Little known Oriental bird: Heinrich's Nightjar

by Iwein Mauro, from OBC Bulletin 37, June 2003.

Walking down from the dismantled Anaso logging camp on the western slopes of Mount Rorekatimbu towards the Palu-Napu main road bisecting the Lore Lindu National Park in Central Sulawesi around dusk on 18 July 1998, I was truly startled by an unidentified flying object nearly colliding with me. Probably as shocked as I was, it hovered for a few seconds at breast height less than half a meter in front of me whilst repeatedly uttering a distinctive tchor and subsequently perched nearby at eye-height on top of low roadside ginger thickets, soon revealing itself as a medium-sized nightjar Caprimulgus species. Fortunately this bird remained happily in the torch beam for about five minutes as I gradually approached it down to point-blank range and I had little hesitation in identifying it as a female Heinrich's Nightjar Eurostopodus diabolicus, one of Sulawesi's most enigmatic and sought-after endemic bird species (overview in Birdlife International (2)).

On 23 July I revisited the site, which is situated at about 1,900 m elevation, accompanied by Bernard and Erika Van Eleghem-Winne, who had received a sound recording of E. diabolicus secured by Mark Van Beirs also at Anaso during a Birdquest the previous year, de facto perhaps the first definite contemporary record of the species (see below). Most likely the territorial male belonging to the previously spotted female, responded dramatically for a short period around dusk by flying several times low overhead and calling back regularly. Between 15 and 18 December 1998 together with Raf Drijvers I taped out a pair on three occasions around the former Anaso clearing at about 2,000 m elevation and further located two different pairs along the main road at an altitude of about 1,750 m near Lake Tambing.(1,2) With the exception of the Anaso pair, which perched briefly though rather distantly, the birds were mostly seen in flight, however they allowed for their vocalisations to be sound recorded. On 13 July 2000 I heard and briefly saw what was undoubtedly Heinrich's Nightjar in ridgetop lower montane forest at 1,650 m within the Gunung Ambang Nature Reserve, Bolaang Mongondow, North Sulawesi.

Great Eared Nightjar Eurostopodus macrotis constitutes the prime confusion species and numerous unsubstantiated claims of Heinrich's Nightjar persist, originating from the fact that few observers seem to realise that the subspecies macropterus endemic to Sulawesi is strikingly different from the huge harrier-like E. m. cerviniceps of mainland South-East Asia that they have usually become acquainted with previously. It definitely remains odd that many observers claiming diabolicus actually do not see Great Eared Nightjar at all on Sulawesi despite the latter's generally much greater relative abundance. Coates and Bishop(3) briefly summarised the main distinguishing features of Heinrich's Nightjar and the species' identification has been thoroughly elucidated by Cleere and Nurney.(4) In contrast, Bishop and Diamond5 incomprehensibly perpetuate some of the previously reigning misconceptions by talking about a much larger E. m. macrotis with harrier-like flight when discussing differences with diabolicus.

Only the unique female type specimen of Heinrich's Nightjar exists and there are slight discrepancies in available published measurements. Based on those provided by Cleere and Nurney4 and under the assumption that sexual biometric differentiation within diabolicus is marginal, as in the closely allied Archbold's Nightjar Eurostopodus archboldi of montane New Guinea (see below), estimated size disparity between the two forms could be up to about 20% in total body length, 26% in wing length and about 20% in tail length. Naturally, some caution is appropriate given that only the holotype is available for analysis and in other members of Eurostopodus the male is known to be the larger sex, in which case the discrepancies may be even further reduced slightly. Nonetheless figures above are consistent with my personal live perception of both taxa and although substantial, to the inexperienced observer such differences may, however, not always be easy to assess correctly in the field, especially in the dim light conditions that characterise many nightjar sightings.

Sulawesi Nightjar Caprimulgus celebensis and Savannah Nightjar C. affinis share with diabolicus a similar biometry, making flight identification without additional supporting evidence problematical. (4) It should be fairly straightforward that the former taxon's imperfectly known, allegedly fragmentary distribution on Sulawesi is likely to reflect inadequate coverage rather than genuine ecological and distributional processes and whilst it may be argued that the latter, due to its different habitat requirements, not normally coexists with diabolicus, both these species at all times require earnest consideration during identification.

On Sulawesi, Great Eared Nightjar appears more attenuated, slimmer and obviously longer- and stiffer-winged, with a more pointed hand resulting in a leisurely, buoyant and purposeful flight mode with frequent long and steady glides and only occasional fluttering when actually feeding. It is a typically aerial feeder, usually hunting above the forest canopy and sometimes congregating in loose flocks of up to about 10 birds when actively feeding. However, on several occasions I have also observed this species feeding singly or in pairs very low above the ground in a manner highly reminiscent of diabolicus. Heinrich's Nightjar is a rather compact, short-tailed, short- and noticeably blunt-winged species, with rapid, fluttering wing beats and an erratic flight only interspersed by short sweeping glides, that forages singly or in pairs and nearly always keeps very low to the ground. In fact, I have personally never observed diabolicus flying higher than mid canopy (contra Bishop and Diamond (5)). As noted for Archbold's Nightjar(4) (and pers. obs. author), this species also catches prey by repeatedly sallying from exposed branches situated low above the ground in small clearings or from atop roadside thickets (pers. obs. author).

In summary, Heinrich's Nightjar plumage-wise is best identified on the combination of 1. its overall dark blackish-brown dress featuring 2. a relatively rounded head lacking erectile eartufts, 3. an obvious greyish-brown supercilium running from the forehead to the nape, 4. a narrow buffish band across the lower throat gradually broadening and darkening towards cheeks before petering out on the side of the hindneck, 5. its blackish-brown breast extensively spotted buffish-chestnut, 6. the very dark, almost blackish arrow-shape centred wing coverts prominently edged rufous in fresh plumage, becoming paler buffish with wear, 7. the complete absence of a rufous hindneck collar, and 8. its uniformly blackish-brown outer primaries with variable spotting on P2-5. (3,4)

One apparently previously overlooked, though diagnostic feature, readily allowing identification under very poor conditions is that the eyes of Heinrich's Nightjar consistently glow purplish in the spotlight (pers. obs. author; pers. com. R. Drijvers; pers. com. M. Van Beirs; co ntra Bishop and Diamond (5)) while those of Great Eared Nightjar, at least on Sulawesi, reflect yellowish and Sulawesi Nightjar's shine bright red. I feel this is a very reliable feature and for that reason suggest that Bishop and Diamond's rediscovery of Heinrich's Nightjar(5) be treated with the necessary caution. A conclusion reinforced by the fact that the described behaviour of the birds does not fit in with my knowledge of the species and that provided plumage details are rather crude and unconvincing.

R. Drijvers and I originally noticed that one member of the territorial pair observed repeatedly, though briefly, in December 1998 (see above), displayed relatively extensive whitish markings in the outer wings. Images recently made available equally strongly suggest that E. diabolicus is sexually dimorphic, sexes varying at least in the amount of white in the outer primaries and colouration and patterning of the lower scapulars. Presumed males exhibit whitish elongated spots on P2–5 (numbered ascendantly) and relatively plain, mainly greyish, indistinctly patterned lower scapulars forming a pale panel, whilst females have the whitish in the wing restricted to exceedingly small spots on P3–4 and buffish lower scapulars boldly vermiculated with dark brown. Additional work is necessary to gain further insight into this taxonomically significant issue.

The male advertising call is highly distinctive and may best be transcribed as a loud, somewhat bubbling series of up to 36 kweik notes, more or less delivered at constant pitch and speed and lasting about 4.7 seconds. Contact calls include a soft, mellow wheet, often quickly repeated up to three times, and distinctive, rolling kreep notes uttered at variable pitch. Calls are given very infrequently and perhaps mainly around dusk. Around this time the species may respond dramatically to tape playback by repeatedly calling back and by flying back and forth, often passing overhead at very close range. However at other times of night birds proved surprisingly lethargic and no reaction was noted at all. All evidence suggests that the species calls exclusively in flight. When agitated, as the bird that nearly flew against me was, a guttural, churring tchor is uttered repeatedly. I stress here that none of these vocalisations are consistent with those previously tentatively ascribed to the species. (2,5,6)

Clearly the affinities of Heinrich's Nightjar lie with the Australopapuan members of Eurostopodus and definitely not with the truly eared, Oriental species – which should perhaps be retained in a separate genus Lyncornis 4 – and the absence of Eurostopodus from the Moluccas and Lesser Sunda islands chain remains puzzling in this respect. Pending further study into the intrageneric limits of Eurostopodus, I therefore recommend continued use of the vernacular name Heinrich's Nightjar (3,4) over Sulawesi Eared Nightjar (2). The presence of white spots on the outer primaries in both sexes of diabolicus, constitutes a highly significant feature, suggesting it groups with the Spotted E. argus and White-throated Nightjar E. mystacalis. However these species exhibit many morphological characteristics that are not attributable to Heinrich's Nightjar, produce highly divergent vocalisations and have different ecological requirements, comprising essentially non-forest species.(4) During opportunistic bird surveys in the Arfak mountains, Vogelkop peninsula, Papua (formerly Irian Jaya) province, Indonesia I learned that the local population of the closely related Archbold's Nightjar E. archboldi there – only discovered as recently as 19957 more than 600 km away from its nearest known breeding grounds in the Snow mountains, Papua province, Indonesia – exhibits exceedingly similar advertising calls to those of diabolicus. I failed to sound record archboldi in the Arfaks though readily lured and caught two different individuals in a mist net with a recording I had made of diabolicus. Given this vocal analogy and many similarities in external morphology including sexual dimorphism, it might perhaps be argued that Archbold's Nightjar is the closest living congener of diabolicus. Archbold's Nightjar too remains a noticeably poorly known taxon: the westernmost boundary of its distribution along New Guinea's central cordillera is imperfectly known and in its historic range its vocalisations remain to be adequately described and documented4 (and B. Beehler in litt., 24 January 2002).

That Heinrich's Nightjar – until recently known only from foothills at approximately 250 m elevation at the base of Mount Klabat in the Minahassa peninsula of North Sulawesi – should now suddenly be regularly encountered in montane forest up to at least 2,000 m (pers. obs. author) more than 750 km away from the type locality, strongly suggests that the species ranges widely across forested Sulawesi and occupies a broad altitudinal zonation. Taking its retiring, crepuscular and nocturnal habits and generally silent nature into account E. diabolicus is probably greatly overlooked rather than rare and is quite likely at least locally reasonably common. Furthermore, as it appears to be partly tolerant to habitat degradation, its treatment as Vulnerable 2,8 may perhaps be downgraded to Near-threatened.

Acknowledgements
It is my pleasure to extend my sincerest thanks to Serge Hoste for kindly preparing the sonagrams, to Jimmy Chew obligingly putting at my disposal his slides and to Soon Chye Ng for kindly facilitating this, to Nigel Cleere, Raf Drijvers, Morten Strange, Mark Van Beirs and Dominique Verbelen for providing literature and useful discussion and to Bapak Rolex and Bapak Valentinus and family at the Kamarora post, Lore Lindu NP for excellent hospitality.

References

1. Mauro, I. (1999). Preliminary report on birds recorded from Wallacea. Sulawesi, Moluccas and Lesser Sundas. Unpublished report.

2. BirdLife international (2001). Threatened Birds of Asia: The Birdlife International Red Data Book. Cambridge: Birdlife International.

3. Coates, B. J. and Bishop, K. D. (1997). A Guide to the Birds of Wallacea. Sulawesi, The Moluccas and Lesser Sunda Islands, Indonesia. Alderley: Dove Publications.

4. Cleere, N. and Nurney, D. (1998). Nightjars. A Guide to Nightjars and Related Nightbirds. Pica Press.

5. Bishop, K. D. and Diamond, J. M. (1997). Rediscovery of Heinrich's Nightjar Eurostopodus diabolicus. Kukila 9:71–73.

6. Holmes, P. and Wood, H. (1980). The report of the ornithological expedition to Sulawesi. Ruslip, Middx., privately published.

7. Gibbs, D. (1996). Mountain Eared Nightjar in Arfak Mountains, Irian Jaya: range extension and first description of nest and egg. Dutch Birding 18:246–247.

8. Stattersfield, A. J., Crosby, M. J., Long, A. J. and Wege, D. C. (1998). Endemic Bird Areas of the World. Priorities for Biodiversity Conservation. Cambridge: Birdlife International.

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