Table Of ContentTHE RAFFLES BULLETIN OF ZOOLOGY 2001 49(2): 173-180
© National UniversityofSingapore
FORAGING BEHAVIOUR OF TWO TAILORBIRDS IN SINGAPORE:
HABITAT, MORPHOLOGICAL AND TEMPORAL COMPARISONS
Malcolm C. K. Soh
DepartmentofBiologicalSciences, National University ofSingapore, BlkS2Science Drive 4, Singapore 117543.
ABSTRACT.- I compared the morphological traits ordifferences in habitatcharacteristicstostudy their
effects on the foraging behaviourofthe Dark-necked Tailorbird (Orthotomus atrogularis) and the Ashy
Tailorbird (0. rujiceps). I also made seasonal comparisons for the former. The results show that despite
differencesinthewinglengthbetweenthesetwotailorbirdspecies,theirfrequencyuseofdifferentforaging
maneuversdidnotdiffer. Thedifferences inheightofthevegetationbetweenthehabitattypes (mangrove
versusrainforest)probablycontributedtothedifferencesintailorbirdforagingheights.However,theforaging
maneuversandsubstrateusedby theDark-neckedTailorbird weresignificantlydifferentwhencompared
betweendifferentperiods(May-September1997andDecember1997-March I998).Therefore,myfindings
suggestthat the foraging behaviourofthese tailorbirds may be affected by variables such as season and
certain habitat variables.
KEY WORDS. - Tailorbird, Orthotomus, foraging behaviour, seasonal variation, morphology.
INTRODUCTION behaviour of two closely related species of birds (see
beyond).Threepredictions were madefrom this study.The
Themorphologyofabirdisconsideredasanimportantfactor first prediction was that differences in the morphological
inrestrictingtherangeofforagingmaneuversitcanperform traitsofthetwo speciesofbirds wouldresultindifferences
(Martin&Karr, 1990).Moermond(1990)suggestedthatany in foraging maneuvers. The second prediction was that the
subtledifferencesinmorphologicaltraits,suchasthelength differenthabitattypesofthebirdswouldresultindifferences
ofthewing, tarsus andtoes ofbirdscouldresultindifferent in the bird foraging heights and the substrates from which
foraging maneuvers. However, there are also other the prey was taken. The third prediction was that the bird
ornithologists who claimthat the habitat ofthe bird can be foraging behaviourwould vary temporally withinthe same
instrumentalininfluencingotherrelatedforagingbehaviours locality.
suchastheheightatwhichitforages andthesubstratefrom
which it obtains its prey (Maurer & Whitmore, 1981; My study focused on two species of tailorbirds Sylviid
Robinson & Holmes, 1982; Holmes & Schultz, 1988). warblers, namely the family representing the Old World
Habitat characteristics such as vegetation height and prey warblers (Baker, 1997).Tailorbirdsaresmall insectivorous
abundance could affect foraging behaviour (Robinson & birds with long bills and rufous caps. Four species of
Holmes, 1982; Holmes & Schultz, 1988). The foraging tailorbirds are found in Singapore (Hails & Jarvis, 1987).
methodsofeven similarspeciesofbirdscandifferbetween TheyaretheCommonTailorbird(Orthotomussutorius),the
areas with different vegetation structures (Maurer & Dark-neckedTailorbird (0. atrogularis), the Rufous-tailed
Whitmore, 1981). Apart from vegetation structure and Tailorbird (0. sericeus) and the Ashy Tailorbird (0.
diversity, seasonal variation can play a major influence on rujiceps). Only the Dark-necked Tailorbird and the Ashy
theforagingbehaviourofbirds(Hejl& Verner, 1990;Miles, Tailorbirdwerestudied.TheDark-neckedTailorbirdisfound
1990; Sakai & Noon, 1990; Keane & Morrison, 1999). predominantly in rainforests while the Ashy Tailorbird
occurs chiefly in mangroves (Hails & Jarvis, 1987).
The objective for this study was to explore the relative
importance of morphological traits, spatial and temporal The need to study their foraging behaviour and habitat use
variationinhabitatcharacteristicsininfluencingtheforaging is critical in light of the diminishing native habitat. The
Received II Aug 2000 Accepted 10 Aug 2001
173
Soh: Foraging behaviour oftailorbirds in Singapore
mangrovehabitatsinSouthEastAsiaareunderseriousthreat Area,coveringanareaofabout 1100ha(Loh, 1997).Much
(Sodhi et aI., 1997). Rapid economic development such as ofthissiteisrepresentedsecondaryrainforest(Turner, 1994)
fish or prawn farming, port, housing and industrial with a small patch of swamp forest which still has a rich
development is responsible for the immense loss ofnative diversity of flora and flora (Ng & Lim, 1992). Nee Soon
mangal habitats (Chou et aI., 1997). An estimate of 1% of forest is sometimes used as an army training ground.
the mangroves including coastal mudflats is all that is left
inSingapore(ChouetaI., 1997).Asfortherainforesthabitats Observations ofthe Dark-neckedTailorbirds were made in
in Singapore, only 5% of native rainforest remain (Turner NeeSoonandMacRitchieforests,whiletheobservationsof
et aI., 1994). theAshyTailorbirdswereobservedinMandaiandPasirRis
mangroves. Most of my field observations of the Ashy
Tailorbirdsweremadeonthelandwardsideofthemangroves
STUDY SITES AND METHODS where the species is most likely to occur. All observations
weremadebetween0700hand 1200honfairweatherdays.
Morphological Measurements Because weather can affect foraging behaviour (Morse,
1989), I did not make any observations on rainy days.
Measurements for exposed portions of the beak, wing
(unflattened) and tarsus lengths were taken for both sexes Foraging Observations
ofthetailorbirdsusingelectronicverniercalipers(±0.1 mm).
The specimens were obtainedfrom the Raffles Museumof Four foraging maneuvers were observed and recorded for
Biodiversity Research (RMBR) in the National University thetailorbirds: glean(birdpicksuppreyfromsubstratewhile
ofSingapore.NinesubspeciesofDark-neckedTailorbirdand perched), snatch (bird obtains prey by short flight or hop
eight subspecies ofAshy Tailorbird are recognised (Baker, fromperch), hover(birdpicks uppreyfrom substratewhile
1997). Only the subspecies, O. a. altrogularis and O. r. hovering) and hawk (bird chases prey in mid-air). The
cineraceusbothfoundintheMalayPeninsulaandSingapore, recordedsubstratesfromwhichtailorbirdsobtainedtheirprey
were measured. wereleaf,twig(partwhichconnectstoleaforleaves),branch
(part which separates from trunk and connects to twig or
Study Site Description twigs), trunk, air (whichincludes spiderwebs) and ground.
Observations took place in four study sites. The mangrove Theheightofthebirdforaging activityontreeorshruband
siteswerelocatedatSungeiMandai(l°28'N, 103°46'E)and the heightofthe tree and shrubon which the foraging took
PasirRis Park (l°23'N, 103°57'E)(Lim, 1992;SodhietaI., placewereestimatedvisually. Anysignificantchanges (> 1
1997)TherainforestsitesincludedNeeSoonandMacRitchie m) in bird foraging height were also recorded during a
forest (l°22'N, 103°43'E, both occuring in the Central foraging sequence.A"foragingsequence"wasdefinedhere
Catchment Area). as all foraging observations recorded 10 seconds after the
moment the bird was first seen until it departs. Only
TheMandaimangal isanisolatedpatchof10halocatedon alternative foraging sequences were recorded for an
the Northwest of Singapore (Sodhi et aI., 1997). Railway individual to reduce statistical dependence and pseudo
construction in the early 1900s caused extensive habitat replication. Each sequence was timed using a stopwatch.
destruction, but it has since recovered well (Murphy &
Sigurdsson, 1990). Mandai mangal is likely to be cleared The foraging activities of both sexes of tailorbirds were
soon for reclamation (Sodhi et aI., 1997). recorded. Although itis possible that the foraging patterns
betweensexescouldbedifferent(MortonetaI., 1993;Sodhi
In contrast to Mandai mangal, the 5 ha mangrove forest at & Paszkowski, 1995),I pooled observations for both sexes
PasirRis was setaside as a nature area (Chou etaI., 1997). for a given species to gain a wider variety of foraging
Anundergroundchannelsuppliesthemangal with asource behavioursandtoobtainalargersamplesizeforcomparisons
offreshwaterfrom arivernearby. Arivuletallowsperiodic between species.
inundation ofsaltwater to the patch. Such measures along
with the replanting of native mangrove plants have Three sets ofobservations for the Dark-necked Tailorbird
encouraged the mangrove vegetation to flourish. A werecarriedoutbetweenMayandJune 1997,betweenJuly
boardwalkcuttingacrossthemangalallowsvisitorstoenter andSeptember1997andbetweenDecember1997andMarch
and view the mangal interior. 1998. Observations for the Ashy Tailorbird were made
between January to March 1998. There were a total of45
The MacRitchieisaforest fragment ofsome530hawithin sequences were recorded in MacRitchie, 43 in Nee Soon,
the Central CatchmentArea (Loh, 1997). The site is amix 22 in Mandai and 21 in Pasir Ris. The total observation
ofsecondaryandsmallpatchesofprimaryrainforest(Corlett, time for the sequences for the Dark-necked Tailorbird and
1991). Surrounding this fragment is a main road, a golf Ashy Tailorbird were approximately 73 minutes and 51
course and a reservoir. A jogging track, heavily utilised minutes respectively.
especially during the weekends, cuts through this forest.
Data were analyzed using SAS, version 6.12. Significant
The Nee Soon forest is also part ofthe Central Catchment differences for all the statistical tests were set at a = 0.05.
174
THE RAFFLES BULLETIN OF ZOOLOGY 2001
Table 1. Mean measured traits ofspecimens ofthe Dark-neckedTailorbirds and Ashy Tailorbirds.±represents standarderror. Sample
sizes are in parenthesis.
Morphological Trait Sex Dark-necked Tailorbird Ashy Tailorbird
Mean Beak Length Male 13.93 ±0.72 (13) 14.64±0.76 (9)
Mean Beak Length Female 13.23 ±0.68 (11) 13.88 ±0.06 (3)
Mean Wing Length Both 45.50 ±3.25 (24) 47.68 ±3.17 (14)
Mean Tarsus Length Both 16.65 ±1.06 (24) 17.33 ±0.93 (14)
Table 2. Results ofMann-Whitney test to compare the measured traits ofthe Dark-necked Tailorbirds and Ashy Tailorbirds. The first
and second figures represent the Zand P values respectively. The figures in bold indicate significant differences. Refer to Table I for
sample sizes.
Dark-necked males and females Ashy males and females Dark-necked and Ashy
Beak -2.00, 0.045 -2.02, 0.043 1.71, 0.09 for males
1.48, 0.14 for females
Tarsus -0.40, 0.69 -0.94,0.35 1.90,0.06
Wing -1.87, 0.06 -1.25, 0.21 2.00,0.046
Data for foraging maneuvers and substrates were analysed otherthan gleaning, there were also nine snatches and one
usingChi-squaredtestforindependence.Ihadtopoolcertain hover recorded.
variables for a specific comparison to satisfy requirements
of the Chi-squared test. Mann-Whitney U-tests were used TheforagingmaneuversusedbytheDark-neckedTailorbird
to compare the vegetation and bird foraging heights and atMacRitchiebetweenMay-September1997andDecember
measured data from RMBR specimens. It should be noted 1997-March 1998 werealso significantly different (Table
that vegetation and bird foraging heights from July to 3). As withNeeSoon,the Dark-neckedTailorbirdgleanon
September 1997 were not recorded. average more from May - September 1997 than from
December 1997 - March 1998 (Fig. 2). Also, apart from
gleaning, four snatches and hovers and one hawk were
RESULTS observed from December 1997 -March 1998 compared to
May - September 1997 where the tailorbirds were only
Bird Morphology observedtoglean.Thus,theforagingmaneuverobservations
ofthe Dark-neckedTailorbirdfrom May-September 1997
Only beak lengths showed significant differences between werenotusedfor subsequentcomparisonsbetweenthetwo
sexes for the Dark-necked Tailorbird and Ashy Tailorbird speciessinceobservationsfortheAshyTailorbirdwereonly
(Tables 1& 2). The wing lengths betweensexes ofthe two recorded from January - March 1998.
species were not significantly different. The tarsus length
too was not significantly different. Measurements for wing TheforagingmaneuversoftheDark-neckedTailorbirdfrom
length and tarsus length for both sexes were thus pooled. December 1997 - March 1998 did not differ significantly
There were significant differences in wing length between betweenNeeSoonandMacRitchie(Table4),thedatawere
species.TheDark-neckedTailorbirdonaveragehadashorter pooled.TheforagingmaneuversoftheAshyTailorbirdfrom
wing length than the Ashy Tailorbird. There were no January - March 1998 did not differ significantly between
significantdifferencesintarsuslengthbetweenspecies.The Mandai and Pasir Ris (Table 4), the data were pooled.
beak lengths were compared between species butthe sexes
werecomparedseparately. Nosignificantdifferencesinbeak Finally,IcomparedthemaneuversusedbytheDark-necked
length between species were found in males and females. Tailorbird occupying the forest habitat and the maneuvers
used by Ashy Tailorbird occupying the mangrove habitat
Foraging Maneuvers (Fig. 3). There were no significant differences in foraging
maneuversbetweenspeciesintheirrespectivehabitats(Table
ThedataforforagingmaneuversfromMay-June 1997were 4).
pooled with data from July - September 1997 to obtain a
largersamplesizeforcomparisonwithdatafromDecember
Table 3. Results ofthe Chi-squaretestto compare the frequency
1997 - March 1998. The foraging maneuvers used by the
of foraging maneuvers and substrate used by the Dark-necked
Dark-necked Tailorbird at Nee Soon between May
Tailorbirds between May -September 1997 and December 1997
September 1997 and December 1997 - March 1998 were
- March 1998. The figures represent the X2, df and P value
significantlydifferent(Table3).TheDark-neckedTailorbird respectively. The figures in boldindicate significantdifferences.
gleaned more on average in May -September 1997than in
December 1997 - March 1998 (Fig. 1). Also, gleaning Maneuvers Substrate
seemedto be the onlyforaging maneuverused during May Nee Soon 10.5,2, 0.005 43.09, 2, 0.001
-September 1997,butfrom December 1997 -March 1998, MacRitchie 13.64,3, 0.003 9.07,3,0.03
175
Soh: Foraging behaviour oftailorbirds in Singapore
100 100
90 90
80 80
70 70
60 ~~~ 60
50 ~'~" 50
40 ~ 40
30 0~- 30
20 20
10 10
glean snatch hover hawk glean snatch hover hawk
Foragingmaneuver Foragingmaneuver
.May-Sep1997oDec1997-Mar1998 •NeeSoon0MacRitchie
Fig. I. Seasonal comparison of the foraging maneuvers used by Fig.3.ForagingmaneuversusedbyDark-neckedTailorbirdinNee
Dark-necked Tailorbird in Nee Soon forest. Total number of SoonandMacRitchieforestfromDecember1997toMarch 1998.
sequencesrecorded from May to September 1997 and December TotalnumberofsequencesrecordedforNeeSoonandMacRitchie
1997 to March 1998 were 17 and 26, respectively. were 26 and 31, respectively.
100 70
90
60
80
70
~
60
'~"
~ 50
~~ 40
0~-
30
20
20
10 10
Cl
glean snatch hover hawk leaf twig branch trunk air ground
Foragingmaneuver Foragingsubtrate
• May-Sept97EjDec97-Mar98 •May-Sept970Dec97-Mar98
Fig. 2. Seasonal comparison ofthe foraging maneuvers used by Fig.4. Seasonalcomparisonofforaging substratesused byDark
Dark-necked Tailorbird in MacRitchie forest. Total number of neckedTailorbirdinNeeSoon forest. Totalnumberofsequences
sequences recorded from May toSeptember 1997andDecember recorded from May to September 1997 and December 1997 to
1997 to March 1998 were 14 and 31, respectively. March 1998 were 17 and 26, respectively.
Foraging Substrates Table4. Results ofthe Chi-squaretest to compare the frequency
of foraging maneuvers and substrate used by both tailorbirds
I also pooled the foraging substrate data from May - June between sites and habitat types. The figures represent the c2, df
1997 with data from July - September 1997. The foraging and P value respectively. The figures in bold indicate significant
differences.
substratesused by the Dark-neckedTailorbirdat Nee Soon
weresignificantlydifferentbetweenMay-September 1997
Maneuvers Substrate
and December 1997 - March 1998 (Table 3). The Dark
Nee Soon & MacRitchie 5.02, 3, 0.17 9.07,3,0.03
neckedTailorbirdusedmoretwigsandbranchesinNeeSoon
Mandai & Pasir Ris 1.39,2,0.5 4.24, 4, 0.38
from May - Sept 1997 but more leaves were used from
Forest & mangrove 1.71, 3, 0.64
December - March 1998 (Fig. 4). The foraging substrates
MacRitchie & mangrove 8.19, 4, 0.09
usedbytheDark-neckedTailorbirdatMacRitchiewerealso
Nee Soon & mangrove 1.56, 4, 0.816
significantlydifferentbetweenthoseperiods(Table3).More
twigs than leaves were used during the May - September
Tailorbird occupying the different forest study sites
1997periodandviceversaforDecember1997-March1998
(MacRitchieandNeeSoon)andthesubstratesusedbyAshy
(Fig. 5). The observations for foraging substrate from May
Tailorbird occupying the mangrove habitat (Fig. 6). There
- September 1997 were not used for further analysis.
were no significant differences in foraging substrate used
betweenMacRitchieandmangrovehabitat(Table4).There
Theforaging substratesoftheDark-neckedTailorbirdfrom
werealsonosignificantdifferencesintheforaging substrate
December1997-March1998differedsignificantlybetween
used between Nee Soon and mangrove habitat (Table 4).
Nee Soon and MacRitchie (Table 4). More leaves and less
twigswereusedbytheDark-neckedTailorbirdinNeeSoon
Vegetation Heights and Bird Foraging Heights
than in MacRitchie (Fig. 6). Since the foraging substrates
usedbytheAshyTailorbirdfromJanuary-March 1998did
Novegetationandbirdforagingheightswererecordedfrom
not show any significant differences between Mandai and
May -June 1997and July -September 1997. Birdforaging
Pasir Ris (Table 4), the data were thus pooled.
heights and the vegetation heights used by Dark-necked
Tailorbird to carry out foraging (Table 5) recorded from
Last, I compared the substrates used by the Dark-necked
176
THE RAFFLES BULLETIN OF ZOOLOGY 2001
Table5. Results ofMann-Whitney testtocomparethebirdforaging and vegetationheightsofboth tailorbirds. Thefigures representthe
Z, dfand P values respectively. The figures in bold indicate significant differences.
Bird foraging height Vegetation height
Nee Soon & MacRitchie -0.26, 31, 26, 0.80 0.15,31,26,0.88
Mandai & Pasir Ris 0.04, 22, 21, 0.97 1.58, 22, 21, 0.11
Forest & mangrove -4.83,57,43,0.0001 -4.28,57,43,0.0001
90 90
80 80
70 70
~ 60 m60
~
~50 8lso
$ $
~ 40 i 40
~ ~
0. 30 0. 30
20 20
10 10
leaf twig branch trunk air ground leaf twig branch trunk aIr ground
Foragingsubstrate Foragingsubstrate
•May-Sep1997oDec1997-Mar1998 •NeeSoon0MacRitchie~Mangrove
Fig. 5. Seasonalcomparison offoraging substratesusedbyDark Fig.6.ForagingsubstratesusedbyDark-neckedTailorbirdinNee
neckedTailorbirdinMacRitchieforest.Totalnumberofsequences Soon and MacRitchie forest and Ashy Tailorbird in mangrove
recorded from May to September 1997 and December 1997 to habitat from December 1997 to March 1998. Total number of
March 1998 were 14 and 31, respectively. sequences recorded for Nee Soon, MacRitchie and mangrove
habitat were 26, 31 and 43, respectively.
December 1997 - March 1998 showed no significant 18 1
16
differences between Nee Soon and MacRitchie. These two
14
datasetswerepooledforsubsequentanalysis.Birdforaging
heights and the height of vegetation used by the Ashy 12
Tailorbird to carry out foraging (Table 5) did not differ g~'"10
significantly between Sungei Mandai and Pasir Ris. These .g::o 8
two data sets were thus pooled.
I
Lastly, the bird foraging and vegetation heights were
comparedbetweenthe two species oftailorbirds, the Dark -
neckedTailorbirdoccupyingtheforesthabitatandtheAshy Height(m)
•VegetationheightDBirdforagingheight
Tailorbird occupying the mangrove habitat (Figs. 7 & 8).
Fig.7.VegetationheightsandtheforagingheightsofDark-necked
Bird foraging heights and the vegetation heights (Table 5)
Tailorbird in the forest (both Nee Soon and MacRitchie). The
showedsignificantdifferencesbetweenforestandmangrove
samplesizeforvegetationheight(numberoftrees orshrubs) and
habitat.Thevegetationintheforesthabitatwereonaverage bird height (number offoraging sequences) is 57..
higher (Mean = 6.93 ± 0.59 m) than the mangrove habitat
(Mean = 3.31 ± 0.27 m). Similarly, the Dark-necked 35'1--------
Tailorbird foraged on average higher (Mean =5.11 ± 0.49 30 I
m) than the Ashy Tailorbird (Mean =1.81 ± 0.20 m).
25
'"
~20
DISCUSSION .0t-15
10
Foraging Maneuvers
Closelyrelatedspeciesthatsharesimilarmorphologicaltraits
0.00-2.00 2.01-4.00 4.01-6.00 >6.01
are likely toshow similarforaging maneuvers(Robinson&
Height(m)
Holmes, 1982). The measurements of the two species of • VegetationheightDBirdforagingheight
tailorbirdstakenforbeak,wingandtarsuslengthrevealthat Fig. 8. Vegetation heights and the foraging heights of Ashy
Tailorbird in the mangrove (both Mandai and Pasir Ris). The
they were only morphologically different in wing length
samplesizefor vegetation height(numberoftrees orshrubs)and
betweenspecies(P=0.046).Itshouldbenotedthatalthough
bird height (number offoraging sequences) is 43.
thetarsuslengthswerenotsignificantlydifferentthePvalue
wasveryclosetosignificantlevel(P=0.06).Unfortunately,
amoreaccurateassessmentofthewingmeasurementscannot
177
Soh: Foraging behaviour of tailorbirds in Singapore
be made due to the smallcollectionoftailorbirds available Foraging Substrates
at the RMBR. Although there were differences in
morphology between the two species in wing length and Seasonalvariationsalsoseemtoaffectthetypesofsubstrates
possibly tarsus length, there were no corresponding usedbyforagingbirds.Thiscouldaccountforthesignificant
differencesintheforagingmaneuversbetweenspecies.This differencesinsubstrateusedbytheDark-neckedTailorbird
result is contrary to the first prediction that differences in inbothNeeSoonandMacRitchiebetweenMay-September
the morphological traits ofthe two species ofbirds would 1997andDecember1997-March1998.Itmaybethatduring
result in differences in foraging maneuvers. Therefore, we therainyseasonespeciallyduringthemonthsofNovember
canprobablyconcludethatslightdifferencesinwinglength andDecember(Chia& Foong, 1991),theproportionofleaf
betweenthesetailorbirdsdonotinfluencethemethodsused eating insects, predominantly Lepidopteran larvae would
to capture prey. increase(Holmes& Schultz, 1988).Thiscouldbethereason
for leaves used as thepreferredforaging substrateto twigs,
Perhapsanothermeasureofmorphologyforthesebirdssuch branchesandotherpartsofatreeduringtheDecember1997
as wing length to bird weight ratio would make a better - March 1998 (Figs. 4, 5). However, this is based on the
comparison. This is based on the assumption that the assumption that caterpillars orother leaf-eating insects are
tailorbirdsforageinamannerthatmaximisesthenetenergy the preferred prey of tailorbirds.
gained per unit time (Morse, 1989). Ifthe wing length to
weight ratio for a particularbird is high, it could afford to The seasonal variations could also be attributed to the
use more energetically expensive maneuvers such as migratory season. Theperiod ofobservations for the Dark
hoveringorsnatchingtocapturecertainprey.Unfortunately, neckedTailorbirds startedin December 1997-March 1998,
this ratio cannot be applied my study since the weight was which coincides with the period in which most Palaeartic
not given for from the museum specimens in the RMBR. migrants appear in Singapore (Hails & Jarvis, 1987).
Observations taken from May - September 1997 mark the
Althoughmorphologyhasanimportantinfluenceonforaging period when the migrants would be absent. Migrants like
behaviour,otherfactors couldplayagreaterrole(Robinson the Inornate Warbler (Phylloscopus inornatus), Eastern
& Holmes, 1982; Gustafsson, 1988; Martin & Karr, 1990; crownedWarbler(P. coronatus)and theArcticWarbler(P.
Vanderwerf, 1994; Whelan, 2001). One such factor is borealis) (Strange, 1993; Hails & Jarvis, 1987) could
seasonalvariation. Seasonalvariationinforagingbehaviour competewiththeresidenttailorbirdsforsamefoodresources.
canoccuratthreelevels:withinseason(Hejl& Verner, 1990; Noske (1995) discovered that the Arctic Warbler and the
Miles,1990;Sakai& Noon,1990;Keane& Morrison, 1999), AshyTailorbirdwereverysimilarinforagingbehaviourwith
betweenseasons(Cale, 1994;Wiedenfeld, 1989)andyearly a mean overlap of 70% in substrates used for foraging.
variation (Hejl & Verner, 1990; Miles, 1990; Keane & Interspecific competition between Dark-necked Tailorbird
Morrison, 1999).Sinceoursamplesizeswerenotsufficient andinsectivoresofsimilarsizelikethemigratory warblers
totestifthevalidityofwithinoryearlyvariationsinforaging as mentioned may affect its foraging behaviour (could also
behaviour, "seasonal variation" as defined in this study is apply for foraging maneuvers).
the change in the foraging behaviour (either in foraging
maneuversorsubstratesused)betweentwodifferentperiods There was also interspecific variation in substrates used by
spanningafewmonths. Comparingtheforaging maneuvers Dark-neckedTailorbirdbetweensites.FromDecember1997
between different periods (May - September 1997 and -March1998,theDark-neckedTailorbirds usedsignifcantly
December 1997 - March 1998) for the Dark-necked moreleaves andlesstwigs in NeeSoonthaninMacRitchie
Tailorbird, significant differences were apparent. For both (Fig. 5). Although both sites are mostly composed of
foresthabitats(NeeSoonandMacRitchie)theDark-necked secondary forest, they may, however, be different in terms
Tailorbirds were observed only to glean from May thedegreeofhumanintrusionormodification.Forinstance,
September1997whereasfromDecember1997-March1998 Vanderwerf(1994)observedthattheElepaioinmorealtered
the birds not only gleaned less but a greater variety of habitats (Chasiempis sandwichensis) used more leaves and
maneuvers were used (snatching, hovering and hawking). lesstwigs moreoftenthanthebirdsin undisturbedhabitats.
Other than habitat modification, the variation in substrate
IcanonlyspeculatewhytheDark-neckedTailorbirdsadopt usecouldalsobeattributedtomicrohabitatdifferencessuch
amorevariedapproachtoforagingforthelatterperiod.One asthevegetationphysiognomy(Whelan,2001). Totestthis
possibility could be due to seasonal fluctuations in their hypothesis, we would need data on the types ofvegetation
preferred prey. Suppose the Dark-necked Tailorbird's used by the Dark-necked Tailorbirds in different localities
preferred prey experienced a population decline from to determine if there are indeed differences in foliage
December1997-March 1998,itisprobablethattheycould structure between them. If so, such distinct foraging
turn their attention to feeding on alternative food sources microhabitatscouldaccountforthedifference in substrates
whichmayrequiresuchenergeticallyexpensive maneuvers used by Dark-necked Tailorbirds between Nee Soon and
tocaptureprey.However,totestsuchahypothesis, wemust MacRitchie.
firstestablishthattailorbirdsshowapreferencetoaparticular
group of insects or food source.
178
THE RAFFLES BULLETIN OF ZOOLOGY2001
Bird Foraging Heights comparedbetweendifferentperiods(May-September1997
andDecember1997-March1998).Preyabundance,thetype
Thesignificantdifferencesinbirdforagingheightsbetween of prey available, competition with migrant birds vary
the two species is likely to be attributed to corresponding seasonally and these in turn may affect the foraging
differences in the vegetation heights. Generally, mangrove behaviouroftheDark-neckedTailorbird.Inshort,thisstudy
trees are shorter than trees found in a rainforest due to the showsthatseasonalvariationsareaimportantconsideration
unstable nature of the mangrove soil and the constant in foraging studies.
exposuretowindsandwaves(Hutchings& Saenger, 1987).
This result is consistent with the second prediction that
different habitat structures would probably result in ACKNOWLEDGEMENTS
differences in bird foraging heights.
The author wishes to thank Navjot Sodhi for sharing his
Contrary to observations on the Dark-necked Tailorbird creativeideasandreadingthemanuscriptcritically.Thanks
beingconfinedtomostlydenseundergrowth(Hails& Jarvis, are also due to Yang ChangMan and Kelvin Limfrom the
1987), several observations in this project revealed that RMBR for assisting in providing the specimens for
foragingactivityofthisspeciescouldoccurfairlyfrequently measurement. I am also grateful to Hugh Ford and an
inthetreecanopy(Fig.7). Thiscouldbeduetocompetition anonymousrefereeforreviewingthemanuscriptandoffering
with other insectivorous birds that typically forage in the insightfulsuggestions.Thestudywaspartiallysupportedby
shrubs such as the very common Striped Tit-Babbler aresearch grantfrom the National University ofSingapore
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