j RaptorRes. 33(2):102-109 ¸ 1999 The Raptor ResearchFoundation, Inc. THE GOLDENEAGLE ( CHRYSAETOS) IN THE BALl MOUNTAINS,

MICHEL CLOUET 16 Avenue des Charmettes,31500 Toulouse,

CLAUDE BAm•u 58 Chemin des C6tes de Pech David, 31400 Toulouse,France

JEAN-LouIsGOAR 11330 VillerougeTermeries, France

ABSRACT.--Westudied Golden (Aquila chrysaetos)in the afro-alpinearea (elevation3500-4000 m) of the Ball Mountains in Ethiopia. We monitored seventerritories from 1-5 successiveyears for a total of 26 territory-years.Home rangesvaried from only 1.5-9 km2, the smallestsize recorded for the .This wasprobably due to the abundanceof prey, mainly haresand grassrats, that made up 50% and 30% of prey, respectively.Despite this, productivityof these Golden Eagleswas quite low averagingonly 0.28 youngper occupiedterritory (N = 25). This wasdue to a large number of unmated territorial adultsand poor breedingperformance by pairs (0.4 youngper pair per year,N = 17). The high densityand frequent interspecificinteractions with Verreaux'sEagles (Aquila ver•eauxii)were key factorsaffecting the dynamicsof this Golden population.The unusualcoexistence of thesetwo closelyrelated specieswas a novel componentof the rich predator guild in the area that includedfive other winteringor residenteagle species. This richnesswas related to the high densityof rodentsand lagomorphs,a characteristicof the Ethiopian afro-alpineecosystem. K•Y WOP,DS: GoldenEagle;, Aquila chrysaetos;Ver•eaux's Eagle;, afro-alpine ; Ethiopia; p• altondance, interspecificcompetition; p•vductivity.

El aguila dorada (Aquila chrysaetos)en las montanasBall de Etiopia Po!sUmEN.--Estudiamosel •tguila dorada (Aquila chrysaetos)en el grea afro-alpina(elevacion 3500-4000 m) de las montafiasBall en Etiopia. Monitoreamossiete territorios de 1-5 arioscontinuos para un total de 26 territorios/afio.Los rangosde hogar variaronentre 1.5-9 km2. Los mas pequefiosreportados para la especie.Esto probablementedebido a la abundanciade presas,principalmente liebres y ratas de pastizales,las cualesrepresentan el 50% y el 30% de las presasrespectivamente. A pesarde estola productividadde las gguilasfue muy baja con un promediode 0.28juveniles por territorio ocupado(N = 25). Esto se debi6 al gran n6mero de adultosterritoriales sin parejay al pobre desempefioreprod- uctivo de las parejas (0.4 juvenilespor pareja por afio, N = 17). La alta densidady las frecuentes interaccionesintraespecificas con las •guilas Verreaux (Aquila verreauxii)fueron factoresque afectaron la dingmicade estapoblaci6n de gguilasdoradas. La inusualcoexistencia de estasdos especiesestre- chamente relacionadases un componente novedosode la estructurade depredadoresen el grea que incluyeotras cinco especies migratorias o residentesde gguilas.Esta riqueza estuvo relacionada con la alta densidadde roedoresy lagomorfos,una caracteristicadel ecosistemaafro-alpino Etiope. [Traducci6nde C6sarMJxquez]

The recent discoveryof a (Aquila alpine region which supportsthe largestmountain chrysaetos)population in Ethiopia (Clouet and Bar- moorland and grasslandhabitat on the continent. rau 1993) has provided a unique opportunity to This rich and unique ecosystem(Dorst and Roux study this speciesin a new biogeographicalarea. 1972, Hillman 1986) supportsan aviancommunity The population occursin the Ba16Mountains lo- dominated by a predator-scavengerguild (Clouet cated in the southern part of the Ethiopian high et al. 1995). The Golden Eagle is part of a unique , eastof the Rift .It occursat the afro- assemblageof eagle speciesand coexistswith the

102 JUNE 1999 GOLDENE^GLE •N ETHIOPIA 103

'-.-:- . -;' " n in

Figure 1. Location of the Bal• Mountain study area in Ethiopia.

Verreaux's Eagle (Aquila verreauxiO.To our knowl- -.::•'::"' -:;..•.,,...... edge, this assemblagethat includes the southern- r-• Valleyfloor • ,-• Plateau most extent of the Golden Eagle has never before Figure 2. Distributionof Golden and ¾½rrcaux'sEaõl½s been studied. in the 200-km2 studyarea in the Bal• MountainsNational Park, Ethiopia, 1993-97. STUDY AREA AND METHODS The study area was in Bala Mountains National Park in the upper Web River Valley and a portion of its trib- utaries above line (3500-4000 m) (Fig. 1). The area not breeding or when it was occupiedby an un- consistsof a continuousnetwork of cliffsstretching from paired , we used the arithmetic center of the the north bank of the Web River to the top of the (Sanetti plateau). The afro-alpine has a tropical known unoccupiednests in the territory. The av- tempered by the altitude and characterizedby an erage distancebetween centers was 4.7 km (range alternating wet season that lasts from March-October = 2.5-7 km) (Fig. 2). Individual territories were and a dry seasonthat lastsfrom November-February. monitored for 1-5 yr totaling 26 territory-years We searchedfor breedingraptors in a 200 km2 area of (Table 1). Single adultswere observedin three out potential nesting and hunting habitat over the course of sevenexpeditions covering five successivebreeding sea- of the sevenmonitored territories and during 4 of sons: August 1993, May 1994, March-November 1995, the 5 yr of the study (i.e., seven (27%) of the 26 March 1996, and February-August 1997. One to three territory-years).Nonterritorial singlebirds were re- observerswalked transectsthrough Golden Eagle terri- corded only twice (1 adult and 1 immature eagle). tories.Observations were made continuouslyfor periods of 2-11 hr, recording the activitiesof the eaglesand any Topographically, the territories included a sec- interspecificbehavior involving other raptorsspecies. All tion of cliff where perches and nestswere located eagle flights were plotted on a map to calculate their and a part of the neighboringplateau. The slope distances and areas covered. The total observation time located at the foot of the cliff was comprisedof for the seven expeditionswas 210 h. ,, and bushes,and it provided a va- We estimateddiet by observingkills and collectingprey itemsin nestsduring the fledgingperiod. Diet diversitywas riety of potential prey including (Procavia calculatedusing a Shannon Index (Delibes et al. 1975, capensis), ( LepusstrarkO, and (Fran- Clouet 1981, Fernfindez1991). Productivity(number of colinusspp.). In the valley bottom, there were also young per occupiedterritory) was calculated by observing coloniesof mole (Tachyoryctesmacrocephalus) youngin neststhat were older than 51 d of age (Steenhof 1987) (exceptin one casewhen a nestlingwas only about and grass rats (Arvicanthisspp. and Lophuromys 35-d old) or in flight with adultsduring the postfledging spp.). stage(August 1993, May 1994,and August1997). We observednine successfulkills by Golden Ea- gles.All were made either on low altitude flights RESULTS by eaglesclose to the slopesat the foot of cliffs We identified seventerritories occupied by - (once) or within 200-2000 m of perches (eight en Eagles. We assumedthat territories were cen- times). Golden Eagleswere alsoseen robbing prey tered on the occupied nest or, when the pair was from an Augur (Buteoaugur), Pallid Har- 104 CLOUET ET AL. VOL. 33, No. 2

Table 1. Territory occupancyand productivityof Golden Eaglesin the Ba16Mountains National Park, 1993-97.

TERRITORY

YEAR 1 2 3 4

1993 Single adult Adult pair Single adult -- (6-13 August) 1 young flying 1994 Adult pair Adult pair Single adult (5-17 May) Adult pair 1995 Singleadult 1 young flying Adult pair Singleadult (21 March-5 April) Adult pair 1996 Single adult Adult pair Adult pair Single adult (4-15 March) 1997 Adult pair Adult pair Adult pair Adult pair (3-12 February) 1 5-wk-oldyoung (9-18 August)

rier (Circus macrourus), Lanner Falcon (Falco biar- Other breeding raptors were observed in every m•cus),and Eagle (Aquila nipalensis). Golden Eagle territory we observed,including one Undulating displayflights and attackson intrud- or two pairs of Augur ,one pair of Lanner ers of other raptor specieswere performed by both Falcons and one pair of Common Kestrels (E tin- paired or unpaired Golden Eagles,both near and nunculus).Frequent mobbing behavior of these <1 km from perches, cliffs and nests.In the case raptors triggered attacks by eagles. We observed of one unpaired territorial adult, undulating Tawny Eagles (A. rapax) which were residents flights accountedfor up to 51% of the total flying throughout the year and Greater Spotted Eagles time (115 min) in November at the beginning of (A. ), Lesser Spotted Eagles (A. pomarina), the breeding seasonsuggesting that the function and Steppe Eagles which were either migrating of this flight was to displayits territory. through or spentthe winter in the area flying over The mean area of the home ranges, estimated Golden Eagle hunting areas. The latter were very from observationsof hunting foraysand territorial numerous and on several occasionsapproximately flights, such as undulating displaysand attackson 30 were recorded simultaneouslyin flight in Feb- intruders, was 3.6 km2 (range = 1.5-9 km2, N = ruary. Steppe Eagleswere observedattacking Gold- 7). Home rangeswere smallerfor singleadults (1- en Eagles on their prey (3 cases). Golden Eagles 1.5 kma, N = 2) and appeared to be larger where were also seen chasing off intruding Tawny and there was no colony or where scrub (Erica Steppe Eagles (3 cases). spp.) wasextensive (estimated to be about9 kma). The survey area contained seven territories of Observations of kills (N = 9) and identification Verreaux's Eagles,all occupied by pairs. All pairs of prey items brought to nests(N = 41) showeda raised at least one young during the studyperiod. predominance of which accounted for The averagedistance between occupied nestswas 86% of the prey: 25 hares (50%), 3 hyraxes(6%), 5.2 km (range = 3-7 km). Observationsat six ter- 2 giant mole rats (4%), 13 grassrats (26%), 5 Ar- ritories showed that Verreaux's Eagles traveled vicanthisblicki (10%), 1 Lophuromysmelanonyx (2%), from 12-13 km2 on hunting foraysand territorial and 7 unidentifiedprey (14%). Smallspecies were flights and they establishedcommon boundaries probably underestimatedbecause they were often between their territories. The mean distance be- entirely consumedby eagles. accounted for tween the nearest occupied nest or territory center a smallerpart of the diet (14%): 4 Moorland Fran- of a Golden Eagle and a Verreaux's Eagle was 3.4 colins (Francolinuspsilolaemus, 8%), 2 Blue-winged km (N = 12, range = 0.65-6.0 km). Territories of Geese (Cyanochencyanopterus, 4%), and 1 uniden- the two specieswere mutually exclusive.Interac- tified bird. Mammals accounted for 90% of the to- tions we recorded most often were attacksby Gold- tal preybiomass and haresalone made up 78% of en Eagleson Verreaux'sEagles (N = 21). Only one the biomass. attackby a Verreaux'sEagle on a Golden Eaglewas JUNE 1999 GOLDENEAGLE IN ETHIOPIA 105

Table 1. Continued. ic encounters observed occurred when two Golden Eagle territories were located between two Ver- TERRITORY reaux's Eagle territories and where inter-nest dis- tances were short. In this situation, the number of 5 6 7 Golden Eagle flights per hour of observation (38 -- hr) was 1.74. Territory defenseflights and undu- lating flightsmade up 32% and 24% of all flights Adult pair (N = 66), respectively.When the specieswere far- 1 10-wk-oldyoung ther apart (N = 4 territories), the number of Adult pair flights per hour of observation (44 hr) was 1.98 1 7-wk-oldyoung Adult pair and the number of undulating flights accounted I 7-wk-oldyoung Adult pair for only 6% of the total number of flightsobserved Adult pair 1 9-wk-oldyoung (N = 87). incubating Adult pair Adult pair Golden Eagles began building nestsin Novem- I young flying ber and unsuccessfulbreeding pairs often contin- with adults ued building until February. We observed five nests containing a single eaglet between March and May and immatures were seen flying together observedand it involvedan attempted piracyat the with adultsin May and August (Table 1). Judging boundary of two territories. from the age of the young we observed,we esti- We recordedthree typesof behaviorby the Gold- mated that laying occurred from mid-November to en Eagle towardVerreaux's Eagles (N = 26). One mid-January, and that fledgling occurred from behavior we classified as tolerance. It occurred mid-March through the end of May. The begin- ning of the breeding seasoncorresponded with the when one or a pair of Verreaux'sEagles flew at high beginning of the dry seasonand the young left the altitudes(>500 m) (N = 5) aboveGolden Eagles. nestat the beginningof the rainy season.This phe- However,we observedbirds of each speciesperched nology correspondedto the breeding seasonsof out of sight of each other on the same cliff at dis- most other nesting raptors in the Ba16Mountains. tances of <400 m. Flight and undulating display The nestingseason for Verreaux'sEagles started a flight behaviorswere triggered by a Verreaux'sEa- few weeksearlier than that of Golden.Eaglesand gle approachingwithin 500-1000 m of a Golden Ea- the first females began incubation in November. gle nestor perch (N = 7). Aggressiveflight behavior Young Verreaux's Eagles left their nests from toward an intruding Verreaux's Eagle occurred March-June. Productivity of Golden Eagles was when the intruding eagle camewithin 500 m of an 0.42 young per territorial pair per year (N = 19, occupiednest or perch used by a Golden Eagle (N Table 2). = 14). Aggressiveflights usually ended when the intruder withdrew. In two cases, the interaction re- DISCUSSION suitedin grapplingof talons. Our observationsof Ethiopian Golden Eagles The most frequent and most intenseinterspecif- showed similaritieswith holarctic populations but

Table 2. Golden Eagle productivityin the Ba16Mountains National Park and locationsof neighboringVerreaux's Eagles.

DISTANCE FROM LOCATIONS OF NEIGHBORING NEIGHBORING YEARS PRODUCTMTY VERREAUX'S EAGLE VERREAUX'S EAGLE TERmTORY OF STUDY (YOUNG/YEAR) PAIR(S) (km) PAIR(S) 2 5 0.40 2.7 different valley 3 4 0.25 0.65 same cliff 3.0 samevalley 5 3 1.0 1.2 different valley 4.0 samevalley 106 CLOt:}•T ET AL. VOL. 33, NO. 2 differed in terms of their use of space,population Zubiri 1990). These values are more than 20 times regulation, and tire of predator community the available biomass of above the tree-line into which they integrated. Even though our prey on the northern slopesof the Pyr•n•es where the sample was limited, Ethiopian Golden Eaglesap- size of Golden Eagle territories is roughly 20 times peared to selectterrestrial prey of similar size to larger than in Ethiopia (Clouet 1991). that taken by Golden Eagles in European and Prey abundance influences breeding perfor- North American populations (Brown and Watson mance both in and in 1964, Murphy 1975, De!ibeset al. 1975, Clouet and (Murphy 1975, Smith and Murphy 1979, Clouet Goar 1980, Haller 1982, Steenhof and Kochert 1981, Haller 1982, Thompson et al. 1982, Tjern- 1988, Fern•ndez and Purroy 1990). The number berg 1983, Jenny 1992, Watson et al. 1992, Steen- of small-sizedprey (<2 kg) such asgrass rats, which hof et al. 1997). Productivityof Golden Eagles was likely underestimatedby our sampling meth- ranging from 0.79-0.82 young per territorial pair od, showedthat these Golden Eagleswere oppor- per year have been reported in long-term studies tunisticpredators taking small prey items when in North America (Phillips et al. 1990, Bats and they became availablein sufficientquantities (De- Moretti 1994, Steenhofet al. 1997). Productivity libes et al. 1975, FernSndez 1991, Watson et al. has been lower in most European studiesranging 1993). Becausehares were predominant prey, diet from 0.48-0.53 young per territorial pair per year diversitywas low (H' = 1.44) in comparisonwith in the (Haller 1996) and Pyr•n•es (Clouet GoldenEagles in the Pyr•n•es (H' = 2.77)(Clouet 1988). Watson (1997) found a significantnegative 1981). correlation betweenbreeding successand diet di- The densityof Golden Eaglesin the Bal• Moun- versity.Where Golden Eagleshad a narrow feeding tains matched the highest figures recorded in the niche they tended to breed more successfullythan western highlands of (one pair per 38 where the niche was broad. In Ethiopia, despite km '•, Watson et al. 1992) and in North America high prey abundance and low diet diversity,pro- (one pair per 29-36 km'•, Phillipset al. 1990). The ductivitywas low. This wasprobably due to the fact home range size we recorded was the smallestre- that territorial pairs did not breed successfullyev- ported for the species. ery year and only raised a singleeaglet per brood In Europe, territory size can range from 40-160 and, in some years, certain territories were occu- km'• (Clouet 1988). In the SwissAlps, the home pied by only singleadults (27% of the total 26 ter- range of four breeding pairs ranged from 22-48 ritory-yearsstudied). The latter may have resulted km2, and core areasused for hunting that ranged from a lack of surplusbirds in the population. Sur- from 6-16 km2 (Haller 1982) were larger than the plus birds have often been noted in Golden Eagle areas used in Ethiopia. Variation in density be- populations (Haller 1982, 1996, Marzluff et al. tween different areasin Europe wasassociated with 1997) and can composeup to 30% of the popu- food availability (Tjernberg 1985, Watson et al. lation in the Alps (Clouet and Couloumy 1994). 1992). The home range size documented in North The lack of surplus birds in the Bal• Mountains America was influenced by the amount of favor- could have been due to human , but able prey habitat and became smaller where high we have no data to support this. It may also have quality prey habitat was abundant (Dixon 1937, been due to the absence of recruitment of individ- Collopyand Edwards1989, Marzluff et al. 1997). uals from outside the Bal6 Mountain range. Be- The abundanceof prey and its availabilitythrough- causethe population is sedentary,there is little op- out the year in the Bal• Mountains was probably portunity for interaction with populationslocated an important factor in explaining the high density 2000 km further to the north (Thiollay and Du- and the small home ranges we observed. Both hautois 1976). No Golden Eagleshave been ob- hares and rodents occurred in high numbers in served on the banks of the during migra- the afro-alpinemoorland and grasslandof the Bal• tory movements (Bruun 1985, Welch and Welch Mountains. Their abundancewas estimated by Got- 1989). Finally, insufficient productivity could ex- telli and Sillero-Zubiri (1990) during their studyof the absenceof surplusbirds. Our estimated the Ethiopian (Canis simiensis). density 0.42 young fledged per territorial pair per year is was 32 individuals/km'• for a biomassof 120 kg/ very low for a Golden Eagle population. In other km2. The biomassof mole and grassrats wasesti- African eagle populations,breeding successis not mated at 3000-4000 kg/km s (Gottelli and Sillero- related to food availabilitybut is highly densityde- Jt•4E 1999 GoLI)E•4E^GLE I•4 ETHIOVI• 107 pendent (Thiollay and Meyer 1978, Gargett1977, and breed at the sametime of the year. Evidently, 1990, Simmons 1993). The effect of density has the two speciesescape from interspecificcompeti- also been documented in Golden Eagle popula- tion by establishingmutually exclusive ranges and tions in Europe (Hailer 1982, 1996, Jenny 1992). they use undulating display flights to advertise The reduced productivity (0.48 young per pair per their territory boundaries(Harmata 1982, Collopy year) recorded in the SwissAlps has been inter- and Edwards 1989). The only niche parameter preted as the consequenceof a density-dependent which actuallydistinguishes the two speciesis diet. regulation process.The major limiting factor in Dietary studies in show that Verreaux's Ea- breeding successin the Alps is the frequency of gles prey almost exclusivelyon hyraxeswhich interactions with unpaired nonterritorial surplus represent up to 98% of their food intake (Brown birds causing a negative feedback effect. Regular et al. 1982, Gargett 1990). The prey remains that interactions with settled birds increasesthe pro- we collected under Verreaux's Eagle nests con- portion of nonbreeding pairs and depressessuc- firmed that hyraxes also predominate in the diet cessfulincubation (Hailer 1982, 1996,Jenny1992). of Verreaux'sEagles in the Bal• Mountains.On the Perhaps, in the Bal• Mountains, interactions be- other hand, hyraxesrepresented only a very small tween Verreaux's and Golden Eagleshave the same part of the diet of Golden Eagles. negative effect on reproduction as surplusGolden We believe that the unusual coexistence of Gold- Eagles in the Alps. Productivity of Golden Eagle en and Verreaux's Eagles is possible because the pairs appeared to be influenced by the proximity Ethiopian afro-alpine region supports a rich and of neighboring Verreaux's Eagle pairs. For the clos- dense rodent community (Yalden 1988) that est nests (650 m apart on the same cliff), neither makespossible the assemblageof the most diverse species succeeded in breeding simultaneously. guild of raptors ever found at such a high altitude. Such interactions with other speciesof eagles are exceptional, making the situation in Ethiopia rath- ACKNOWLEDGMENTS er unique. We would like to thank Alex Clamens and Jeff Watson Golden Eagle coexistencewith other eagle spe- for comments on a first draft of this , Elisabeth cies such as with Bonelli's Eagles (Hieraaetusfascia- Goar for the English translationand Mike Kochert, Dawd tus) has been reported in the Mediterranean re- Ellis,and GaryR. Bortolottifor their stimulatingremarks. gion. Apparently, the two speciesbecome eco- LITERATURE CITED logically isolated through their territoriality and diets (Brosset1961, Cheylan1977, Jordano 1981, BAHAT,O. 1989. Aspectsin the ecologyand biodynamics Clouet and Goar 1984, Parellada et al. 1984, Fer- of the Golden Eagle (Aquilachrysaetos homeye•i) in the arid regions of . M.S. thesis,Tel Aviv Univ., Tel nandez and Insausti 1986, Bahat 1989). In Israel, Aviv, Israel. apparentlycompetition betweenthe two speciesre- BATS,J.W. ANDM.O. MORETTI.1994. Golden Eagle (Aquz- suits in a lower density of Golden Eaglesin areas la chrysaetos)population ecology in eastern . where Bonelli's Eagles are abundant and where Nat. 54:248-255. both speciesmaintain exclusivehome ranges (Ba- BROOKE,R.K., J.H. GOBLER,M.P. STU•qT IRWIN AND P hat 1989). STEYN.1972. A studyof the migratoryeagles Aquzla In Ethiopia, the Golden Eagle is integrated into nipalensisand A. pomarina (Aves: ) •n the largesteagle assemblageknown which includes southernAfrica with comparativenotes on other large five other Aquila species.Here, ecologicalisolation raptors. Occas.Pap. Matra. Mus. Rhod.B5:61-114. betweenTawny Eagles (resident) and Steppe,Less- BROSSET,A. 1961. ]•cologie des oiseaux du Marocorien- er, and Greater Spotted Eagles (winter only) de- tal. Trav. Inst. Sci. Chb•if Rabat, . velops through specialized use of habitat (less BROWN,L.H. ANDt. WATSON.1964. The Golden Eagle •n relation to its food supply.Ibis 106:78-100. rocky than that of the Golden Eagle), temporal --, E.K. Uvo3AN AND K. NEWMAN. 1982. The birds of separationof breeding periodsand partly through Africa. Vol. 1. Academic Press, London, U.K. diet. The Verreaux's Eagle is morphologicallyand BRUUN,B. 1985. Raptor migration in the Red Sea area ecologicallyvery similar to the Golden Eagle and ICBP Tech. Publ. No. 5:251-255. it is generallyregarded to be its African equivalent. CHEYL•N,G. 1977. La placetrophique de l'aigle de Bo- Both speciesare of similar size, have a similar pre- nelli (Hieraaetusfasciatus) dans les bioc•nosesm•di- dation potential (Voous1970, Brookeet al. 1972), terran•ennes. Alauda 45:1-15. feed on terrestrial prey, use similar nestinghabitat CLOU•T, M. 1981. L'Aigle royal dans les fran- 108 CLOUET ET AL. VOL. 33, NO. 2

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