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| Map of Southeast Asia showing type localities (large flag markers) and collection localities of museum specimens (small circles) for the various taxa described under Cyanoderma erythropterum and C. bicolor. in Teo, Sin, Nieves et Rheindt, 2025. Images of Babblers modified from Eaton et al. (2016). |
ABSTRACT
Quantitative analysis of colouration is an essential tool for subspecies delimitation but has always posed a challenge in avian taxonomy. In this study on the Chestnut-winged Babbler (Cyanoderma erythropterum) species complex from tropical Southeast Asia, we made use of colour measurements taken with digital cameras and applied two methodologies—(1) the 75% subspecies rule on quantitative colourimetric variables, and (2) the CIEDE2000 colour distance method to generate phylograms, which has probably never been applied in taxonomy before. Given its large number of described subspecies, many of which have been synonymised in modern taxonomies, the species complex serves as an appropriate model to test subspecies validity. Our data indicate that one synonymised subspecies (C. e. apega), from the islands of Bangka and Belitung, requires re-instalment and recognition, whereas one widely recognised subspecies (C. e. fulviventre), from the Banyak Islands, should be synonymised. Our approach also allowed us to redraw geographic subspecies boundaries. Our work indicates that current subspecies taxonomies of many poorly known tropical species may remain error-ridden and highlights the importance and viability of large-scale taxonomic revisions targeting avian subspecies globally while incorporating quantitative colourimetric approaches.
Keywords: Chestnut-winged Babbler, Colour distance, Subspecies boundaries, Subspecies rule, Taxonomy
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| Map of Southeast Asia showing type localities (large flag markers) and collection localities of museum specimens (small circles) for the various taxa described under Cyanoderma erythropterum and C. bicolor. Flag markers are positioned precisely at type localities except in the case of C. e. pyrrhophaeum, whose type locality is ill-defined as ‘Sumatra’. Subspecies that are widely synonymised are shown with the same colour (both in font and type locality flag) as the taxon under which they are usually subsumed. Underlined names refer to subspecies that have widely been accepted prior to this study. Red dots indicate the localities of specimens from the Lee Kong Chian Natural History Museum (Singapore), while light-blue dots indicate the localities of specimens from other museums. Black stippled lines represent subspecies boundaries widely followed prior to this study, while yellow stippled lines represent revised subspecies boundaries proposed in this study. The dark-grey shaded area in Borneo reflects remaining uncertainty around the boundary between C. b. bicolor and C. b. rufum. Images of babblers modified from Eaton et al. (2016). |
Conclusions:
Our results validate the current synonymisation of neocarum and sordidum with erythropterum, and of pellum with pyrrhophaeum. Our data also show that a widely recognised subspecies, fulviventre from the Banyak Islands, is indistinct in plumage and should be synonymised with pyrrhophaeum. Subspecies apega from the Bangka and Belitung islands is widely synonymised in modern taxonomies, but emerged as distinct in its crown colouration, exhibiting a pattern that would render it intermediate between C. erythropterum and C. bicolor. We advocate a resurrection of this subspecies, and confirm its placement within C. erythropterum on the basis of vocal data. Our examination of specimens indicates that the subspecies boundary between rufum and bicolor is much further south than widely assumed, although we do not rule out the possibility that rufum may form part of a cline of crown colour across Borneo.
The following is our proposed subspecies taxonomy for the Chestnut-winged Babbler (Cyanoderma erythropterum) complex:
● Cyanoderma erythropterum (Blyth, 1842):
Chestnut-winged Babbler
○ C. e. erythropterum (Blyth, 1842) – Type locality: Singapore
■ Range: Thai-Malay Peninsula, Singapore, Riau Islands, Natuna Islands
■ Synonyms: sordidum Baker, 1917; neocarum Oberholser, 1932
○ C. e. pyrrhophaeum (Hartlaub, 1844) – Type locality: “Malacca and Sumatra” (here restricted to Sumatra, as Malacca was a trading hub)
■ Range: Sumatra, Batu and Banyak Islands
■ Synonyms: pellum Oberholser, 1912; eripellum Oberholser, 1922; fulviventre Richmond, 1903
■ Main diagnosis: Grey breast darker than erythropterum
○ C. e. apega Oberholser, 1922 – Type locality: Tanjong Tedong, Bangka Island
■ Range: Bangka Island and Belitung Island
■ Main diagnosis: Incomplete chestnut crown with front half being slate grey unlike complete chestnut crown in erythropterum and pyrrhophaeum
● Cyanoderma bicolor (Blyth, 1865):
Bicoloured Babbler
○ C. b. bicolor (Blyth, 1865) – Type locality: Labuan (Malaysian Borneo)
■ Range: Northern and central Borneo, Banggi Island
○ C. b. rufum (Chasen & Kloss, 1927) – Type locality: Sampit, south coast of Kalimantan
■ Range: Southern Borneo, likely found only across the Central Kalimantan Depression
■ Main diagnosis: Rufous colouration extending to the nape, and even to some extent to the crown, unlike the grey colouration observed in bicolor
Shen Han Teo, Yong Chee Keita Sin, Mayjean Marie Ines Nieves and Frank E. Rheindt. 2025. Birds of a feather: Comprehensive Plumage Colour Analysis for A Revised Subspecies Classification of the Chestnut-winged Babbler (Cyanoderma erythropterum) species complex. Avian Research. In Press, 100307. DOI: doi.org/10.1016/j.avrs.2025.100307 [4 October 2025]
