Chapman Et Al. 1989 - Variability in Parrot Flock Size- Possible Functions of Communal Roosts

7/29/2019 Chapman Et Al. 1989 - Variability in Parrot Flock Size- Possible Functions of Communal Roosts

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The Condor91:842-8470 The CooperOrnithologicalociety 989

VARIABILITY IN PARR OT FLO CK SIZE:POSSIBLE FUNCTIONS OF COMMUNAL ROOSTSC. A. CHAPMA N, L. J. CHAPM AN AND L. LEFEBVREDepartment of Biology,McGill University,1205, av Dr. Pen jield,Montreal, Quebec,Canada H3A IBI

Abstract. This study docum ents lock size in mixed-species arrot/parak eetlocks ntropicaldry forestof CostaRica . Variability in flocksizewasquantified n a diurnal basis,between esting nd nonnesting eriods, and during departures rom roost sites n the earlymorning. Flock size was greater when an imals were congreg atingnear the roost site neardusk than d uring the rest of the day o r w hen the birds were departing the roost. Flocks weresmallest n the first half of the dry s easonwhen they are repo rted to nest. Three hypotheses(Information Ce nter [ICI, Diu rnal Ac tivity Center [DA C], and a general oraginghypothesis)all mak e predictionsrelating the size of specific ypes of flocks o the den sity and distributionof food resources.As p redicted, by the foraginghypothesis,diurnal flock size was positivelyrelated to the density of po tential fruit resources ,and flocks were largest when resourceswere uniformly distributed. How ever, contrary o predictionsof the IC and DAC hypotheses,roosting flock size was not related to the density and distribution of food reso urces.Th esize of flocks departing roost sites in the morning w as small, and evidence suggestedha tthese flocks may have been avoiding following the flocks that left prev iously.

Key words: Comm unal roosts; arrots;group iving.

INTRODUCTIONThe ecological literature contains a number offield and experimental stud ies which have con-sidered determinants of animal group size (Alt-mann 1974, Pulliam and Caraco 1984). Theo-retical and em pirical evidence suggests hat thedensity and distribution of resourcesmay con-strain the size of animal groups by influencingthe number of animals that can efficiently foragetogether (Bradbury and Vehrencamp 1976,Leighton 1986, S tacey 1986). With respect toroosting flocks of birds, there are two major hy-pothe ses hat m ake predictions concerning ffocksize and the density and distribution of reso urces.The Information Center (IC) h ypothesis (Wardand Zahavi 1973) suggestshat birds congregatein large communal roosts at night to facilitatethe exchang e of information between memb ersregarding the location of feeding sites. Ward andZahavi (1973) sugge st hat the number o f birdsattending th e ro ost should be largest when theinformation concerning the location of feedingsites s most valuable. This may occur when foo dresourcesare at a low density and are clumped.Cacc amise and Mo rrison (1986) present an al-ternative view of communal roosting based onmovem ents of individually marke d European

Received 18 Janu ary 1989 . Final accep tance 8May 1989.

Starlings (Sturnus vulgaris). These authors sug-gest that individuals only leave their own for-aging area to come to communal roosts,and theassociated feeding areas, when doing so m orethan co mpen sates or the co st of travelling to theroost. They suggest hat this occurs when thereare clumped resources near the roost (see alsoCaccamise et al. 1983, Fischl and Caccam ise1985, Morrison and Caccamise 1985). With re-spec t to the relationship between roos t size andresourcedistribution, one of th e predictions thatthis hyp othesis shareswith the IC hypothe sis isthat as food resourcesbecome more clumped,the size of communal roostsshould ncrease.Thisprediction can be con trasted to the one m ade bythe general foraging iterature which sugg estshatwhen depletable food resources are rare andclumped, daytime foraging ffocks will be small(Bradbury and Vehrencam p 1976). If a groupsforaging activity results in the depletion of theresources n the p atches they use, the foragingliterature suggestshat an increase in g roup sizewill increase the area to be s earche d. With anincrease in the time spent travelling some pointwill be reach edat which the energy spent n travelexce eds he energy obtained from the environ-ment, and a smaller group size will be advan-tageous.

The objective of this study is first to d ocumen tthe variability in the size of neotropica l mixed-species locks of parrots and parakeets, and sec-


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COMMUNAL ROOSTS N PARROTS 843ondly to consider predictions relating the densityand distribution of food resources o variationin flock size. Parrots are a suitable taxonomicgroup to examine these predictions since theyare highly gregariousanimals and feed primarilyon the seedsand fruit pulp of a small number oftropical fruiting trees (Janzen 19 84). Fruit treestend to represent isolated patches (Chapman1988 ) and it is relatively ea sy o determine theirlocation and distribution. During a IO-monthfield study in Cos ta Rica, we quantified the vari-ability in p arrot flock size on a diurnal and sea-sonal (nesting vs. nonnesting) basis and docu-mented th e size, timing, and direction of flocksdeparting a major roosting site in the morning.METHODSSTUDY SITEField observations were condu cted from May1987 to March 1988 in Santa Rosa National Park,Costa Rica. The area in which the study wasconductedwas originally tropical dry forest. Overthe p ast 300 years, large areas of the upper pla-teau were cleared for p asture, but w ith the es-tablishment of the Santa Ro sa secto r as a na-tional park in 197 1, some areas have gradu allyreverted to wo ody vege tation. The vegetation inthe Santa Rosa Sector area consistsof a mosaicof grassland Hyparrhenia rufa), dry successionalsemideciduou s ores t dominated by Leuhea spe-ciosa,Burserasimaruba, Cecropiapeltata, Spon-dias mombin, and Guazum a ulmifolia, and near-ly-pristine semievergreen forest with trees suchas Man ilkara chicle, Hym enaea courbaril, an dMastichodendroncapiri (Janzen 1986).

The climate of the region is characterized bytwo distinct seasons; wet season rom late Mayto December and a dry seasonencompassing h eremainder o f the year. The annual rainfall in theSanta Rosa sector averages 1,527 mm. Little ifany rain falls in the dry seasonand the m ajorityof the trees in th e dry successionalsemidecidu-ous forest lose their leaves.FLOCK COUNTSWhenever a pa rrot/parakeet flock was encoun-tered, the following information was recorded :date, time of day, flock size, activity (if feeding,the food item being consumed), and location.When possible, the species compo sition of th eflock was determined. B ecau se of the ir color-

ation, the pa rrots were extremely difficult to seein trees with leaves. Thus, counts were only re-corde d for flying flocks , flocks that were flushedfrom trees, or flocks in trees with little or nofoliage. To ensure that counts were as indepen-dent a s possible,counts of flocks seen n the sam earea as a flock just counted, but w hich h ad beenlost from sight, we re not cons idered.

One of the major commu nal ro ostsof the par-rots was near the administration area of the park.This roost wa s particularly am enable for o bser-vations, as there was a sm all cleared hill directlyoverlooking the roost site. Nea r dusk and dawn ,flocks near the roo st were very active and vocal.The size of flocks arriving at roost sites tendedto be large, and o ften the number of animals thatthey contained had to be estima ted. For analyt-ical purposeswe classified flocks into three dif-ferent categories; eparting flocks, oosting locks,and day time foraging flocks. Departing flockswere those locks seenearly in th e morning (priorto 06:30) that a ppea red to be leaving th e area ofthe ro ost site. Roosting flockswere meant to rep-resent the number of animals attending the com-munal roost. The size of roosting flocks was dif-ficult to determine since they con tained manyindividuals that ap proac hed the area from anumber of different anglesas daylight was fading.How ever, estimating their size wa s facilitated bycharacteristic behaviors of the parrots near theroost. Just prior to d usk, the parrots would oftenfly as a single flock, circling the area before set-tling into the ro ost tree. The num ber of birdsattending the roo st on a particular night w as con-sidered to be the larges t estimated number ofbirds seen circling the roost site. Although thismay be an underestimate, we believe this is asuitable method for estimating relative changein the size of the roosting flock. Daytime foragingflocks were considered as any flock seen awayfrom the roost.To examine if som e birds followed other birdsleaving the roost in the morning, two observe rswere stationed on the hill a bove the roost on 12occasions.For ea ch lock ofbirds leaving the areanear the roost, we recorded he time of departure,flock size, the direction of their departure , andif possible, the speciescomposition of the flock.We followed all departing birds until they wereout of sight (approximately 70 0 m ). Often de-parting flocks divided after approximately 300m. When this occurred the size and direction ofeach of the new flocks were recorded.


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844 C. A. CHAPMAN, L. J. CHAPMAN AND L. LEFEBVRE

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05 6 7 6 9 10 11 12 13 14 15 16 17 18

TIME O F THE DAYFIGUR E 1. The mean size of parrot/parakee t flockson an hourly basis n Santa Rosa National Park. CostaRica.

M J JL A S 0 N D J F

ECOLOGICAL PARAMETERSThe density and distribution of potential foodresourceswere derived from an ongoing study of29 fruiting speciesand the foraging strategiesofthree speciesof primates (Chapman and C hap-man, unpubl.). A variety of frugivores, includingparrots, were observe d feeding on m any of thesetree speciesover the course of the study. Datawere derived from three 4-ha grids divided into10-m by 10-m cells. The location and size (di-ameter at breast height, dbh) of all trees of the29 tree speciesgreater than 5 c m in diameterwere recorded. Approximately once every 3 or4 weeks, tree species were enumerated for thepresenceof fruit. The spatial distribution of thetrees bearing fruit in a particular month was rep-resentedby the coefficientofdispersion (CD; Pie-lou 19 69, Sokal and Rohlf 1981). On average,considering he species nd sizesexamined, therewere 33 trees/habearing fruit in any given month,but this value ranged from 6.6 to 48.1 trees/ha(Chapm an 1988). Since the dbh o f tropical fruit-ing tree s has been sho wn to accurately reflect thefruiting capacity of individual fruiting trees (Pe-ters et al. 1988) we represented potential foodabundance as the density o f the trees fruiting ina month, weighted by their dbh.RESULTSDESCRIPTION OF FLOCK-SIZE VARIABILITYOver the 1 0 months of the study, 432 flockswerecounted. Flocks contained individuals of fourspecies;White-fronted Parrots (Amazona albi-from), Orange-fronted Parakeets (Aratinga ca-

MONTHFIGURE 2. The mean sizeof parrot/parake et lockson a monthly basis n SantaRosa National Park, Costa

Rica from M ay 1987 to February 198 8 (flocks seenearly in the morning include som e flocks leaving theroost, flocks seen late at night include flocks congre-gating at the roost but not the total roosting flock; thedotted line represents the flock size expected if the;I~;~jbution of flock sizes between months was uni-

nicularis), Orange-chinned Parrots (Brotogerisjugularis), and Yellow-crowned Amazon parrots(Amazona ochrocephala).Over the duration ofthe study the proportion of the b irds identifiedas being one of the four species did not varybetween months for any of the speciesby morethan 6% (n = 89).

The sizeof the flocksobservedat different hoursof the day was highly variable (range = 4-48,Fig. 1). The mean size of roos ting flocks (55.4individuals) was much larger than daytime for-aging flocks 8.1 individuals) and departing flocks(13.6 individuals). The sizes of all three flockcategorieswere significantly different from eachother (Kruskal-Wallis and a po steriori multiplecomparisons test P < 0.05, Conover 1980). Theearly morning observations at th e roo st site sim-ilarly suggesthat the large roosting flocksrapidlybreak up into smaller flocks as they leave theroost site.Flock size exhibited considerable month-to-month variability (range = 3-29, Fig. 2). Theobserved monthly distribution of flock sizeswastested against a uniform distribution based onthe overall mean flock size. The observed dis-tribution differed significantly from uniform (x2= 69.8, P < 0.001, df = 9). January and Februarywere the months with the smallest mean flocksizes. n these months at least one of the sp ecies(Brotogeris ugularis) is reported to be nesting(Janzen 1984).


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COMMUNA L ROOSTS N PARROTS 845TABLE 1. Correlations etween oo d abundancedensityof foodresources eighted y the size ree),distri-bution of food resources,nd the size of three ypesof parrot flocksobservedn Santa RosaNational Park,CostaRica.

Departing lockDaytime foragingRoostingDeparting lockDaytime foragingRoosting

Total sample Mea n flock NonnestingFood abundancer = 0.119, P = 0.336 13.6 r = -0.089, P = 0.617

r = 0.280,P -c 0.001 8.1 r = 0.195,P = 0.001r = -0.04, P = 0.847 55.4 r = 0.033,P = 0.800

Distributionof foodresourcesr = -0.089, P = 0.472 13.6 r = 0.290,P = 0.825r = -0.184, P < 0.001 8.1 r = -0.082, P = 0.127r = 0.078,P = 0.706 55.4 r=0.117,P=0.603

ECOLOG ICAL DETERMINANTS OF FLOCK SIZEEcological theory predicts that when animals arefeeding on depleting resources ,group size will belargest when resources are abundant and uni-formly distributed (Bradbury and Vehrencamp1976). n contrast, one prediction of both the ICand DAC hypotheses s that roosting flock sizewill be largest when resources are rare andclumped. To examine these hypotheseswe con-sidered the relationships between ecological pa-rameters and flock size for departing, daytimeforaging, and roosting flocks separate ly.The sizeof daytime flocks was related to both the densityand distribution of fruit resources Table 1). Incontrast, neither departing flocks nor roostingflockswere related to theseecologicalparameters(Table 1). When the months during which par-rots have been reported to nest (January andFebruary) w ere excluded from the analyses, thesize of the daytime foraging flockswas not relatedto the distribution of resou rces. How ever, de-parting and roos ting flocksshoweda similar non-significantpattern and daytim e foraging flock sizewas again positively related to food abundance.ROOST DEPARTURESThe pattern with which parrots dispersed fromthe roos t site was documented on 12 m ornings.The birds tended to restrict their dep arture routesby not flying large distances over stretch es ofopen grassland.The area near the roost site com-prised patchesof forest connectedby narrow for-ested strips. The parrots almost exclusively usedthese strips to travel between forested areas. Asa result, there were generally six rolttes usedwhendeparting the roost. We categorized he directionof every flock leaving th e general area of the roost(follower) relative to the flock that left imme-diately prior to it (leader). If flocks were leaving

the roost at random, one would expect one inevery six leaving flocks to h ead in the sa me di-rection as the preceding flock. W e assume thatall rou tes were equally attractive to the birds, asthe choice of which of th e six directions to takewhen leaving the roost did not d iffer from ran-dom (x2 = 5.65, P > 0.25, df = 5). Of 41 flocksseen eaving the roost site, only two of the flocksfollowed th e preceding one w hich differs signif-icantly from random departure (x2 = 4.1, P


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846 C. A. CHAPMAN, L. J. CHAPMAN AND L. LEFEBVREleave in small groups and, it w ould appe ar, in adispersive fashion. If, when leaving a ro ost, par-rots avoid following preceding groups, the prob-ability of arriving at a fruit tree that has consp e-cific co mpetitorsor that hasalready been depletedmay be lower than if departure routes were sim-ilar among flocks. Thus, roosts may serve to fa-cilitate dispersion of fora gers and minimize in-traspecific competition for foo d resource s.

This does not preclude other functions forroosting.Our Foraging Dispersion Hypothesis andthe IC and DAC hypothesesare clearly not mu-tually exclusive. Following may occur betweenindividuals in a roost while successive locks de-parting a roost avoid the preceding flocks de-parture route.The relationship between the size of the diur-nal flock s and the density and distribution offood resources sugge sts hat diurnal flock sizemay have been influenced by foraging efficiency(Altmann 1974, Bradbury and Vehrenca mp 197 6,Leighton 1986, Stacey 1986). Animals must for-age over an a rea that meets their energetic re-quirements. If resources re depleted, an increasein g roup sizewill increase he area to be searched.With an increase in the time spent travelling,some point will be reachedat w hich energy spentin travel excee ds he energy obtained from theenvironment, and a smaller group size will beadvantageous . Following this logic, and a ssum-ing that p arrots delete the patche s hey u se, con-ditions that wo uld increase the need to travel,would decreasegroup size. The diurnal parrotflockswe observedwere sm allest when fruit den-sity was low and clumped. This sugge sts hatwhen parrots have to travel a long distance be-tween feeding sites, they attem pt to reduce thedistance they must travel by dec reasing he num-ber of animals feeding at the site. This may allowanimals to stay longer in each patch they visit,as it would take longer for the flock to depletethe tree.

The findings of this study sugges t hat the s izeof the parrot flocks may reflect individuals re-sponding to different ecologicalpressuresat dif-ferent times of the day. In the day, parrots maybe ado pting a flock size which is suitable to main-tain a low level of feeding competition. At nightother factors may determine the number of an-imals attending the roost, but at daw n the roostmay function to reduce potential feeding com-petition by dispersing foragers.

ACKNOWLEDGMENTSThis research was funded by NSER C Post-Doctoraland Post-Grad uate Fellowships, a Province of AlbertaScholarship,and a University of A lberta DissertationFellowship to CA C, an NSER C Post-Graduate Schol-arship to LJC. and NSER C operating grants to L. Le-febvre and D.Krame r. We wish to thank D. Kohn andD. Bevan for assistancen the field. and D. H. Janzenfor allowing us use of his m ouse &ids and ass istancein other reas swell.This manuscript enefited reatlyfrom the critical comm ents rovidedby F. Joyce, .-A. Giraldeau, P. W eatherhead, and two anonymousreviewers.LITERATURE CITEDALTMANN,S. A. 197 4. Baboon s, space, time, andenergy. Am. Zool. 14:221-248 .BRADBURY,. W., AN D S. VEHRE NCAM P. 976. Socialorganization and foraging in emballonurid bats.II. A model for the determination of group size.Behav . Ecol. Sociobiol. 1:383- 404.CACCAMISE,. F., AN D D. W. MORRISON.1986. Avi-an commu nal roosting: mplications ofdiurnal ac-tivity centers. Am. Nat. 128 :191 -198 .CACCAMISE, D. F., L. A. LYON,AN D J. FISCHL. 1983.Seasona l atterns n roosting locksof starlings ndCommon Grackles. Condor 85474 -48 1.CHAPMAN,C. 1988. Patch use and patch depletionby the spider and how ling monkeys of S anta RosaNational Park, CostaRica. Behaviour 150:99-l 16.CONOVER,W. J. 1980. Practical nonparametric sta-tistics. John Wiley and S ons, New York.CROOK , . H. 196 5. The adaptive significance f avi-an social organizations. Symp. Zool. Sot. Lond.14:181-218.FISCHL, ., AN D D. F. CACCA MISE. 985. Influence ofhabitat and seasonon foraging flock compo sitionin the Eu ropean starling(Sturnusvulgaris).Oeco-logica 67532-53 9.DE GROOT,P. 198 0. Information transfer in sociallyroosting weaver birds (Quelea quelea;Ploceinae):An experimental study. Anim. Behav. 28:1249-1254.JANZE N, . H. 1984. B rotogerisjugdaris, . 548-550.In D. H. Janzen ed.], Costa Rican natural history.Chicago Univ. Press, Chicago.JANZEN,D. H. 1986. Guanacaste National Park:Tropical, ecological,and cultural restoration. Ed-itorial Universidad Estatal A Distancia, San Jose.K~EBS, . R. 197 4. Colonial nestingand social eedingas strategies or exploiting food resources n theGreat B lue Heron (Ardeuherodieu).Behaviour 5 1:99-134.LEIGHT ON, . 1986. Hombill socialdispersion:Vari-ation on a monoaamous heme, D. 108-l 30. In D.I. Rubens tein and R. W. Wran gham [eds.], Eco-logicalaspectsof socialevolution. Princeton Univ.Press,Princeton.MORRISON,D. W., AND D. F. C ACCA MISE. 1985.Ephem eral roosts and stable patches:A radiote-lemetry studyof comm unal roostingstarlings.Auk102:793-804.


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COMMUNAL ROOSTS IN PARROTS 847PETERS, R. H., S. CLO UTIER , . DUBE, A. EVANS,P.HASTINGS,H. KAISER, D. KOHN, AN D B. SAR-WER-FONER.1988 . The allometry of the weightof fruit on treesand shrubs n Barbados.Oecologia74612-616.PIELOU, . C. 1969 . An introduction to mathematicalecology.Wiley, New York.PULLIAM,H. R., AND T. CAR ACO . 1984 . Living ingroups: Is there an optimal group size?, p. 122-147. In J. R. Krebs and N. B. D avies [eds.], Be-havioural ecology.Sinauer, Sunderland.SOKAL,R. R., ANDF. J. ROH LF. 198 1. Biometry. W.H. Freeman and Co., San Francisco.

STACEY, P. B. 1986 . Group size and foraging effi-ciency in y ellow baboons.Beha v. Ecol. Sociobiol.18:175-187.WALTZ,E. C. 19 87. A test of the information-centrehypothesis n two coloniesof comm on terns, Ster-na hirundo. Anim. Behav. 35:48-59.WARD,P., AN D A. ZAHAV I. 19 73. The importance ofcertain assemblagesof birds as Information-Centres for food finding. Ibis 115 :s17-534.WEATHERHEAD,. J. 1983 . Two principal strategiesin avian communal roosts. Am. Nat. 121:237-243.

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Chapman Et Al. 1989 - Variability in Parrot Flock Size- Possible Functions of Communal Roosts