Turdus poliocephalus sukahujan, Rheindt & Prawiradilaga & Ashar & Lee & Wu & Ng, 2020

Rheindt, Frank E., Prawiradilaga, Dewi M., Ashar, Hidayat, Lee, Geraldine W. X., Wu, Meng Yue & Ng, Nathaniel S. R., 2020, A lost world in Wallacea: Description of a montane archipelagic avifauna (supplement), Science 36, pp. 1-104 : 63-67

SM10:

Turdus poliocephalus sukahujan , subspecies nova

(Taliabu Island Thrush;

urn:lsid:zoobank.org:act:44D4876F-2524-4C59-80E8-9462CF9D00A8

) Frank E. Rheindt, Dewi M. Prawiradilaga, Hidayat Ashari, Suparno

Holotype

MZB.Ornit.34.374 ( fig. S14 ); adult male collected 10 Dec 2013 near Waiyo dinahana Camp (~ 1200m) on Taliabu Island (S 01⁰ 47.614 '; E 124⁰ 48.216 '). Collected by the Rheindt / LIPI field party, including tissue samples from breast muscle and liver; skin prepared by Suparno; field number Tbu41; no molt; low fat; weight 67g; wing length 11cm; wing spread 33.3cm; total length 22.7cm; bill 2.2cm; tail 8.8cm; tarsus 3.2cm.

Description of holotype

Mantle, rump and crown generally dark cold-brown (5Y 2/2), becoming duller and paler on the forehead, auriculars and lores (5Y 3/2). Upperwing coverts consist of seemingly fresh lesser and median coverts roughly concolorous with mantle as well as more worn, less cold- brown greater coverts (2.5Y 3/2), some of which show traces of paler margins. Remiges (which mostly appear fresh) are dull brown (5Y 3/2), with the edges of outer primaries distinctly warmer-brown. Undersides of remiges are uniformly grey-brown (10YR 4/2). Rectrices are blackish-brown (2.5Y 2/2), slightly paler on underside. Throat dull brown (5Y 2/2) becoming slightly paler towards chin. Breast and upper belly slightly darker than throat (2.5Y 3/2), with cinnamon edging on most feathers (7.5YR 3/6), while the lower belly, flanks and vent are darker greyish-brown (2.5Y 2/1). Iris extremely dark brown with a prominent bright orange-yellow bare orbital ring. Bill and gape in life uniformly bright orange-yellow with no darker bill tip. In life, tarsi and toes (including toepads) are bright yellow, slightly less orange than bill and orbital ring; claws are cream-lemon, paler than toes.

Diagnosis

A smallish, all-dark Turdus thrush with conspicuous yellow-orange bare parts and eye ring; obviously a new subspecies of the Island Thrush T. poliocephalus because it is strikingly different from all Island Thrush subspecies in close geographic proximity, although more closely resembles several subspecies that are geographically distant.

The new subspecies differs from T. p. loeseri (North Sumatra) in its lack of strong blackish tones in the plumage. It conspicuously differs from the larger, longer-winged celebensis and hygroscopus from nearby Sulawesi as well as from Greater Sundaic indrapurae, javanicus, stresemanni, biesenbachi, fumidus, seebohmi and whiteheadi in its lack of solid white or rufous on the belly; its uniform dark-brown coloration in which only the underparts show a faint warm tinge; and its unmarked dark-brown vent. The new subspecies has a darker hood and breast than the above-mentioned taxa except for the intensely rufous-bellied fumidus and perhaps hygroscopus. It also differs from deningeri from Seram in its paler, browner (less black) body color, with a darker non-contrasting hood, and differs from schlegelii and sterlingi (both from Timor) in its less olive overall coloration and lack of rufous belly and vent.

Within Indonesia, the new subspecies most closely resembles the much larger and longer-winged versteegi and papuensis from Papua but is dark slaty-brown (versus black) in overall body color and has an unmarked vent. The lower underparts have more extensive and paler-cinnamon feather margins in the new subspecies versus narrower, more rufous feather margins in the Papuan forms.

Among geographically more distant Island Thrush subspecies from outside of Indonesia, the new subspecies most closely approaches three taxa from the Solomon Islands (bougainvillei, kulambangrae, sladeni) and two taxa from the Bismarck Archipelago (heinrothi, beehleri), but lacks those subspecies’ blackish overall body color. It also superficially resembles nigrorum from Negros (Philippines), but the latter subspecies’ lower belly grades into whitish on the vent while the new subspecies is uniformly dark on those parts. Furthermore, another Philippine subspecies, kelleri from Mt Apo (Mindanao), is similar to the new subspecies but becomes darker brown towards the front of the head while the new subspecies has a nearly uniform head coloration becoming slightly paler on the forehead.

Etymology

The subspecies name, translated from Bahasa Indonesia “it likes the rain” (=suka hujan), refers to the circumstances surrounding the bird’s discovery. On FER’s first expedition to the highest parts of Taliabu ( 48), heavy persistent rain impeded bird observations, leading to a decision to abort the expedition ahead of schedule. On his premature descent, FER then spotted a hardy individual of the new thrush, perched in the pouring rain, leading him to change plans yet again and mount a subsequent second and eventually third expedition to these elevations in better weather. This latter third expedition then allowed for a more detailed documentation of Taliabu’s distinct novel montane avifauna ( 19, 48). The name sukahujan is treated as an inalienable noun in apposition.

Individual, sex and age-related variation within the taxon

All nine specimens procured resemble one another considerably in coloration. The greatest variation occurs in the tone of the lower breast and abdomen, which is intermediate in the holotype but ranges from all-drab brownish grey throughout with almost no warm edging (in MZB.Ornit.34.416) to more extensive cinnamon feather edging (in most adult females, especially MZB.Ornit.34.409), the latter leading to an overall warmer underparts impression. Females are variably slightly less slaty on upperparts and crown, and one adult individual (MZB.Ornit.34.416) of unknown sex has a male-like slaty tone on upperparts but a female- like crown color that is more suffused with dull brown. Two juveniles (MZB.Ornit.34.410; MZB.Ornit.34.417) exhibit noticeable mottling on their mantle and underparts coloration, caused by dark brownish-black feather margins contrasting against the paler (on underparts sometimes warm cinnamon) feather centers.

History of discovery

This new taxon was first discovered on 8 and 9 April 2009 by FER near its type locality on the occasion of a hike to the highest elevations on Taliabu that was cut short by heavy rainfall. Subsequent sightings on 14 and 15 April 2009 were made in the same area on the occasion of a return visit ( 48). We found this population again during our collecting trip to this area from 6-16 Dec 2013 ( 19) and collected the type along with the accompanying specimens.

Distribution and status

T. poliocephalus sukahujan is similar in distribution to Locustella portenta in that it inhabits a peculiar type of dwarf forest in the highlands of Taliabu above ~1,050m ( fig. S2 ), and in that its occurrence on other islands in the Sula Archipelago is unlikely because of the lower elevations of these islands. Similarly, extensive field work around the highest elevations of Peleng in the neighboring Banggai Archipelago have failed to produce any records of Turdus thrushes ( 19, 48). Not unlike L. portenta , this new thrush taxon must be considered threatened with habitat destruction throughout its small range, which is unlikely to exceed 15,000 ha (see L. portenta under SM3 for more details), although we found it at high densities in suitable forest interior. Although T. poliocephalus is known to inhabit natural alpine grassland on other islands (e.g. New Guinea), our surveys showed that it does not occur in the extensive grasslands in the highlands of Taliabu that are the result of recent forest fires.

Taxonomic rationale

With approximately 50 subspecies, the Island Thrush Turdus poliocephalus is arguably the most polytypic bird species on Earth (13 5). Attempts to separate this variable species complex into smaller species-level lineages [e.g. (13 6)] have been called into question by genetic data showing that plumage characters are not a good predictor of close relationships in this complex (13 7). Hence, modern sources generally continue to unite all or most subspecies into one polytypic species. Not unlike some Philippine subspecies previously under study (13 7), the new subspecies from Taliabu is phenotypically extremely different from neighboring forms but more closely resembles certain geographically distant subspecies. It is presently best accommodated at the subspecies level. Comprehensive genomic and phenotypic inquiries are needed in the future to assess whether any internal species limits must be drawn within the complex. Such future analyses may well elevate the Taliabu Island Thrush to species level. However, such analyses are currently hampered by a lack of crucial comparative DNA material.

Figures

Fig. S14. Holotype of Turdus poliocephalus sukahujan (MZB.Ornit.34.374).

Fig. S2. Google Earth satellite map of Peleng and Banggai (top) and the western half of Taliabu (bottom). Peleng and Banggai are the two largest islands of the “Banggai Archipelago”. Our main collection site, an unnamed locality “above Kokolomboi [village]”, is mapped with a yellow pin. Areas above 800m elevation are circled with a red line, indicating the presence of montane vegetation. The island of Taliabu is the largest member of the “Sula Archipelago”. Our main collection locality “Waiyo dinahana” is mapped with a yellow pin. Areas above 1,050 m elevation are circled with a red line, indicating the presence of a peculiar montane stunted forest vegetation. Within the montane zone above 1,050m, areas degraded by logging activity as reflected on Google Earth satellite imagery are mapped in turquoise and areas burnt down by extensive fires in the 1980s are mapped in dark-green. Given the outdated status of some of the satellite imagery, present-day forest degradation is likely more extensive than shown.