Nestling Feeding-Space Strategy in Arabian Babblers

The Auk 116(3):651-657, 1999 NESTLING FEEDING-SPACE STRATEGY IN ARABIAN BABBLERS RONI OSTREIHER 1 Departmentof Zoology,Tel Aviv University,Hazeva Field Study Center, D.N. Arava86815, Israel ABSTRACT.--Maintainingindividual spaceis a foraging tacticwidely usedby many adult animals.However, this behaviorhas not been describedfor altricial nestlingbirds. In this study,nestling Arabian Babblers(Turdoides squamiceps) defended individual spacesaround themselvesand obtainedfood in relationto the sizeof thesespaces. Each nestling created two circularfeeding zones around itself: (1) an internalone (termed exclusive space), within whichit usuallysucceeded in preventingnest mates from obtaining food; and (2) anexternal one(shared space), within whichthe nestling competed for foodwith siblingsaccording to its relativestrength. The first indicationof the imminentdeath of a nestlingwas when sib- lingsbegan receiving food insideits exclusivefeeding space. Stability of feedingspaces was testedby addinga fifth nestlingto thenest. None of theexclusive feeding spaces decreased, and feedingrates within them remainedconstant. In contrast,feeding rates in the shared feedingspaces diminished as a consequenceof their reductionin size.It seemsthat a nestling'ssurvival depends on its abilityto defendits exclusivefeeding space. This tactic may enableArabian Babbler nestlings to obtainfood and survivein their cooperativeas well as competitivenest environmentin a relatively nonaggressivemanner. Received22 December 1997,accepted 13 November1998. AVlAN BROOD REDUCTION has been the sub- tionsof siblingcompetition. All of thesebehav- jectof muchempirical and theoreticalresearch iors, however, seem to be directed toward the sincethe appearance of O'Connor's(1978) sem- feedingadults. In the Blue-footedBooby (Sula inal paper. So-called altruistic behaviors,as nebouxii; Drummond and Garcia Chavelas well as selfishones, were explainedthrough 1989), Great Blue Heron (Ardeaherodias; Mock broodreduction using a wide rangeof theories 1985,Mock and Parker1986), Great Egret (Ar- suchas inclusivefitness, the handicapprinci- dea alba; Mock 1984, 1985), American Kestrel ple, parent-offspringconflict, and honestsig- (Falcosparverius; Anderson et al. 1993),Arabian naling. Babbler(Turdoides squamiceps; Ostreiher 1997), Unlikeits theoreticalexplanations, the mech- andothers, food distribution among nest mates anisms of brood reduction have attracted less is determinedby the outcomesof nestlingin- attention.In nonpasserines,sibling aggression teractions.It seemsunlikely that in passerines, is very commonand leadsto both unevenfood no direct interactionshave been developed distribution and brood reduction (Cash and amongthe nestmates during the evolutionof Evans 1986, Mock et al. 1987, Anderson 1990, nestling competition. Forbes1991, Drummond 1993,Ploger 1997). In Arabian Babblersare cooperativebreeders passerines,however, aggressive interactions (Zahavi 1990).The modal clutchsize is four, amongnest matesare rare. Althoughnonag- and incubationusually starts after the lastegg gressivebrood reduction has been widely doc- has been laid such that broods hatch almost umented(Magrath 1989, Teather 1992, McRae synchronously,with only6 to 34 h passingbe- et al. 1993, Kacelnik et al. 1995, Price and Yden- tween hatchingof the first and last chicks. berg 1995,Leonard and Horn 1996,Ostreiher Feedingrate is negativelycorrelated with 1997),its mechanismsare still enigmatic.Beg- hatchingorder, nestlings compete for food,and ging calls (Harper 1986,Smith and Montgo- brood reductionis frequent (Ostreiher1997). merie 1991), jockeying for a better position Peckingamong the nestlingshas neverbeen (Gottlander1987, McRae et al. 1993),body pos- seen(based on morethan 1,100h of observation tures (Kilner 1995),or a combinationof these fromhatching to fledgingin 42 nests),but nest behaviors (Kacelnik et al. 1995, Kilner 1995) matesmay struggle by pushingeach other. The have been considered as the main manifesta- aim of the currentresearch was to studymech- anismsof siblingrivalry usedby ArabianBab- E-mail: [email protected] bler nestlings. 651 652 RONIOSTREIHER [Auk, Vol. 116 METHODS as the nestling'sfeeding-space center. Distance from eachsquare in whicha food item wasobtained to all The studywas carried out at the ShezafNature Re- four nestlingfeeding-space centers was measured in serve in the Arava Valley and around the Hazeva mm from the square'scenter to the feeding-space Field Study Center,about 30 km southof the Dead center. Sea,in southeasternIsrael. These babblers are part of Eachnestling's ability to control the nest surface an ongoingstudy that was startedin 1971 (Zahavi arounditself was measured by countingthe squares 1990).The currentstudy wasconducted over the five within whichfood was obtained. Each square on the breeding seasonsfrom I Februaryto 1 June,1992 to nest surfacewas scoredfor eachnestling into three 1996. categories:(1) squareswithin whichit receivedfood The study area containedabout 35 groupscom- exclusively;(2) squaresin whichboth it and one or prisingmore than 240 individuals.The birds are ac- moresiblings received food; and (3) squareswithin customedto human presenceand tolerant of close which it did not receive food. This division was valid observations.Each group was observed at leasttwice as long as the nestlingstayed in the samelocation. a week, and groupsthat had started nest building When a nestlingchanged its place,the classification were observeddaily. Nestswere visited every day, of squaresalso changed.Accordingly, I prepared and eachegg was marked to establishlaying order. separatemaps for eachplace in which at least one During the lasttwo daysof the 14-dayincubation pe- feedingwas received.Each nestling's feeding maps riod, nestswere checkedevery hour and were ob- were placedone abovethe other,keeping the feed- servedcontinuously from first to lasthatching. Each ing-spacecenter and nestling direction constant. nestlingwas marked with a coloreddot immediately Each nestling thus accumulatedreference points afterhatching. A coloredcollar was fitted to its neck with every feeding event around a constantpoint. and a coloredwire to oneleg on day 4 afterhatching. Combiningall of the referencepoints enabledthe The collarswere exchangedamong nestlings at the mapping of eachnestling's feeding space. Squares end of eachobservation to preventpossible feeding that were not used, but were surrounded on all four biasassociated with certaincolors. Each nestling was sidesby squaresbelonging to a singlecategory, were bandedwith a four-colorcombination at 9 or 10 days assignedto that samecategory. old, and the collarwas removed on day 12 or 13,one To examinethe flexibilityof feedingspaces, a for- or two days before fledging. eign nestling,which was older than the nestinhab- Observations of nests were made from 0.5 to 2 m itants by one or two days,was introducedinto each distance.Many groupsbecame so habituated that of 10 different four-nestlingnests. The experiment most or all of the adults did not mob experimenters wascarried out in 10 of the original19 nests,on the as they handled nestlings.Daily 3-h observations, ninth or tenth day posthatching,following the 3-h startingat first light, were madeat 42 nestsin which morning observationperiod. Feeding rate was re- four nestlingshad hatched.Data presentedhere are cordedfor 3 h, beginning1 h after the nestlingin- of 285observation hours on days8 to 12posthatching troduction.The five nestlings'feeding spaces were for 19 nestscontaining four nestlingsthat success- measuredand comparedwith their feeding spaces fully fledged. An additional 65 observationhours prior to the experiment.At the end of eachtrial, the were carriedout for five nestsin which one nestling foreignnestling was returned to its natal nest.Each died. trial involved a different foreign nestling.Adding a Activity in the nestwas recorded with a SVHSvid- fifth nestlingfor 4 h did not changethe feedingrates eo cameralocated vertically above the nest. Videos on the next morningbecause feeding rates during 3 were downloadedto a computerusing Fast Movie h on the following morning did not differ between MachinePro hardware and software and analyzed in the 10 experimentaland 9 nonexperimental(same- slow motion to an accuracyof 1/25 s. At eachnest, aged nestlings)nests (Wilcoxon two-sample test, n• a glasswith a grid of squares(each square 0.5 x 0.5 = 10, n2 = 9, U = 42.5, P = 0.21). cm) wasplaced briefly on the nestsurface when the When an adult landed on the nest rim, the nes- adults were away. One video frame taken with the tlings rose,turned their beaksupward, gaped,and glassgrid in placewas usedas a referenceto locate called. In the courseof this extension,they often in the computeran overlappingtransparent grid, pushedeach other so that someindividuals were made in Corel Draw software, above the nest surface. forced to subside or even lose their balance and read- The locationsof all feedingswere then mappedon inessto be fed. Extensionsand pushingevents some- the nestsurface using coordinates of the grid. times occurred independent of adult landings. ! Nestlingsreceived food only when in a staticpos- measuredpushing efficiency of individual nestlings ture, not while moving.The centerof the openbeak as the proportion of pushesthat were followed by while receivingfood was determinedas a reference feedingsto the pusher.A feedingwas considered to point. Eachtransfer of a prey item from feederto be a consequenceof a pushif the sequenceof feeder nestlingcontributed

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