Monitoring populations of Red-footed Boobies Sula sula and frigatebirds Fregata spp. breeding on Aldabra Atoll, Indian Ocean

11 Pages·2001·4.6 MB·

by  Alan E Burger

MichaelWalters 236 Bull. B.O.C. 2001 121(4)
Hancock, J & Elliot H. 1978. Herons ofthe world. London Editions Ltd., London.
Hodgson, B.H. (no date) Icones ined. Grail, in library ofBritish Museum, London.
Hodgson, B.H. 1844. Catalogue ofNipalese birds, collected between 1824 and 1844. In J.E. Gray,
1831-44, Zoological Miscellany, 6: 86.
Hume, A.O. 1878. A revised list ofthe birds ofTenasserim. Stray Feathers, 6: 470 [1-524].
Inskipp, T, Lindsay, N & Duckworth, W. 1996. An annotated check list ofthe birds ofthe
Oriental Region. Oriental Bird Club, Sandy.
International Commission on Zoological Nomenclature, 1999. International Code ofZoological
Nomenclature. Fourth edition. London.
Raffles, S. 1822. Second part ofthe descriptive catalogue ofa zoological collection made in the
Island of Sumatra and its vicinity, Transactions ofthe Linnean Society ofLondon, 13: 325
[227-340].
Ripley, S.D. 1982. A synopsis ofthe birds ofIndia and Pakistan, second edition, Bombay Natural
History Society.
Sharpe, R.B. 1898. Catalogue ofthe birds in the British Museum, vol. 26, p. 70. London.
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Press, New Haven & London.
Address: Michael Walters, Bird Group, The Natural History Museum, Tring, Herts HP23 6AP.
© British Ornithologists' Club 2001
Monitoring populations of Red-footed Boobies
Sula sula and frigatebirds Fregata spp. breeding
on Aldabra Atoll, Indian Ocean
&
byAlan E. Burger Michael Betts
Received29June2000
Frigatebirdsandboobieshaveexperiencedconsiderablepersecutionanddisturbance
inthewesternIndianOceanoverthepastcentury, andduringthis timepopulations
ofboth groups declined or disappeared on many islands (Feare 1978, Carboneras
1992, Orta 1992, Cheke 2001). Some colonies are now protected but the size and
population trends are poorly known for most Indian Ocean colonies. We report
counts offrigatebirds andboobiesmade onAldabraAtoll inMarch-May2000, and
compare them with previous counts to assess population trends. We also make
recommendationsforfuturemonitoring, andcommentontheuseofsmallboatsasa
censusingplatform for seabirds nesting inmangroves.
Aldabra Atoll supports the largest breeding population of frigatebirds in the
Indian Ocean, and the second largest in the world, with c. 6,000 pairs ofLesser
FrigatebirdFregafaarieland4,000pairsofGreatFrigatebirdF.minor(Reville 1983).
TheonlyotherbreedingsiteinSeychellesisnearbyCosmoledoAtollwhichsupports
200-400 pairs ofeach species (Rocamora & Skerrett 2001). Despite their global
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Alan E. Burger MichaelBetts 237 Bull. B.O.C. 2001 121(4)
importancetheAldabrafrigatebirdcolonieshavebeencensusedonlytwice, in 1967
(Diamond1971, 1975)andin1976-1977(Reville 1980, 1983).
Aldabra also supports a large breeding population ofRed-footed Boobies Sula
sula,estimatedat6,000-7,000breedingpairs(Diamond1971, 1974).WithinSeychelles
and the western Indian Ocean this is exceeded only by the Cosmoledo population
(10,000-12,000pairs;Rocamora&Skerrett2001).TheAldabrapopulationhasbeen
censusedonly once, in 1967-1968 (Diamond 1974). This is the onlybooby species
known to nest on Aldabra.
Methods
Between 12-20March2000AEBvisitedAldabraandcensusedalltheknownfrigatebird
coloniesandmostoftheRed-footedBoobycolonies,coveringmorethanhalfofthe
m
lagoonshorelineoftheatollina4 longoceankayak(K-lightfold-up,Feathercraft
Ltd.,Vancouver,Canada).Thekayakpermittedaccess tomostofthesmallchannels
amongthemangrovesandbetweensmallislets. Between20Marchand9May2000
MB
surveyedtheremaininglagoonshoreusingasmallmotorboat, andR. Wanless
countedboobiesaroundlieauxCedres.Wegenerallyremained8-1 mawayfromthe
edge ofthe mangroves and shore, except where forced to pass through narrower
channels,andplottedourlocationson 1:25,000orthophoto-mapsaswemovedalong
the shore. Ahand-heldtally counterwas usedto count frigatebirdnests and, where
common, booby nests. Binoculars were usedto check andcount a fewnests which
m
couldnotbeapproachedwithinc. 30 because ofshallowwater.
Red-footedBoobieswereintwostagesofbreeding,whichwerecountedseparately.
Mostnestscontainedanadultapparentlyincubating.Wecheckedc. 20nestsandall
containedeggs,butsomeotherbirdsmighthavebeenbroodingsmallchicks.Colonies
also contained large nestlings or newly fledgedjuveniles from an earlier nesting
stage(hereaftercalledfledglings)sittinginornearnests. Thesewereseparatedfrom
older second-year immature birds by appearance: the fledglings had duller, more
uniformbrownplumage, sometimeswithtraces ofdown, paleranddullerpinkfeet
andlegs,andblackbills(Diamond 1974).Afewlargedownychickswerepresentin
March andincluded inthe fledgling count.
To include boobynests deep in the canopy, where adults on and offnests could
notbe distinguished fromthe boat, Diamond (1974) multipliedthe total numberof
adults seen by an "incubating index", which was the ratio ofadults seen on nests
(incubating orbrooding) dividedby those definitelynot on anest. We chose notto
applythis correctionforseveralreasons. Adultswere oftenroosting inareaswhere
there were no nests. The ratio would need to be re-calculated for each section of
colony to account for spatial and temporal variations in the attendance ofoff-duty
adults.Weencounteredrelativelyfewsituationswherewecouldnotdeterminewhether
an adult was on anest ornot, although this sometimes requiredminor shifts ofthe
boatpositionandtheuseofbinoculars. Consequently,wemadenoefforttoestimate
thenestswhichmighthavebeenassociatedwiththesmallnumbersofadultboobies
&
Alan E. Burger MichaelBens Z3S Bull. B.O.C. 2001 121(4)
" Aldabra Atoll . = .= .r r __ i-
POlymnie GioonetColony MddeCampcotony
3-a-: =asse ^K/^S^I—-^jL__ ^ PasseHouareau
,<3
Picarc -^~i^__^0r " ^-^^IteauxCedres
-:a.e:c:~. e',':-a t,^^^ """^BrasTakamaka
\_^e>"e Vc_s: :—_es ^^
.,G'~~5' aJ>—~~ ^-- v-^_
10km
5
46°10-E -f::: e 46°30-E '--.-: s-
i i
Figure 1. Map ofAldabra Atoll, showing islands and other features mentioned in the text and the
locations ofthe three frigatebird colonies (black areas).
see::deep inthecanopy. Thenumbersgivenareouractualcountsofvisiblyoccupied
nests. From the boats we couldusually see deep into the canopy andour subjective
impression was that we missed fewer than 10°o ofthe booby nests, except for the
coloniesinandnearBrasTakamaka. Boobynests alongthemangrovefringe inBras
Takamaka a::d around fie aux Aigrettes were counted using binoculars from the
oppositeshore.0.8-1.2kmaway, andwereprobablyunder-estimated.Wewereunable
to visitallthe small channels inthemangroves ofBras Takamaka (Figure 1 ). where
Diamond(1974)hadreportedseveralhundredboobynests,butMB andexperienced
rangers collectively estimated that there were c. 100 nests there, based on their
experience during several explorations overthepasttwo years.
Arthetuneofthe censusbothspecies offngatebiidshadlargechicks. On.Aldabra.
fledglingsofthetwo speciescanbedistinguishedusmgheadcolour(whiteorcreamy
in Great, andreddish in Lesser: Diamond 1975i. but this takes experience (Reville
1983:69 andwasnotattempted. Countsofbothspecieswerepooled.About 1-2%of
)
thenests containedanadultbird,whichmighthavebeenshadingasmallerchick or
i
lesslikelyan^gg orinanemptynest. Thesewereincludedinthetotalcount. Reville
i
(1983) estimatedthatboat counts recorded 80-90°o ofthe frigatebirdnests present.
rased on comparisons with counts from blinds inside colomes.
Results and discussion
Red-footedBooby
This species shows plumage polymorphism in the Indian Ocean (Le Cone 1999.
Cheke2001 ).butwe sawonlywhitemorphadults, andnowhite-tailedbrov.nmorphs.
Boobies nested in small clusters ofnests scatteredalongthemangrove fringe ofthe
lagoon, with a few nests also mPemphis acidula shrubs and othertrees. Nearly all
::es:s were on the lee shores ofislands or inlets, sheltered from the SE wind (the
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Alan E. Burger MichaelBens 239 Bull. B.O.C. 2001 121(4)
strongest winds). We counted 4,095 nests with a sitting adult and 838 fledglings
(Table 1).Mostofthese(71%ofthenestswithadultsand54%ofthefledglings)were
in areas not used for nesting by frigatebirds. Those boobies which did nest within
the frigatebird colonies usually chose trees orbushes with no frigatebird nests, but
this was not quantified. The majority of boobies therefore nested apart from
frigatebirds, although frigatebirds often roosted nearby.
ItisdifficulttocompareourcountswithDiamond's(1974)censusin 1967-1968
because his counts were made at a different time ofyear (between October and
February),andthescaleofDiamond'smapmakesitdifficulttodeterminetheboundaries
ofhis countareas. Thetype ofboatusedforthe counts (dinghyinDiamond's study,
kayak in ours) should not affect the counts. Diamond counted the "Main Colony",
TABLE
1.
Distribution and numbers ofRed-footed Booby nests in March-May 2000, compared with data
from Diamond's (1974) census made between Oct 1967 and Feb 1968.
Thisstudy 1967-1968(Diamond1974)
Island Location Adults Fledglings Adults Calculated Fledglings Dateof
on and on no.of and Diamond's
nests juveniles nests nests juveniles survey
Picard LagoonandGrandPasse 28 7 11 12 2** 25Nov
Malabar Gionnetfrigatebirdcolony 10 20 42** 11-12Oct
Malabar GionnettoCampFrigatecolony 114 9 - - - -
Malabar CampFrigatefrigatebirdcolony 628 164 159 187 2 31Jan
Malabar CampFrigatetolieVerte 1,022 110— 90 125 10 15Feb
Malabar lieVertetoMiddleCampcolony*** 965 156
—
Malabar MiddleCampfrigatebirdcolony*** 534 200_ 1,912 2,412 613** 12Feb
GrandTerre PasseHouareau,lieauxCedres 569 172 81 157 9 8Nov&
(westend) andun-namedisland 4Jan
GrandTerre lieauxAigrettesand 125* .* - - - -
(southwestend) outerBrasTakamaka
GrandTerre BrasTakamaka 100* _* 11 52 240** 13-14Feb
(southwestend) innerchannels
GrandTerre GrosIlotCavalier 13 13 1 8Dec
(eastend)
Total 4,095 838 2,277 2,958 919
Withinfrigatebirdcolonies 1,172 384 2,071 2,599 615
Outsidefrigatebirdcolonies 2,923 454 206 359 304
MalabarIslandtotals 3,273 659 2,161 2,724 667
* Roughestimateofadultsonnests,noestimateoffledglings
** Diamond'scountfortheseareasincludedallimmatures,notjustfledglingsandjuveniles
*** Diamond's"MainColony"coversthesetwoareas
&
Alan E. Burger MichaelBetts 240 Bull. B.O.C. 2001 121(4)
between Middle Camp and lie Verte, and the Bras Takamaka colonies twice, in
October-NovemberandagaininFebruary. He counted all otherareas once, most in
February. ConsideringonlytheFebruarycounts,whichwereclosertothemonthsof
ourcounts,Diamond'stotalforAldabrawas2,277adultsonnestswhichheestimated
represented 2,958 nests, including those not visible in the canopy (Table 1). Our
countofnestswithadultsin2000wasthus 80%higherthanthe 1967-1968 count,if
weapplynocorrectionforinvisibleneststoeitherdataset. ComparingonlyMalabar
Island,whichcontainsthebulkoftheboobiesandwaswellcoveredbybothcensuses,
the count of 3,273 nests with adults in 2000 was 51% higher than in 1967-1968
(Table 1).
Diamond's (1974) February 1968 estimate for "Main Colony" (1,912 adults on
nestswithno correctionforinvisiblenests)washigherthanthe 1,499nests counted
inMarch2000(Table 1),butthisdifferencemightbepartlyduetoourinterpretation
ofwheretheboundaryof"MainColony"lay. In2000therewasnoclearboundaryof
the colonies; small clusters of nests were scattered along most of the southern
Malabar shore. The biggest difference between the two counts was in the west of
Malabar, including the Gionnet and Frigate frigatebird colonies. We found 1,774
nestsherewhereasDiamond(1974),whoexploredtheareathoroughly(pers.comm.),
reported only 249 nests with sitting adults in 1967-1968. Red-footedBoobies have
greatlyexpandedtheircoloniesinthispartofMalabarIsland,probablyasaresultof
thecessationofhumanexploitationanddisturbancewhichwas commonthere until
1967(Diamond1975,Reville 1980).
Thecountsoffledglings appearedtobehigherin 1967-1968thanin2000(Table
1). This comparison is not meaningful, however, because breeding success might
havedifferedamongtheyears;Diamond(1974)sometimespooleddatafromfledglings
and older, second-year immatures; and at the time ofourMarch 2000 census some
fledglings had already left the colonies to roost elsewhere andwere not counted.
WefoundnoevidenceofboobiesnestingonPolymnieIsland, lieMoustique, lie
Esprit, lieMichelle, PasseDubois, alongthe southernlagoon shores ofGrandTerre
fromPasse Duboisto BrasTakamaka, oronanyislets inthese areas (Figure 1). The
smallcolonyof13nestsatGrosilotCavalierontheshoreofGrandTerrein 1967-1968
(Table 1)wasnotoccupiedin2000.
Frigatebirds
Frigatebirds were found in only three colonies on the lagoon shores of Malabar
Island: Gionnet(whichincludedGrosIlotGionnet),Frigate,andMiddleCamp(Figure
1).TheseweretheonlycoloniesactiveduringReville's(1980, 1983)extensiveresearch
in 1976-1977. Atthattime allthree colonies containedboth species, butthebulkof
theGreatFrigatebirdsnestedatMiddleCampandtheLessersatFrigate(Table2).We
hadinsufficienttime in2000to identifyorcountallthe adultbirdsto confirmthese
distributions,excepttonotethattheproportionsofeachspeciesinthethreecolonies
seemed similarto those in 1976-1977. Inthe 1960s frigatebirds also nested inBras
Takamakainthe southeastcornerofthelagoon(Diamond 1971, 1975),butthisarea
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Alan E. Burger MichaelBern 241 Bull. B.O.C. 2001 121(4)
wasnotusedinthe 1970s(Reville 1980, 1983)orsincethen(MB,pers. obs.).
Inthe 1960sthefrigatebirdsonMalabarIsland,particularlyintheFrigatecolony,
werefrequentlydisturbedandpredatedbypeople, andReville(1983)suggestedthat
the lownumbers inthatcolony, greateruse oftheMiddle Camp colonyandnesting
inBrasTakamakawereallconsequencesofthisdisturbance. Since 1967thecolonies
havebeenprotectedandhumandisturbancegenerallylimitedtooccasionalvisitsby
tourists in boats, restricted to the Gionnet colony and supervised by the Aldabra
staff. The Frigate colony expanded, apparently due to immigration of Lesser
Frigatebirdsfrom MiddleCampandBrasTakamaka, andthelattercolonyceasedto
beused(Reville 1983).
Wecounted3,100activefrigatebirdnests,nearlyallcontaininglargechicks(Table
2). Itisdifficulttomakecomparisonswithpreviouscountsin 1967(Diamond 1975)
and 1976-1977(Reville 1983)becausethecountsweremadeatdifferenttimesofyear,
and both ofthese authors used various factors to convert counts ofadults or nests
into the estimatedtotal breeding population. We have usedthe leastmodified data,
wherepossible, to comparewithourcounts (Table2).
Diamond(1975)didnotcountallthenestsinthelargeMiddleCamp colony,but
sampled sections of mangroves and extrapolated the results to estimate the total
colony. He provided no direct estimates ofactive nests, but statedthatthe count of
adultfemales intrees correspondedcloselywiththenumberofnests, andthis count
issummarisedinTable2.Usingvariousconversionfactors.Diamond(1975)estimated
themaximumnestcountat 1820Greatand5352LesserFrigatebirds,andweapplied
his conversionfactorsto estimate thenumbers inthe fourcoloniesusedatthattime
(Table2).
Reville(1980,1983)didnotusesamplingoradultnestratios,butcounteddirectly
allthenestsineachcolonyfromaboatwhentherewasmaximumoccupancybetween
August and November. He found little difference between 1976 and 1977 in the
numbersanddistributionofthetwospeciesinthethreecolonies(Table2).Thetotals
giveninReville(1983)areslightlyhigherthanthosegiveninhisthesis(Reville 1980),
and we assumed that the former are the revised, accurate counts. He found no
significant difference between years in the breeding success of monitored sub-
coloniesofeitherspecies(Reville 1980).Consequently,weappliedhisfledgling:peak-
nest-count ratio (0.67 for Great and 0.25 for Lesser; Reville 1983) to estimate the
average number offledglings in 1976 and 1977 (Table 2). This provides a crude
estimate ofthe number ofoccupied nests expected late in the season, to compare
withourMarch2000data. Ourcountwas29%higherthanthisestimateoffledglings
forbothspecies. The greatestnumerical differencewas intheFrigate colony.
Reville (1983) concluded that there had been little change in the numbers of
frigatebirds between 1967 and 1976-1977, andthatthe differencesbetweenhis and
Diamond's (1975) estimates were due to differences in methods ofcounting nests
andinestimatingmaximumnestcounts.Thedistributionandrelativeproportionsof
the two species had, however, changed, and were explained by the cessation of
human persecution, especially at the Frigate colony (Reville 1983). The relatively
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Alan E. Burger MichaelBern 242 Bull. B.O.C. 2001 121(4)
TABLE 2
Counts of frigatebirds at the Aldabra colonies in 1967 (Diamond 1975), 1976-1977
(Reville 1983) and March 2000 (this study). See the text for methods used to adjust
previous counts to allow comparison with the March 2000 count.
Colony
Dateandmeasure Gionnet Frigate MiddleCamp BrasTakamaka Total
Oct-Nov1967:estimatednestsfromfemalecount
GreatFrigatebird 46 42 583 60 731
LesserFrigatebird 1,428 257 1,685
Bothspecies 46 42 2,010 317 2,415
%percolony 1.9 1.7 83.2 13.1 100
Aug1967:estimatedmaximumnestcount
GreatFrigatebird 160 87 1.554 19 1.820
LesserFrigatebird 4.660 692 5,352
Bothspecies 160 87 6,214 711 7,172
%percolony 2.2 1.2 86.6 9.9 100
Aug-Nov1976and1977:maximumnestcount
1976 GreatFrigatebird 234 976 1,508 2,718
LesserFrigatebird 131 2,817 93 3,041
Bothspecies 365 3.793 1,601 5,759
1977 GreatFrigatebird 192 545 1.511 2248
LesserFrigatebird 116 2,369 355 2,840
Bothspecies 308 2,914 1,866 5,088
Mean1976-1977 GreatFrigatebird 213 761 1.510 2,483
LesserFrigatebird 124 2,593 224 2,941
Bothspecies 337 3354 1,734 5,424
%percolony 6.2 61.8 32.0 0.0 100
1976-1977:estimatednumberoffledglings
Mean1976-1977 GreatFrigatebird* 143 510 1.011 1.664
LesserFrigatebird** 31 648 56 735
Bothspecies 174 1,158 1,067 2399
%percolony 7.2 48.3 44.5 0.0 100
March2000:countofactivenests
Bothspecies 283 1,668 1,149 3,100
%percolony 9.1 53.8 37.1 0.0 100
*Estimatednumberoffledglingsassuming67%breedingsuccessinGreatFrigatebirds(Reville1983)
**Estimatednumberoffledglingsassuming25%breedingsuccessinLesserFrigatebirdsfReville1983)
smalldifferencesbetween 1976-1977and2000couldbeexplainedbyvariationsinthe
proportion ofadults breeding (see Reville 1980, 1983), or differences in breeding
success prior to the fledgling census.
Ourconservative conclusionis thatthe numbers and distribution offrigatebirds
in2000weresimilarto,orslightlyhigherthanthosein 1976-1977.Obviously,amore
detailed investigation is neededto showifthesepopulations are indeedstable. This
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Alan E. Burger MichaelBern 243 Bull. B.O.C. 2001 121(4)
should include identificationofthespeciesineachcolonyandideallycoverseveral
years to account for variations in breeding success. The work ofDiamond (1975)
and Reville (1980, 1983) provides avaluable baseline for such comparisons.
Changesinboobyandfrigatebirdpopulations
The available data do not permit firm conclusions about changes in the Aldabra
populations ofboobies andfrigatebirds. The fewcountsuseddifferingmethodsand
sampled at different times in the breeding seasons. There are insufficient data to
accountforvariationsintheproportions ofadultsbreedingandinbreedingsuccess.
The number of pairs and the spatial distribution of Red-footed Boobies nesting
NW
duringthe monsoonseasonhavebothincreasedsince 1967-1968.Theareawith
the most complete coverage, Malabar Island, showed a 51% increase in adults
NW
incubating during the monsoon season (Table 1). Part, or maybe all, ofthis
increase canbe attributedtothe cessationofpredationandreductionofdisturbance
bypeople after 1967. Although Feare (1978) thoughtthathumans were unlikelyto
have affected boobies on Aldabra, it is difficult to imagine that the boobies were
sparedthepredation and disturbance from fishermenwhowerekillingfrigatebirds
forfoodonAldabra, especiallyonwesternMalabar(Diamond 1975, 1979, Stoddart
1984). Most of the colonies of Red-footed Boobies in the Indian Ocean have
disappeared as a result ofhuman disturbance, introducedpredators, habitat change
anddirectpredation(Feare 1978,Carboneras 1992).
As discussedabove,therewere dramatic changesinthe distributionandrelative
proportions ofthe frigatebird colonies between 1967 and 1976, but Reville (1983)
concludedthatthe total breedingpopulationhadchangedlittle duringthatinterval.
The differencesbetweenthe 1976-1977 and2000 censuses appearrelativelyminor
and might be explained by variations in the proportions of adults breeding or in
breeding success. Itis surprisingthatthenumbers ofbreedingfrigatebirdshavenot
increasednoticeably since the period ofexploitation and disturbance byhumans in
the 1960s.
Recommendationsforfuturemonitoring
Direct counts of both booby and frigatebird nests from a small boat provide an
efficientcensusingmethod. Thecomplexsamplingtechniquesusedforbothgroups
byDiamond(1974, 1975) arenotnecessary. Reville (1980, 1983)reachedthe same
conclusion, and used direct counts ofall nests to census frigatebirds. Estimates of
thetotalbreedingpopulationsrequireextrapolationsfromnestcountstoaccountfor
nests missed, asynchronous laying and laying ofreplacement clutches. Long-term
monitoring, however, is best done using the actual nest counts, with minimal
extrapolation or correction ofdata. With suitable planning, logistical support and
tidal cycles, most ofthe booby andall the frigatebirdnests inAldabraAtoll canbe
simultaneously counted from a small boat in 10-14 days. Estimating year-round
populations ofboobies will require several counts to cover the different phases of
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Alan E. Burger MichaelBern 244 Bull. B.O.C. 2001 121(4)
breeding.
Aerial photography ofcolonies has been used to census seabird colonies, with
mixedsuccess(Harris&Lloyd 1977,Buckley&Buckley2000).Thismethodmight
be suitable for frigatebirds and boobies onAldabra, when aircraft are available at
the landing strip onnearbyAssumptionIsland. Simultaneousboat samplingwillbe
needed to ground-truth aerial photographs and identify the ratios ofbreeding and
non-breeding birds appearing in the photographs.
MonitoringofRed-footedBoobies
Laying appears to occurin at leasttwo fairly synchronisedpulses throughthe year
on Aldabra. Some pairs have eggs while others have large chicks or dependent
fledglings. Ideallyone shouldcountthe numbers ofnests duringincubationineach
breeding pulse. Monitoring of laying chronology will be needed to plan these
censuses fortheoptimaldate. Atleastonecomplete census canbe done atthe same
timeasthecompletecountoffrigatebirdsinmid-FebruarythroughMarch(seebelow).
Counts twice a year will also allow an index ofbreeding success to be calculated
(ratio oflarge chicks orfledglingstothe earliercountofnestswitheggs).
Counting roosting adults in proximity to the nests should be considered, but is
notessentialunlessDiamond's (1974)methodofusingnest:adultratio is appliedto
estimate nests obscured by the tree canopy. When reporting census data a clear
distinction should be made between direct counts and estimates made using this
method.
Monitoringoffrigatebirds
The most complete counts shouldbe done atthe peak ofincubation, after laying is
completedbutbeforemanynestshavefailed.Forfrigatebirdsthisraisestheproblem
ofdisturbance,becauseduringearlyincubationnestswhicharevacatedaregenerally
plundered by males seeking nesting material (Reville 1980). Clearly any census
undertaken during this time should be limited to parts ofthe colony which can be
m
counted from aboat 30 ormore fromthe nests to minimise disturbance andnest
desertion. Counts made during the late chick-rearing stage, such as our count in
March, are far less likely to cause nest failure because there are very few adults
seekingnestingmaterialatthistime, andevenifadultsareflushed,thecountsfocus
onthe large chicksremaining inthe nests.
As a compromise we suggest the following protocol for monitoring both
populations and breeding success of frigatebirds at Aldabra. The method can be
adapted for use in other areas. First, make annual counts of nests during early
incubation in sections (sub-colonies) ofeach ofthe three colonies. These sections
m
should be selected to allow counting from a boat at least 30 offshore to avoid
disturbingthebirds.Identifythespeciesofadultineachnestduringtheseincubation
countstomonitorchangesintherelativeproportions ineachcolony. Second,repeat
these counts ofselectedsub-colonies atintervalsthroughthebreeding season, orat
leastduringthelatechick-rearingphase,toprovideanestimateofbreedingsuccess.
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Alan E. Burger MichaelBens 245 Bull. B.O.C. 2001 121(4)
Third,makeacompleteannualcountoflargechicksinallthreecoloniesduringlate
chick-rearing(mid-FebruarythroughMarch). Thiscountcanbeconvertedtoatotal
nestingpopulationbyapplyingthe nest success inthe selectedsub-colonies, orcan
becomparedwithsimilarcountsinotheryearswithoutanyconversions. Identifyall
adults present during the late chick-rearing census to confirm the distribution of
species in the colonies. Monitoring the effects oftourists at the Gionnet colony
should continue.
Monitoringofthebreedingchronologywillbenecessarytoplanthetimingofthe
census. Most laying in Great Frigatebirds occurs between mid-August and mid-
November, but laying in Lesser Frigatebirds is more variable and prolonged, with
mosteggs laidbetweenJune andNovember(Reville 1983). Otherseabirds, suchas
White Terns Gygis alba nesting inthe mangroves, canbe counted atthe same time
as the counts ofboobies and frigatebirds, although additional effortwillbe needed
to include tern nests not near these colonies.
Usingkayaks forcensusing
Anoceankayakprovedtobeanidealplatformforcensusingfrigatebirdsandboobies
nestinginmangrovetrees. Theshallowinletsandnarrowsaroundthecolonieswere
easilynegotiatedwiththe kayak, whichhad a draught of10-20 cm, andit couldbe
towedorcarriedacrossshallowerbanks. Thekayakcouldbeusedoveramuchwider
tidalrangethana largerboatwith a deeperdraughtormotor, thusproviding longer
accessto colonies. AtlowtidesnoboatscouldbeusedinAldabraAtoll. Thekayak
could manoeuvre through the narrow channels around the colonies faster than a
powered boat, reducing the time needed for censusing. A motor boat was used to
carrythekayakto distantsites andprovide a safetyback-up. Finally, thekayakwas
virtuallysilent,greatlyreducingthelevelofdisturbancecomparedwithamotorboat.
Reville(1980)foundthatthenoiseofthemotorwasasignificantcauseofdisturbance,
androwedorpuntedadinghyaroundthefrigatebirdcoloniestoreducedisturbance.
Windandroughwatercanlimittravelbykayak, eveninthelagoon. Mostofour
censusing was done along sheltered shores but required travel across exposed or
openwater.An oceankayak, withacovered deck and sealed spray-skirt, permitted
travel in choppy water where an open canoe or small rowing boat would be risky.
Kayakingexperience,atwo-wayradio,andsupportingmotorboatareessentialsafety
requirements forthiswork.
Acknowledgements
Research by AEB in Seychelles was funded by a grant from the Second Dutch Trust Fund to
BirdLifeSeychelles.MBwasthewardenofAldabra,employedbytheSeychellesIslandsFoundation.
We thank the following for their important contributions to this project: logistics manager Susan
Barclay, boatman Philip Baccus, ranger Tony Jupiter, and other staffand volunteers on Aldabra
for logistical support, boat transport, advice and friendship; Ross Wanless (University ofCape
Town), Rachel Wiseman, and Jacques Harter for assistance and companionship at the field camp;
Ross Wanless for help in counting boobies; David Rowat and the crew ofthe M.V. "Indian Ocean
Explorer" for a very pleasant and rewarding voyage by AEB; the director, Nirmal Jivan Shah,