The Auk

A QuarterlyJournal of Ornithology Vol. 114 No. 4 October 1997

The Auk 114(4):553-561, 1997

BROOD DIVISION IS ASSOCIATED WITH FLEDGLING DISPERSION IN THE BLUETHROAT (LUSCINIA S. SVECICA)

KRISTINANTHONISEN, CHRISTIN KROKENE, AND JAN T. LIFJELD• ZoologicalMuseum, University of Oslo,Sars gate 1, N-0562Oslo, Norway

ABSTRACT.--Inmany bird specieswith biparentalcare, each parent takes the exclusive care of someof the youngafter fledging.Some of the hypothesesthat havebeen put forth to explainbrood division behavior state that it is advantageousfor a particularparent to care for a particular"type" of young,e.g. with respectto sex,size, or parentage.Other hypoth- esesclaim a benefitto the parents(e.g. reduced foraging costs or risksof predation)only whenthe youngare spatially dispersed. In thispaper, we describebrood division in a Nor- wegianpopulation of Bluethroats(Luscinia s. svecica).In general,brood division arose once theyoung became spatially dispersed after fledging. The only exceptions to therule occurred whenthe male was polygynous and provisioned the young at a lowrate. No brooddivision wasfound when the young were still in the nest,nor when they were physically prevented from spacingout by an enclosurearound the nest.Young fed by the sameparent were more clusteredthan young fed by differentparents. Experimental switching of youngamong sin- gle-pargntgroups suggested that parents were able to recognizeindividual offspring outside the nest.However, there were no indicationsthat parents divided the brood by sex,size, or geneticparentage. Our data are consistentwith hypothesesthat assumea parentalbenefit frombrood division when the youngare spatially dispersed. Received 13 September1996, ac- cepted7 April 1997.

IN MANYBIRD SPECIES with biparentalcare, 1978,Moreno 1984, Harper 1985,McLaughlin the parentsdivide the broodafter fledging so and Montgomerie1985, Kopachena and Fails that eachparent takes sole care of someyoung 1991). (Skutch1976). This phenomenonis known as In many altricial species,particularly open brood division(Smith 1978,McLaughlin and nesters,the youngleave the nestearly (Maher Montgomerie 1985), and the associationbe- 1964),often beforethey are completelydevel- tween a parent and its young is termed a "fam- oped and able to fly (Tinbergen1939, Skutch ily unit" (Nolan 1978).Many reportson brood 1976, Knapton 1978, Kopachenaand Fails division are anecdotaland provide no data. 1991).Early nestdeparture coupled with spa- However, some studies of brood division are tial dispersionof young is viewed as a strategy moredetailed and well documented(e.g. Smith to minimize the risk of predation(Tinbergen 1939,Maher 1964,Willis 1972,Knapton 1978, Nolan 1978)and/or to help parentsreduce the Addresscorrespondence to this author.E-mail: energeticcosts of parental care (McLaughlin [email protected] and Montgomerie1989a,b). Spatial dispersion

553 FRONTISPIECE.• male Bluethroatwith food for its young.This male was part of an experimentalstudy showingthat the colorof leg bandsaffects the male'smate-guarding intensity during the female'sfertile period (Johnsenet al. 1997).Photo by JohnnySteen. 554 ANTHONISEN,KROKENE, AND LIFJELD [Auk, Vol. 114

of young after fledginghas been reportedin in relationto variouscharacteristics of young, severalspecies that exhibitbrood division(e.g. includinggenetic parentage. Hann 1937, Marler 1956, Nolan 1978, Moreno 1984,Kopachena and Falls 1991),and it could METHODS be a proximate factor that initiatesbrood divi- sion (Smith and Merkt 1980, Moreno 1984, Fieldwork was carried out in 1992 and 1993 in Linkhartand Reynolds 1987). It is importantto Ovre Heimdalen (61ø25'N, 8ø52'E),which is located distinguishbetween dispersion,which is a east of the JotunheimenMountains in southernNor- characteristicdescribing positions in space, way at an elevationof about1,100 m. The vegetation of the studysite was dominatedby clumpsof dwarf and brood division, which is a behavioral char- birch (Betulanana), willow (Salixspp.), and juniper acteristicdescribing how parents allocateef- (Juniperuscommunis). The openhabitat made it rela- fortsamong their young. Hence, brood division tively easy to locate and observethe parents and doesnot necessarilyrequire that theyoung are their young after nest departure. The estimated spatiallydispersed. breedingdensity in our study area was 38 breeding Several alternative hypotheseshave been pairsper km2. For a moredetailed description of the suggestedto explain the benefitof brood di- study area see Vik (1978). vision.Some of theseassume that the young are TheBluethroat is a predominantlysocially monog- spatiallydispersed (either singly or in groups amous,territorial, ground-nestingpasserine. Both fed by eachparent), whereas other hypotheses parentsfeed and defendthe offspringbefore and af- ter fledging(Cramp 1988). The young leave the nest are neutral to any spacingpattern. One hy- beforethey are ableto fly (Harrison1975, Arheimer pothesisfor dispersedyoung suggests that par- 1982),at which time they usuallybecome spatially entsincrease their foragingefficiency when di- dispersed(Peiponen 1960, Theiss 1973, Koch 1983). viding the brood (Simmons1974, Smith 1978, Juvenilesare sexuallymonomorphic (Cramp 1988). Moreno 1984, McLaughlin and Montgomerie The speciesis single-broodedin the northernpart of 1985, Byle 1990), and anotherstates that brood its range, which includes Norway (Harrison 1975, divisioncan minimize the effectsof predation Cramp 1988). (Smith 1978, Harper 1985, McLaughlin and Adult birds were caughtin mist nets and given Montgomerie1985, Byle 1990).Other hypoth- uniquecombinations of colorbands. Eight to 10 days esesstate that it is advantageousfor a partic- after hatching,nestlings were individuallymarked with a colorband on eachtarsus and a spotof acrylic ular parent to feed a particular "type" of paint of similar coloron the forehead.Several mor- young,i.e. with respectto sex,size, or parent- phologicalmeasurements were taken of adults and age (Harper 1985,McLaughlin and Montgom- young,including body mass(- 0.1 g), tarsuslength erie 1985;see also Price and Gibbs 1987, Byle (+ 0.1 mm), and wing length(+_ 0.5 mm). We also 1990).Because these hypotheses do not assume collected blood samples for DNA fingerprinting anything about the spatial distribution of analysis.See Krokene et al. (1996)for detailson the young,brood divisionmight well occurwhile methodsand resultsof the parentagestudies. the youngare still in the nest.No hypothesis To determine whether brood division occurred hasproved to be of generalvalidity, and Harp- while the youngwere in the nest,five broods(four er (1985) pointed out that the variousexpla- of which reachedfledging) were videotapedfrom 3 nations for brood division are not exclusive and to 9 h betweenday 10 posthatching and fledging. The camerawas placedon a tripod 0.5 to 1.5 m from the do not need to be equally applicableto all spe- nest.We recordedidentity of the parentand the par- cies. ticular young that receivedfood during all feeding In this paper,we reporton the occurrenceof visits. If a parentfed more than one youngduring brood division in the Bluethroat (Luscinias. sve- the samefeeding visit, eachfeeding was counted cica). In particular, we examined whether separately. brood divisionwas relatedto the spatialdis- In order to examine whether brood division was a persion of young, and we looked at parental consequenceof the young becomingspatially dis- provisioningto individual young both in the persed, enclosureexperiments were carried out at threenests. A wire-meshenclosure, approximately 1 nest and after fledging.We alsocarried out an m in diameterand 0.35 m high, was placedaround experimentto see how parentsallocated their the nestsbefore the youngleft the nest,preventing provisioningwhen the fledglingswere pre- themfrom dispersing after nest departure. Food pro- vented from dispersingfrom the nest. In ad- visioningto individual fledgedyoung was observed dition, we examined how broods were divided from a blind close to the enclosure. There were two October 1997] BroodDivision in the Bluethroat 555

observationperiods per brood,each period ranging dication of brood division while the young from 45 to 155 min. The enclosure was removed im- were in the nest.The only caseswhere the par- mediately after the last observationperiod, which ents seemedto favorparticular young or had ended 25 to 50 h after the young left the nest. different feeding distributionsoccurred in two After the youngfledged we collecteddata on the parental feedingpatterns by observingthe parents nestsof a polygynousmale (Table1). and their offspringfrom distancesless than 50 m. Nestlingsleft the neston average11.6 days The first observationperiod occurred on the day the after hatching(range 10 to 13, n = 17). These young fledged (day 0) or the next day. We tried to are minimum estimates because some of the observebroods on at least three different days.Ob- nestdepartures may havebeen induced by us. servationperiods (n - 62) lasted from 20 min to 4 h In threenests, all youngfledged during 3, 4, or (œ 125 min), which generallywas long enoughto 7 h, respectively.In four othernests, young left see severalfeedings per offspring (averagenumber the nestasynchronously, with as muchas 48 h of feedings per young per observationwas 5.75, elapsingbetween departure of the firstand last range 1 to 18). Observationswere performed between 0800 and young. 2200h and were carriedout by two to five observers. Provisioningafter fiedging.--Most of the With onlytwo observerswe had to followone parent Bluethroatparents divided their broodsafter each. With more observers we were able to monitor nest departure (see Tables 1 and 2). Generally, eachyoung. Fledglings usually remained at the same brood division was not observed unless the location for long periods of time, and they often young were spatially dispersed.In six families madebegging calls. When we were reasonablysure whereparents and someor all youngwere ob- of the positionof a fledgling,we oftenapproached it servedmore than one day afterfledging, stable carefully to determine its identity. Otherwise, we waited until the end of the observation session to brood division was observedduring the first observationperiod on day 0 or day 1 (family avoiddisturbing it. When the watchwas finished, we 2/92, 6/92, 13/92, 15/92, 57/92, and 61/93; estimatedall inter-youngdistances and the distances from eachyoung to the nest. Table 2). By "stable brood division" we mean Sixof thenests were not founduntil afterhatching. that the parentalfeeding pattern, i.e. whichoff- The age of young from thesenests was estimated springeach parent fed, was the sameduring from growth curves(body massand wing length) eachobservation of that particularbrood. drawn from a sampleof broodswith known hatch- In five other families where parents and at ing dates(Rangbru 1994). Altogether, 58 offspring leastsome of theyoung were observed for more from 17 broodswere observed after fledging,among than oneday, stable brood division was not ob- which41 offspringfrom 13broods were observed for serveduntil oneto threedays after fledging.In more than one day. Blood samplesfrom 10 of these three of these families, however, broods were families were analyzed by means of DNA finger- printing to assignparentage (Krokene et al. 1996). divided at the first watch,but the divisionpat- Becausefledglings are mobile and sometimesmix tern changedduring the following days. The with other family units, we would expectparents to time elapsed until division became stable co- be able to recognizetheir young. Hence,we made incided with increasesin dispersal distance two experimentswith a fledgedbrood (brood2/92). from the nestand inter-youngdistance during In the firstexperiment, we swappeda male-attended the daysfollowing fledging. On average,fledg- and a female-attendedfledgling. The distancebe- lingswere located farther from the nestduring tween the two positionswas approximately60 m. In the secondexperiment, one of the female-attended the last than during the first watch (Table3). fledglingswas first replacedwith, and then released Likewise, the mean distancebetween young togetherwith, an unrelatedfledgling from another fed by different parentsincreased significantly brood (7/92). Nest 7/92 was locatedapproximately with time, and the increase in distance between 100 m from nest 2/92, behind a small hilltop. youngfed by the sameparent was nearly sig- nificant (Table 3). RESULTS The mean dispersaldistance from the nest did not differ betweenyoung fed by male and Provisioningin the nest.--In general, males female parents (Wilcoxon matched-pairs and females did not differ in provisioning signed-ranktest, Z = -0.87, n = 10, P = 0.39; rates,and eachparent appearedto distribute Fig. 1). However,young fed by the sameparent food evenly among their young during the stayedcloser to eachother than to youngfrom nestlingperiod (Table1). Thus,there was no in- the other family unit (Z = -2.55, n = 9, P = 556 ANTHONISEN,KROKENE, AND LIFJELD [Auk, Vol. 114

TABLE1. Provisioningpatterns in five Bluethroatfamilies during the nestlingperiod, insidea wire-mesh enclosure,and after dispersalfrom the nest.Values are total numberof feedingsobserved. Offspring are listed in order of decreasingsize. Feeding Offspring Location parent A B C D E F P• pb Brood 14/92 (Primary nest of polygynousmale) Nest Female 26 19 19 19 25 9 0.09 Male 8 11 10 5 3 11 0.22 0.02 Dispersed Female 35 11 5 41 0 dead 0.0001 Male 0 0 4 0 24 dead 0.0001 0.0001 Brood 16/92 (Secondarynest of polygynousmale) Nest Female 32 22 42 18 0.007 Male 3 1 0 1 Dispersed Female 19 16 11 dead 0.34 Male 0 0 5 dead 0.007 0.002

Brood 17/93 Nest Female 30 22 19 26 24 0.58 Male 14 27 24 15 23 0.17 0.75 Enclosure Female 5 6 4 6 4 0.94 Male 1 2 1 2 2 0.95 0.97 Dispersed Female 13 0 14 0 17 0.0001 Male 21 23 0 7 0 0.0001 0.0001

Brood 59/93 Enclosure Female 2 5 5 2 6 0.48 Male 3 8 4 4 2 0.29 0.46 Dispersed Female 0 31 7 5 2O 0.0001 Male 11 0 18 5 2 0.0001 0.0001

Brood 61/93 Nest Female 15 22 -- -- 0.25 Male 10 10 -- -- 1 0.49 Enclosure Female 6 0.76 Male 2 1 0.88 Dispersed Female 31 0 0.0001 Male 0 20 0.0001 0.0001

X • testfor randomnessof feedingby eachparent. X 2test for independenceof feedingdistributions between parents. Expectedfrequencies too small for X • test.

0.01;Fig. 2). The idea that brood divisionwas only I m apart and were fed by both parents. promotedby the spatialdispersion of young In brood 59/93, two fledglings dispersed wasillustrated by the enclosureexperiment. In quicklyaway from the nestand werefed exclu- all threeenclosed broods, both parentsfed all sivelyby oneparent each. The remainingthree youngoutside the nest cup, as they had done fledglingsdid not movevery far from the nest when the youngwere in the nest(Table 1). Par- or from eachother initially, but asthe distance ents showedno tendencyto feed particular among them increased,the feeding pattern younginside the enclosures(Table 1). On the changedfrom no brood division to complete other hand, the parentsdivided the brood as brood division. soonas the youngbecame dispersed after re- Brood division was observed in four other moval of the enclosure(Tables 1 and 2). broods (53/92, 54/92, 58/92 and 69/93; Table A causal link between brood division and 2), but none of the young was observedfor brood dispersionalso was illustratedby a cou- more than one day after nest departure.In ple of casestudies. In brood 79/92, the whole brood 5/92 only one young was seen after broodwas completely divided between the two fledging,and it wasfed by both parents(Table parentsduring the firstwatch, whereas during 2). In the secondarybrood of the polygynous the secondwatch two fledglingswere sitting male (brood 16/92), the male was observed October1997] BroodDivision in theBluethroat 557

TABLE2. Provisioningpatterns (no. of youngfed by 160 ' female, male, or both parents) in 17 Bluethroat familiesrevealed during the last observationpe- •.• 140' riod of a young. • 120' Parent oE100 ' No. young Nest Female Male Both missing" 2/92 2 I 0 0 • 80 5/92 0 0 I 4 6/92 3 I 0 0 • 40 13/92 2 I 0 0 14/92 3 I I 0 15/92 2 1 0 2 16/92 2 0 1 0 2/92 6/92 13/92 14/92 15/92 57/92 79/92 17/93 59/93 81/93 53 / 92 3 0 0 3 Brood number 54/92 0 1 0 5 57/92 1 2 0 0 FzG.1. Mean distancebetween fledged offspring 58/92 3 0 0 4 and the nest from all observations of 10 Bluethroat 79/92 3 2 2 0 broods.White bars indicateyoung fed by females, 17/93 2 3 0 0 and black bars indicateyoung fed by males. 59/93 2 3 0 0 61/93 1 1 0 0 69/93 3 2 0 1 70/93 2 0 0 2 ranked accordingto four size categories:body Total 34 19 5 21 mass,wing length,tarsus length, and the three Young not seenafter fiedging. measurements combined (i.e. mean rank). No significantdifferences were foundbetween fe- male-attended and male-attended offspring providingonly five feedings,all to oneyoung, (Wilcoxon tests, Z-values between -0.83 and whereasthe femalewas observedfeeding all -0.27, all Ps > 0.40). Thus, we concludethat threefledglings, for a total of 46 feeds(Table 1). the parentsdid not divide thebrood according In summary,brood division was the typical to sizeof young.In fourcases the identity of the form of postfledgingcare, and it appearedto be first young to fledge was known. In two cases inducedby the spatialdispersion of the young. the fledglingwas subsequentlyfed by the fe- No cleardivision rules.--Among monogamous male,in the two other casesit was fed by the pairs,males tended to carefor fewerfledglings male. Hence, at least within this restricted sam- than did females(Z = -1.85, n = 15,P = 0.064; ple, there was no indicationthat one sex was Table2). However,this result shouldbe treated morelikely than the other sex to takecare of the with caution because in each of four broods one first young to fledge. parent and severalyoung were not found, and A few offspring were recapturedand sexed, we arenot confidentthat they were not present. accordingto Lindstr6m et al. (1985), before When excludingthese cases, we found no in- they left the breedingsite in late Julyor early dicationthat males cared for fewer fledglings August(T. Aarvakpers. comm.). As a resultof than did females(Z = -1.16, n = 11, P = 0.25). theserecaptures, we know that two sonswere Within each brood, the offspring were fed by their mother,whereas one daughter was

TABLE3. Distances(m) betweennest and offspringand betweenoffspring from the sameand different family units in Bluethroatbroods; values are • _+SE.

Observation

Distance between FirsP Last b Z c n P Nest and offspring 36 -+ 5 116 -+ 19 -3.20 14 0.001 Offspring from samefamily unit 33 +- 5 53 -+ 8 -1.84 9 0.07 Offspring from different family units 51 _+7 94 +_19 -2.31 10 0.02 •0 to 3 days after young left nest. b3 to 15 days after young left nest. ßWilcoxon matched-pairs signed-rank test. 558 ANTHONISEN,KROKENE, AND LIFJELD [Auk, Vol. 114

180 ß severaltimes in rapid succession.There was no •' 160- suchhesitation in feedingafter the releaseof cO• 140' her own young.The next day we observedthe female feeding only the two young from her own nest, whereasthe young from the other i 100- nestwas fed by its ownfather Evidently, he had recoveredhis lost young. • 80'

•5 4o- DISCUSSION

We found no evidence for brood division while the young were still in the nest. More- Brood number over, parents distributed food evenly among brood members. Our findings are consistent FIC. 2. Mean distancebetween fledged offspring from all observations of nine Bluethroat broods. with the generalpattern for birds with bipa- White barsindicate offspring belonging to the same rental care,although in somespecies females family unit, i.e. fed by the sameparent, and black havebeen reportedto feed the smalleryoung bars indicateoffspring belonging to differentfamily in the nest preferentially (Gottlander 1987, unitswithin a brood,i.e. fed by differentparents. Stampset al. 1987,Lifjeld et al. 1992).Almost no studies have reported strict division of broodsduring the nestlingstage, as pointed fed by her fatherand anotherby her motherat out by Reed (1981) and Weatherhead and the stageof brood division.Due to the small McRae(1990). Apparently, the only indication samplesize, these findings preclude a firm con- of brood division in the nest occurred at a Great clusionabout brood divisionby the sex of off- Tit (Parusmajor) nest where the male and fe- spring,but at leastthey indicate that parents do male parent fed the nestlingsfrom different not divide the broodstrictly by sex. positionsin the nest on the last day of obser- In threebroods with knownparentage (Kro- vation, and as a result the brood became divid- keneet al. 1996),some or all extrapairoffspring ed (Bengtssonand Ryden 1981). were seenafter fledging. In onebrood (13/92), In most of the Bluethroatfamilies, the par- the femaletook exclusivecare of the only ex- entsdivided the brood after nestdeparture in trapair offspring.In anotherbrood (15/92), the sucha way that eachparent fed certainyoung malefed only onefledgling, which was one out almostexclusively. Among the few exceptions of four extrapairoffspring. Only oneoffspring was the secondarybrood of the polygynous in that brood was sired by the residentmale, male, where complete division of the entire and it was fed exclusivelyby the female.In the brood was not observed,probably because the third brood (5/92), the only offspringseen af- male provided very little care.Another excep- ter fledgingwas the only extrapairoffspring in tion was a brood where probably all but one that brood, and it was fed by both parents. young were depredated.Consequently, brood Thus, in two casesa male fed a fledglingto divisionafter fledgingappears to be the rule in whom he was not geneticallyrelated, and in Bluethroats. These results are in accordance one case he took exclusive care of such an off- with many detailedstudies that have found spring. brood division to occur in nearly all broods, Offspring-recognitionexperiments.--The two e.g. Northern Wheatears (Oenantheoenanthe; switchingexperiments were carriedout with a Moreno 1984), Lapland Longspurs(Calcarius brood that had showed stable brood division lapponicus;McLaughlin and Montgomerie for two days.In the first experimentboth par- 1985), Flammulated Owls (Otus fiammeolus; ents, after approximately 1 min of hesitation, Linkhart and Reynolds1987), AmericanRob- adoptedthe "new" youngthat previouslyhad ins (Turdus migratorius;Weatherhead and beenfed by the otherparent. In the secondex- McRae 1990), and White-throated Sparrows perimentthe femaleclearly refused to feed the (Zonotrichiaalbicollis; Kopachena and Falls unrelated young for 25 min, whereafter the 1991).Other studieshave reportedbrood di- young started to beg intensively and was fed vision in only someof the broods,e.g. in Eu- October 1997] BroodDivision in the Bluethroat 559 ropean Robins (Erithacusrubecula; Harper (4.8% fledglingslost per day). In our estimate 1985),Medium Ground-Finches (Geospizafortis; of predationrates, all youngthat disappeared Price and Gibbs 1987), and Cactus Finches were presumeddead; consequently,the pre- (Geospizascandens; Price and Gibbs 1987). In dationrate after fledgingmay havebeen over- specieswith morethan one nesting attempt per estimated.Thus, shortening of the nestlingpe- season,brood divisiontends to occurmostly in riod may yield considerablefitness benefits, broodsthat arenot followedby anotherbreed- whichprobably explains why theyoung leave ing attempt, e.g. Five-striped Sparrows (Am- the nestbefore they are fully capableof flying. phispizaquinquestriata; Mills et al. 1980),Euro- A study of anotheropen nester in our study pean Robins (Harper 1985), Eurasian Black- area, the Willow Warbler (Phylloscopustrochi- birds (Turdus merula; Edwards 1985), and lus),indicated that a majorfunction of malepa- Northern Mockingbirds(Mimus polyglottos;rental careis to promoteearly fledging(Bjorn- Zaias and Breitwisch1989). stad and Lifjeld 1996). The onset of brood division in the Bluethroat The main predatoron Bluethroatnestlings varied betweennests and amongyoung from was the adder (Viperaberus; T. Amundsenand the same nest. Some broods were divided on J. T. Lifjeld unpubl. data). Many snakesuse ol- the day of fledging,and otherswere not com- factionto locateprey (Dowling 1986).Because pletelydivided until four daysafter fledging. the odorprobably is strongerwhen the young Thetime between fledging and stable brood di- are gathered,it may be more profitablefor vision ranges from 0 to 4 days in Dunnocks young to becomespatially dispersedthan to (Prunellamodularis; Byle 1990), 0 to 5 days in stay together after nest departure.Moreover, Song Sparrows (Melospizamelodia; Smith and spatiallydispersed young are lessconspicuous Merkt 1980),3 to 8 daysin NorthernWheatears than gatheredyoung, regardlessof whether (Moreno 1984), 0 to 10 days in EurasianBlack- they are detectedby sound, sight, or smell. birds (Edwards1985), and lessthan 1 day in Therefore,spatial dispersion of youngmay re- Prairie Warblers (Dendroica discolor;Nolan ducethe risk of predationfrom otherpotential 1978) and Lapland Longspurs(McLaughlin predators as well. For instance,we witnesseda and Montgomerie1985). stoat(Mustela erminea) take only one fledgling In our study, the onset of brood division when several others from the same brood were seemedto be closelycoupled to the spatialdis- in closeproximity. The stoatsearched the area persionof young.Stable brood division was ob- for at least30 min withoutdetecting any of the servedonly when the younghad becomespa- other brood members. tially dispersed,as demonstratedin the enclo- Brood division assumesthat parentsare ca- sure experiments. Likewise, brood division pable of some sort of offspring recognition couldbe relaxedor changedif two youngfed (Hotsfall 1984,Edwards 1985,Kopachena and by differentparents came into secondarycon- Falls1991). It hasbeen suggested that parents tact. Theseobservations strongly suggest a can recognizefledglings from their begging causal link between brood division and the calls (Smith and Merkt 1980,Harper 1985) or spatialorganization of the young,an idea that from the actual locationof the fledglings previouslywas put forth by others(Smith and (Harper 1985,Kopachena and Falls1991). Our Merkt 1980,Moreno 1984, Linkhart and Reyn- switchingexperiments suggest that parents olds 1987). useboth individual begging calls and location We did not attempt to explain why Blue- of dispersedyoung as cuesfor offspringrec- throat fledglingsbecome spatially dispersed ognition. However, the fact that one female afterthey leave the nest, but otherworkers (e.g. startedto feed a foreignyoung suggeststhat Tinbergen 1939, Willis 1972, Knapton 1978) the recognitionis not perfectand that the cost have suggestedthat suchbehavior is an anti- of refusingto feedone's own offspring is higher predatorstrategy and/or helpsparents reduce than the costof feedingan unrelatedindivid- the energetic costs of parental care (Mc- ual. Moreover, parents often had difficulty Laughlin and Montgomerie1989a, b). In our finding a particular fledgling when it had study,predation was high duringthe nestling moved between two visits and did not make period (8.2%nestlings lost per day), and losses beggingcalls when the parentcame to feed it. appeared to decrease after nest departure Our data on brood division with respectto 560 ANTHONISEN,KROKENE, AND LIFJELD [Auk, Vol. 114 offspring sex, size, and paternity were few; as has been reportedin other species(Nolan consequently,no strict conclusionscan be 1978, Moreno 1984). drawn. However,we do know that the parents We concludethat brood division is the typi- did not divide the brood strictlyaccording to cal form of postfledging parental care in these characteristics. Because brood division Bluethroats. Brood division occurred once the seemedto occurin responseto spatialdisper- youngbecame spatially dispersed. In this set- sion of the young, it is likely that brood divi- ting, brooddivision is likely to enhancethe for- sionis beneficialonly when the youngare dis- agingeconomics of the parents.Our studydid persed. not revealany rulesas to how the maleand fe- Two majorhypotheses concerning brood di- maleparents divided their broods. vision are basedon the assumptionof spatial dispersion.The Predation Hypothesis states ACKNOWLEDGMENTS that brood division can preventloss of the en- tire brood when the male-attended and the fe- We thank T. Amundsen, J. Ekman, P. Krokene, T. male-attended young are widely separated Part, E. Roskaft,T. Slagsvoid,L. A. Whittingham,an (Harper 1985, McLaughlinand Montgomerie anonymousreviewer, and the BehavioralEcology Groupat the ZoologicalMuseum in Oslofor helpful 1985;see also Byte 1990). Thus, at best,a pred- discussionsand commentson the manuscript.We ator that tracksa parentto its youngwill find also thank E Krokene,K. Rigstad,E Brobakken,A. onlyhalf the brood (McLaughlin and Montgom- Johnsen,B. Rangbru,and T. Aarvak for help in the erie 1985). This explanationseems plausible field. provided that the brood memberstended by oneparent are clustered.If, on the otherhand, LITERATURE CITED the groupmembers are spatiattydispersed, as in the Bluethroat,the antipredationbenefit is ARHEIMER,O. 1982. Blghakans Lusciniasvecica hiick- lessobvious. In addition,both parentsmay de- ningsbiologii fjallbjiSrkskogvid Ammarniis.Var fend all of their young even if they feed only Fggelvarld 42:249-260. some of them (Willis 1972), so that predators BENGTSSON,H. B., AND O. RYDEN.1981. Develop- shouldhave no difficultylocating both parents ment of parent-younginteraction in asynchro- as long as the parentsdetect the predator.This nously hatched broods of altricial birds. Zeit- schriftffir Tierpsychologie56:255-272. was probablythe casewith the Bluethroatsin BJORNSTAD,G., AND J. T. LIFJELD.1996. Male paren- our study. The reduced predationrate after tal carepromotes early fiedgingin an open-nest- fledging may have resulted from spatial dis- er, the Willow WarblerPhylloscopus trochilus. Ibis persionof thebrood rather than brood division 138:229-235. per se. BYLE,P. A. E 1990. Brooddivision and parental care The Improved-ForagingHypothesis states in the period betweenfiedging and indepen- that parental foraging is more efficientwhen dence in the Dunhock (Prunella modularis).Be- broodsare divided. First, it may be easierfor haviour 113:1-20. parentsto locateindividual young, and thereby CRAMP,$. (Ed). 1988. The birds of the westernPa- spendless time searching for them(Smith 1978, learctic,vol. 5. OxfordUniversity Press, Oxford. DOWLING,H. D. 1986. Reptiles and amphibians. Harper 1985, McLaughlin and Montgomerie Pages120-135 in All the world'sanimals (K. Ad- 1985, Byte 1990). Brood division also may en- ler and T. R. Halliday, Eds.).Torstar Books, New hanceforaging efficiency by reducingthe par- York. ents' foraging route or travel costs(Moreno EDWARDS,P. J. 1985. Brood division and transition 1984,McLaughlin and Montgomerie1985). For to independencein Blackbirds Turdusmerula. instance,if food itemsare patchilydistributed Ibis 127:42-59. and differentpatches do not containenough GOTTLANDER,K. 1987. Parentalfeeding behaviour food to sustainall of the young,then dividing and sibling competitionin the Pied Flycatcher Ficedulahypoleuca. Ornis Scandinavica18:269- the brood may help parents to reduce their 276. travel time and becomemore efficientprovid- HANN,H. W. 1937. Life historyof the Ovenbirdin ers (Moreno1984, Byle 1990).Such a division southernMichigan. Wilson Bulletin 49:145-237. would result in young fed by the sameparent HARPER, D. G. C. 1985. 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