A survey on mass movements of the vulnerable plain-pouched hornbill in the Belum-Temengor forest complex, Peninsular Malaysia

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THE RAFFLES BULLETIN OF ZOOLOGY 2011
A SURVEY ON MASS MOVEMENTS OF THE VULNERABLE PLAIN-POUCHED
HORNBILL IN THE BELUM-TEMENGOR FOREST COMPLEX, PENINSULAR MALAYSIA
Ravinder Kaur
Malaysian Nature Society, JKR 641, Jalan Kelantan Bukit Persekutuan, 50480 Kuala Lumpur
Teresa Ong
Malaysian Nature Society, JKR 641, Jalan Kelantan Bukit Persekutuan, 50480 Kuala Lumpur
Kim Chye Lim
Malaysian Nature Society, JKR 641, Jalan Kelantan Bukit Persekutuan, 50480 Kuala Lumpur
Chin Aik Yeap
Malaysian Nature Society, JKR 641, Jalan Kelantan Bukit Persekutuan, 50480 Kuala Lumpur
E-mail: hod.conservation@mns.org.my
ABSTRACT. – Annually, hundreds of Plain-pouched Hornbills, Aceros subrufi collis, are seen in the
Temengor Forest Reserve between the months of August and September. The Malaysian Nature Society
(MNS), an environmental non-government organization (NGO), conducted a 64-day survey with assistance
from volunteers to count the daily numbers of A. subrufi collis present at dawn and dusk during their mass
movements over Pos Chiong. The Plain-pouched Hornbill fl ocks were observed heading north-east during
the dawn surveys and south-west during the dusk surveys. The highest number of A. subrufi collis counted
during the survey was 3,261 individuals whilst the lowest number was 595 individuals in a single count.
This paper was presented at the 5th International Hornbill Conference jointly organised by the National
Parks Board (Singapore) and the Hornbill Research Foundation (Thailand), in Singapore on 22nd–25th March
2009.
KEY WORDS. – Plain-pouched Hornbill, Aceros subrufi collis, Malaysia, conservation.
THE RAFFLES BULLETIN OF ZOOLOGY 2011 Supplement No. 24: 171–176
Date of Publication: 30 Mar.2011
© National University of Singapore
INTRODUCTION
The Belum-Temengor Forest Complex, which is located in the
northern part of Peninsular Malaysia, consists mainly of the
Royal Belum State Park (117,500 ha) and Temengor Forest
Reserve (148,670 ha). It is the second largest contiguous
partially protected rain forest in Peninsular Malaysia and is
linked to two protected areas in southern Thailand, the Hala-
Bala Wildlife Sanctuary and Bang Lang National Park. Within
this forest complex exists the Temengor Lake (172 km2), a
consequence of the 1970’s damming of several rivers for the
purposes of irrigation, water catchment and hydro-electric
power generation (Yeap et al., 2005). This forest complex
is also one of Malaysia’s Important Bird Areas (Yeap et al.,
2005). The forest complex is also home to local indigenous
people of the Jahai and Temiar groups.
The Malaysian Nature Society (MNS) organized two
scientifi c expeditions into the forest complex in 1994 and
1998 to record its abundant biological diversity, and proposed
for it to be gazetted as a protected area. Subsequently, the
Forestry Department Peninsular Malaysia (FDPM) organized
a third expedition in 2003, which further consolidated earlier
fi ndings. These expeditions revealed that within this forest
complex, there were healthy populations of large mammals
such as Asian elephants (Elephas maximus), tigers (Panthera
tigris), Malayan tapir (Tapirus indicus), gibbons (Hylobates
spp.) and leaf-monkeys (Presbytis spp.), birds (e.g., pheasants,
hornbills, trogons, broadbills, etc.), reptiles and amphibians
and many others. In addition to that, plants were also found
to be highly diverse and some are endemic to the forest
complex.
Ten species of hornbills occur in the Belum-Temengor
Forest Complex, six of which are globally near-threatened,
namely the Rhinoceros Hornbill (Buceros rhinoceros), Great
Hornbill (Buceros bicornis), Helmeted Hornbill (Rhinoplax
vigil), White-crowned Hornbill (Berenicornis comatus),
Wrinkled Hornbill (Aceros corrugatus) and Black Hornbill
(Anthracoceros malayanus). Three other species, the Oriental

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Kaur et al.: Mass movements of Plain-pouched Hornbill
Pied Hornbill (Anthracoceros albirostris), Bushy-crested
Hornbill (Anorrhinus galeritus) and Wreathed Hornbill
(Aceros undulatus) are of least concern whereas the Plain-
pouched (or Tenasserim) Hornbill (Aceros subrufi collis) is
globally vulnerable (Birdlife International, 2001).
Through these expeditions, MNS had unearthed the
astonishing movements of thousands of Plain-pouched
Hornbills. The species is very similar to the Wreathed
Hornbill but it is smaller, has a shorter bill with brownish
base and lacking corrugations, and has a black streak on the
gular pouch (Robson, 2000).
Mainly seen during their dawn and evening fl ights, these
hornbills moved in continuous ‘waves’ that generally lasted
between one and a half to two hours. At the time of the
MNS expeditions, the hornbill population was estimated to
be about 2,500 individuals.
Thus, recognizing the importance and value of Belum-
Temengor, MNS embarked on a long-term conservation
programme using hornbills as a fl agship species in 2004.
Through this program, the Society is involved in hornbill
research, education and awareness activities, policy and
lobbying activities for greater, more comprehensive
protection for the forest complex.
In 2008, seeing that a concerted effort to monitor and
document the large number of individuals in the fl ocks was
needed, MNS initiated a pilot volunteer programme spanning
from 1st August to 26th September 2008. A total of 34 people
volunteered to participate in this programme under the
supervision of four experienced MNS Coordinators.
METHODS
Observers were positioned at strategic locations in Pos
Chiong, Temengor, an area that has been identifi ed as the
Fig. 1. Survey locations within the Belum-Temengor forest complex.
The Thai-Malaysia boundary is represented with the thick yellow
line. The thinner yellow line represents Important Bird Area
boundaries. The red points represent surveyed areas.
habitual fl ight path of the Plain-pouched Hornbills. Surveys
were conducted twice a day throughout the months of
August and September 2008, in the morning from 0700 to
0900 hrs followed by the evening survey at 1730 to 1930
hrs, to record the number of individuals seen. Additional
information such as the fl ight direction, sex, and weather
were documented whenever possible. Field equipment such
as binoculars (Bushnell 8 x 42 Field 6.0º 105m/1000m)
tally counters and spotting scopes (Leica APO–TELEVID
77, T77: 20x–60x, T62: 16x–48x) were used to assist in
counting, species identifi cation and sexing of individuals.
As a guiding tool, to ensure the observers recorded all the
crucial information as stated above, a standard data sheet
was distributed to each observer.
RESULTS
The survey was conducted over a span of 64 days. During
that time, observers experienced 11 days of poor weather.
At the beginning of the survey in August, the observers
experienced poor visibility due to haze.
Based on Fig. 2, in the month of August the morning counts
ranged between 731 individuals (on 7th August) and 2844
individuals (on 29th August). In the month of September,
the morning counts ranged from 3261 individuals (on
14th September, the highest number documented during
any morning survey) and 1,520 individuals (on 20th
September).
Based on Fig. 3, in the month of August the evening counts
ranged from 1,135 individuals (on 18th August) to 2,383
individuals (on 22nd August). In the month of September,
the evening counts ranged from 529 individuals (on 4th
September) to 2681 individuals (on 14th September, the
highest number documented during any evening survey.
Thus, the number of individuals seemed to build up steadily
in August and reached a peak in the middle of September.
Then there was a gradual decrease in numbers towards the
end of September.
Generally, the fl ocks were observed to head north-east during
the morning surveys and south-west during the evening
surveys. The fl ocks were seen moving in a continuous stream
Fig. 2. Morning fl ight surveys of the A. subrufi colollis fl ocks in Pos
Chiong, Temengor during the months of August and September
2008.

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THE RAFFLES BULLETIN OF ZOOLOGY 2011
Fig. 3. Evening fl ight surveys of the A. subrufi colollis fl ocks in Pos
Chiong, Temengor during the months of August and September
2008.
with different fl ight formations: ‘V’ formation (Fig. 4), single
line formation (Fig. 5) and dispersed (Fig. 6). At times fl ocks
fl ew directly over observers, seemingly undisturbed by their
presence. During the survey, observers noted that the calls
used were either two short notes ‘ehk-ehk’ or three short notes
‘ehk-ehk-ehk’, and this may be important in distinguishing
them from Wreathed Hornbills.
Eight other species of hornbills were sighted and/or heard
during the 64 days, including all those known to be present
in Belum-Temengor (and in Peninsular Malaysia as a whole)
except for the Wrinkled Hornbill.
Several diffi culties were faced by the observers such as
poor visibility due to the appearance of haze, an occurrence
present during the beginning of August. Many fl ocks of
Plain-pouched Hornbills were not visible and thus not
counted, resulting in very low counts. It was observed that
under such conditions the hornbills tended to fl y low under
the mist and haze, closer to the tree line and through valleys,
more distant birds thus fl ying out of sight. Surveys conducted
during clear and sunny weather on the other hand, revealed
them to fl y higher than usual.
A few volunteers were inexperienced in handling equipment
such as binoculars and spotting scopes. This sometimes
Fig. 4. Plain-pouched Horbills in a ‘V’ shape formation.
interfered with their ability to count the hornbill fl ocks. Thus,
the MNS coordinator would handle the counts for the fi rst day
or two, while the volunteer observers became accustomed to
Fig. 5. Plain-pouched Hornbills in a line formation.
Fig. 6. Plain-pouched Hornbills in a dispersed formation.

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Kaur et al.: Mass movements of Plain-pouched Hornbill
the methods. To avoid disruption in the counts, in the event
a volunteer was unable to perform, the MNS coordinator
would replace the volunteer and continue the counts on the
volunteer’s behalf.
DISCUSSION
History. – The mass movements of hornbills of the genus
Aceros were first discovered in August 1992 when 300
individuals were seen at Tasik Kenering, during dawn
and evening flights (Ho & Sutari 1993). Prior to 1999,
the fl ocks were generally accepted as Wreathed Hornbills
(A. undulatus), a common species in the Malay Peninsula
(Ho & Supari 2000). However, there were speculations
that the fl ocks may have been Plain-pouched Hornbills (A.
subrufi collis) based on their three-note calls (Davison et al.
1995); calls of the various species are described by Lekagul &
Round (1991). Then during the MNS’s Heritage and Scientifi c
Expedition to Belum in 1993–1994, 2421 hornbills in fl ight
were recorded followed by 1277 hornbills in 1994 (Yaacob,
1994). In 1998, during the MNS Belum Expedition Phase II
to an area further north, fl ocks of up to 233 hornbills were
observed (Lim, 2000).
Plain-pouched and Wreathed Hornbills are very similar
morphologically, and in the past the Plain-pouched has
sometimes been considered the juvenile plumage of the
Wreathed Hornbill (Rasmussen, 2000). The indigenous
people in Temengor, the Jahai, were unaware of the
differences between these two species and thus have only
one name in their native language to describe both species,
Sang Kor.
The Plain-pouched Hornbill lacks corrugations on its bill and
it does not possess a black bar on the gular pouch, unlike
the Wreathed Hornbill that possesses both these features
(Wells 1999). Even a nestling Wreathed Hornbill possesses
a black bar on its gular pouch (Frith & Douglas, 1978). The
Plain-pouched Hornbill is smaller, with longer and narrower
wings than the Wreathed (Kemp & Woodcock, 1995). The
Plain-pouched also possesses a more rufous head (Kemp,
1988). All these features could be confi rmed repeatedly in
the population in the Belum-Temengor area.
In 1999, species in these fl ocks were revealed by Ho Hua
Chew and Sutari Supari to be Plain-pouched Hornbills.
This record was then submitted to the Records Committee,
Bird Conservation Council, Malaysian Nature Society and
subsequently was accepted as the tenth species of hornbill
for Malaysia (Malaysian Nature Society, 2000).
Possible reasons for seasonal fl ocking. – There are three
kinds of hornbill fl ocks; ephemeral fl ocks, foraging fl ocks
and communal roosting fl ocks. Ephemeral fl ocks occur when
individuals congregate at a fruiting tree, though they may
arrive and leave separately. Foraging fl ocks occur when a
group of individuals continually travel together as a social
unit in search of food. The communal roosts occur when
foraging fl ocks convene in the evening to roost in a particular
area (Kinnaird et al., 2007). Such behaviour of foraging in
fl ocks and communal roosting are typical behaviour of the
Plain-pouched Hornbill. Oates in 1883 reported large fl ocks
of Plain-pouched Hornbills in their hundreds, seen in the
morning and evening, travelling long distances to feed and
roosting with Wreathed Hornbills in bamboos (Kemp &
Woodcock, 1995).
Mass movements of hornbills are not regarded as true
migration, as the proportion of individuals involved and
direction are not necessarily the same each year. Though
hornbills are omnivorous, these movements seemed to be
governed by fl uctuations in the availability of fruiting trees.
The Wreathed Hornbill for example has been reported to
cover an area spanning 100km² in search of food in a single
day. They have even been reported to fl y between islands
such as Java and Bali and to travel 30km across open country
(del Hoyo et al., 2001).
The Plain-pouched Hornbill flocks have been observed
yearly in high numbers in Temengor during the months of
August and September. This occurrence may be largely due
to the fact most wild fruit trees in Temengor from the family
Anacardiaceae, Bombacaceae, Euphorbiaceae, Meliaceae,
Moraceae, Rutaceae, Sapindaceae are fruiting during these
two months (A. Latiff et. al., 1995). Wreathed Hornbills
have been reported to be able to track short term peaks in
fruiting of lipid rich species in local habitats over a large area
(Leighton, 1982; Kinnaird et al., 1996). In Borneo, fl ocking
by Aceros hornbills was attributed to their specialised diet
of lipid rich fruits. Lipid rich fruiting trees are patchy in
space and time, therefore Aceros hornbills may track the
food resources over a greater area than do hornbills of other
genera (Leighton, 1982).
The Jahai people of Temengor have also commented that the
presence of the hornbill fl ocks coincides with the fruiting
season. However, the understanding of the fruiting phenology
of Belum-Temengor is currently incomplete, making clear
correlations difficult. The movements of the hornbills
coincides with the emergence of thousands of mayflies
(Ephemeroptera) from the Temengor lake, and the hornbills
have been seen engaged in aerial feeding on these newly
emerged mayfl ies by MNS observers (Yeap et al., 2005).
The arrival of large fl ocks of hornbills in August and September
each year, followed by their apparent disappearance from
the Belum-Temengor Forest Complex, may be due to their
breeding cycle. In Thailand this species has been reported
to nest between January and May; nine nests studied were
within this date range (Poonswad et al., 1998). Their nesting
time and ecology have yet to be documented in Malaysia
(Wells, 1999).
As for their behaviour of moving in large numbers, this
may prove benefi cial in terms of foraging effi ciency, as
individuals within the group are able to take advantage of
the whole group’s foraging ability and experience of recent
food sources (Ward & Zahavi, 1973).

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Fig. 7. Previous surveys on A. subrufi collis carried out from the
year 2004 to the year 2007 by Malayan Nature Society.
Possible Roosting Site. – Plain-pouched Hornbills are known
to prefer lowland river valleys in tall evergreen and mixed
deciduous hill forest (Kemp & Woodcock, 1995; Round,
1988; del Hoyo et al., 2001). These birds have been recorded
up to 1,000 m above sea level (Yeap et al., 2005).
In 1993, up to 2421 Aceros individuals were documented
during their evening fl ights fl ying towards the south and
south-west, along the Perak River (Davison et al., 1995).
In this study, it had been observed that the Plain-pouched
Hornbills tended to head north-east during the morning
surveys and south-west during the evening surveys. Based
on their consistent evening fl ight direction over the years
since 2004, the MNS team estimates that the roosting site
may be located south of Sungai Jut towards Gunung Ramin
(Yeap et al., 2005).
During this survey in 2008, a few hornbills were seen fl ying
in other directions, away from the large flocks. Closer
observation through the spotting scope revealed that the
individuals were in fact Wreathed rather than Plain-pouched
Hornbills.
Overview of Previous Surveys 2004–2007. – MNS has
previously conducted surveys from 2004 to 2007 to document
the number of Plain-pouched Hornbills to observe their fl ight
path and their general behaviour (Fig. 7). Throughout these
years, fl ocks occurred mainly at Pos Chiong, Temengor, and
were seen in large numbers during August and September.
The highest numbers of individuals during this period of
time were selected for comparison.
In 2004, the highest counts for morning surveys revealed 1,072
individuals, and for the evening surveys 989 individuals. In
2005, the highest counts for morning surveys revealed 191
individuals. In 2006, the highest counts for morning surveys
revealed 1,549 individuals, and for the evening surveys
1,555 individuals. In 2007, the highest counts for morning
surveys revealed 203 individuals, and the evening surveys
76 individuals.
The low numbers documented during 2005 and 2007 (Fig.
7) may have been due to disturbance to the forested area
around Pos Chiong. Disturbance in the form of land clearing
by indigenous people, and logging by commercial concerns,
may have infl uenced the hornbills into changing their fl ight
path, hence leading to a lower count than usual.
Recommendations. – Contrary to what had been observed by
Ho & Sutari (2000), the MNS coordinators did not fi nd the
Plain-pouched Hornbills wary of human presence. In fact,
fl ocks fl ew directly over the observers based in a village,
day after day without dispersing. Some individuals tilted
their heads to view the observers but continued to fl y over
the noisy village without showing any signs of fear. One
evening, the MNS coordinators were surprised to fi nd two
Helmeted Hornbills fl y in and perch on a tree very near to the
noisy village area. The behaviour of these hornbills seems to
indicate the lack of fear, suggesting that threats from human
beings such as hunting, only occur on a small scale.
Interviews with the Jahai revealed that the Jahai are
opportunistic people and will hunt and consume almost any
wild animal. There has also been the discovery of a partly
charred bill of a Plain-pouched Hornbill obtained from the
Jahai at Pos Chiong, strong evidence that hornbill hunting
exists (Ho & Supari, 2000).
The major threat faced by the hornbills in Temengor is
deforestation. Currently, the northern region of the Belum-
Temengor Forest Complex, the Royal Belum State Park is
a protected area however the southern region, Temengor
Forest Reserve, is mainly a production forest. Selective
logging removes large trees that are required for hornbill
nesting purposes (larger than one metre in diameter), hence
severely affecting the larger species of hornbills such as
Buceros and Aceros. As an example of how intolerant the
hornbills are to deforestation, Rasmussen’s visit to Toungoo
in the Sittang plain, Myanmar in year 2000, the species’
former stronghold, revealed no sightings of hornbills, after
conducting observations over several days. The original forest
cover had apparently been cleared for the purposes of rice
cultivation. (Birdlife International, 2001).
MNS strongly recommends that the Temengor section of
the forest complex is protected as this would then create a
single large trans-boundary protected area spanning southern
Thailand and northern Peninsular Malaysia. Many species
will benefi t greatly residing in a large contiguous rainforest
such as the Plain-pouched Hornbills which apparently depend
on such vast areas while foraging.
Lastly, the mass movements of Aceros in Temengor hold
a national and international importance, as they have been
considered unique to this area (Davison et al., 1995). It is
an awe-inspiring and an exciting phenomenon, hence it has
great potential to be marketed as a tourist attraction for the
purpose of monetary gain.
ACKNOWLEDGEMENTS
Thus, recognizing the importance and value of Belum-
Temengor, MNS embarked on a long-term conservation
programme using hornbills as a fl agship species in 2004.

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Kaur et al.: Mass movements of Plain-pouched Hornbill
Through this program, the Society is involved in hornbill
research, education and awareness activities, policy and
lobbying activities for greater, more comprehensive
protection for the forest complex. In 2008, seeing that a
concerted effort to monitor and document the large number
of individuals in the fl ocks was needed, MNS initiated a
pilot volunteer programme spanning from 1st August to
26th September 2008. A total of 34 people volunteered to
participate in this programme under the supervision of four
experienced MNS Coordinators.
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