumatra. The species is believed to be highly dependent on intact primary forests, yet in recent decades these habitats have undergone extensive conversion into plantations, infrastructure, and settlements, resulting in significant habitat fragmentation (BirdLife International, 2021).

In Aceh, although important landscapes such as the Leuser Ecosystem and Ulu Masen Ecosystem still remain, they are increasingly threatened by logging, oil palm expansion, and road development, all of which continue to degrade potential lowland habitats for this species. Another critical threat is hunting and the songbird trade, widely recognized as a major driver of songbird population declines in Indonesia, despite the species being legally protected and listed under CITES Appendix II. At the same time, the lack of systematic surveys and the limited number of targeted studies in potential habitats have created substantial data gaps, leaving the species’ current status in the wild uncertain (BirdLife International, 2021).

Search Efforts and Future Prospects

Efforts to rediscover Cyornis ruckii face significant challenges, particularly in field identification, as no verified sound recordings are currently available for scientific reference. This limitation represents a major obstacle, given that the species is considered cryptic and exhibits strong morphological similarity to other members of the genus Cyornis, where vocal differences are often the most reliable means of identification (Collar, 2020). Although there have been no confirmed records since 1918, several unverified observations offer some hope. For instance, reports of a pair observed in Jambi between 2013 and 2014 resembled this species visually, although these sightings remain unconfirmed and may instead refer to Cyornis concretus (Assiddiqi et al., 2020).

In Aceh, similar reports from local communities are frequently encountered, yet most lack sufficient photographic or audio documentation to support identification. Optimism regarding the possible persistence of this species is strengthened by recent rediscoveries of birds once thought lost, such as Malacocincla perspicillata and Carpococcyx viridis, demonstrating that rare species may still survive in underexplored habitats.

Looking ahead, citizen science approaches are becoming increasingly important, particularly through platforms such as eBird and Burungnesia, which enable public participation in biodiversity data collection. Nevertheless, targeted research supported by intensive surveys in lowland primary forests, especially within the Leuser Ecosystem and Ulu Masen Ecosystem, remains a crucial step toward scientifically clarifying the current status of this elusive species.

Reference

Assiddiqi, Z., van Balen, S., & Collar, N. J. (2020). Mystery flycatchers in Central Sumatra—Rück's Blue-flycatcher or White-tailed Flycatcher? Kukila.

BirdLife International (2021). Species factsheet: Rück's Blue Flycatcher Cyornis ruckii. Downloaded from https://datazone.birdlife.org/species/factsheet/r%C3%BCcks-blue-flycatcher-cyornis-ruckii. on 27/02/2026.

Collar, Nigel J. (2020). Rück’s Blue-flycatcher Cyornis ruckii: the evidence revisited. Kukila 23: 14-18.

Oustalet, É. (1881). Description de nouvelles espèces d’oiseaux. Bulletin de la Société Philomathique de Paris.

Robinson, H. C. & C. B. Kloss. 1919. On a collection of birds from N.E. Sumatra. J. Straits Branch Roy. Asiat. Soc. 80: 73-133.