Breeding biology of the
Grey-headed Bulbul Pycnonotus priocephalus (Aves: Pycnonotidae) in the Western Ghats, India
Peroth Balakrishnan
Division of
Conservation Ecology, Sálim Ali Centre for Ornithology and Natural History,
Anaikatty, Coimbatore, Tamil Nadu 641108,
India & Wildlife
Research and Conservation Trust, c/o. Anupallavi, Chungathara, Nilambur, Kerala
679334, India
Current
address: Department of Zoology, NSS College, Manjeri, Kerala 676122, India
Email:
baluperoth@gmail.com
Date
of publication (online): 26 January 2011
Date
of publication (print): 26 January 2011
ISSN
0974-7907 (online) | 0974-7893 (print)
Editor: Ignacy Kitowski
Manuscript
details:
Ms # o2381
Received 02
January 2010
Final received
05 November 2010
Finally
accepted 20 December 2010
Citation: Balakrishnan, P. (2011). Breeding biology of the
Grey-headed Bulbul Pycnonotus priocephalus (Aves:
Pycnonotidae) in the Western
Ghats, India. Journal of
Threatened Taxa 3(1): 1415-1424.
Copyright: © Peroth Balakrishnan 2011. Creative
Commons Attribution 3.0 Unported License. JoTT allows unrestricted use
of this article in any medium for non-profit purposes, reproduction and
distribution by providing adequate credit to the authors and the source of publication.
Author
Details: Dr.
P. Balakrishnan was a Research Fellow at SACON, Coimbatore. He
is also associated with the Wildlife Research & Conservation Trust,
Nilambur. His research interests
include ecology and conservation of threatened and fragmented populations,
wildlife–habitat relationships, human and climate change impacts on the
demography and life-history strategies of birds.
Acknowledgements: This study was funded by the Ministry of
Environment and Forests, Government of India. Logistical support and permission to use field stations were
provided by the Forests and Wildlife Department of Kerala. I am grateful to Drs. V.S. Vijayan, L.
Vijayan, R. Sankaran (late), P.A. Azeez, P. Pramod, T.V. Sajeev, L.D.C.
Fishpool, K.S.A. Das, S. Manchi and D. Mukherjee and M. Vimal for generous
support and discussions during the study. I am obliged to Drs. K. Swarupanandan, N. Venkatasubramanian, K.
Kunhikannan, V.S. Ramanchandran and T.S. Nayar and P.S. Jothish and S. Suresh
for help with plant identification. Karuppusamy, Jose, Mohandas, Mahesh, Sainudheen, Kaliappan and Mari
provided field assistance. I am
grateful to Drs. V.S. Vijayan, Will Cresswell, N.S. Sodhi, G. Ritchison, N.V.
Joshi, K.S.A. Das and A.P. Zaibin, T.N. Bindu and anonymous reviewers for
helpful comments on earlier versions of the manuscript.
Abstract: The breeding biology of the endemic
Grey-headed Bulbul Pycnonotus priocephalus was studied from 2003 to 2005 in Silent Valley National Park,
Western Ghats, India. Nests were
located during three field seasons from the arrival (December) to the dispersal
of the birds (June) and collected data on various breeding parameters,
availability of fruits and weather conditions. All nests were found in mid-elevation evergreen forests
ranging from 900 to 1,400 m elevation. Breeding occurred in the drier months (January–May), which
coincides with high fruit availability. Nest building lasted 3–8 days. Majority of the nests (>72%; n = 39) were built on two
plant species (Ochlandra travancorica and saplings of Syzygium sp.) and the mean nest height was 1.52 ± 0.80 m (n = 52). Nests were randomly oriented around the
nesting plants with a mean vector of orientation equaling 160.450. The clutch size averaged 1.53 ± 0.50
eggs (range = 1–2; n = 47). Incubation and nestling periods were 13 ±
0.87 (n = 9) and 12 ± 0.50 (n = 9) days, respectively. Overall nest success was 10.79%. Nest
success rates varied among incubation and nestling periods. Grey-headed Bulbul exhibit life-history
traits associated with low productivity such as short breeding season, low
clutch size, fewer broods per year and high predation rates indicating that
deterioration of breeding habitats might seriously hamper the long-term
survival of the species.
Keywords: Breeding biology, Grey-headed
Bulbul, life-history traits, nesting success, Pycnonotus priocephalus, Silent Valley, tropical forests, Western Ghats.
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Introduction
Information
on life history traits, especially for rare species, are essential for
estimating and understanding population growth rates (Stahl & Oli 2006),
and predicting responses to environmental changes to be able to develop
appropriate conservation management strategies (Martin 1996; Newton 1998;
BirdLife International 2000; Both & Visser 2005). Yet, breeding biology and life history traits of most
tropical birds are poorly known and large groups like bulbuls are no exception
(Stutchbury & Morton 2001; Fishpool & Tobias 2005).
The
family Pycnonotidae(bulbuls) comprises
about 140 species and 355 taxa, widespread in southern Asia, Africa, Madagascar
and islands of the western Indian Ocean (Sibley & Monroe 1990; Fishpool
& Tobias 2005; Woxwold et al. 2009). The reproductive traits of only a few widespread and lowland pycnonotids
have been studied in Asia and Africa (e.g., Liversidge 1970; Vijayan 1975,
1980; Walting 1983; Ali & Ripley 1987; Hsu & Lin 1997; Kruger 2004;
Fishpool & Tobias 2005).
Here, I
report the first study of the reproductive biology of the Grey-headed Bulbul, Pycnonotus priocephalus (Image 1), one of the 16
restricted-range bird species of the Western Ghats, southern India
(Stattersfield et al. 1998). It
has a very limited distribution in the heavy rainfall areas along the
southwestern side of India from Belgaum and Goa south through Kerala including
the Nilgiris, Palnis, western Mysore and Coorg from plains to 1,400m (Ali &
Ripley 1987; Balakrishnan 2007). Although
Grey-headed Bulbuls are reported from the moist deciduous, scrub, and evergreen
forests in the rain shadow areas, the breeding of the species is restricted to
the mid-elevation west coast tropical evergreen forests (c. 50km2)
between 700m and 1,400m in Silent Valley National Park (Balakrishnan
2007). The species was listed as
Least Concern (BirdLife International 2008) based on the qualitative
descriptions in Ali & Ripley (1987) and Grimmett et al. (1998). A recent survey of the Grey-headed Bulbul along the Western
Ghats revealed rarity and natural patchy occurrence within evergreen forests and
seasonal altitudinal movements (Balakrishnan
2007). Following this the species
has been uplisted to Near Threatened category (BirdLife International
2010). Owing to these attributes
together with continuing habitat loss and degradation, it is essential to
understand key life history traits including developmental periods and survival
rates of Grey-headed Bulbul. The
objectives of the present study was to obtain information on the breeding
season, nesting plants, nest placement, clutch sizes, developmental periods,
nesting success and causes of nest failures and compare this information with
available data for other bulbuls.
Materials
and Methods
Study area
This
study was conducted in the core area of Silent Valley National Park (11000’-11015’N
& 76015’-76035’E, area: 89.52km2,
elevation: 658-2,383 m) in the Western Ghats, India, during January 2003
through May 2005. The climate is
typically tropical, with mean annual rainfall above 5,000mm, which falls mostly
during the south-west monsoon period (May–September). January to March arecompar-atively drier months. From
June to December the relative humidity is often high, around 95%. The mean maximum and minimum
temperatures at Silent Valley during the study period were 25.80C
and 19.80C, respectively. The general vegetation in the area is typical wet evergreen with montane
sholas (forests) and grasslands at higher elevations. Within the study sites, the distribution and breeding of
Grey-headed Bulbul was recorded only in the evergreen forests. The vegetation in the breeding habitat
is dominated by large evergreen trees such as Cullenia exarillata, Canarium strictum, Calophyllum
elatum, Elaeocarpus serratus, Myristica dactyloides, Mesua ferrea, Elaeocarpus
munronii, Syzygium spp., Palaquium ellipticum, Persea
macrantha and Poeciloneuron sp. The sub-canopy and understorey is dominated by species such as Clerodendrum viscosum, Maesa
indica, Chloranthus brachystachys, Ochlandra travancorica and several Strobilanthes species.
Field methods
I located
and monitored nests of Grey-headed Bulbul during three breeding seasons from
the arrival (December) to the dispersal of the birds (June) in 2003 through
2005. I recorded 20, 23 and 25
pairs of Grey-headed Bulbul from the intensive study area during 2003, 2004 and
2005 breeding seasons respectively and a total of 54 nests during this
period. Since the species is
extremely shy and finding nests from the large stretch of understorey patches
was very difficult, most of the nests were located by observing the behaviour
of adult birds (carrying nest materials and food and frequent visits to certain
patches; Martin & Guepel 1993). Breeding seasonality was determined from the nesting records of each
month during three years. To
examine whether the seasonal variation in fruit availability influences the
timing of breeding, I monitored 25 plant species that comprised >90% of
their fruit diet in two transects of 2,000 x 20 m. These plants include 15 trees (Antidesma menasu, Callicarpa
tomentosa, Clerodendrum viscosum, Allophyllus cobbe, Litsea floribunda, Litsea
stocksii, Olea dioica, Oreocnide integrifolia, Persea macrantha, Symplocos
cochinchinensis, Symplocos racemosa, Syzygium cumini, Syzygium sp., Viburnum sp. and Ziziphus rugosa), six shrubs (Chloranthus brachystachyus, Lantana camara, Leea
indica, Maesa indica, Psychotria nigra andPolygonam chinense),
three lianas (Rubia
cordifolia, Rubus ellipticusandSmilax sp.) and
one epiphyte (Scurrola
parasitica). I quantified the number of species and
percent of individuals fruiting per month as an indicator of fruit
availability. Weather data were
collected from the Walakkad forest station of the Kerala Forests and Wildlife
Department.
The
breeding status including the dates of nest construction, egg laying,
incubation and nestlings were recorded every day for each nest found. Nest contents were determined by using
a mirror and pole for inaccessible nests. Nests that fledged at least one young were considered successful. Observations of fledgling in or near
nest, or parents feeding new fledglings in the general area of the nest were
taken as evidence of a successful nest. Depredation was assumed when eggs or nestlings (when too young to
fledge) disappeared (Martin & Roper 1988). Various aspects of nest placement such as nest plant
species, nest height (height of the nest above ground), relative height (height
of nest above ground divided by the height of the plant) and height of nesting
plant were measured in the field immediately after the fledging of the young. Orientation of the nests relative to
the main stem was recorded for all nests. Compass bearings of nests were
recorded to the nearest degree using a Suunto MCA-D compass.
Data analysis
The
influence of temperature, rainfall and availability of fruits on the breeding
season was tested using Spearman rank correlation (Zar 1999). Nesting success of Grey-headed Bulbul
was estimated from 47 intensively monitored nests by different methods. First, I calculated the apparent
nesting success (number of successful nests divided by the total number of
nests found). Second, I calculated
the reproductive success as an index of the chick fledged versus eggs laid (see
Vijayan 1980; Jehle et al. 2004), and finally, I used the Mayfield estimator
(Mayfield 1975) to calculate the daily mortality and survival rates and nesting
success. The daily mortality rate (m) was calculated by dividing the number of
clutches that failed to survive by the total number of days all nests were
under observation and exposed to loss. The daily survival rates (DSR) were calculated as, 1-number of failed
nests/number of exposure days. Then, to obtain an estimate of nest survival over the entire nesting
period, the daily survival rate is raised to the power equivalent to the
average number of days (d) in the nesting period as, nest survival = (DSR)d. Nest survival for the entire nesting period and for
incubation and nestling periods were calculated separately. The variance (v) and
standard error (SE) are approximated for the estimator of daily survival
probabilities by following Johnson (1979) and Hensler (1985).
The
null hypothesis of uniform distribution of nest orientation in all directions
was tested by using Rao’s spacing test (Zar 1999). Watson-Williams F test was used to test the variations in the
orientations of moss and vine nests and successful and failed nests (Batschelet
1981; Bergin 1991; Zar 1999). Circular statistics were computed using the statistical package Oriana (Kovach Computing Services 2004,
Version 2.01c) and other analyses were performed using SPSS (SPSS Inc. 1999,
Version 10.0). Results are reported as Mean ± SE values and a probability level
of ≤ 0.05 was considered statistically significant and ≤ 0.01 was considered
highly significant. The summary statistics for circular data are presented as
mean vector (µ ± SE0).
Results
Breeding season
The
Grey-headed Bulbul began arriving at the study sites by the third week of
December. The breeding activities
were commenced in January and the first egg layingdates were 2, 10 and 26 January for 2003, 2004 and 2005 breeding seasons
respectively. I found 47 nests of
Grey-headed Bulbul, 14, 15 and 18 nests during 2003, 2004 and 2005 breeding
seasons respectively from Silent Valley. Additionally, seven more nests were located in the surrounding reserved
forests (Nilambur and Mannarkkad forest divisions), but not included in the
analysis except for nest placement attributes. Peak egg layingwas observed in early April during 2004 and 2005 while it was in late March
during 2003 (Fig. 1). The breeding
season (January-May) of the species was positively correlated with maximum
temperature (Spearman’s r = 0.660, n = 29, p < 0.01), number of species with
fruits (Spearman’s r = 0.895, n = 27, p < 0.01; Fig. 2) and % of
individuals monitored fruited (Spearman’s r = 0.761, n = 27, p < 0.01; Fig.
2) and inversely correlated with the monthly rainfall (Spearman’s r = –0.373,
n = 29, p < 0.05; Fig. 3). Although a negative trend exists, number of nests
per month was not statistically correlated with the number of rainy days per
month (Spearman’s r = –0.350, n = 29, p = 0.063; Fig. 3).
Nest structure, placement and
orientation
Three
types of nests were constructed based on the variation in the microhabitat. The
dominant (n = 35) typical bulbul nests (hereafter: vine nests) were made mainly
of vines and grasses and seen mostly in Strobilanthes patches. Second type of nests (hereafter: moss nests) constructed mainly with Ochlandra leaves and green moss (n = 18) were
located mostly in reed-bamboo (Ochlandra
travancorica)
patches. A single nest, of a third
type made with fresh green leaves was also recorded (Table 1). The construction time varied
considerably between the two major nest types, 3–5 days for the vine
nests (n = 4) and 6–8 days for moss nests (n = 2).
Grey-headed
Bulbul used 12 plant species for nesting. Nests were located on live plants except the four nests placed on dead
branches of Ochlandra
travancorica, Strobilanthes foliosus and sapling of Syzygium sp. Ochlandra travancorica (n = 21, 38.89%) and saplings of Syzygium sp. (n = 18, 33.33%) were
the most used plant species, followed by Calamus pseudo-tenuis (n = 3), Lasianthus jackianus, Thottea siliquosa, and an un-identified shrub (two each) Oreocnide integrifolia,
Antidesma menasu, Saprosma glomerata, Strobilanthes foliosus, Sarcococca
coriacea and an
unidentified sapling (one each). All the moss nests except one recorded on Strobilanthes foliosus were on Ochlandra travancorica. Of the 35 vine
nests, 18 were on saplings of Syzygium sp.
The nests
were 1.52 ± 0.80 m (range = 0.52–4.8 m, n = 52) above the ground, and at
a mean relative height of 0.61 ± 0.20 (range = 0.18–1.00, n = 52). All the nests were placed in the
junctions of multiple branches, closer to the central stem except for the nests
placed on Oreocnide
integrifolia and Sarcococca coriacea. In general, there was no significant
difference in the nest height (ANOVA: F2,48= 0.737, p = 0.484), height of nesting plant (ANOVA: F2,48 = 0.353,
p = 0.705), and relative nest height (ANOVA: F2,48 = 0.879, p =
0.422) between the breeding seasons. Nest placement attributes significantly varied between the moss and vine
nests. The moss nests had higher
nest heights (2.29 ± 0.20 m vs 1.08 ± 0.05 m; ANOVA: F1,48= 51.540, p < 0.001) and were placed on taller plants compared to the vine
nests (4.17 ± 0.33 m vs 1.90 ± 0.15 m; ANOVA: F1,48 = 51.281, p <
0.001). However, there was no difference in the relative nest heights between
the moss and vine nests (0.59 ± 0.05 vs 0.63 ± 0.03; ANOVA: F1,48 = 0.939, p = 0.337).
Nests
were significantly non-uniformly oriented with clear avoidance of the north
side of the nesting plants (mean vector, µ ± SE = 160.45 ± 9.230;
Rao’s spacing test: U = 227.5, n = 48, p < 0.01). Nest orientation deviated
significantly from random for both moss (Mean vector, µ ± SE = 190.88 ± 14.500; Rao’s
spacing test, U = 180, p < 0.05) and vine nests (Mean vector, µ ± SE = 147.01 ± 10.910; Rao’s
spacing test, U = 207.5, p < 0.01) and slightly differed between two nest
types (Watson-Williams test: F1,45 = 5.56, p = 0.023). There was no
variation in nest orientation between the successful (Mean Vector, µ ± SE = 141.26 ± 27.250) and
failed nests (Mean vector, µ ± SE = 164.13 ± 9.720; Watson-Williams
test: F1,46 = 0.864, p = 0.358). In general, the nests
were placed in the leeward side of the nesting plants.
Clutch size and developmental
periods
The
average clutch size of Grey-headed Bulbul was 1.53 ± 0.50 eggs (range = 1–2
eggs, n = 47), with 53% of nests with two and remaining with one egg. Of the 31 vine nests monitored, 71% of
nests were with one egg and remaining with two eggs each. All the moss nests monitored (n = 15) produced two eggs each. There was a significant reduction in
the clutch size by advance of the breeding season (Spearman’s r = –0.457,
n = 46; p < 0.001). One egg was
laid per day, and incubation started with the last egg.
The mean
incubation period for all the nests for which a complete record is available
from the clutch completion to hatching was 13.00 ± 0.87days (range = 12–14
days, n = 9). Nestlings spent an average of
12 ± 0.50 days (range = 11–13 days, n = 9) in the nest. The overall
period for incubation to fledgling lasted a mean of 25 ± 0.89 days (range = 24–26
days, n = 6). Thus the entire breeding cycle
including the nest construction, egg laying and developmental periods completed
within a month.
Nest success
Nine of
47 nests (19.15%) monitored fledged young, with successful nests producing 1.67
± 0.50 young/nest. Apparent
nesting success (successful nests/total nests found) varied significantly
between the breeding years. The
highest percentage of nesting success was in 2003 (42.86%), while it was 13.33%
and 5.56% during 2004 and 2005, respectively. A total of 15 chicks were fledged
from 72 eggs of 47 nests (20.83%). Egg mortality was quite heavy (66.67%) owing to high predation and
varied significantly between breeding seasons. Out of 72 eggs laid, only 24 (33.33%) were hatched. Nine chicks disappeared from the nests
due to predation (Table 1). Using
Mayfield’s method, the daily survival rates were 0.899 ± 0.017 and 0.958 ±
0.018 during the incubation and nestling periods, respectively, and 0.915 ±
0.013 overall. The Mayfield nest success
was 24.97% and 60.01% during the incubation and nestling periods, respectively,
and 10.79% for the overall nesting period. Nest success differed among years,
32%, 5.69% and 4.53% respectively, for 2003, 2004 and 2005 breeding seasons
(Table 2). There was no
significant variation in the egg survival among the moss (25.42%) and vine
nests (22.33%). However, chicks of
moss nests had a significantly better chance of surviving the nestling period
(100%) than vine nests (34.55%). Consequently, the overall nesting success of moss nests (16.66%) was
significantly higher than the vine nests (6.52%, Table 3). Apparent nesting success also varied
between the moss nests (26.67% of 15 nests) and vine nests (12.9% of 31
nests). The single leaf nest found
in 2003 successfully fledged two chicks. More than 90% of the nest failures
were due to predation and trampling. Nest predation was characterized by complete loss of the clutch or
brood.
Discussion
Breeding
of open-country Pycnonotus species have been recorded in all
months and some are known quite commonly to raise 3–5 broods in a year
while breeding activities in the montane forest species tends to be suppressed
during the wet and coldest months (Ali & Ripley 1987; Fishpool & Tobias
2005). Breeding of Grey-headed
Bulbul is highly seasonal (January-May; Fig. 1) and coincides with high fruit
availability and absence of high rainfall (Fig. 2, 3). The general breeding patterns of the
bird community at Silent Valley was also highly seasonal during the study
period (Das 2008; P. Balakrishnan pers. obs.). Moreover, the breeding season of south Indian passerines is
strongly related to the pre-monsoon during May to June, a month before the peak
monsoon (Pramod & Yom-Tov 1999), so that the peak food demand of chicks
coincides with the arrival of the monsoon (Ali & Ripley 1987). The heavy rainfall during longer south-west (June–September) monsoon may also restrict
the breeding of open-cup nesting species to the drier months. The general fruiting pattern in Silent
Valley National Park shows a bi-modal pattern with a higher peak during the
late summer-early south-west monsoon (March–May)
and a smaller peak in the early north-west monsoon (November-December)
(Balakrishnan 2007). On the other
hand, food plant species of Grey-headed Bulbul showed a single peak which coincides with the first peak of general fruiting
and breeding season of the study species. Thus, fruit availability is an important factor deciding the breeding
season of Grey-headed Bulbul however, further studies
on the availability of other food sources such as caterpillars are required to
understand relative importance of weather conditions and food abundance in
determining the timing of breeding.
Nesting
plant selection by Grey-headed Bulbul seems to be adaptive. Of the 12 plant species used for
nesting, two species (Ochlandra
travancorica and
saplings of Syzygiumsp.) together bear
about 72% of nests. Besides giving
enough support for nest placement, high foliage cover on the Ochlandra provides a camouflaging background to
the moss nests. On the other hand,
the liana draped Syzygium saplings with a dull background form abetter camouflaged environment for the vine nests
(background matching hypothesis: Martin 1988; Filliater et al. 1994; Hansell 2000). The differential nest placement
attributes of Grey-headed Bulbul in Ochlandra and Strobilanthes patches are also adaptive to the
respective microhabitats. The moss
nests in the Ochlandra patches are placed above 2m well
inside the foliage and thus camouflaged from the predators. On the other hand, the nests in the Strobilanthes patches are placed around one meter
height in a pale background surrounded by dry stems and lianas. However, in both the habitats, nests
were placed in the middle of the nesting plants. The non-uniform orientation of nests around the nest plants
was towards the leeward directions of the nest sites and orientation did not
affect the outcome of the nests.
The nest
construction period of Grey-headed Bulbul (3–8 days) was similar to that
of other species (Red-vented Bulbul Pycnonotus cafer: 2–5 days, Yellow-throated Bulbul Pycnonotus xantholaemus: 3–8 days, Cape Bulbul Pycnonotus capensis: 2–10 days; see Ali & Ripley
1987; Fishpool & Tobias 2005). Similarly the incubation (13 days) and nestling periods (12 days) of
Grey-headed Bulbul fall within the range of the developmental periods reported
for pycnonotids (11–14 days and 10–13 days for incubation and
nestling periods, respectively; Vijayan 1975, 1980; Ali & Ripley 1987;
Fishpool & Tobias 2005; P. Balakrishnan pers. obs.). In most of the African and Asian
species of bulbuls, the clutch usually consists of two or three eggs and many
species known to lay four or five eggs (Ali & Ripley 1987; Fishpool &
Tobias 2005). The average clutch
size of Grey-headed Bulbul was 1.53 ± 0.50 eggs with half of the nests
producing a single egg (Table 1). This is one of the lowest clutch size reported for the pycnonotids (Ali
& Ripley 1987; Fishpool & Tobias 2005). Intra-seasonal decline in the clutch size, which is a
commonly observed pattern in several tropical and temperate birds (Hamann &
Cooke 1989; Doligez & Clobert 2003), was also observed for the study
species. The role of different mechanisms
hypothesized to explain the clutch size reduction including seasonal variation
in the food availability and predator abundance (Hamann & Cooke 1989;
Martin 1992; Doligez & Clobert 2003) needs further experimental studies.
Since the
bulbuls were not colour marked, the estimation of the nesting attempts per year
was unclear. Available data suggests that Grey-headed Bulbul is a
single-brooded passerine, although, possibility of a replacement brood is not
ruled out. Moreover, in spite of
the extensive search in all territories, the number of nests recorded was fewer
than the total number of pairs recorded in all the breeding seasons.
The
overall Mayfield nest success of Grey-headed Bulbul was low at 10.79% (with
24.97% for egg stage and 60.01% for the nestling stage; Table 2) compared to
that of Yellow-browed Bulbul Iole indica and Square-tailed Black Bulbul Hypsipetes ganeesa which breeds in the same habitat
(Balakrishnan 2009; 2010). Some
authors have reported higher predation rates during the nestling period
(reviewed in Martin 1992). However,
I found higher predation during the egg stage as reported by others (e.g.,
Mermoz & Reboreda 1998). Nest
predation rates of Grey-headed Bulbul appear to be significantly higher than
that of the average rates (71%) recorded for the open cup-nesting tropical birds
(Robinson et al. 2000; Stutchbury & Morton 2001). In southern India, the nesting success of lowland bulbuls
was reported as 13.2% (15 chicks out of 114 eggs) for White-browed Bulbul Pycnonotus luteolus and 8.3% (11 chicks out of 134 eggs) for Red-vented Bulbul
(Vijayan 1975, 1980) and, this figure drops to 8% in the introduced population
of Red-vented Bulbul in Fiji (Walting 1983). However, these studies are conducted in highly disturbed
habitats and the species are known to raise several broods per year (Vijayan
1980; Ali & Ripley 1987; Fishpool & Tobias 2005). The nest losses of Grey-headed Bulbul
were mainly caused by predation and trampling by large mammals such as Asian
Elephant Elephas
maximus and
Sambar Deer Cervus
unicolor. Nests in the Ochlandra patches, which are a major feeding
ground for the elephants, seemed to be highly vulnerable. In the present study the direct
evidence of predation is restricted to a single observation of egg predation by
Jungle Striped SquirrelFunambulus tristriatus. The potential nest predators at the
study site include White-bellied Treepie Dendrocitta leucogastra, Rufous Treepie Dendrocitta vagabunda, Greater Coucal Centropus sinensis, and Asian Koel Eudynamys scolopacea, Indian
Rat Snake Ptyas
mucosa, Common
Vine Snake Ahaetulla
nasuta and
several species of small mammals and snakes. Brood parasitism is a major problem for a number of African
bulbuls (Krüger 2004) and two species of Asian bulbuls (Fishpool & Tobias 2005). However, no brood parasitism was
observed in the Grey-headed Bulbul nests during this study.
The
results of the present study show that timing of breeding and developmental
periods of Grey-headed Bulbul is similar to that of many congeners or other
tropical species. However, they
exhibit several life history traits associated with low productivity such as
relatively short breeding season, low clutch size (lowest in the genus), less
number of broods per year and nesting failures due to predation. These atypical reproductive traits
along with the restricted range, patchiness in occurrence and large-scale loss
of lowland habitats in Western Ghats (Menon & Bawa 1997; Stattersfield et
al. 1998; Mittermeier et al. 1999) suggests
that any further deterioration of the breeding habitats might seriously hamper
the long-term survival of Grey-headed Bulbul. Further research on the life history traits including the
age of first reproduction, adult and juvenile survival and breeding success in
other sites is also required before recommendations on effective conservation
measures can be made.
References
Ali, S. & S.D. Ripley (1987). Handbook of the Birds of India and Pakistan. Compact edition, Oxford University Press, New Delhi, 737pp.
Balakrishnan,
P. (2007). Status, distribution and ecology of the Grey-headed Bulbul Pycnonotus priocephalusin the Western Ghats, India. PhD Thesis. Bharathiar
University, Coimbatore, India, xvi + 223pp.
Balakrishnan,
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