Living on the Edge: The Straw-headed Bulbul in Pulau Ubin

Review contributed by Mr Budak


Straw-headed Bulbul. Picture by Sian, courtesy of Dr. Ho Hua Chew

Singapore tends to be seen as a place where wildlife is barely hanging on. Its native megafauna has long been extinguished, while a handful of endemics cling to the precipice of extinction in the island's fragile central reserves. So it might surprise some that a species of global conservation significance is actually finding sanctuary in Singapore, even as it faces extermination in neighbouring countries.

The creature in question is an unspectacular songbird called the straw-headed bulbul (Pycnonotus zeylanicus), which has found a safe haven in Singapore's wooded areas, in particular Pulau Ubin. Remarkably, there is no record of this species in Singapore prior to 1951, and even till to 1970s, the bulbul was not known to be common, even on Ubin. A bird survey in 1992 counted 50 birds on Ubin, which fell to 30 in 2000. However, the population rebounded to about 32 breeding pairs in 2001, whilst the mainland recorded a estimate of 76-93 birds.

Revealing these figures, Dr. Ho Hua Chew of the Nature Society of Singapore shared the background behind a recent field study he conducted on Pulau Ubin to analyse the habitat preferences and prevalence of the bulbul. The largest bulbul species in Southeast Asia, the straw-headed bulbul is a perky brown-bodied bird boasting a yellow crown, white throat and a black streak across its cheek.

Liquid gold
The bulbul's rich, melodious song, described as liquid gold, is more often heard than the bird itself, and has led to the species' disappearance from of its former range. Once found throughout the Sunda Shelf from Burma to Borneo, the bulbul is now believed to be extinct in peninsular Thailand and Java and near extinction in Sumatera. Veteran birder Ivan Polunin notes that suitable habitats in Southwestern Johor are also devoid of the bird. Habitat destruction such as clearing of secondary forests and mangroves is one reason for this fate, but the widespread practice of trapping songbirds for the pet trade is thought to be a significant factor in the bird's rarity, a fate shared by the once common white-rumped sharma. The bulbul is now classified under the CITES Division 2, which allows for trapping and trade of the species under specified permits and quotas.

A mosaic landscape
To obtain updated data on the bird's population and habitats on Ubin, Dr. Ho conducted extensive field work on the island in 2001-2002. The study sought to determine the extent of the species in Ubin and why the island is proving to be a stronghold for the bird. There is a "need to see how the species fits the island; it's preferred habitats, nest sites, habits etc..," Dr. Ho said at his talk this Wednesday evening at the NSS office in Geylang.

Firstly, it was necessary to develop a profile of Ubin's biophysical environment. The island sits between mainland Singapore and Southern Johor, and according to Dr. Ho, serves as a kind of stepping stone that allows species from Johor to disperse into Singapore.

Ubin spans some 102 hectares and a human population of about 500. The island serves as a refuge for many species once found on the mainland, including the wild boar, oriental pied hornbill, jungle fowl, buffy fish owl and white-rumped sharma. Smallholdings, rubber estates, coconut plantations and orchards used to dot the island, but many of these are now disused and dismissed to the care of mother nature, whose tendrils and creepers are reclaiming what was once her virgin territory.

Dr. Ho pointed out four primary threats to wildlife on Ubin. There is the encroachment of modernisation in the form of future high rise developments and resort centres (e.g. the Lagoon Resort). Land reclamation is also changing the landscape; for instance, a part of Pulau Ketam (an islet off Ubin) has been cleared of its original mangroves and turned into a landfill for no discernible purpose. Large camping grounds also play a role; Dr. Ho notes that the NPCC campground on the northeast of the island required the clearing of nearly 30 hectares of secondary forest. Finally, as visitors to the island and observers of its residents' backyards would know, there is poaching. In Singapore, however, the straw-headed bulbul is not a popular cagebird.

Dr. Ho divides Ubin into three broad biophysical sections. The island's narrow centre is dominated by mangroves, ponds and other wetland, serving as a sort of watery gulf between the extremities. The western side of the island has been largely given over to the Outward Bound School, for better or worse, and is covered by secondary forest (led by Adinandra belukar and Caryota palms), abandoned orchards and wasteland that is being recolonised by fast growing Acacia and Paraserianthes falcataria (aka Albizia) trees. The east end contains higher ground, secondary forest and old rubber plantations. A 'mosaic' landscape is how Dr. Ho terms the island's features. His survey estimates the following land composition: rubber plantations (32%), secondary forest (14%), mangroves (14%), orchards (12%), open ground (10%) and wet areas (17%).


Picture by Sian, courtesy of Dr. Ho Hua Chew

Bulbul ecology and observation
What we know about the straw-head bulbul is that the bird is an omnivore, feeding on fruits, berries, insects, spiders and other small animals. The bird is often seen in territorial pairs, and nests all year round, although February to April appear to be the main breeding months for the Singapore population. The nest, a cup-shaped mesh of leaves, plant fibres, roots and grasses, is built 1-5 metres above the ground and the normal clutch size is two. After the nesting period, the birds may move around in groups of 3-8 individuals.

Describing his methodology in surveying Ubin's habitats, Dr. Ho said he employed three ways to observe the birds: the archaic 'look-see method', territorial mapping of observed pairs and playback of recorded birdsong to register vocal responses. Several variables pertaining to the nature of the habitats surveyed were gathered and the data analysed by a logistical regression programme to ascertain correlations between the bulbul's presence and the characteristics of the habitat. The variables were collated from 30 randomly-selected 'presence' sites (locations where birds have been sighted) as well as 30 random 'absence' sites (which serve as a control).
The result allowed Dr. Ho to build a model that would help predict the probability of the bulbul's presence in a particular habitat, given knowledge of certain variables. After mapping out both presence and absence sites on a map, he determined that at least 32 clusters (one breeding pair each) existed on the island.

The study's rationale is based on the need to clarify the ecology of the bulbul in terms of its population density, breeding success and feeding patterns in carefully evaluated habitats. Information about where the birds feed and nest in relation to the island's habitats would hopefully aid in the design of reserves as well as habitat management and future efforts to secure viable populations of the species. Prior to Dr. Ho's study, there were no ecological studies on the bulbul's population in Ubin and Singapore. On the mainland, the bird is known to exist in scattered groups in Bukit Batok, Bukit Timah and the Botanic Gardens. Ubin was thus chosen for its biogeographic bridging role between Johor and Singapore as well as its relatively high concentration of bulbuls.

Birds on the edge
Dr. Ho observed that in rubber plantations, pairs were more dispersed, compared to birds living in secondary forest, which had more clustered populations. In addition, the birds tended to be found at the edge of habitats, rather than the centre. The birds were commonly seen at the edges of plantations and the fringes of mangrove swamps and secondary forests. In contrast to observations in other countries, the Ubin bulbuls preferred low ground, being absent from areas more than 20 metres above sea level.

The population and density of bulbuls on Ubin were recorded as follows:

Sector	Pairs	Hectare	Density (per Ha)
West	13	350	0.04
Centre	5	333.5	0.02
East	13	336.5	0.04
Total	32	1020	0.03

The specific habitats frequented by the birds were recorded.

Habitat Plantations Orchards Secondary Forest Mangroves

Number of Pairs 10 9 7 6

The variable data collected from the 30 presence and 30 absences sites for logistical regression of habitat preferences were:

• Altitude
• Nearest edge (of habitat)
• Tree density
• Tree richness
• Undergrowth density
• Nearest waterbody
• Nearest coast
• Nearest track/road
• Nearest building

The crunched numbers indicated that only four variables were correlated to the bulbul's presence: altitude, the nearest edge, tree density and tree richness. A mathematical model was formulated to predict the relative probability of the bulbul's presence given data on these four relevant variables. Dr. Ho noted that the birds seem to be fairly tolerant of human activity on Ubin, being indifferent to the existence of tracks, roads and buildings. Ubin's bulbuls were also indifferent to the proximity of waterbodies, in contrast to studies in Borneo which found the bird to be highly riverine, never straying more than 20 metres beyond a riverbank.

Altitude proves to be negatively correlated to the bulbul's presence, with a decreasing chance of sightings as one enters higher ground. This feature may be linked to the species' marked presence for edge habitats; habitat centres on the island also tend to be located on high ground.

Edges, however, appear to be the single most vital feature of bulbul habitats. Dr. Ho notes that the probability of sighting a bulbul rises strongly as one approaches the fringes of a habitat. So what matters is not just the habitat itself, but also the adjacent habitats with which it borders. For instance, a habitat that borders a mangrove swamp would yield a 33% chance of a sighting. Habitats fringed by open ground were also favourable spots (see table below).

Mangrove Open ground Waterbody Secondary forest Seacoast Rubber Plantation

33% 30% 10% 10% 10% 3.33%

Tree richness (i.e. the number of tree species in a habitat) is positively correlated with bulbul presence. Dr. Ho believes this stems from the greater availability of food sources in heterogeneous forests, in contrast to the uniformity of Acacia secondary woodland and rubber plantations. The Ubin bulbuls also prefer a lower tree density, contradicting earlier observations that the bulbul is an early successional species that colonises newly abandoned land.

Conservation strategies
Dr. Ho outlined a number of conservation strategies based on his study's conclusions. Firstly, it is important to preserve tree diversity as well as maintain a varied number of habitats. Thus, Ubin's existing mosaic of landscapes consisting of patches of woodland, orchards, mangroves, old plantations and open areas should be retained in preference to monocultured habitats. And rather than allowing nature to overwhelm the entire island and recreate an endless firmament of forests, the existing orchards and plantations should be managed to control tree density and preserve the borders between varied habitats. Such management would also have to take into account the preferences of other species which may prefer denser habitats, such as the jungle fowl and white-rumped sharma.

The birds' implied indifference to human activity, however, does not mean that the erection of high rise developments would necessarily prove insignificant to the welfare of the island's species. What remains as well is the question of whether the bulbuls in mainland Singapore share the preferences of their Ubin counterparts. Given the species' precarious situation in much of its former range, the maintenance of suitable habitats on Ubin and Singapore may well prove vital to the survival of the straw-headed bulbul.