j. Raptor Res. 23(3):72-84 ¸ 1989 The Raptor Research Foundation, Inc. CENSUSING OF DIURNAL RAPTORS IN A PRIMARY RAIN FOREST: COMPARATIVE METHODS AND SPECIES DETECTABILITY JEAN-MARC THIOLLAY ABSTRACT.--Noreliable method has ever beenproposed to censusa rain forestraptor community.I investigated4 methods in primaryforest of FrenchGuiana and compared results; 1) Mappingterritorial pairs, displayingover canopyand followedfrom dominantlookouts, gives the mostcomplete data on regularlysoaring species (Harpagus, Buteogallus, Spizaetus, Spizastur); 2) meaninstantaneous number of birdsflying over a definitearea in optimumconditions may be a reasonabledensity estimate for vultures (Cathartidae);3) mappingof individualsrecorded along a regularnetwork of trailsin the understory wasonly successful for the highlyconspicuous Red-throated Caracara (Daptrius americanus); 4) density estimatesfrom understorystrip transectswere consistentwith thoseobtained by othermethods for 6 of 8 species.Specific detectability, soaring behaviors and frequencies of displayflights varied widely among speciesand so did time required to assessthe existence of a territorialpair (1-7 d). Marking,radiotracking andplayback of vocalizations are promising techniques but are very time consuming and more appropriate for a detailedstudy of particularpairs than for surveyof a wholecommunity. Use of an abundanceindex, with a distincttechnique for eachspecies, may avoidbiases of densityestimates. Birds of prey are notoriouslydifficult to surveyin many speciesare almost impossibleto find in pri- tropical forests,especially in tall, dense,large un- mary forest,except under extraordinarily lucky cir- broken tractsof humid lowland forest.No complete cumstances.In fact, for many species,the nest has censusof a rain forestraptor community,with den- neverbeen described. Several species, including some sity estimatesover a significantarea, has ever been of the mostcommon neotropical forest raptors, seem publishedand no appropriatemethodology has even to never soar(?) nor even fly over the canopy and been proposed.However, many rain forestraptors very rarely ventureoutside the understory.Although are now threatenedby habitat destruction,distur- not particularly shy, most speciesare very secretive bance or fragmentation(Thiollay 1985b). Raptors and spend long periods perched motionless.Many may be suitableindicators of optimum sizeof a forest are very vocal,but othersare usually silent. Density reserve,because raptors are likely to require areas is often low and distribution very patchy, which larger than most other species.Yet, there is still an further decreases rate of encounters. urgent need of basicdata on natural distribution and Thus, most raptor-specific censusmethods (see density of rain forest raptors becauseof a concern review in Fuller and Mosher 1981), devisedfor tem- about the suitability of many reservesor even na- perate species,cannot be applied directly to tropical tional parks which may well prove to be too small forest species.Nevertheless, I have tried to adapt for long term survival of someraptor speciessup- classicalconcepts to proposeempirical methodsthat ported originally. need further improvementand testingin similar sit- As part of a larger studyon designof a national uations. My attemptsare preliminary, and as such, park in French Guiana, I assessedthe distribution voluntarily unsophisticated.No single method can and relativeabundance of raptorsover the country's be appropriate for every species,and 2 or more dif- 80 000 km 2 rain forest area, and I estimated the ferent techniquesshould always be used concur- densityof everyspecies within a representative100 rently. km2 samplequadrat (Thiollay 1989). In this paper I concentrateonly on comparative Life history and behaviour of most rain forest resultsof complementarymethodologies. Biological raptor speciesare very poorly known, if at all (Thiol- and conservationsignificance of the data are devel- lay 1985a). Often only scantinformation comes from oped elsewhere(Thiollay 1989). marginal habitats,rather openwoodlands or edges where a species'biology may be atypical.After 20 STUDY AREA AND METHODS yrs of personal experiencein both New and Old After 5 yrs of raptor surveysthroughout French World tropical forests,I can testify that nestsof Guiana, I tried to obtain a quantitativeestimate of 72 AUTUMN 1989 CENSUSING RAPTORS IN RAIN FOREST 73 the densityof every raptor specieswithin as large a crossedby foot at least twice, including 35 outside representativearea as possible. Zone II. Around the Nouragues field station, in north-cen- Mapping of SoaringBirds. One groupof species tral French Guiana (4ø05'N, 52ø41'W), a 100 km 2 regularlysoar above canopy and may then be easily quadrat of primary forestwas chosenwithin a much detectedfrom outside the forest or through large larger expanse of similar unbroken lowland rain openings:Gray-headed Kite (Leptodoncayanensis), forest. The area was hilly, crossedby small rocky Hook-billed Kite (Chondrohieraxuncinatus), Ru- watercourses,and ranged in altitude from 27-413 fous-thighedand Double-toothedKites (Harpagus m. Also includedwere sizeabletracts of every forest cliodonand H. bidentatus),Tiny, Bicoloredand Gray- type encounteredin Guiana, exceptcloud forest. The bellied Hawks (Accipitersuperciliosus, A. bicolorand quadrat was centeredaround 2 large natural open- A. poliogaster),White Hawk (Leucopternisalbicollis), ings, with small field stations nearby: a medium- Great Black Hawk (Buteogallusurubitinga), Crested s•zedriver in the southand a large graniticinselberg Eagle (Morphnusguianesis), Black and White Eagle in the northern part provided the most convenient (Spizastur melanoleucus),Black and Ornate Hawk observationsites (Fig. 1). Eagles(Spizaetus tyrannus and S. ornatus).Soaring The surveytook place in the driest season,from behavior performed by adults on their breeding early Septemberto mid-October1986 and 1987 over groundshas mainly a territorial function (surveil- a total of 73 d of intensivefield work with unusually lance and maintenanceof pair bond or territorial fine weather conditions. From the little information limits, Newton 1979). Soaringis often accompanied available (Haverschmidt 1968 and pets. obs.), most by loud calling, nuptial display or instraspecific raptors were either breedingor ending their repro- aggression.I assumedthat adults, especiallywhen ductivecycle (feeding fledgedyoung). All were as- displaying,flew mostly,if not only, over their own sumedto be sedentarywith an extendedperiod of territory. breeding activity and a permanent pair bond and A large, bare, rocky outcrop protruding 200 m territory occupancy. over the surroundingforest offered an ideal vantage Different censusmethods have been adapted to point. Four convenientlookouts were chosenon the the behaviorof individual species.Methods are here outermostparts of the inselberg,each offering an proposedas preliminary suggestions.Comparative unrestricted180 ø view. A total of 167 hr was spent results will be used as a test of reliability. As the overlookingthe forest in fine weather and mainly coveragewas better, more intensive and frequent during the optimal morning hours (0900 H-1200 around field stations and large openings,2 better H). The marked relief facilitated the location of known areas were distinguished(Fig. 1). Zone I (6 flying birds. km2) included the inselberg and its lookouts, the All raptors seenflying over the forest (or perched main field station and its clearing as well as a dense on emergent dead trees) were followed and their networkof trails. Every speciespresent was assumed itinerary carefully mapped (1/50000 scale). Day to be recordedand reasonablywell mapped(no new after day, the data were superimposed,soon giving data during the last 30 d.). ZoneH (42 km2 including a pictureof clearly separatedranges for mostspecies Zone I) was centeredaround the inselbergand cov- So-calledterritories were derivedby the minimum ered a 3-kin tad area around the 4 outmost lookouts convex polygon method (Ford and Myers 1981; of the mountain top from which only soaringspecies Southwood 1966) connectingthe outermostpoints were mapped. A 3-kin tad was the maximum dis- reachedby birds under observation.Territory size tance at which large raptors were accuratelyiden- was determinedusing a planimeterand correcting tified and located with 10 x 40 binoculars and 11- for small samplesize biasesby the methodof Jenn- 33 x telescope. rich and Turner (1969). Adjacent territories were The entire studyarea (100 km2) was visiblefrom discriminatedby simultaneousobservation of the 2 the top of the inselbergbut only the largestraptors pairs involved. could be seenbeyond Zone II. However, most outer Over 20 completeflight circuits were drawn for regionswere in sight abovecanopy within 2.5 km at least 1 pair of the 5 mostcommon and conspicuous of 9 additionallookouts (riverbanks and large treefall species(H. bidentatus,L. albicollis,B. urubitinga,S. gaps on ridges or steep slopes).The quadrat was melanoleucusand S. ornatus).Each pair yieldedrare- divided into 1 x 1 km squares,of which 70 were ly more than 1 circuit/d. No new information was 74 JEAN-MARCTHIOLLAY VOL. 23, NO. 3 + + : ."..../7 + ß I :':..., + + + + / '; '.3 " :"".... +++I i.. + + +l + :.....j + + + ! + + + + + + + + + + + {../'....: •......t..+ ñ.....[' mereram <'.'.7 AUTUMN 1989 GENSUSING RAPTORS IN RAIN FOREST 75 obtainedafter the 5-10 circuitsplotted initially. Thus, rad on a 180ø field, the largest area manageablefor samplesize