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Observations of Isabela Oriole Oriolus isabellae in
the Sierra Madre, Luzon, Philippines, with
descriptions of the call

MERLIJN VAN WEERD and ROB HUTCHINSON
Forktail 20 (2004) SHORT NOTES

The Isabela Oriole Oriolus isabellae is endemic to Luzon in the Philippines. It has been recorded from the Bataan Peninsula and from the Sierra Madre in Cagayan, Isabela and Quirino provinces (Collar et al . 1999). There have only been two recent records:one individual in 1993 in secondary forest at 440 m in the foothills of the Sierra Madre in Quirino province (Gamauf and Tebbich 1995)and two individuals in 1994 on the edge of forest in the Sierra Madre in Cagayan province (Van der Linde 1995), neither of which were considered absolutely certain (Collar 1998). The species is classified as Endangered (under criterion C1)because it has a small population, estimated to number 1, 000 -2, 499 individuals, which is believed to be declining at a rate exceeding 20%in five years/two generations (BirdLife International 2000, 2001).

Little is known of the ecology and of the behaviour of the species (Collar et al . 1999). Its call was hitherto unknown (Kennedy et al. 2000). Independently of each other, we observed a pair of Isabela Orioles in Ambabok, Disulap, municipality of San Mariano, Isabela province on 27 and 29 March 2003 (RH)and 25 and 26 May 2003 (MvW). In April 2004, one Isabela Oriole was captured here using a mist-net and released after documentation.

Isabela Oriole    Isabela Oriole
Plates 1 –3
. Isabela Oriole Oriolus isabellae , April 2004, Ambabok, San Mariano, Isabela, Luzon, Philippines (Photographs by MvM).

OBSERVATIONS

On the morning of 27 March, RH was birding close to the former settlement of Ambabok when he heard anunfamiliar oriole-like call which was clearly different from both Black-naped Oriole Oriolus chinensis and White-lored Oriole Oriolus xanthonotus albiloris . The call was recorded and the bird responded immediately to playback by flying in overhead to within 10 m. The combination of a heavy, broad-based grey bill, blue- grey legs, yellowish underparts washed olive on the breast-sides, plain olive head and upperparts, green wings with brighter yellow fringes, and yellow rather than white lores quickly identified the bird as an Isabela Oriole. A second singing bird which appeared identical in appearance was soon located and the next two hours were spent studying the birds, which were still present and calling strongly when the observer left.

The birds were in an area of degraded secondary forest with extensive areas of bamboo, and they spent most of their time in the canopy of two large mayapis trees Anisoptera thurifera . They were generally unobtrusive and both spent much of their time calling from different perches in the canopy. The birds were not observed actively feeding during the period of observation despite several Blackish Cuckooshrike Coracina coerulescens , Bar-bellied Cuckooshrike C. striata and Balicassiao Dicrurus balicassius feeding in the same area.

Some good sound recordings of the calls were made, generally falling into two categories (Fig. 1, sonagrams 1 and 2). The birds responded strongly to occasional playback by flying in directly overhead, often at close range, and usually gave a rather agitated call (Fig. 1, sonagram 3). This latter call was otherwise only occasionally interspersed with the normal call.

The birds were again present on 29 March and were watched for some two hours during the late after noon. The birds still favoured the same area, but were less mobile, spending much of the time perched on open branches within the canopy of one of the large trees, usually within 5 m and often within 2 m of each other. Again, both birds called regularly throughout the period of observation.

No other Isabela Orioles were located in the area despite regularly playing the recorded calls around nearby Apaya, an old logging camp in selectively logged but disturbed forest, and along the route between the two sites.


Sonograms of the calls of the Isabela Oriole

Figure 1
.Sonagrams of the calls of the Isabela Oriole, March 2003, Ambabok, San Mariano, Isabela, Luzon, Philippines (recordings by RH).

In the afternoon of 25 May, the calls recorded by RH were played by MvW at the site where the species had been observed in March. The large trees in which the birds had been previously seen had very recently been felled, but two orioles responded and flew to surrounding trees, about 50 m apart and 25 m from the observer. The birds were not observed, however, but repeatedly responded to each other and to the tape recorder with exactly the same call as being played back. On 26 May at 10h30, one oriole responded to playback and moved from degraded forest with bamboo stands to a large tree, locally known as Dita Alstonia scholaris , in the company of three Blackish Cuckooshrikes. This bird was seen well for several seconds before disappearing into dense foliage:it was entirely green/yellow with heavily streaked wings, the bill was large and blackish (not reddish as in White lored Oriole which is endemic to northern Luzon)and the white lores were absent (again separating it from White-lored Oriole). It was later seen moving between large trees (Dracontomelon dao, Endospermum peltatum, Anisoptera thurifera and Alstonia scholaris ), always in the canopy, in the company of Blackish Cuckooshrike, Bar-bellied Cuckooshrike, Black-and-white Triller Lalage melanoleuca and Balicassiao. It no longer responded directly to playback and did not approach. However, it did call from time to time and received a response from a second bird on the other side of the river, some 100 m away. Playback was used over 26 -29 May in a variety of habitats near Ambabok, but no additional responses were received.

Three different sounds were recorded:(1)a clear, mournful, slightly descending whistle lasting 0. 5 s, repeated irregularly every 1-2 s, resembling Fig. 1, sonagram 1;(2)a slightly higher, rising whistle, usually with a slight terminal inflection, lasting about 0. 75 s, also irregularly repeated every 1-2 s, resembling Fig. 1, sonagram 2;(3)a harsh, rolling call, not unlike a cricket, lasting about 0. 25 s, usually given at a rate of 2 calls/s and repeated every 2 s (resembling Fig. 1, sonagram 3), usually given as a long series in response to playback but occasionally interspersed with the other calls. In May 2003, an additional call was heard but not recorded:a clear three-tone whistle lasting 1 s with the second tone higher than the first, and the third lower than the first and second. These calls differ considerably from the varied vocabulary of White-lored Oriole in which the commonest two or three note calls are longer and flutier. The typical call of the closely related Philippine Oriole Oriolus xanthonotus , which is absent from Luzon, is a slightly longer, higher-pitched and flutier whistle than Isabela Oriole, and rises notice- ably towards the end.

Ambabok was revisited during 1 -4 April 2004 by MvW. A pair of Isabela Orioles, probably the same as those seen in 2003, was observed in exactly the same area. One individual responded strongly to whistled imitations of the calls described above. On 3 April it was captured in a mist-net. The bird was measured, documented (Plates 1 -3)and released. The description in Kennedy et al. (2000)is accurate with the exception of the colouring of upperparts and underparts;we suggest describing the upperparts as dark olive-yellow (yellow-olive in Kennedy et al. )and the underparts as bright yellow with an olive wash (olive-yellow in Kennedy et al. ). The contrast between upperparts and underparts is rather strong. The main field characters distinguishing the species from the similar White-lored Oriole are the stout greyish bill and the lack of whitish lores. The pair was observed regularly, sometimes also with Black-naped Oriole in addition to the flock- species observed in 2003. On four occasions Isabela Orioles were observed eating caterpillars.

CONSERVATION

Ambabok (17 o 01 ’26 ”N 122 o 10 ’44 ”E) is the location of a former small settlement on the edge of the Sierra Madre mountain range on the banks of the Catalangan River. The village was deserted in 1994 and open grassy fields and small patches of degraded forest and bamboo remain. The elevation is around 200 m. The large open area with forest patches is bordered by selectively logged forest which gradually becomes less disturbed eastwards towards the mountains. Administratively Ambabok is part of barangay Dibuluan in the municipality of San Mariano, Isabela province. Ambabok is situated just inside the Northern Sierra Madre Natural Park (NSMNP), the largest of ten priority protected areas in the Philippines under the National Integrated Protected Area System (NIPAS). Although protected on paper, law enforcement in the Ambabok area (on the western side of the NSMNP)is generally lacking and the lowland forest on this side of the park is under heavy pressure from illegal small-scale logging. During the May 2003 and April 2004 visits, a large number of loggers were active in Ambabok, which is used as a campsite, and in the surrounding forest. Selective logging for the larger and most valuable timber trees has now disturbed all remaining lowland forest on this side of the Sierra Madre. Several internationally funded conservation projects are active in the area. A new project funded by the Netherlands and implemented by WWF Philippines intends to address the illegal logging in the park through a combination of community development, awareness-raising and strengthening of law enforcement.

The Isabela Oriole thus still occurs in Isabela province in the municipality (San Mariano) where the holotype was collected in 1894 (Dickinson et al . 1991) and near Disulap (8 km south-west of Ambabok) where 11 specimens were collected in 1961 when the species apparently was still quite common (Collar et al. 1999). The recording and description of the call will perhaps lead to more observations of this species. However, the facts that (1)many observers have been looking for the species during recent years without success, including a DENR/BirdLife survey team in 1991 and 1992 (Danielsen et al . 1994, Poulsen 1995) plus many keen birdwatchers brought to the area by Tim Fisher's bird expeditions in the Philippines, and (2)the first author did not observe any other Isabela Orioles during three years of extensive fieldwork in the area, indicate that the species is probably genuinely rare rather than merely cryptic.

The reasons for the apparent rarity and patchy distribution of the species are unclear, but the known elevation range is 50 -440 m (Collar et al . 1999), indicating that it could be a lowland forest specialist.

However, all observations in which the habitat has been recorded are from secondary forest, suggesting that this is the species °Øs preferred habitat or that it may be rather adaptable to habitat change. The Isabela Oriole is apparently not now hunted or collected, so the only plausible causes of its apparent decline are habitat loss and/or competition with the closely related White-lored Oriole. Competition and replacement by related species following habitat disturbance is occurring with the Green Racquet-tail Prioniturus luconensis , a Luzon endemic which is being replaced by Blue-crowned Racquet-tail P. discurus in southern Luzon (Collar et al. 1999). Effective habitat conservation is necessary and could become a reality in the near future if conservation initiatives in the NSMNP yield success. The observation of the Isabela Oriole at Mansarong in 1994 (Van der Linde 1995) was not in the NSMNP (contra Collar et al. 1999)and the Mansarong area is not officially protected.

The species has not been observed in Bataan since 1947 (Collar et al . 1999) and in fact only two pairs and one individual have been observed since 1961:the pair described here, a pair in Mansarong in 1994 (Van der Linde 1995) and an individual in Quirino province in 1993 (Gamauf and Tebbich 1995), with the last two observations not considered absolutely certain (Collar 1998). It seems to be defen- sible and suitably precautionary to assume that not more than 250 mature Isabela Orioles survive in the wild. Given the continued destruction and fragmentation of Luzon's lowland forest, the population is likely to be declining, and each subpopulation may number less than 50 individuals. Thus it would seem appropriate to elevate this species to the rank of Critically Endangered, under criterion C2a(i)of the IUCN Red List (i. e. fewer than 250 mature individuals, all subpopulations numbering fewer than 50 individuals and a continuing population decline).

ACKNOWLEDGEMENTS

MvW would like to thank Dominic Rodriguez and Jessie Guerrero who participated in fieldwork in Ambabok in 2003 and Wouter Thijs, Marijn Prins, Edmund Jose and Rio Vinoya who were part of the team that captured the Isabela Oriole in 2004. RH was accompanied by Roldan Dugay during his visit in March 2003. Nigel Collar made substantial comments to an earlier version of the paper.

REFERENCES

BirdLife International (2000)Threatened birds of the world . BirdLife International. Barcelona and Cambridge, U.K. :Lynx
        Edicions and BirdLife International.
BirdLife International (2001)Threatened birds of Asia . Cambridge, U.K.:BirdLife International.
Collar, N.J. (1998)Bill morphology and the identification of Isabela Oriole Oriolus isabellae . Forktail 14:82 -84.
Collar, N.J., Mallari, N.A.D. and Tabaranza Jr., B.R. (1999) Threatened birds of the Philippines. Makati City:Bookmark.
Danielsen, F., Balete, D.S., Christensen, T.D., Heegaard, M., Jacobsen, O.F., Jensen, A., Luns, T. and Poulsen,         M.K.(1994) Conservation of biological diversity in the Sierra Madre mountains of Isabela and southern Cagayan         Province, the Philippines. Manila and Copenhagen:DENR, BirdLife International and DOF.
Gamauf, A. and Tebbich, S. (1995)Re-discovery of the Isabela Oriole Oriolus isabellae. Forktail 11:170 -171.
IUCN (2001)The IUCN Red list of threatened species:2001 Categories and Criteria (v.3.1)Cambridge, U.K.:IUCN/SSC          Red List Programme.
Kennedy, R. S. , Gonzales, P.C., Dickinson, E.C., Miranda Jr., H.C. , and Fisher, T.H. (2000)A guide to the birds of the          Philippines. Oxford:Oxford University Press.
Poulsen, M. K. (1995)The threatened and near-threatened birds of Luzon, Philippines, and the role of the Sierra Madre          mountains in their conservation. Bird Conserv. Internat. 5:79 -115.
Van der Linde, M. (1995)A further record of the Isabela Oriole Oriolus isabellae from Baggao, Cagayan Province,          northern Philippines. Forktail 11:171.

Merlijn van Weerd, Centre of Environmental Science (CML), Leiden University, PO Box 9518, 2300 Leiden, the            Netherlands. E-mail: vanweerd@cml.leidenuniv.nl
Rob Hutchinson, 26 Sutton Avenue, Chellaston, Derby DE73 1RJ, U.K. E-mail:rob_Hutchinson@btopenworld.com