The Auk 114(3):396-404, 1997

FORAGING ECOLOGY OF THREE SYMPATRIC TURACOS IN A MONTANE FOREST IN RWANDA

CHIN SUN • AND TIMOTHY C. MOERMOND Departmentof Zoology,University of Wisconsin,Madison, Wisconsin 53706, USA

ABSTRACT.--Westudied the foragingecology of three sympatrictufacos (Great Blue Tu- raco [Corythaeolacristata], Ruwenzori Turaco [Musophagajohnstoni], and Black-billedTuraco [Tauracoschuettii]) in a tropical montaneforest in Rwandabetween November 1991 and De- cember1992. All three speciesfed primarily on fruit. WhereasBlack-billed Turacos were strictlyfrugivorous, Great Blue and Ruwenzorituracos were partially folivorous;leaves con- stituted25% and 6.3%of their overalldiets, respectively. The overalldietary diversity was highestfor GreatBlue Turacos and lowestfor Black-billedTufacos. Most fruits eaten by tu- racoscame from trees.Although most leaveseaten by Great BlueTuracos also camefrom trees,leaves eaten by RuwenzoriTuracos mainly camefrom lianasand epiphytes.For the Ruwenzoriand Black-billedtufacos, the two territorialspecies, monthly dietary diversity increasedwith increasingfruit abundancein the environment,but this relationshipdid not hold for GreatBlue Turacos. During periods of fruit scarcity,all threespecies depended more heavilyon theirmost frequently used foods; this trend was particularly evident in the two territorial species.Like most frugivores,these turacos probably were generaliststhat ex- ploited a greatvariety of fruit sources.Received 24 June1996, accepted 24 January1997.

TURACOS(FAMILY MUSOPHAGIDAE) are large ecologyof theseturacos between 2,000 and birds endemic to woodland and forested habi- 2,500m, where they coexist.Specifically, we tats of sub-Saharan Africa and are considered addressthe following:(1) Whatis the diet com- to be principallyfrugivorous (Brosset and Fry positionfor eachspecies of turaco?(2) Whatis 1988).Given their large body sizeand poten- the dietaryresponse of eachspecies to tempo- tially frugivoroushabits, forest turacos may be ral changesin resourceabundance in the en- important seeddispersers in African tropical vironment? forests.However, the ecologyof mostturacos is poorly known (Dowsett-Lemaire1983, 1990; METHODS Candy 1984;Brosset and Erard 1986;Brosset and Fry 1988).Here, we examinethe foraging Studysite and birds.--The Nyungwe NaturalForest ecologyof threesympatric turacos in a tropical Reserve is a 950-km 2 montane forest located in south- montane forest in Rwanda. western Rwanda, Africa (ca. 2ø35'S,29ø15'E). The Threespecies of turacosoccur in theNyung- generaltopography of the forestis highlydissected, with steepslopes and few levelareas. The studysite we Forest Reserve, Rwanda. The Great Blue Tu- encompassesan area of approximately3.5 km2 and raco(Corythaeola cristata) exists throughout the rangesfrom 1,950to 2,500 m in elevation.Extensive entire elevationalrange of the forest(1,760 to trail systemswithin the study siteprovide access to 2,950 m; Dowsett-Lemaire1990). The Ruwen- the home rangesof all focalbirds studied. zori Turaco (Musophagajohnstoni) is common The Great Blue Turaco weighs approximately between2,000 and 2,920m and may descendto 1,000g and is the largestspecies in the family.The 1,800 m in some wetter areas (J.-P. Vande Great Blue Turaco lives in social groups of 6 to 20 Weghepers. comm.,C. Sun pers. obs.).The individuals (Brossetand Fry 1988,C. Sun pets. obs.). Black-billedTuraco (Tauracoschuettii) is found Fifteenbirds formed the focalgroup that we studied. up to 2,500 m but is more abundantat lower Ruwenzori and Black-billed turacos each weigh elevations and reaches the lowest limit of the about250 g. Individualsof both specieslive in pairs that defend year-round territories (Brossetand Fry forest (Dowsett-Lemaire 1990, C. Sun pers. 1988, C. Sun unpubl. data). obs.).In this paper we comparethe foraging Between November 1991 and December 1992, C.S. followedfocal groupsof turacosmonthly and re- • Presentaddress: Department of Plant Pathology, corded their activities, movements,and diets. Each University of Wisconsin,Madison, Wisconsin 53706, specieswas followed for two to fivedays each month, USA. E-mail: [email protected] during whichdata were collectedfor 25 to 40 h. On

396 July1997] ForagingEcology ofFrugivorous Turacos 397

A A=0.5 C=0.5 Food scores A--1 A=I B=0.5 B--1 B=I B=I C=1 B=0.5

A A B B C B Feeding pattern I I I I I I I I I I I I I I I I I I I I I Time 00 03 06 09 12 15 16 21 24 27 30 min

Activity FORAGE

B A=0.5 Food scores A=I B=I B=0,5 B=I B=I

AAB B B AB B B Feedingpattern,,

Time 00 03 06 09 12 15 16 21 24 27 30 min

Activity REST I FORAGE--•- F•G.1. Schematicsof activitiesand feedingpatterns of two focalbirds and their food scores. (A) depicts a focalbird activelyforaging, (B) depictsa bird mostlyresting. "Activity" indicates the behavior state of the focalbird during each3-min interval.Dots and linesabove the time barsdepict the feedingpatterns; dots denotefeeding "incident," linesdenote continuous feeding. Letters above each dot or line indicatesthe food type(s)eaten during that feeding episode. "Food scores" records the type of foodeaten and its corresponding scorein each3-min interval. The dashed vertical lines protruding from the time bar (B) divide 3-min intervals into half. average,one group of Great Blue Turacos,2.2 pairs foodtypes eaten by eachbird weregiven a totalscore of RuwenzoriTuracos, and 1.6 pairs of Black-billed of one.If a bird ate two typesof foodwithin a 3-min Turacoswere followedmonthly. Results presented period,each food type was scored0.5 for that bird herewere based on 433,476, and 400 h of systematic duringthat interval(Fig. 1A). Wedid not distinguish observationson one group of Great Blue Turacos(15 how long a bird fed on eachfood type during a 3-min birds), four pairs of Ruwenzori Turacos, and two interval.Birds often ate only onefood type duringa pairs of Black-billedTuracos, respectively. foragingbout and never were observedto feed on Recordingactivities.--We used a focal-animalsam- more than two food typeswithin a 3-min interval. pling procedure(Altmann 1974)to documentthe ac- During long restingperiods, a focalbird occasion- tivity patterns of territorial Ruwenzori and Black- ally tookone or two fooditems. Foods taken at such billed turacos. For the Great Blue Turaco, we used a feeding"incidents," when the bird wasnot "forag- focal-groupscan sampling procedure.We used a ing," were recordedonly if the feeding occurred 3-min instantaneoussampling method to recordthe during the first half of a 3-min interval (Fig. lB). behavior"states" (Altmann 1974) of focalbirds. Be- Foodeaten at a feedingincident that began in thesec- havior "events" of short duration (e.g. calls and ond half of a 3-min interval would have been record- fights) were recordedas frequencieswithin each ed in the subsequentinterval(s) if the feedinginci- 3-min interval (Martin and Bateson1986). dent turned into a continuous bout that extended Recordingdiets.--A food type wasdefined as a dis- into a subsequenttime interval(s). On average,then, tinct kind of item eatenby turacos(e.g. fruits of spe- foodseaten at thesefeeding incidents were recorded ciesA, leavesof speciesB). Fruits, leaves, and flowers only50% of thetime. Foods recorded at thesefeeding of the sameplant specieswere treatedas different incidentswere scored equally as those recorded dur- food types.To quantifythe differentialuse of food ing continuousfeeding (i.e. when birds were "for- types,we usedthe followingprocedure. When focal aging"). birdswere activelyforaging, the behaviorstate (i.e. By consideringfoods eaten at shortfeeding inci- "foraging") and the numberof foragingindividuals dentsequally with thoseconsumed during contin- wererecorded at 3-min samplingpoints. During the uous feeding,we might have biasedour results. 3-min interval following each samplingpoint, all However,this practicewas necessary given that ob- 398 SUNAND MOERMOND [Auk, Vol. 114 serving several birds simultaneouslynecessitated (numberper ha) of reproductivelymature treesof that we recorddata quickly and consistently,and we speciesi. For community-widefruit abundancein- could not predict at the outsetwhether a feeding dices,i includesall speciesof treessampled in the would be an extendedfeeding bout or a shortinci- phenologysurvey. Because animals use only a subset dent. Our procedurerepresented the best compro- of resourcesin their environment,community-wide misebetween neglecting and overestimating the im- fruit abundanceindices may be a poor measureof portanceof food typeseaten at theseincidents. the quantityof resourcesactually available to a par- Becausethe daily overallnumber of foodtypes eat- ticularspecies. Therefore, we calculatedfruit abun- en by a group or pair of turacosusually was small, danceindices specific to eachturaco species, includ- and the foodtypes eaten at feedingincidents usually ing only plant speciesactually consumedby the were not differentfrom thoseeaten during continu- birds. Unless indicated otherwise, all fruit abun- ous feeding, our samplingscheme was unlikely to danceindices applied in the followinganalyses are havemissed recording the rare food types. the indicesspecific for eachturaco species. Estimatingfruit abundance.--Todocument tempo- Data analyses.--Weused the Simpsonindex (D), ral changesin community-widefruit abundance,we the reciprocalof Simpson,soriginal formula (Simp- conductedmonthly surveys on the phenologyof 568 son 1949), to describedietary diversity of each tu- reproductivelymature trees of 49 species.As part of raco species: a study of seeddispersal by frugivores,we focused on tree speciesthat produced fleshy fruits. Although 1 we did not sampleall tree speciesat the studysite, D- • p? (2) our sample included most of the commonspecies (seeSun et al. 1996).During phenologysurveys, the where p•is the proportionof food type i in the total percentageof the crowncovered with fruits in each of food typesconsumed during the periodof inter- tree was estimatedand assigneda scorebetween 0 est. The Simpsonindex (and its derivatives)is sen- and 4 (0 = 0%, 1 = 1-25%, 2 = 26-50%, 3 = 51-75%, sitiveto changesin the commonfood types, whereas 4 = 76-100%). the more widely usedShannon index is more sensi- To determine the relative abundance and size dis- tive to changesin rare food types(Peet 1974). Due to tributionsof tree species,we classifiedand mapped the limited monthly samplingeffort on each focal differenthabitats in the study site and then placed groupof birds, the variationamong months in ob- five to seven0.1-ha sampling plots within eachhab- serveduse of the rarely used food types was more itat type. The locationsof plots were selectedby susceptibleto samplingerrors than was that of the stratified random sampling.All trees in each plot commonlyused food types.We chosethe Simpson with dbh ->10cm were identified to speciesand their indexto minimizethe influenceon diet diversityin- dbh measured.All habitatswere mappedon topo- dicesof the rarely usedfood typesand to emphasize graphicmaps and digitized to calculatetheir relative changesin the commonlyused food types. areas.The densityof eachtree speciesin the study Variationin monthly dietary diversitycould sim- sitewas derivedfrom the sumof the tree densityin ply be due to differencesin samplingeffort or in the each habitat multiplied by the proportionof each quantityof datarecorded between months (Fleming habitatin the studysite. 1986).To examinethis possibleconfounding effect, Evidencesuggests that the reproductivecapacity we examinedthe correlationbetween monthly total of a tree is approximatelyproportional to its dbh. food scoresand monthly dietary diversity for each McDiarmid et al. (1977) found a linear positive re- species;none of the correlationswas significant. lationshipbetween fruit number and dbh in one We used the simplifiedMorisita index (CH;Horn Neotropicalspecies, and Peterset al. (1988) found 1966) to assessdietary overlapbetween speciesor thatfruit-crop size was proportional to dbh to the 1.2 betweenmonths within species: powerin 22 speciesfrom the WestIndies. To account for the different abundance and size distributions of 2 • P,jP•k adult trees amongspecies, we multiplied the mean fruiting scoresfrom eachspecies by the productof the averagedbh and the densityof adult treesfor that speciesbefore integratingfruit abundanceof wherep, and p•krepresent the proportionof food different species.The fruit abundanceindex (Fn0in type i in the diet of bird species(or month)j and k, month m was calculated as: respectively.Morisita's overlap index hasnearly zero biasat all samplesizes (Smith and Zaret 1982).How- r,• = • Sin,x DBH,x D,, (1) ever, we recorded resource use as scores for each food type, not as the numberof individualsas re- where Sin,is the mean fruiting scoreof speciesi in quiredby Morisita'sindex. In this situation,the sim- monthm, DBH,is the averagedbh (cm)of reproduc- plified Morisita'sindex, which usesthe percentage tivelymature trees for speciesi, andD, is the density valuesand is almostidentical to theoriginal Morisita July1997] ForagingEcology of FrugivorousTuracos 399

TABLE1. Overall diet composition(%) and the

numberof plant specieseaten by three speciesof 5O tufacos.

Total Fruits Flowers Leaves Galls" species Great Blue Turaco (11,290) b 73.3 (16)c 0.9 (4) 25.0 (18) 0.9 (1) 28 Ruwenzori Turaco (2,842) 91.9 (19) 1.7 (1) 6.3 (10) 0.2 (1) 25 Black-billed Turaco (1,554) Month 100 (12) ------12

• Tufacos were seeneating galls from Syzygiumparvifolium trees. FIC. 2. Percentagesof leavesin monthly diet of bNumbers in parenthesesdenote the total number food scoreson Great Blue (solid line) and Ruwenzori (dashed line) which results were based. cNumbers in parenthesesdenote the numberof plant speciesin- cludedin eachfood category. tree plants were particularly important food sources for Ruwenzori Turacos at certain times index(Krebs 1989:384), is theappropriate measure of overlap. of the year; in November-December1991, leaves(mainly of lianasand epiphytes)consti- tuted about 25% of their diet, whereas between RESULTS May and July 1992 liana fruits accountedfor nearlyhalf of their diet (Figs.2 and 4). Turacodiets.--Fruit was the major component Fruit abundanceand dietarydiversity.--Fruit in the diet of all three turacospecies (Table 1, abundancefluctuated widely through time, Appendix).The Black-billedTuraco was strict- ly frugivorous.The other two speciesalso ate leaves,with the Great Blue Turacobeing the A most folivorous(Table 1). The temporalpat- 1 oo terns of leaf useby Ruwenzoriand Great Blue 8o turacos differed (Fig. 2). Leaves constituted • Great Blue more than 25% of the diet of Ruwenzori Tura- •m Ruwenzorl 60 cos in November and December 1991, but be- • Black-billed came unimportant thereafter. In contrast, 40 leaves always constitutedat least 9% of the monthlydiet of GreatBlue Turacos. Moreover, 2o the monthlypercentage of leavesin the diet of o Great Blue Turacoswas negativelycorrelated Tree Llano Fig Epl yte Shrub Ground with fruit abundance(rs -- -0.464, P < 0.05, B Vegetation n = 14), indicatingthat GreatBlue Turacos in- creased their use of leaves as fruits became 1 oo scarce. Overall,the majorityof fruit eatenby turacos • 8o came from trees, with an additional 5 to 12% from stranglerfigs (Ficusoreodryadum). How- ever,a relativelyhigh percentage of fruitseaten .•: 40 by the Ruwenzori Turaco came from lianas 20 (Fig. 3A). Of the leaveseaten by the GreatBlue 0 Turaco, roughly half (51.2%) came from trees Tree LlanoFig EpiphyteShrub Ground and the rest from lianas and epiphytes(Fig. 3B). In contrast,only a smallproportion (2.3%) Vegetation of the leaveseaten by the RuwenzoriTuraco FIG. 3. Fruit and leaf sources in diets of turacos. camefrom trees, and the majority (92.9%) came (A) shows the fruit and (B) shows the leaf sources from lianas and epiphytes.In summary,non- from plantsof differentgrowth forms. 400 SVNAND MOERMOND [Auk, Vol. 114

8O TABLE2. Dietary diversity (overall and monthly mean), correlationbetween monthly dietary di- versity and fruit abundance (rs), and between- monthdietary overlapfor threespecies of turacos. Values in columns 2 and 4 are œ _+ SD.

Between- Overall month diver- Monthly mean dietary sity diversitya rs overlapb Great Blue Turaco 8.82 3.02 +- 1.22 -0.257 0.67 +- 0.33 Month Ruwenzori Turaco FIG.4. Monthly percentagesof liana fruits in the 5.81 3.41 -+ 0.72 0.587* 0.71 +_ 0.28 diet of Ruwenzori Turacos. Black-billed Turaco 2.10 1.97 + 1.00 c 0.550* 0.83 +- 0.22

with a peak betweenMay and July1992 (Fig. *, P < 0.05. 5). The community-widefruit abundancein- • n = 14 monthsfor eachspecies. bBetween-month dietary overlap did not differ among species dices were positively correlatedwith fruit (Mann-WhitneyU-tests, all Ps > 0.05). abundance indices for Great Blue and Black- ßMonthly dietary diversity of Black-billedTuraco significantly dif- ferent from that of the other two species(Kruskal-Wallis test, P < billed turacos(Great Blue:rs = 0.697,P < 0.01; 0.003). Black-billed: rs = 0.604, P < 0.05, n = 14), but not for Ruwenzori Turacos (r s = 0.015, P > 0.05,n = 14).Of all speciessampled in thephe- lated with fruit abundance for Ruwenzori and nology study, 13 speciesof fruits were con- Black-billedturacos, but no suchrelationship sumedby at least one speciesof turaco.For was found for Great Blue Turacos (Table 2). these13 fruit species,the monthlyfruit abun- Whenmonthly food types were ranked by their dancewas positivelycorrelated with monthly importance(percentages) in the diet, thecom- fruit diversity(described by Simpson'sindex; rs bined percentagesof the two highest-ranked = 0.477,P < 0.05,n = 14),suggesting that both foodtypes in the dietsincreased with decreas- quantity and diversity of fruits increaseddur- ing fruit abundancefor all turacospecies. This ing the periodof fruit abundance. negativecorrelation was significantfor Ruwen- Dietarydiversity overall was highest for the zori and Black-billed turacos, but not for Great Great Blue Turaco and lowest for the Black- BlueTuracos (Table 3). Theseresults suggested billedTuraco (Table 2). Monthlydietary diver- two trends: (1) as fruits becameabundant, tu- sity for Black-billedTuracos was significantly racostended to diversifythe foodtypes they lower than thosefor the other species,but no used; and (2) as fruits becamescarce, turacos difference was found between Great Blue and becameincreasingly dependent on the most Ruwenzorituracos (Table 2). The monthlydi- frequentlyused food types.Both trendswere etary diversity indices were positively corre- TABLE3. Mean monthly percentageof the two 5on0 highest-rankedfood typesin the diet of threespe- ciesof turacos,and the correlation(rs) between the 4000 summed percentageof the two highest-ranked food types and fruit abundancein the environ- 3000 ment(n = 14 monthsfor eachspecies of turaco).

2000 % of highest- ranked food 1000 Species typesin diet rs Great Blue Turaco 74.0% -0.380 0 Ruwenzori Turaco 70.7% -0.528' Nov91 Jan92 Mar May Jul Bep Nov Black-billed Turaco 71.6% • -0.603* Month *, P < 0.05. FfG.5. Seasonalpatterns of community-wideand Value representsthe first-rankedfood item only for Black-billed turacospecies-specific fruit abundanceindices. Tufaco. July1997] ForagingEcology ofFrugivorous Turacos 401

Great Blue in mammalian herbivoresthat the quality of foods consumed often decreases with increas- ing body size (Geist 1974, Demment and Van

37 Soest 1985, Gordon and Illius 1996). Leavesappeared consistently in the diet of GreatBlue Turacos (>9% in all monthlydiets), regardlessof fruit abundance.In addition,the negativecorrelation between fruit abundance and leaf consumptionby Great Blue Turacos indicates that leaves were eaten more often

11 when fruits were scarce.Leaves may be essen- tial as well as partial substitutesfor fruits in 31 0.221 this turaco. The Ruwenzori Turaco consumed Ruwenzori Black-billed more leaves in November-December 1991, and again in December1993 (Fig. 2; H. Kraeuter FIG.6. Dietary overlapamong turacos. Numbers pers.comm.), when Syzygiumparvifolium fruits within trianglesindicate the number of food types alsowere important in the diet (C. Sununpubl. sharedby eachspecies-pair; the 10 food types shared data, H. Kraeuter pers. comm.).Thus, when by Great Blue and Black-billedturacos also were sharedby all three speciesof turaco.The number certainkinds of fruits are predominantin the nextto eachbird diagramindicates the totalnumber diet, RuwenzoriTuracos may eat leavesto bal- of foodtypes used by that species.Numbers parallel ance their nutritional intake. to the sideof the triangleindicate dietary overlap be- Dietary diversityamong species.--Although tween species-pairs.Bird diagrams are based on the territorial Black-billed and Ruwenzori tu- platesby Martin Woodcockin Fry et al. (1988). racosare similarin body size and homerange size (Brossetand Fry 1988, C. Sun unpubl. evident in Ruwenzori and Black-billed turacos, data),dietary diversity of Black-billedTuracos was much lower than that of Ruwenzori Tura- the two territorial species. Intra- andinterspecific dietary overlap.--The in- cos(Table 2). Thiscould have resulted from the dices of between-monthdietary overlap did smallermean monthly sample size for the for- not differ among species(Table 2). The be- mer (1.6 pairs) versusthe latter (2.2 pairs). tween-monthdietary overlap was not correlat- However,Black-billed Turacos foraged less of- ed with fruit abundancefor any of the turaco ten than Ruwenzori Turacos(8.5 vs. 14.6 % of species.Among all species,the Black-billed Tu- daily activity budget; C. Sun unpubl. data). racoshared the highestpercentage of its over- Thus, the differencein dietary diversity be- all food types with other species.However, tweenthe two speciesprobably reflected a true when the percentageof eachfood type in the differencein their biology. diet was considered, Great Blue and Ruwenzori The high overall dietary diversity of Great turacoshad the highestdietary overlap among BlueTuracos likely wasdue to the presenceof all species-paircombinations (Fig. 6). leavesin its diet. Althoughthe focalgroup of Great Blue Turacos that we studied had a home

DISCUSSION range50 timesthe size of an averageterritory for the othertwo species(C. Sununpubl. data), Frugivoryand folivory.--Although fruit was thesebirds did not feed on a greater diversity the majorcomponent in the diet of eachturaco of fruits (Table1). Given that fruits in tropical species,Great Blue and Ruwenzori turacos also forestsare patchily distributedin spaceand ate leaves. Because leaves contain lower con- time,only a limitednumber (and hence diver- centrationsof lipids and solublecarbohydrates sity) of fruit patches(e.g. trees of a few species than fruits and often are loaded with second- in heavyfruit) maybe largeenough to accom- ary compounds,they are probablypoor food modatean entireforaging group or flockof fru- sourcesfor frugivores(Milton 1981,Cork and givores at any given time (Diamond 1975, Foley1991). The GreatBlue Turaco, being the Chapman 1990, Fleming 1992). Although a largestof the threespecies, was the mostfoli- largehome range might ensure the availability vorous.Our result is consistentwith findings of some food resources at all times, it does not 402 SUNAND MOERMOND [Auk,Vol. 114 necessarilyguarantee more diverse food re- competition(MacArthur 1972). However, our sourcesat any given time. resultswere derived from small samplesizes Fruit abundanceand dietary diversity.--Fruit and thusare not appropriatefor examiningin- abundanceand monthlydietary diversitywere terspecificcompetition. A long-termstudy that positively correlatedfor Ruwenzori and Black- incorporateslarger sample sizes and extends billed turacos(Table 2). Turacosprobably are into elevationsbeyond which all threespecies similar to most frugivores,being generalists coexist will provide opportunities to further and capableof exploitinga greatvariety of fruit examinespecies interactions and mechanisms sources (Herrera 1984, Moermond and Den- underlying the different elevationaldistribu- slow 1985,Wheelwright 1985, Fleming1986). tions of these three turacos. Because Ruwenzori and Black-billed turacos were territorialyear-round and foragedalmost ACKNOWLEDGMENTS exclusivelywithin their territories(C. Sun un- We are greatly in debt to our Rwandan field assis- publ. data),the diversityof foodsthey atemay tants:J.-B. Gakima, F. Ngayabahiga,E Nkurunziza, well have been limited by the availability of G. Ndiramiye, F. Manirafasha,I. Semahoro,G. Ka- fruits in their territories. When fruits are abun- marampaka,and E. Ngabonziza.We alsothank our dant, turacosmay exploitthe whole rangeof Rwandan host institutions: I.R.S.T. and O.R.T.P.N. fruit resources available in their territories, in- P.C.EN.provided logisticsupport during all phases creasingtheir dietary diversity. In contrast, of this study,and SwarovskiOptik, EastpakCom- during periods of fruit scarcity,turacos may pany, Patagonia,and VasqueBoots generouslydo- have to live on the few reliable food sources nated field equipment. Mr. Martin Woodcockand that are availableyear-round (i.e. top-ranked AcademicPress kindly grantedus permissionto use the bird diagramsin the figure.Drs. P. Arcese,J. R. food types), as supportedby the significant Baylis,T. J. Givnish,and C. T. Snowdonprovided negative correlation between overall fruit valuable comments on an earlier draft of the manu- abundanceand the proportionof top-ranked script.C. Sunwas partly supportedby a JohnJeffer- foodtypes in the dietsof Ruwenzoriand Black- sonDavis summerfellowship from the Department billed turacos (Table 3). of Zoology,University of Wisconsin-Madison.This Interspecificaggression.--Although trees were study was supportedby grants from U.S.A.I.D. Pro- the majorsources of fruits for all threeturacos, gram for Science and Technology Cooperation a significantproportion of fruits eatenby Ru- (DHR-5542-G-SS-9033-00), U.S.A.I.D. Mission to wenzori Turacosfrom May to July camefrom Rwanda,and the Wildlife ConservationSociety. lianas(Fig. 4). The territoriesof RuwenzoriTu- racosusually had complexvegetation structure LITERATURE CITED and often aboundedwith epiphytesand lianas. ALTMANN,J. 1974. Observationalstudy of behavior: In contrast,the vegetationwithin Black-billed Samplingmethods. Behaviour 49:227-267. Turaco territories was relatively simple struc- BROSSET,A., AND C. ERARD. 1986. Les oiseaux de R•- turally and frequentlylacked lianas (C. Sunun- gionsForesti•res du Nord-Est du Gabon.Soci•t• publ.data). Areas with abundantlianas might Nationale de Portection de la Nature, Paris. providea higherdiversity of fruits and thusbe BROSSET,g., ANDC. H. FRY.1988. Order Musophag- favoredby both of the territorialspecies. How- iformes. Pages26-57 in The birds of Africa (C. H. Fry, S. Keith,and E. K. Urban,Eds.). Academ- ever, the Ruwenzori Turaco was the dominant ic Press,San Diego, California. species during interspecificinteractions (C. CANDY,M. 1984. Habits of breedingbiology of the Sun unpubl. data). Thus, Black-billedTuracos Great Blue Tufaco. Journal of East African Nat- might not be able to settlein their preferred ural History Society190:1-19. habitatsin the presenceof RuwenzoriTuracos. CHAPMAN,C. g. 1990. Ecologicalconstraints on Focal pairs of Ruwenzori Turacos often group size in three speciesof Neotropicalpri- chasedand fought with GreatBlue Turacos that mates.Folia Primatologica 55:1-9. trespassedwithin their territories.Similarly, CORK,S. J., AND W. J. FOLEY.1991. Digestive and metabolicstrategies of arborealmammalian fo- membersof the focal group of Great BlueTu- livores in relation to chemical defenses in tem- racosoften were chasedby RuwenzoriTuracos perate and tropical forests.Pages 133-166 in whenthey foraged in fruitingtrees or traveled Plant defensesagainst mammalianherbivory throughthe forest(C. Sunpers. obs.). Interspe- (R. T. Palo and C. T. Robbins,Eds.). CRC Press, cific aggressionusually underlies resource Boca Raton, Florida. July1997] ForagingEcology ofFrugivorous Turacos 403

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APPENDIX.The relativeimportance of eachfood type (% in diet) for three speciesof turacos.Food types are listedin descendingorder based on their relativeimportance in the diet of GreatBlue Turacos.

Food Great Blue Ruwenzori Black-billed Species parta Turaco Turaco Turaco Growth form Balthasarea schliebenii FR 19.14 35.10 1.03 Tree Syzygiumparvifolium FR 19.07 12.88 5.47 Tree Ilex mitis FR 12.90 3.10 4.05 Tree Olinia rochetiana FR 8.92 2.50 1.54 Tree Maytenusacuminata LV 7.81 <0.1 • Tree Ficusoreodryadum FR 6.62 4.93 11.90 Fig Embeliaschimperi LV 3.96 2.92 -- Liana Phragmantherarufescens LV 3.29 -- -- Epiphyte Ekebergiacapensis FR 2.80 7.64 67.31 Tree Dalbergialactea LV 2.35 -- -- Liana Psychotriamahonii LV 2.21 -- -- Tree Ocotea michelsonii LV 1.50 -- -- Tree Salacia erecta LV 1.29 1.02 -- Liana Scheffieragoetzenii FR 1.07 8.01 0.39 Liana Scheffieragoetzenii LV 1.00 <0.1 -- Liana Canthiumspp. A FR 0.93 -- -- Liana Syzygiumparvifolium Gall 0.89 0.21 -- Tree Strombosiascheffieri FR 0.70 -- I Tree Canthiumspp. A FL 0.70 -- -- Liana Bersamaabyssinica LV 0.60 -- -- Tree Podocarpusmilanjianus FR 0.43 0.99 <0.1 Tree Ekebergiacapensis LV 0.40 -- -- Tree Prunusafricana FR 0.35 <0.1 6.11 Tree Embeliaschimperi FR 0.25 11.81 -- Liana Canthiumspp. A LV 0.23 -- -- Liana Syzygiumparvifolium LV 0.15 -- -- Tree Parinari excelsa FL 0.10 -- -- Tree Olea hochstetteri FR <0.1 0.70 0.84 Tree Rapaneamelanophloeios FR <0.1 0.70 -- Tree Alanglumchinense FR •0.1 -- I Tree Beilschmiediatroupinii FR <0.1 -- -- Tree Symphoniaglobulifera FL <0.1 -- -- Tree Olinia rochetiana FL <0.1 -- -- Tree Olinia rochetiana LV <0.1 -- -- Tree Sericanthe leonardii LV <0.1 -- -- Tree Allophyllusspp. LV <0.1 -- -- Liana Aquatic plants LV <0.1 -- -- Groundc Galinieracoffeoides FR -- 1.88 -- Shrub Carapagrandifiora FL -- 1.65 I Tree Orchids LV -- 1.16 -- Epiphyte Sericanthe leonardii FR I 0.84 -- Tree Lichensor mosses LV -- 0.33 -- Epiphyte Maesa lanceolata FR I 0.28 1.09 Tree Tapinanthusspp. LV -- 0.28 -- Epiphyte Psychotriamahonii FR I 0.18 -- Tree Galinieracoffeoides LV -- 0.12 -- Shrub Ferns LV -- 0.11 -- Ground c Salacia erecta FR I 0.11 -- Liana Casearia russorica FR -- <0.1 I Tree Allophyllusspp. FR -- <0.1 -- Liana Sericostachysscandens LV -- <0.1 -- Groundc Tricalysianiamniamensis FR -- -- 0.13 Tree Memecylonwaliklense FR ! I -- Tree Ixora burundensis FR -- -- I Shrub Brideliabrideliifolia FR -- -- I Tree Harunganamontana FR -- -- I Tree Total no. food typesa 37 31 12

* FR, Fruits; FL, flowers; LV, leaves. b--, food type not eaten;I, food type observedeaten during incidental observations but not duringsystematic sampling. 'Plants suchas forbs, herbs, or creepinglianas that grow nearground level. dNot includingfood typesobserved eaten only duringincidental observations.