The diet of Yellow-vented Bulbul (Pycnonotus goiavier) in oil palm agroecosystems

Abstract

The Yellow-vented Bulbul (Pycnonotus goiavier) is one of the most sighted birds in oil palm plantation. A study on their dietary habits was conducted at the Durafarm Oil Palm Plantation from February 2011 to May 2013 to determine the reason behind their abundance. In this study, 45 individuals of P. goiavier were dissected for stomach content analysis. Ivlev's electivity index (E) was used to measure the degree of food selection by P. goiavier. The results showed that this bird species mainly selected the Order Coleoptera (mostly pollinating weevil) (E= +0.97) and Homoptera (E= +0.87) as their main food sources in the oil palm plantation. This bird also selected Order Diptera (E= -0.30), Hemiptera (E= -0.43) and Hymenoptera (E= -0.92) as a prey based on their abundance. However, this species mainly avoided feeding on insects from Order Odonata, Orthoptera, Dictyoptera and Lepidoptera (E=-1.0 each) in oil palm plantation. There is a weak negative correlation found for the distribution of P. goiavier with the abundance of insects in oil palm plantation. Further investigation is needed on this bird species towards predating the oil palm pollinating weevil, since it could possibly affect the population density of the pollinating weevil and subsequently the oil palm fruit set.

Full text

417
Journal of Oil Palm Research Vol. 27 (4) December 2015 p. 417-424
ABSTRACT
The Yellow-vented Bulbul (Pycnonotus goiavier) is one of the most sighted birds in oil palm plantation.
A study on their dietary habits was conducted at the Durafarm Oil Palm Plantation from February 2011
to May 2013 to determine the reason behind their abundance. In this study, 45 individuals of P. goiavier
were dissected for stomach content analysis. Ivlev’s electivity index (E) was used to measure the degree of
food selection by P. goiavier. The results showed that this bird species mainly selected the Order Coleoptera
(mostly pollinating weevil) (E= +0.97) and Homoptera (E= +0.87) as their main food sources in the oil
palm plantation. This bird also selected Order Diptera (E= -0.30), Hemiptera (E= -0.43) and Hymenoptera
(E= -0.92) as a prey based on their abundance. However, this species mainly avoided feeding on insects
from Order Odonata, Orthoptera, Dictyoptera and Lepidoptera (E=-1.0 each) in oil palm plantation. There
is a weak negative correlation found for the distribution of P. goiavier with the abundance of insects in
oil palm plantation. Further investigation is needed on this bird species towards predating the oil palm
pollinating weevil, since it could possibly aect the population density of the pollinating weevil and
subsequently the oil palm fruit set.
Keywords: diet, Yellow-vented Bulbul, oil palm, Coleoptera.
Date received: 17 April 2015; Sent for revision: 21 April 2015; Received in nal form: 18 September 2015; Accepted: 25 September 2015.
THE DIET OF YELLOW-VENTED BULBUL
(Pycnonotus goiavier) IN OIL PALM AGRO-
ECOSYSTEMS
BETTYCOPA AMIT*; ANDREW ALEK TUEN**; KHALID HARON*; MOHD HANIFF HARUN*
and NORMAN KAMARUDIN*
* Malaysian Palm Oil Board,
6 Persiaran Institusi, Bandar Baru Bangi,
43000 Kajang, Selangor, Malaysia.
E-mail:bettycopa@mpob.gov.my
**
Institute of Biodiversity and Environmental Conservation,
Universiti Malaysia Sarawak,
94300 Kota Samarahan, Sarawak, Malaysia.
INTRODUCTION
The Yellow-vented Bulbul (Pycnonotus goiavier) is
the most common of all bulbuls and best-known
garden bird in Malaysia (Davison and Chew,
2008). This common resident bird can be found in
almost all habitats except the deep forest, starting
from mangrove to secondary forest and from rural
to urban areas (Wee, 2009). According to Tan and
Ria (2001) the success of P. goiavier in almost all
habitats is due to their diverse diet, consisting of
both plants and animals. In terms of food selection,
P. goiavier is considered a generalist where they feed
on owers, nectar, fruits, insects and even carrion
(Ward, 1969; Fishpool; Tobias, 2005; Wells, 2007),
foraging near ground level (Myers, 2009). P. goiavier
in oil palm plantations at Bah Jambi and Bukit
Maradja were reported to have parts of Lepidoptera,

Journal of oil Palm research 27 (4) (DecemBer 2015)
418
and Coleoptera (Curculionidae) and seeds in their
gut content (Chenon and Susanto, 2006). Now, they
have successful adapted themselves to become one
of the most abundant birds in cultivated areas such
as the oil palm plantation (Chenon and Susanto,
2006; Amit et al., 2011; Azman et al., 2011). This bird
species was also reported to contribute as a pest
control agent in the oil palm ecosystem (Chenon and
Susanto, 2006). The pollinating weevil (Elaeidobius
kamerunicus) was introduced into Malaysia to
overcome the inconsistencies of oil palm pollination
(Syed et al., 1982). The introduction of this species
increased pollination and fruit production from
20% to 30% (Syed, 1982; Basri et al., 1983). Now, this
seemingly exclusive diet of the bulbul towards the
pollinating weevil raises a new concern to the oil
palm planters; whether the abundance of this bird
species contributes to the low fruit set, as reported
by some oil palm plantations in peat areas.
The diet of birds has been studied through
examination of regurgitated samples (presumably
from the crop), stomach samples analysis and faecal
samples (Chapman and Roseberg, 1991; Major, 1990).
Theoretically, prey content in regurgitated samples
should be easier to identify because the samples
represent recent feeding and has not undergone
much digestion. This would be followed by stomach
samples which represent slightly older feeding and
last faecal samples which represent undigested
food. There are several methods to extract prey
content in birds such as through stomach ushing
method (Martin and Hockey, 1993; Chou et al., 1998),
non-lethal potassium antimony tartrate method
(Zduniak, 2005; Asokan et al., 2009), faecal analysis
method (Rodway and Cooke, 2002; Parrish 1994),
and dissection method (Chenon and Susanto, 2006).
The method of inserting solution from oesophagus
to the stomach using syringe through exible plastic
tube and then removing the tube slowly leading
to the regurgitation of at least part of the stomach
content is similar between stomach ushing method
and potassium antimony tartrate method but
dierent in terms of the solution used. Stomach
ushing method (Martin and Hockey, 1993) used
water or saline solution while the other method used
potassium antimony tartrate (Zduniak, 2005). In this
study, the gut contents were determined through
dissecting method, to assess the diet of this bird,
which are available in oil palm ecosystem (Chenon
and Susanto, 2006).
Our knowledge of the diet of P. goiavier in oil
palm plantation on peat is still inadequate. Available
studies include the analysis of gut contents from
the Yellow-vented Bulbul in Indonesian oil palm
plantation (Chenon and Susanto, 2006) on mineral
soils and some observations on the behaviour of
P. goiavier (Wee, 2009). So far there is no detailed
information on the diet of P. goiavier in oil palm
plantations. The aim of this study is to determine the
diet of P. goiavier in oil palm plantations on peat via
dissection method.
MATERIALS AND METHODS
Location and Description of Study Site
Sampling was conducted 10 times between
2011 to 2013 at the Durafarm Oil Palm Plantation
(DFM) (Figure 1), located in Block 88 (N 01
o
23.827’
E 111
o
24.845’), Betong, Sarawak. This plantation
belongs to WTK Sdn Bhd with palms from 6 to 13
years old. It has an area of about 5022 ha and located
approximately 35 km from Betong town. Adjacent
Figure 1. Map of the sampling site at the Durafarm Oil Palm Plantation, Block 88, Betong, Sarawak, Malaysia.

419
THE DIET OF YELLOW-VENTED BULBUL (Pycnonotus goiavier) IN OIL PALM AGRO-ECOSYSTEMS
to this plantation is a peat swamp forest which was
logged about 20 years ago and has been earmarked
for oil palm development.
Field Method
Mist-net. In order to capture the bulbul, more than
10 mist-nets were set up at DFM. The nets were
deployed in three rows with ve nets per row. The
nets were operated from 6.00 am until 6.00 pm
for three days during each sampling periods and
checked at every 2 hr interval (Rahman and Tuen,
2006). The captured Yellow-vented Bulbul (Figure 2)
was placed in a cloth bag.
Laboratory Technique
Analysis of stomach content. The stomach contents
were classied into insect parts and others such
as plants, and arthropods. Identication of insect
species present in the stomach was done by
examining the samples under microscope and the
prey items found inside the stomach were identied
up to the Order level. The insect parts were identied
based on Borror et al. (1954) and Triplehorn and
Johnson (2005). The insect parts detected were head,
mouthparts, elytra, mandibles, body structure, legs
and wings. The insect parts were observed using
a compound microscope with a camera attached.
The images were taken by using Motic Image Plus
Version 2.0 software with resolution of 1600 x 1200
pixels and the magnication lens were adjusted
between the range of 2.0 – 7.5. The identied insect
parts were sorted and recorded up to their respective
Orders.
Statistical Analysis
Statistical analysis was carried out using
statistical package for social sciences (SPSS-Version
17). Pearson’s correlation coecient (r) measures the
closeness of the relationship between the abundance
of insect per month and the distribution of Yellow-
vented Bulbul per month in DFM. The value for a
Pearson’s can fall nearer to +1 or -1which shows the
perfect relationship between the two variables.
Ivlev’s Electivity Index
Ivlev’s electivity index (E) was used to measure
the degree of food selection by the Yellow-vented
Bulbul in DFM towards a particular prey species.
It is commonly used to compare the feeding habits
of predator with the availability of potential food
resources in the natural habitats (Strauss, 1979). The
relationship is dened as:
r
i
- p
t
E = ,
r
i
+ p
t
where,
E = measure of electivity;
r
i
= relative abundance of prey item i in the gut
(as a proportion or percentage of the total gut
contents); and
p
t
= relative abundance of the same prey item in the
environment.
The possible range of the index is - 1 to + 1. The
negative value indicates avoidance or inaccessibility
of the prey item, zero indicates random selection
Figure 2. Yellow-vented Bubul captured in the Durafarm Oil Palm
Plantation, Betong, Sarawak, Malaysia.
Out of 56 individuals recorded in this site, 45
individuals (80% of the total individual captured in
DFM) were dissected for stomach content analysis
and another 11 individuals were released back to
DFM. The stomach of each bird was dissected open
on site, washed and the contents rinsed into a vial
containing 70% ethanol for laboratory analysis
(Seefelt and Gillingham 2006; Durães and Marini,
2005). The samples were transported to a laboratory
in the Universiti Malaysia Sarawak (UNIMAS),
Sarawak, Malaysia for further analysis.
Distribution of food resources. A transect survey
for insects and fruits was carried out by walking
along the harvesting paths at Block 88, DFM. Three
line transects (50 m length for each transect) were
established at each row where the nets were set
up. This survey was carried out during the same
sampling day. All the insects and fruits available
along transect were recorded. Identication for
insects was done to the Order level only. Species of
fruit found along the trail which cannot be identied
on site were collected and brought to UNIMAS
laboratory for further identication.

Journal of oil Palm research 27 (4) (DecemBer 2015)
420
from the environment and positive values indicate
active selection (Strauss, 1979).
RESULTS AND DISCUSSION
Availability of Food Resources (insects and plants)
in Oil Palm Plantation
Agricultural ecosystem such as oil palm
plantation provides a source of food to many birds
in the form of grain, seeds, insects and rodents
(Asokan et al., 2009). Insects are one of the major
preys consumed by birds in oil palm plantation
(Chenon and Susanto, 2006; Koh and Wilcove,
2007). However, there is limited information on the
diversity of insects in oil palm plantation in relation
to bird diversity. Turner et al. (2008) and Turner
and Foster (2009) have reported on the impact of
conversion of forest to oil palm plantations on
inverterbrate biodiversity.
In this study, the distribution of food resources
(insects and fruits) in DFM was carried out by
walking along the line transect where the mist-
nets was set up. The results showed that a total
of 687 individuals from nine Orders of insects
were recorded for 10 times of sampling (Figure
3). The Order Hymenoptera (60.67%) especially
from the family Formicidae was most abundant,
followed by the Orders Diptera (13.50%), Odonata
(9.00%) and Hemiptera (6.17%). The abundance
of Hymenopterans from the family Formicidae
in oil palm plantation supported the ndings by
Turner and Foster (2009) and Bawa et al. (2011).
Family Formicidae is very important to the oil palm
ecosystem, aiding in decomposition, pollination and
preying on other pests besides acting as food sources
for predators such as birds (Turner and Foster, 2009).
The Order Homoptera was the least abundant insect
Order in DFM plantation.
In terms of fruit survey, Melastoma sp. was the
only fruit plant species recorded at the study site
besides oil palm fruitlet. In plantation management,
this noxious weed (Melastoma sp.) has to be
controlled due to competition with the oil palm for
nutrient, moisture and light (Barnes and Luz, 1990).
Plant diversity in oil palm plantation is restricted
to several adaptable natural ground cover or local
vegetation such as ground and epiphytic ferns (Koh,
2008).
Distribution of P. goiavier in DFM
A total of 56 individuals of P. goiavier were
captured in DFM through mist-netting methods.
The distribution of this bird in DFM throughout the
study period is shown in Figure 4. In this study, the
Yellow-vented Bulbul was abundant in February.
This is due to the breeding season for this bird,
being early in the year between January to March
(Myers, 2009), and they actively go out to search for
food to feed the nestlings. However, in November,
lesser number of this species was captured due to
the rainy season at the end of the year. The capture
rates of birds decreased during the rainy day
because the birds seldom forage for food during the
rain (Tuen et al., 2006; Gouk, 2009). The presence of
insect in oil palm plantation play an important role
to support the survival of birds that prey on insects
(Chenon and Susanto, 2006; Koh and Wilcove, 2008).
In this study, there is a weak negative relationship
between the distribution of Yellow-vented Bulbul
and abundance of insect in oil palm environment
(r= -0.218). This value indicates that as the number
of Yellow-vented Bulbul increases, the number of
insect in oil palm environment decreases.
Yellow-vented Bulbul and its Diet in Oil Palm
Plantation
In the study on the dietary of birds, there are some
factors that should be included in our explanation
such as foraging height, use of plant species,
habitat selection and competition (MacArthur, 1958;
Morse, 1968; Perrins and Birkhead, 1983). Hence,
the presence of prey items in the stomach and
availability of insect diversity in the environment
can be used to determine the diet selection of birds
at dierent habitats.
Figure 3. Percentage of individuals of each order of insect recorded in
the Durafarm Oil Palm Plantation (DFM) for 10X sampling through
line transect survey.
Figure 4. Total individuals of P. goiavier in the Durafarm Oil Palm
Plantation (DFM) at dierent months and years.
Percentage (%)
Hymenoptera
Diptera
Odonata
Hemiptera
Orthoptera
Lepidoptera
Coleopetra
Dictyoptera
Homoptera
Months
Feb-11 May-11 Jul-11 Nov-11 Jan-12 May-12 Jul-12 Nov-12 Feb-13 May-13
00 20.00 40.00 60.00 80.00
60.67
13.50
9.00
6.17
5.00
3.83
1.33
0.33
0.17
12
8
5
3 3
6
4
0
10
5

421
THE DIET OF YELLOW-VENTED BULBUL (Pycnonotus goiavier) IN OIL PALM AGRO-ECOSYSTEMS
A total of 45 individuals (80% of the total
individuals of P. goiavier recorded in DFM) of
P. goiavier were dissected to extract their stomach
samples. The insect parts found in the stomach were
grouped according to the Orders. Table 1 shows
the number of species and individuals of plant,
annelids and insects according to dierent Orders
and families found in the stomach of Yellow-vented
Bulbul at DFM. Some insect parts that had been
digested and could not be identied were recorded
as unidentied parts. Examination of stomach
content of P. goiavier showed that they feed on insects
as well as on annelids, small berries, leaets and oil
palm fruitlets (Figures 5, 6 and 7).
The Ivlev’s electivity index was used to measure
the degree of selection by the predator towards a
particular prey species. Table 2 showed the Ivlev’s
electivity index for dierent insect Orders by Yellow-
vented Bulbul.
The results showed that the Orders Coleoptera
(E= +0.97) and Homoptera (E= +0.87) were mainly
selected as prey items by P. goiavier and this result
reected that these Orders as the main food sources
for this adaptable species in oil palm plantation. This
bird seems to exclusively select as their food from the
Coleopteran family Curculionidae (the pollinating
weevil, Elaeidobius kamerunicus). This nding is
similar to that of Chenon and Susanto (2006) who
conducted their study at Bah Jambi and Bukit
Maradja, Indonesia. Elaeidobius kamerunicus is one of
the most important weevils in oil palm plantation.
This species play an important role to increased
pollination and fruit production from 20% to 30%
(Syed, 1982; Basri et al., 1983). Other Coleopterans
TABLE 2. IVLEV’S ELECTIVITY INDEX OF DIFFERENT ORDERS
No. Order of insect Relative abundance of
prey item in the gut, ri
Relative abundance of the same
prey item in the environment, pi
Measure of
electivity, E
1 Coleoptera 85.37 1.33 +0.97
2 Homoptera 2.44 0.17 +0.87
3 Diptera 7.32 13.50 -0.30
4 Hemiptera 2.44 6.17 -0.43
5 Hymenoptera 2.44 60.70 -0.92
6 Odonata 0.00 9.00 -1
7 Orthoptera 0.00 5.00 -1
8 Dictyoptera 0.00 0.33 -1
9 Lepidoptera 0.00 3.83 -1
TABLE 1. NUMBER OF SPECIES AND INDIVIDUALS OF PLANT, ANNELIDS AND INSECTS FOUND IN THE GUT OF
YELLOW-VENTED BULBUL
Order Family Number of species Number of
individual
Plant species - - 8 11
Annelids Haplotaxida - 2 2
Insects Diptera Culidae 3 3
Hemiptera Unidentied 1 1
Hymenoptera Formicidae 1 1
Homoptera Cicadellidae 1 2
Coleoptera Staphylinidae 1 1
- Scolytidae 1 1
- Curculionidae 1 30
Unidentied - 1 1

Journal of oil Palm research 27 (4) (DecemBer 2015)
422
Figure 7. Other samples found in P. goiavier gut: (A) class annelids; earthworm, (B) order annelids, (C) small berries, (D) oil palm bres,
(E) oil palm bres and grass, and (F) grass part.
Figure 5. Elaeidobius kamerunicus parts found in P. goiavier: (A) rostrum and antenna, (B) body, (C) elytra and (D) legs.
Figure 6. Some of the insect parts found in P. goiavier gut: (A) Order Coleoptera; family Staphylinidae, (B) Order Coleoptera; family Scolytidae,
(C) Order Homoptera; body part, (D) Order Homoptera; mouth part, (E) Order Diptera; family Culicidae, (F) Order Diptera; family Culicidae,
(G) Order Hymenoptera; family Formicidae, and (H) Order Hemiptera; hind wings.
consumed by this bulbul are the bark beetles
(Scolytidae) and rove beetles (Staphylinidae) with
2.2% each from the total Order Coleoptera recorded
in their gut. Other Orders such as Hemiptera (E=
-0.43) and Diptera (E= -0.30) was selected by P.
goiavier as prey based on abundance. P. goiavier tries
to avoid preying on the Order Hymenoptera (E=
-0.92) in the oil palm environment. This bird also
avoids the Orders Odonata, Orthoptera, Dictyoptera
and Lepidoptera. These Orders were present in the
DFM but were absent in the birds gut.
This bird also feeds on oil palm fruitlets as
revealed by the presence of some palm fruit bres
in stomach content; and this was not reported
by Chenon and Susanto (2006). In terms of fruit
survey along the line transect in oil palm plantation,
Melastoma sp. was the only species of plant that was
recorded to produce fruit besides oil palm. A total

423
THE DIET OF YELLOW-VENTED BULBUL (Pycnonotus goiavier) IN OIL PALM AGRO-ECOSYSTEMS
of eight individuals of this bird fed on unidentied
plant species in oil palm plantation. This result
shows that Melastoma sp. is not the only fruiting
plant in the oil palm plantation because there were
other unidentied berries recorded in their stomach
content.
CONCLUSION
In an oil palm habitat, P. goiavier mainly select
the Order Coleoptera (specically from family
Curculionidae, Staphylinidae, and Scolytidae) as
their prey, followed by Homoptera (Cicadellidae),
Diptera (Culidae), Hemiptera and Hymenoptera
(Formicidae). This species avoids feeding on insects
from the Order Odonata, Orthoptera, Dictyoptera
and Lepidoptera. Even though the oil palm habitat
is providing food sources for this bird species to
continue their survival, further study is needed
on the eect of this bird population on oil palm
pollinating weevil and seasonal variation in their
diet.
ACKNOWLEDGEMENT
The authors would like to thank the Director-
General of MPOB for permission to publish this
article. They also wish to thank the sta from
TROPI, MPOB and the Institute of Biodiversity and
Environmental Conservation (IBEC), UNIMAS for
their great support and invaluable assistance.
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