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Isabela Oriole Oriolus isabellae
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Justification
Recent records of this species suggest that it is likely to have an extremely small and fragmented population which is rapidly declining owing to extensive habitat loss. It is therefore listed as Critically Endangered.

Taxonomic source(s)
Sibley, C. G.; Monroe, B. L. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, USA.

Identification
20 cm. Medium-sized, green and yellow oriole. Olive-green head and upperparts, olive-brown wings, yellow underparts. Large, heavy dark grey bill, dark red iris, dark grey legs. Similar spp. White-lored Oriole O. albilorus has white lores, shorter tail and shorter, more slender dark red bill. Voice Three calls have been recorded: a lsightly descending whistle lasting 0.5 s and repeated at intervals of 1-2 s; a higher, rising whistle repeated at similar intervals, and a harsh rolling call, not unlike a cricket repeated at 2 calls/s in a long series (van Weerd and Hutchinson 2004).

Distribution and population
Oriolus isabellae is endemic to Luzon in the Philippines (Collar et al. 1999). It is known from three localities in Bataan province and five in the north-east of the island. Eleven birds were collected near San Mariano in the Sierra Madre mountains in 11 days in 1961, indicating that it may not have been particularly rare in suitable habitat. However, in the north-east it has been recorded recently at just three localities (in Quirino province and Cagayan province, and in 2003-2006 and February 2012 at Ambabok and nearby Dunoy, San Mariano municipality, Isabela province [van Weerd and Hutchinson 2004, M. van Weerd in litt. 2007, 2012]), despite an increasing number of fieldworkers attempting to locate it. There have been no observations in Bataan since 1947 and surveys in relatively undisturbed forest along the eastern coast of the Sierra Madre in 2006 failed to locate the species (M. van Weerd in litt. 2007). This indicates considerable rarity and that its distribution is probably patchy.

Population justification
The population is precautionarily estimated to number fewer than 250 mature individuals, and so it is placed in the band 50-249 mature individuals. This equates to 75-374 individuals in total, rounded here to 70-400 individuals.

Trend justification
The species is suspected to be declining at an unquantified rate owing to the extensive loss of forest habitat on Luzon.

Ecology
It frequents the canopy and middle storeys of forests, especially thick bamboo forest, but also forest edge, from 50-440 m. Constraints on its distribution and relative abundance are unknown, but it appears to tolerate secondary growth. Two of the localities that have produced recent records (Ambabok and Dunoy) are characterised by degraded and secondary forest (M. van Weerd in litt. 2012). It was recently observed in mixed-species flocks in fruiting trees.


Threats
It may be a lowland specialist and lowland forest destruction is assumed to be its primary threat. Since the 1930s, forest cover in the Sierra Madre has declined by 83% and by the late 1980s only 24% of Luzon was estimated to remain forested, with most remaining areas under logging concession. There is virtually no forest left near Disulap, a key historical site, and one of the recent records was from a degraded forest tract of only 100 km2, isolated from the Sierra Madre forests and offering no long-term prospects for the species. Illegal logging and agricultural encroachment in the western part of the Northern Sierra Madre Natural Park (NSMNP) threatens remaining habitat, and habitat protection and law enforcement are very weak in San Mariano municipality  (M. van Weerd in litt. 2012). Competition with the closely related White-lored Oriole O. albiloris may impact this species (van Weerd and Hutchinson 2004).


Conservation Actions Underway
Ambabok, where the species was recorded in 2003, 2004 and 2012, is located within the NSMNP, although there is no active law enforcement in this area. The Bataan Natural Park/Subic Bay protected area, which supports up to 50 km2 of forest, probably encompasses one of the historical localities. Exploratory surveys in 2007 of the eastern side of the NSMNP upstream of the Palanan River and along the Pacific coast in Palanan, Divilacan and Maconacon, at 0-1,000 m, failed to find the species (M. van Weerd in litt. 2012). In early 2012, a new survey project (by students at the University of the Philippines, Isabela State University and the Mabuwaya Foundation) was planned for 2012 and 2012, with the aim of visiting all historical locations for the species in the Sierra Madre and Bataan, and to survey suitable habitat in San Mariano municipality in the Sierra Madre. This project is to include an awareness-raising campaign targeting government officials, park management authorities and local communities in the Sierra Madre, with information about the species and its status. The project is expected to produce a conservation strategy for the species, to be implemented by national and local government departments (M. van Weerd in litt. 2012).
Conservation Actions Proposed
Conduct extensive surveys, particularly in bamboo forests, around historical sites in Bataan, the Mariveles Mountains and Mt Cetaceo, around Mansarong, and also Diffun (Quirino Province) where it has recently been reported. Propose key sites found for formal protection. Lobby for active on-the-ground protection of the NSMNP. Raise awareness among local people and local government about the species in an effort to implement conservation measures.

References
Collar, N. J.; Mallari, N. A. D.; Tabaranza, B. R. J. 1999. Threatened birds of the Philippines: the Haribon Foundation/BirdLife International Red Data Book. Bookmark, Makati City.

Van Weerd, M.; Hutchinson, R. 2004. Observations of Isabela Oriole Oriolus isabellae in the Sierra Madre, Luzon, Philippines, with descriptions of the call. Forktail 20: 133-136.

Further web sources of information
Detailed species accounts from the Threatened birds of Asia: the BirdLife International Red Data Book (BirdLife International 2001).

Explore HBW Alive for further information on this species

Search for photos and videos, and hear sounds of this species from the Internet Bird Collection

Text account compilers
Benstead, P., Bird, J., Davidson, P., Harding, M., Peet, N., Symes, A., Taylor, J.

Contributors
van Weerd, M.

IUCN Red List evaluators
Butchart, S., Symes, A.

Recommended citation
BirdLife International (2014) Species factsheet: Oriolus isabellae. Downloaded from http://www.birdlife.org on 22/08/2014. Recommended citation for factsheets for more than one species: BirdLife International (2014) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 22/08/2014.

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

To provide new information to update this factsheet or to correct any errors, please email BirdLife

To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums.

Key facts
Current IUCN Red List category Critically Endangered
Family Oriolidae (Orioles and figbirds)
Species name author Ogilvie-Grant, 1894
Population size 50-249 mature individuals
Population trend Decreasing
Distribution size (breeding/resident) 90,000 km2
Country endemic? Yes
Links to further information
- Additional Information on this species