Individual Differences in Feeding Habits in a Newly Established Great Egret Casmerodius albus Population: Key Factors for Recolonisation

摘要

AfterhavingalmostcompletelyvanishedfromTheNetherlandsbythe19thcentury,fromthe1970sonwardstheGreatEgretisincreasinginnumbersagain,particularlyinthenewlycreatedwetlandofOostvaardersplassen.Duringthefirst25years,therateofre-colonisationhasbeenextremelylow,theresidentpopulationneverexceedingfivebreedingpairs.From2000onwards,however,followingtheextensionofthesurfaceareaofshallowmarshland,thenumberofbreedingbirdsincreasedto45pairs.ThispaperdealswiththefeedingecologyoftheGreatEgretduring1976a€“99,whennumberswereatalowlevel.Individualdifferencesinfoodchoiceandfeedinghabitswerestudiedinordertogaininsightintothekeyfactorsforestablishingandmaintainingahealthypopulation.Observationsonforagingbirdswerecarriedoutduring1987a€“92.Thebirdsforagedmostlywithinthereserve.FromMarchtoJulytheypreferredforaginginditches,changingtoReedbordersofshallowpoolsanddrygrasslandsfromAugustonwards.Intheditches,Three-spinedSticklebackGasterosteusaculeatus,othersmallfishesandtadpoleswerethemainprey(91.4%ofallpreyitems).Intheshallowwaterbodiestheegretscaughtalmostexclusivelysticklebacks,andingrasslandsCommonVolesMicrotusarvalis.Onebirdwasobservedfeedingnearfishingboats,takingPerchPercafluviatilisandRoachRutilusrutilusof10a€“20cmlength.Whenfeedingonsticklebacks,aGreatEgrethadtoforagefor1.5to3hourstofulfilitsdailyneeds.Whenscavengingonlargerfisharoundfishingboatsonly15minutessufficed,buttheenergyexpenditurewaslikelytobehigherduetocommutingflightsandinter-specificcompetitionwithGreyHeronsArdeacinerea.However,forthemajorityofGreatEgretstheshallowandtransparentwaterinditchesandpoolswithahighabundanceofyoungsticklebacksandsomecoveragewithemergentvegetationsprovedtobethemostprofitablefeedinghabitat.ThedecisivefactorunderlyingthespectacularincreaseinrecentyearshasprobablybeenthecreationofclearandshallowfreshwaterpoolsandinundatedgrasslandsintheformerlydryborderzoneofOostvaardersplassen.Theseforaginggroundslieclosetoundisturbedbreedingsites.ThisfindingmayproveusefulforwetlandrestorationelsewhereinTheNetherlands.