The status and distribution of Changeable Hawk-eagle (Nisaetus cirrhatus) in Singapore K. H. Tan Blk 264C Compassvale Bow #05-58 Singapore 543264 (Corresponding author email: kokhuitan@yahoo.co.uk) Abstract The Changeable Hawk-eagle is the largest forest raptor in Singapore and the subspecies found here is the only one that is polymorphic. Based on active nest sites located, the total breeding population is estimated to be 9 pairs. Most of the nests located are on Albizia trees in secondary woodlands, and hence for the continued survival of this species in Singapore, secondary woodlands in urban areas have to be conserved. Introduction Raptors in Singapore are generally not well studied. The Changeable Hawk-eagle, a nationally threatened raptor here, is presently listed as an uncommon resident breeder 1. Being at the top of the food chain, it is an indicator of our ecosystem health, both in terms of prey density and habitat suitability. It is the largest forest raptor in Singapore but not much is known about its breeding population. The Changeable Hawk-eagle used to be known as Spizaetus cirrhatus but is now known as Nisaetus cirrhatus 2. The genus Spizaetus is now used for some New World hawk-eagles. The Old World hawkeagles are now placed in the genus Nisaetus 3. There are six subspecies of Changeable Hawk-eagle 4 (some treat N. c. floris as a full species 5 ). The one found in Singapore, N. c. limnaeetus, is the widest ranging of them all, namely from North India and Nepal through Burma, West and South Indochina, and Malay Peninsula to Greater Sundas and West and Southeast Philippines 6. It is the only subspecies known to be polymorphic, exhibiting both pale and dark morphs. The other subspecies are not known to have a dark morph. Past records Prior to 1927, there has been no Changeable Hawk-eagle nesting record in the whole of Peninsula Malaysia, including Singapore. However, the species is widely distributed throughout Peninsula Malaysia and also occasionally on the larger islands. Although it frequently occurred in forests and forest edges, it was also found in open country, where it was known to rob poultry 7. By 1992, the resident population in Singapore had been estimated at 12 birds based on sightings at various locations, namely Bukit Timah Nature Reserve, Central Catchment Nature Reserve, Pulau Tekong and Sembawang. It was feared that only the birds in the central forests were doing well and that the clearing of cultivated plantations would adversely affect the species 8. By 1999, there were only a few pairs persisting in Singapore 9. However, there was still no confirmed nesting record in Singapore. It was listed at a rare resident (breeding not proven) 10. From the 21 st century onwards, sightings of this raptor have increased and the first nesting record was finally observed in Singapore. Based on the Annual Bird Census over 10 years from 1996 to 2005, the Changeable Hawk-eagle sightings exhibit an increasing trend 11.
Methodology The present study seeks to estimate the Changeable Hawk-eagle breeding population in Singapore. The methodology is to locate active nest sites in a single season and estimate the total population based on population density and suitable habitats. At certain sites, no nest was located, but pairing behaviour was observed. These would be taken into consideration in the estimation, provided they are sufficiently far from active nest sites and there is a suitable habitat area. The nest consists of sticks pile up on the fork of a tree, typically Albizia. Albizia is among the tallest trees that are commonly found in secondary woodlands. This would explain why most of the nests located were on Albizia. Figure 1: Nest consisting of sticks on the fork of an Albizia tree at Neo Tiew Lane 2. Figure 2: An pale morph adult in nest at Mandai West Camp on 22 Apr 2011. An dark morph adult laid low in its nest at Mt. faber on 23 Jan 2011, possibly incubating.
Results Figure 3 shows all the observations recorded from Aug 2010 to Jul 2011. Figure 3: Location of sightings. Red: active nest sites. Yellow: pairing behaviour noted. Purple: no activity detected at nest. Green: Immature sighted. Blue: sightings without indication of nesting. Six active nest sites were located (refer to Table 1). Most of them were in the north-western part of Singapore Poyan, Neo Tiew, Mandai West Camp and Woodlands. There was one in the south (Mt. Faber) and one in the east (Changi). Besides these active nest sites, pairing behaviour was also noted at three other sites: Sarimbun (per comms. Yong D. L.), Seletar Camp (per comms. Serene Chng) and Singapore Quarry (per obs.). There were also a few nests which seem to be inactive (refer to Table 2). There was one each at Pasir Ris, Khatib, Dairy Farm and Temenggong Road, as well as two each at Woodlands and Bt Batok. Table 1: Active nest sites Site Remarks Reported by Changi Cove Nest on unidentified tree Yong D. L. Mandai West Camp 1 pale and 1 dark in nest on Albizia Con Foley, Danny Lau, Tan K. H. Mt. Faber 1 pale (the other unknown) in nest on Tan K. H., Yong D. L. unidentified tree Neo Tiew Lane 2 1 pale (the other unknown) on Albizia Tan K. H. Poyan 1 pale and 1 dark on Albizia Con Foley, Danny Lau, Tan K. H. Woodlands Road Adult not seen, chick in nest on Albizia Yong D. L. Table 2: Nest sites with no nesting activity observed
Site Remarks Reported by Bt. Batok St 21 Was active last season, but not this season William IP Bt. Batok Town Park Nest looked degraded Tan K. H. Dairy Farm Nest on Albizia Tay S. L. Khatib Bongsu Nest on Albizia, 1 heard calling nearby Tan K. H. Pasir Ris Farmway 1 Nest on Albizia, 1 dark morph seen perched elsewhere Tan K. H. Temenggong Road Nest looked degraded Danny Lau Woodlands Drive 91 Nest on Albizia, 1 heard calling nearby Tan K. H. Woodlands Town Garden Nest on Albizia, 1 seen perched nearby Lau J. S. Discussion Based on active nest sites (6) and pairing behaviour (3), we estimate that there are 9 breeding pairs of Changeable Hawk-eagle in Singapore. This estimate is based on the closest distance between active nest sites being 3.72 km (that between Woodlands and Mandai West Camp) and the availability of suitable habitats at the respective sites where pairing behaviour was observed: Western Catchment, Dairy Farm and Seletar Camp. There could be other breeding pairs which had gone undetected, particularly in the Central Catchment and protected areas (e.g. Khatib Bongsu, Simpang, behind Pasir Ris Farmway 1). Future studies could be conducted to cover more extensive grounds by getting permit to survey protected areas and having more manpower to monitor various potential sites. Offshore islands are also not covered in this project. The north-western part of Singapore seems to be the main breeding ground for this species. Main reasons could be the proximity to Malaysia, availability of habitat, suitability of nesting trees (particularly Albizia) and a viable population of prey items. Changeable Hawk-eagles are known to prey on Changeable Lizards (per obs.) and mammals (e.g. rat 11, squirrel 12, colugo 13 and long-tailed macaque 14 ). The population of prey items seem to be able to support the present number of Changeable Hawk-eagles. Further studies could be conducted to assess whether the present population of prey items could sustain the population of Changeable Hawk-eagles, as well as what the other prey items are. Conclusion The Changeable Hawk-eagle, although listed as an uncommon resident breeder in Singapore, is getting more common with an estimated 9 breeding pairs presently. Its adaptability to our urban landscape is the key to its success. However, being an apex predator, its number will never be high on such a small island as Singapore. As such, the conservation of the secondary woodlands all over Singapore is imperative for the continued survival of this species. Acknowledgements I would like to show my appreciation to Yong Dong Li for getting me to start on this project, as well as The Nature Society (Singapore) for funding it. I would also like to thank my birding pals, Con Foley, Danny Lau and Lau Jia Sheng for the company throughout the project. Last but not least, thank you to the following people for reporting their sightings: Alan Owyong, Alvin, Arjun Krishnan,
Bernard Bok, Clement Ong, Eugene Wong Hwee Lee, Jim Yeo Gim Seng, Lai Yeu Huan, Lee Mee Yee, Nancy Loh, Serene Chng, Tay Soon Lian, Thomas, William IP. References 1. Lim, K.S., 2007. Pocket checklist of the birds of the Republic of Singapore (Second Edition). Bird Group Records Committee, Nature Society (Singapore), Singapore. 2. Gjershaug, J. O., 2006. Taxonomy and conservation status of hawk-eagles (Genus Nisaetus) in South-east Asia. Ph.D. dissertation, Norwegian University of Science and Technology, Trondheim, Norway. 3. Gamauf, A., J.O. Gjershaug, K. Kvaløy, N. Røv, and E. Haring. 2005. Molecular phylogeny of the hawk-eagles (genus Spizaetus). Zoologische Mededelingen Leiden 79-3(21):179-180 4. Gamauf, A.; Gjershaug, J. O.; Røv, N.; Kvaløy, K. & Haring, E. (2005): Species or subspecies? The dilemma of taxonomic ranking of some South-East Asian hawk-eagles (genus Spizaetus). Bird Conservation International 15(1): 99 117. 5. Gjershaug, J. O., Kvaløy, K., Røv, N., Parry, S. J., Prawiradilaga, D. M., Suparman, U. and Rahman, Z. (2004). The taxonomic status of Flores Hawk-eagle Spizaetus floris (Hartert, 1898). Forktail 20: 55 62. 6. Del-Hoyo, J., Elliot, A., Sargatal, J., 1994. Handbook of the birds of the world, Vol. 2 New World vultures to guinea fowl. Spain Barcelona: Lynx Editions. p. 203. 7. Robinson, H. C. (1927). The Birds of the Malay Peninsula. Vol. I: The Commoner Birds. Witherby: London. pp. 69-71. 8. Lim, K.S. (1992). Vanishing birds of Singapore. Nature Society (Singapore), Singapore. pp. 25-26. 9. Wells, D. R. 1999. The birds of the Thai-Malay Peninsula. Volume One: Non-passerines. Academic Press. London. 10. Lim, K.S., 1999. Pocket checklist of the birds of the Republic of Singapore. Bird Group Records Committee, Nature Society (Singapore), Singapore. 11. Lim, K.C. & Lim, K.S. (2009). State of Singapore s Wild Birds and Bird Habitats: A Review of the Annual Bird Census, 1996-2005. Nature Society (Singapore), Singapore. 12. Lim, B. K., 2009. Changeable Hawk-eagle catches a rat. BESGroup.com (Accessed by http://besgroup.talfrynature.com/2009/05/25/changeable-hawk-eagle-catches-a-rat/) 13. Tsang, K. C., 2008. Three raptors in one morning. BESGroup.com (Accessed by http://besgroup.talfrynature.com/2008/02/04/three-raptors-in-one-morning/)
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