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J. RaptorRes. 34(1):37-41 ¸ 2000 The Raptor ResearchFoundation, Inc.

SUMMARYOF PHILIPPINEEAGLE REPRODUCTIVE SUCCESS, 1978-98

HECTORC. MIRANDA,JR. PhilippineEagle Foundation, Garnet cot. Diamond Street, Marfori Heights, Davao City, 8000 Philippinesand University of thePhilippines-Mindanao, Ladislawa Avenue, Buhangin, Davao City,8000 Philippines

DENNISI. SALVADOR,JAYSON C. IBA•EZANr• GLICERIA A. BALAQUIT-IBA•EZ PhilippineEagle Foundation, Garnet cot. Diamond Street, Marfori Heights,Davao City,8000 Philippines

KEYWORDS: PhilippineEagle,, Pithecophagajefferyi; breed- RESULTS AND DISCUSSION •ngsuccess;, population decline,, forest fragmentation. Prior to 1970, only one nestingpair of Philippine Ea- gleswas located (Gonzales1971). From 1978-83, several The PhilippineEagle (Pithecophagajefferyi)is one of the nesting pairs were intensivelystudied within the logging rarest eaglesin the world. Its present statushas always concessionof the Paper Industries Corporation of the been regarded as critically endangered. Previousesti- Philippines (PICOP) in Surigao del Sur and Davao Ori- matesof the Philippine Eagle population havebeen spec- ental provinces,and within the Mount Apo National Park ulative (Alvarez 1970, Gonzales1971, Rabor 1971). The (Kennedy 1981, 1985). This wasa period when intensive most recent estimate suggeststhat the total population logging operations occurred on Mindanao Island and consistsof between300-500 individuals(Kennedy 1977, many nesting areas were logged or altered by slash-and- 1985). Philippine Eagleslay a single egg and have a 2-yr burn farmers. Eight breeding attempts by 6 pairs failed cyclebetween successive breedings when pairs breed suc- (72.7%) out of a total of 11 attemptsduring this period cessfully,but in caseswhen breedingattempts fail, adults (Fig. 1). One nestling wasretrieved from a nest at Mount breed the followingyear. Sincework began in earneston Apo National Park and is currently being kept at the Phil- this species,a large amount of information on nesting successes and failures in Mindanao have been amassed. ippine Eagle Center in Davao City. From 1984-88, the PEF and the Department of Envi- Here, we report this information basedon records col- ronment and Natural Resources (DENR) monitored the lected over the past 20 yr, and provide insightsas to the key reason for the decline of the population. breeding population. Of the eight pairs monitored, there were 11 breedingattempts. Four failed (36.4%) and four METHODS young (36.4% nestingsuccess) were produced.This rep- We compiled all existinginformation on the reproduc- resentedan 18.2% increasein fledging successcompared rave successof Philippine Eagles based on published to the previousperiod. (Kennedy 1985) and unpublished documentsgathered From 1989-93, 11 breeding attempts by nine pairs re- by the Philippine Eagle Foundation (PEF) between sulted in eight fledglings(72.7% nestingsuccess) and, 1978-98. We defined a successfulbreeding attempt as from 1994-98, 17 breeding attemptsby 12 pairs resulted those with young eagles that survived until fledging. in a higher successrate (88.2%). The increasein breed- Nests were located by daily surveillancefrom vantage ing pairs was mainly due to an increased awarenessby points, usuallyalong mountain ridgesand in areaswhere local people and increased observer effort and was not eagle presencewas reported by local settlers.Observers stayedin these areasfor about a week during the breed- indicativeof the recoveryof the population. Other strat- ing seasonbetween August-Decemberand from 0600- egies such as the reward systemand media-basedinfor- 1500 H. Blinds were built in trees adjacent to nest trees, mation campaign have also been widely used by the PEF usually about 50-100 m away.Life history information since the early 1990s to increase the information on the wasobtained and daily activitieswere recorded. A reward number of breeding pairs in the population. The in- systemfor reporting occupiednests was initiated in 1981. creasein breeding pairs during the last decade may also From 1985 to the present time, the reward systemwas have been due to increasing fragmentation of lowland intensifiedand coupledwith other on-siteprograms such as the development of community-basedinitiatives and dipterocarp rainforest that result in increased contact conservation education activities. Reports of sightings with settlers. were improved further by forging partnership arrange- Breeding successbased on eight pairs with •1 nesting ments with broadcastingstations in Mindanao Island. attempt wasestimated at 0.38 ___0.14 (_SD) young/pair/

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16 Preciseassessment of the causesof breeding failure is

ß Successful difficult. Birds exposed to food shortagesand distur- bancesduring critical periods of the nesting cycle may ==lO [] Failure abandoneggs or nests(Newton 1979). Our summaryof 8 causesof nestingfailures (Table 1) wasnot completebe- [] Unknown causefield methodsvaried over the years.Moreover, our E= 2 resultsshowed that manyof the breedingpairs were dis- turbed by logging operations,slash-and-burn farming 0 1978-83 1984-88 1989-93 1994-98 and by the observersthemselves. Three of 15 failures (6 pairs) (8 pairs) (9 pairs) (12 pairs) (20.0%) were due to removalof youngfrom nestsor fell- ing of nest treeswith young.Most individualscurrently RelaUve number of successful rs. failed nesUng at10empts kept at the Philippine Eagle Center in Malagos,Davao Figure 1. Summary of the successand failure of Phil- City were either confiscatedor surrenderedas juveniles. ippine Eagle breeding attemptsbased on 7-yr intervals. There were alsothree cases(23.5%) wherein eggswere addled and/or abandoned, but the causes of nest aban- donment were unknown. yr and nestingsuccess averaged 76.3% (Table 2). This The information we obtained may also have been was a conservative estimate since we did not take into based on the most conspicuousor accessiblebreeding accountthat eaglesmay have nestedin followingyears pairs and, therefore, it may not be indicativeof the true after previousbreeding attempts failed, instead of their productivityof the populationof PhilippineEagles. Some typical2-yr cycle.Many pairs had only one nestingrecord Philippine Eagle pairs may be more experiencedbreed- and thesewere excludedin calculatingpercent breeding ers and may alsobe overrepresentedin our samplewhich successto minimize bias.One suchpair at Rd. 6P PICOP, could account for the high reproductive successwe re- Surigao del Sur was documented to have bred three corded.Also, the high breeding successmay alsoreflect times during which one nesting attempt failed and the the diminishingpersecution of PhilippineEagles by the fate of the other two attempts were unknown. Some local people. Despite the limitations of the data we col- pairs, especiallythose within the Bukidnon province,had lected, we believe that it providesimportant baselinein- 100% breedingsuccess rates while otherslike the pair at formation to help focusfuture researchand conservation Mount Apo, Toril, Davao City had a 33.3% successrate effortson the Philippine Eagle. and a productivityof 0.17 young/pair/yr. These differ- The current statusof the PhilippineEagle as Critically encesmay havebeen due to variationin food supplybe- Endangered is based mainly on the fact that this is a tweenthe areas,differences in the agesof the breeding large-sizedbird requiring a large territory and adapted birds (Newton 1979), or simplyan artifact of the small to a tropicalrainforest ecosystem that is fastdisappearing samplesize. The overall successof Philippine Eaglesav- in the Philippine archipelago.Theoretically, the assess- eragedabout 58.0% for 50 breedingattempts by 29 pairs ment of raptor population stabilityinvolves integration from 1978-98. Basedon the assumptionthat eachbreed- of reproductivedata with survivaldata for variousage ing attempt had equal probability of successor failure, classes(Henny et al. 1970), but the lack of information and that no regionaldifferences existed among different on survivalof Philippine Eaglesafter fledging limits the pairs or subpopulations,we consideredthis productivity preciseassessment of their populationstatus. Although it to be high and not indicativeof a populationsuffering is clear that the major threat to tropical birds of prey is from breeding failures. forest destruction (Thiollay 1985, 1989, 1992), it wasun-

Table 2. Breedingrates of PhilippineEagle pairs with more than one recordednesting attempt.

No. BREEDING BREEDING RATE BREEDINGPA•R LOCATION ATTEMPTS % SUCCESS (YOUNG/PA•R/yr) Dalwangan,Malaybalay, Bukidnon 5 60 0.30 Mlnlanga Range, La Paz,Agusan del Sur 2 100 0.50 Freedom, Cabanglasan,Bukidnon 2 100 0.50 Guilang-guilang,Manolo Fortich, Bukidnon 3 100 0.50 Mount Apo, Toril, Davao City 6 33.3 0.17 Amabel, Magpet, North Cotabato 2 50 0.25 Mount Sinaka,Arakan Valley,North Cotabato 2 100 0.50 Salaysay,Marilog District, Davao City 6 66.7 0.33 Mean 76.3 0.38 MAaCH 2000 SHORT COMMUNICATIONS 41 clear whether the population decline of the Philippine co Polo-Davao, Standard Chartered Bank, Cincinnati Mu- Eagle is mainly due to reproductive failures or to in- seum Center, Regional Eagle Watch Teams, Parks and creasedmortality of juveniles, subadultsand/or adults.A Wildlife Bureau of the Department of Environment and high rate of nestingfailures could explain the population Naturai Resourcesof the Republic of the Philippines, and Philippine Eagle Working Group for their unwaver- decline becausePhilippine Eagles lay a single egg and ing support.We are grateful to W.A. Burnham and J.-M. normally breed once every two years. Our data indicate Thiollay for their valuablecomments on the manuscript. that it is not nestingfailures that are responsiblefor the We alsothank the long-timesupporters of the Philippine population decline but that decreasedsurvival of juve- Eagle Foundation, R.S. Kennedy,W.A. Burnham andJ.W. niles and subadults and their inability to disperse be- Grier. Lastly,we thank the many participantsand sup- tween forest islandsto establisheventual breeding terri- porterswho over the yearshave contributed in one way tories may be limiting the number of breeding pairs in or another to the conservationefforts for the Philippine this population. Past studieshave suggestedthat the sta- Eagle. bility of breeding raptor populationsis not related to pro- LITERATURE CITED longed good production of young but could be main- ALVAREZ,J.B., JR. 1970.A report on the 1969 statusof the tained by immigration or dispersal(Mebs 1964, Ratcliffe 1972, Newton 1979, Grief 1980). Nevertheless, forest monkey-eatingeagle of the Philippines.Pages 68-73 in Papers and Proc. Int. Union Conserr. Nat. Resour., fragmentation has untold effects on large tropical forest Vol. II. l lth Technical Meeting, New Delhi, India. raptors such as the Philippine Eagle. Future research GONZALES,R.B. 1971. Report on the 1969 statusof the should focus on aspects of metapopulation dynamics monkey-eating eagle on Mindanao Island, Philip- such as survival and dispersalstudies in a highly frag- pines. Bull. Int. Counc.Bird Pres• 11:154-168. mented habitat, continued monitoring of reproductive GRIER,J.W. 1980. Modeling approachesto Bald Eagle performance of known breeding pairs in Mindanao and population dynamics. Wildl. Soc.Bull. 8:316-322. initiation of basic population ecologystudies in other is- HENNY,CJ., W.S. OVERTONAND H.M. WRIGHT. 1970. De- lands of the archipelagowhere Philippine Eaglesare his- termining parametersfor populationsby using struc- toricallyknown. tural models.J. Wildl. Manage.34:690-793. KENNEDY,R.S. 1977.Notes on the biologyand population RESUMEN.--EI•xito reproductivo total del figuila de las statusof the monkey-eatingeagle of the Philippines. Filipinas (Pithecophagajefferyi)promedio 58.0% de los in- Wilson Bull. 89:1-20. tentos reproductivospor 29 parejasdesde 1978-98. E1 1981. Saving the Philippine Eagle. Natl. Geog•: •xito reproductivocon baseen ocho parejascon mas de 159:847-856. un intento reproductivo fue estimado en 0.38 +- 0.14 ß 1985. Conservationresearch of the Philippine (-+SD) juvenil/pareja/afio y el •xito reproductivopro- Eagle. Natl. Geog•:Soc. ires. Rep. 18:401-414. medio 76.3%. Hubo 15 fracasosreproductivos, tres de los MEBS,T. 1964. Zur biologie und populationsdynamikdes cfialesse debieron a la remoci6n de juveniles del nido o mausebussard( Buteobuteo). J. Ornithol.105:247-306. ca/das del nido, yen tres casoslos huevosfueron infer- NEWTON,I. 1979. Populationecology of raptors.T. 8cA.D. tiles o abandonados.Nuestro anS,lisis sugiere que los fra- Poyser,London, U.K. casosreproductivos no son un factor mayor en la decli- RABOR,D.S. 1971. The present statusof conservationof naci6n poblacionaldel/tguila de las Filipinasy apunta a the monkey-eatingeagle of the Philippines. Phihpp un decrecimiento de la sobrevivenciade los juveniles y Ge0grrJ. 15:90-103. subadultosy su inhabilidad para dispersarseentre los RATCLIFFE,D.A. 1972.The peregrinepopulation of Great fragmentosde bosquecomo la causade la declinaci6n Britain in 1971ßBird Study19:117-156ß num•rica de esta especie. THIOLLAY,J.-M. 1985. Falconiformes of tropical rainfor- [Traducci6n de C6sarM•quez] est: a review. Pages 155-165 in I. Newton and R.D. ACKNOWLEDGMENTS Chancellor [ED$.], Conservationstudies on raptorsß ICBP, Cambridge, U.K. We would like to thank R.S. Kennedyand the members ß 1989ßArea requirements for the conservationof of ER.E.E. Ltd. whose pioneering work laid the founda- rainforest of rainforest raptors and game birds in tion for the conservationresearch of this magnificent French Guianaß Conserv. Biol. 3:128-137. raptor. We are grateful to the other members of the Field Research component of the Philippine Eagle Founda- ß1992. Influence of selectivelogging on bird spe- tion; Donald S. Afan, Glenn Lovell, L. Bueser, Kharina cies diversityin a Guianan rainforestßConserv. Biol. 6: M. Gatil, Adriano M. Oxales and the rest of the staff. We 47-63. thank the Peregrine Fund, John D. and Catherine T. MacArthur Foundation, Oxbow Power Corporation, Mar- Received 5 December 1998; accepted 28 October 1999