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POPULATION of the SILVER GULL Larus Novaehollandiae on THE

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POPULATION of the SILVER GULL Larus Novaehollandiae on THE Corella,1988, 12(4): 113-118 POPULATIONOF THE SILVERGULL Larusnovaehollandiae ON THE CAPRICORNAND BUNKERISLANDS. GREAT BARRIER REEF T, A. WALKER OueenslandNational Parks and Wildlife Service. P.O. Box 5391.Townsvillc Mail Centre.Oueensland 4810 Re(eired29 S.ptembet. 1985llevis<l 6 May, 1967 SilverGulls were censusedirom January1983 to March 1985 on the Capicorn and Bunker lslands.Gulls moved between the islands,congregating at siteswhere food scaapsand peoplewere most abundant.Such siles were permanent,as at Heronlsland. or temporarywhen campersand lrsrrermenwere presenlal otherislands. On wtnterdays up to 75% of gullsin the areasought food scrapsal the Heronlsland resort. In summerthe numberpresent was reducedto essthan 25% when secludedislands were soughl lor nesting.Gu ls are an importantpredator at lern and boobycolon es andthis p.edatiorrs increas,ngas a resuko, ruman aclv lies. INTRODUCTION as a result of human activities (Hulsman and Kikkawa 1984). Gulls scirvenge table scraps. The prescnt century has been a time of grcat grrhrgc and tishingoffal. I here rrrenu quantita- population expansion for many speciesof gulls tive data on the importance of thesefood sources around the world. Thc increasc is whollv or locallv but somc cstimatesfrom other parts of the prrtirtllydrre to provi'ionot lood hr man r uiban. world suggestlhat gulls and.\avenging \(Jbirds maritimc and rural activitics (Bourne 1983. obtain as much food from commcrcial fishins offal Burger 1983, Blokpoel and Tessier 1985). The asthey ohtain nirlurally {Bourne I9R.1).Thc \ingle Silvcr Gull Larus novaehollardi..reof Australia. most important source of food for gulls on the New Zealand and South Africa is no exception Capricorn-Bunkcr Islands appeais to be and it has cxpanded substantially over rcccnt foodscrapsand garbage at Hcron'Island. decades (Wheeler and Watson 1963. Blakers, Daviesand Reilly19t14). Breeding gulls on the Five Islands in New South Wales, for examplc. incrcasedin number from about 2 000 in 1940t<r STUDY AREA AND METHODS over 100 000 in 1978(Gibson 1979).In Australia the Silvcr Gull is most abundant in tcmperale The Capricorn Group consistsof nine islands. regions but also occurs in the tropics. Ncsting Thc Bunker Group lies to thc south and has three occurs on islands on the Great Barrier Reef. islands t*'o of which (Fairfax and Hoskyn) are notablv those of the Capricorn and Bunker double islandsmaking a total of 14 in the region. (Jroups. Ail are wooded sand or shingle ca1'sand ntany have Pisonia grandis loresl. The isiands are on The Silver Gull population is consideredro be coral reeli at the southern end of the Great unnaturall.vhigh on the Capricorn-BunkerIslands Barrier Recf (Fig. 1). I A. l4la/kerSilver Gulls on Great BarrierHeef Cote )a 1214) obsen,cd,1l1,ingto and irorn thc surroun(ling TrrY f r' Irljrrrl..Ml.\iutunt nult.\1.r.,,.turr..rl,ltrrini 1' r.1 t,;t:r ;t a w,int.rnrt'nrh. \\hcn Lr lF urrl(,,,,,'.,,i1.,ra;,. 1,,,.*.;;;- raf. ,lr',nj'c\J lr)tI ;)rllll I'ci'r\\ 1lq11i111rlr. .lrrrrr]]..r. KIkkir\;r(l()-{l) l(l,,ft,-,1 i. nr,,r||nrirl],,1ltrrr5rrll. :rl I lcl,,Dl\l. l(l in ll)r I;rt,I,r,,rr'. ,rD,l .,,tr,id..r...1 thrs to bc an incrcltscor'cr. I.lr.crious tcars. Hr r.rrrl.irrrr.l i. rh, t.,,;rln,,ilrl I,,f I\,,l,uli,tr,,n. a "l 11,)tlrpf,)l'lr':rrr,l Silrt.r (,trll. irr rlr.:rr..:,. lrhrLr N an arcll ol approxintatclrl9 ha lrnd supportsll rounst [esort lrnr]ir Lrnivcrsilrhelcl statioD. rr[]'i/ I t . Thc 'j, rlcstcnl hall is corcrctl r.Lri l]1)i,/.i,lrrs I , sitlt iruilclingslnri r.oacls itnd ttre eastern hall is iirrestecl.In lgg"l the hLlnranpopLllati()n itrcragcd itl)oul 190 hut roscto Ii{urc I 1-or.,rrrr)rrarrrTr roughlv380 tt linlc\. Lach clavhrotlscraps lronr thc Icso atc duntpccllrt sclr.Hurrtl|ccls-0f gLtlis ;ru'aitthis e\'ent ()n the l)cacltitntl lollo\ the dunrprngboit! oui to 1ced.(iull chicksbandcd on Sihcr Ciulls \\'crc countcd al llcron Islald other (lapricornantl Bunkcr Isllrntlshare bcen sercral tirner each mitnth during l9lt.l. (.ounts loun(iilt I Icron Islanclu ithin ntonthsof brinding. wcrc tirkcniluring hith or nriclticlc to cnsurcthal llrc lIt-ds \\'efe not out fola-qiDgon the exprtscd Balrctt ( lt)10)lotcd tltarSijvcr GiLlls hatl ncsrs rcc'1.Most .sullsoccurrcri ori thi bcach.prirticu- at Hclon Island in a)ctobcr l9l0 brrl C'oopcr. (19-13) lltllr atountithc uesterrthltll ol the islanri.SnIrll \'is;tcdIlc'ron Island in Dcccnbcr li).l(r nunrbcrsalso ll-cquclttc(l kitchcn ancl dinilg tuca-s irndrcportcd thitt lis il r-csrlltoi thc cstatrlishment qhclc food \crap\ \\'ercarlrilable. ( ountiirgrvls ol il lourisl ccntrc thf gLrllsanrl ground-ncstiItg .rntl\lif(.1l'\ llr( r.,.1th..r rrrll. n!\(r \\.rrii t,, 1r'rnsno longerbre(i therc. Sinccthen therehavi lhc Iore\l bccn occilslonaifcports ol pails ol gLrllsncsting on IIeron Islill)d (Wheeler ancl Watson (iulls l9(r-1. uclc c()untcd scvcrlll timcs l)ctwccn Ilrrlsnlrn irnclKikkil\\'l l9ll-1)but brccrlingbircls .lrtnuarvl()fil ancl\'lrircir 1985 on all islandsin the lnvariairlYseck lhc ntoresecludecl islands to nesi. thc rcgione\ccpt Onc Trcc Isllnd. Ciruntsu'cr-c- madc on thc bcachb) lalkin! nrrrntlthc perinl- ctcr ()l cachislancl. Llnfrtrcstctl alcas ancl cantping ilrelis\\'etc i so rnspccicd. 1O00I Dail\'()ccuputioltoj thc islancjshv cenpcr-sirr l9SJ \\ls clrlcrrlirlcdlront pcrnrits issrLcdbv OuccnslandNational Prirksancl Wilcllilc Scrvicc lrnd nroclilicclbr tie-ldobserrations ol irctual - 700 ciurlpcrsprcscnl. Dailv attcnrlanccol tcssclsat 3 ooo thc islanclsancl ussociatedreefs \\'asdelived front trr'icc$ ccklt ofrsc'rrations irt surrcillancc llights .100 throughoul l9fi-1.The lotiil numbersoi vcsscls ,j rc'cordcrllt cach rccl clLrringthc \cnr wcrc 300 ciiliclccibr the numi)crol llightsto cive un iittli- 200 cati!c diiil\ \ aluc. 100 RESULTS AND DISCUSSION o 'Ihc rcsultso1 gull coiLnlsa1 IIeron lslandurr stroun in Figrrlc l. Nrrnrbcrslluctuatcd \\'idcl\ Irtllurc i \i,,,ff ,1 ,,1.f,1, ,) (;tll\ d llu)tt !\ ltrl tlutitryt:].lr llom dar 1oria\ lrnddLrring thc rinr as gullsu,crc December.1988 T. A. Walker silvet Gr s on Great Barr er Reef Nunrben of gulls jntcrtidal 'f at thc other islandsare sum- zonc. As rvcll thcre is somc consurrrlt- nurizccl in ablc l. Dctailcd examination of tion of insects (F-mbur! 1933. Cooper. 19,18. counts rcveiLlcdthrec lacts.Firstlv. numbersof Hulsn'tirrr 197.1). bcrries (c g. .Solanunr gullson an islancioften varicd greatlvover short ttnericanrut) and tLLrtlehatchlings $,hcn lllcsc are IlnlLrruri,rLlr. Sr;, rr,llr.nr,,r,.nicnt ,,1 !:trll. k,,m scasonallvavailablc. lt appearsthat if thc abovc islanclto island *as stronglv influenciaib_v the sourccsof foocl arc the onlv attritctionof thc lrr(\e1Lr','lI\(',l\l(. Tllrrrll\. Ih(,qntrner,l( r:r( ir.( islanclthcn thc nuntbcrol gullsislo\\'. Ihis is bc\t iu gLrllnLrrrhers ilt LIcron Islandcoincide'd with dcmonstrirtcdat North ReefIsland which hacltlrc lr) rncrcirsein gull numberson manv of the nest- lowcstgull visitationoi unv islarrd(lablc l). At inq islancls.An cstimatefor thc total avcragcgull this island.lvian fooclsources (seabirci colonies) population 'lirtalon the islandsin l9ll.lis approxirnately are feu' (Walkcr ancl Dolnrn l9116):thcrc arc no I 100. nun]bcrs lluctuatc as a rcsult of gull nestingsites ulcl the lvailabilitv ol human mo\enlent to and fronr the rnainland(Hulsrnan tir,',1.(rill..r. rrrin,'r.,- errJr.rrcr',1Irr rlrt l,'u 1979.also bandinglcturns). Thc closestisland is hurran visilation(Table l). little more thrn an hour's flight from shore (.15 knr). Silvcr Gulls flr'up to three tincs this dis- Avitn Iootl sottrtes lirnccto ieed on clurrps (van 'l'cts larbagc in Svdnev Gulls arc thc major nlturiil prcdator ol eggs 1969). luntlchicks ilt tcrn and boobv colonies in thc Ciulls arc lttritcted to u particularrislancl litr rcgion. IIigh clutch mortalilv iLrpto 100'11,)has feccling anclkrr.brcedinq. In thc Capricorn- bccn obscrvcd at smiill tcrn colonies(Emburv Bunker rcqion iour tvpcs of attractantcan bc 1933.Coopcr 19,{E.Llulsman 1977). Ciulls also dcplivc chicks of lirocl bv robbing fish lronr thc dcscribe(l:natural non-iivian foocl sourccs; avian 'l ioo(i sources:human litod sourccs:and ncsting lriir(|lt. llllrl\nl.,lt It)-r'] hesq .r!llviti\.s iilL sitcs. scasoniLlirnd are rlost cxtt:nsivca1 ciist Fairfax Islnnd. u'lrichhas thc onlv largc Brorvn Booby Srtlu lurtogLtstcrcolonv in the region. Brown ,\ttlureI tkDt-utittrt lltod sourte.; Boobicsbrccd mrinlv from August to April. Egg t.-it1urlrl footl consistsprirnarilv t)f smtll irlvcrtc- or fish robbing bv gr.rllsis highlv clcvclopecl.par- burtcs and fish foragccl fronr thc rccf flai iLncl ticularlvwhcn boobiesrrc disturbedat thcir ncsts TABLE I Visitation l(JCapricorn a d Uunkcr Islandsbv campcrs(198,1). vessel\ (t981) and Silver Gulls (.tanuar). 1983March l9E5). Campcr\/day Ve\.elJdi,y Silvcr (;ul1s lsland (na) !lean Max. Reef Me an Melln Min Max (n) North Rccf I 10 1 :l ll livon ll ll 30 l.; 19 L: 120 13 105 l: 150 1.I 5l '1 11',7 20 5l :5 0.5 51 6 120 12 0 70 .1 230 t2 Nhstherd l0 t9 s0 ]E I t0 l-i 17 t2 Erskine l- t7 l0 70 1l Floskvn(ensl) l 0.5 16330 I t{o\kvn (lvcst) ) 69 : 200 lJ Frirlar (east) .100 0.u 133 2E l0 .+- ) :1 6 .10 6 Lrdt Uusgrave 75 6.2 100 17 260 t6 116 T.A. Walker:Sitvet Gulls on Great Barrierueef Corella12(4) r:r.n...,)t pconte,Atrer Heronlsland, 160 tll.ll^,1..,f numbcrsut gullswer( presentat easr I,rrrlaxf,gh::, (Tahle l\land ll. Thisis a|lribulcblero r40 o o {ne presenccof thehoob} colony because human ""r relari\elvlou andhcnce tbotlscraps ;,:i,i,ll.tj| o 120 O J J 100 f a o O E El) LIuman Jood sources u.l o'o 60 a .
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