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f Raptor Res. 27(2):111-112¸ 1993 The Raptor ResearchFoundation, Inc.

EAGLE OWL PREDATION ON EGYPTIAN VULTURE ANDNORTHERNGOSHAWK:POSSIBLE EFFECT OF A

DECREASE IN EUROPEAN RABBIT AVAILABILITY

Jos•. L. TELLA• AND SANTI MAiqIOSA
2
Departamentode BiologœaAnimal, Faculdadde Biologœa,Universidadde Barcelona,
Avda.Diagonal,6,I5, 08028, Barcelona,Catalonia,Spain

Eagle Owls (Bubobubo)occasionallyprey upon small
and medium-sizedbirdsof prey (Mikkola 1983). Here we
reportobservationsof EagleOwl nestpredationon Egyptian Vulture (Neophronpercnopterus)
and Northern Goshawk (Accipitergentilis)madeduringtwo intensivestudiesof these diurnal raptors in the Ebro basin, northeastern
Spain. We monitored 178 breeding attemptsof 58 pairsof Egyptian Vultures from 1980-91 and 101 breedingattemptsfrom 26 pairs of goshawksfrom 1987-90.
Eagle Owl predationaffectedsevennestsof EgyptianVultures. On two occasions,
nestlingsolder than 35-40 dwere beheaded,which is a characteristicbehaviorof EagleOwls (Cramp 1985, J.A. Donfizar pers. comm.). Only
their headsand somewing featherswere found in the nest.
The remainsof a decapitatedadult bird, probablykilled
while incubatingor broodingverysmallchicks,werefound
50 m from another nest. Nestlingsin the other four nests
were very young and disappearedwithout leaving any
trace.The nestswere inaccessible
to mammalianpredatorsthat can kill nestlings(e.g., the red fox Vulpesvulpes;
Donfizar and Ceballos1989, Tella and Torre 1990), andEagle Owl nestsor roostingsiteswere detectednearby.
Eagle Owl predationwas the mostplausibleexplanation
for these losses.

Four goshawknestswith youngbetween13-38 d weredepredatedby Eagle Owls. In one case,a nestling aged
38 d was taken from a nestwhich containedthree young.
The remains and leg band of that bird were found in anEagle Owl nest 3 km away. On another occasion,theremains of a 17-18 d old goshawk chick were found in
the sameEagleOwl nest,2 km away from a goshawknest
that had lost its 15-20 d old chicks on the same day.

• Presentaddress:Estaci6nBio16gicade Dofiana (CSIC),
Avda. M a Luisa s/n, Pabel16nde Per6, 41013 Sevilla,Spain.
2 Present address:The Game Conservancy,Fordingbridge, Hampshire, SP6 1EF, United Kingdom.
111

Although this chick was not marked, no other goshawk
nestin the area lost nestlingsof that particular age at thesametime. The othertwo nestswere foundempty,except
for a few feathers

left in one of them.

In nests wherenestlingsstarvedor were killed by humans,we never detectedthe actionof scavengers
or the parentsremovingthe
carcasses.
Moreover,no signsof human robberyor mammalian predationwere found,leavingEagle Owl predation
as the most plausibleexplanationfor theselosses.In oneof thesetwo nests,which containedthree young,the predationof two chicks was detected 4 d after the first one

had beentaken.This is a commonbehaviorof Eagle Owls,
who can take severalconsecutivenightsto kill and remove
nestlingsof a wholebrood(Olsson1979 in Cramp 1985).
No caseof Eagle Owl predationon the Egyptian Vulture hasbeenreporteduntil now, and only a few caseson
the goshawk(Mikkola 1983). In the studyarea, 31% of
Egyptian Vulture nestswere <200 m from Eagle Owl
nestsor roostswhich may haveincreasedthe risk of predation (Real and Mafiosa 1990). Nevertheless,no depredationswere reportedbefore 1989 (N = 52), while 7.1%(N = 99) of the nestswith youngwere depredatedafterthat year (Fisher's exact test, upper tail = 0.0478, lower
tail = 0.0483), accountingfor 50% of brood losses(Tella1991). For goshawks,6.1% (N = 33) of nestscontaining
nestlingswere affectedby Eagle Owl predationin 198788, and this percentageincreasedto 18.2% (N = 11) in
1989 (Fisher's exact test, upper tail = 0.0000, lower tail= 0.2565).

In the Ebro Valley, the European rabbit (Oryctolaguscuniculus)is the main prey of the Eagle Owl (Don•zar1989), as it is in other Mediterranean areas (Don•zar etal. 1989). Yearly peak abundancesof European rabbits
in the area were determinedby following a 19.7 km transect at dusk 1-5 times each month in a vehicle at a max-

imum speedof 40 km/hr. All the rabbits seen on thetransectwere recordedand the monthly averagenumberof rabbitsper km was calculated.Peak rabbit abundanceswere 2.1 rabbits/km in 1987 and 2.9 in 1988, but numbers

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decreased to 0.7 in 1989 and 1990, and to 0.4 in 1991,

probably due to the viral haemorrhagicdisease(Tella1991, Mafiosa 1992). During this period, the diet of an
Eagle Owl pair was determinedby collectingprey remains
at their nest during and after the nestling period. We
detectedan increasein the percentageof diurnal raptors
consumedfrom 3.5% (N = 144 prey) in 1986-87 to 9.3%
(N = 43 prey) in 1989 (Fisher's exact test, upper tail =0.0897, lower tail = 0.1250).
Althoughour resultsdonot provideconclusive
evidence,
we suggestthat a diet shift of the Eagle Owl in responseto the scarcityof its main prey may be responsiblefor theincreasedpredation on nestsof other speciesof raptors
after 1989. A similar effect has been reported by McInvaille and Keith (1974) for the Great Grey Owl (Strixnebulosa)and Red-tailed Hawk (Buteojamaicensis).This
may be an important factor in the managementof someendangeredspeciesof birds of prey, as well as for thedynamics of the predator-prey communities involved

LITERATURE

CITEDCRAMP,S. [ED.]. 1985. The birds of the western palearctic. Vol. 4. Oxford Univ. Press, Oxford, U.K.

DON3.ZAR,J.A. 1989. Variacionesgeogrfificasy estacionales en la dieta del Bfiho Real (Bubo bubo) enNavarra.

--Ardeola 36:25-39.

ANDO. CEBALLOS.1988. Red fox predationon
fledglingEgyptianVultures. J. RaptorRes.22(3):88.
, F. HIRALDO, M. DELIBES AND R.R. ESTRELLA.

1989. Comparativefoodhabitsof the Eagle Owl Bubobuboand the Great Horned Owl Bubo virginianusin
six Palearcticand Nearcftc

biomes.

Ornis Scand. 20:

298-306.

MAI•OSA,S. 1992. Incid•nciadela pneum6niaviricadel
conill sobrela comunitatde rapinyaires segarrencs.ElMediNaturaldel Vallis

3:141-149.

MCINVAILLE, W.B. AND L.B. KEITH.

1974.

Predator-preyrelationsandbreedingbiologyof the Great HornedOwl(Rohner and Doyle 1992, Polisand Myers 1989).
RESUMEN.--Sedescribendiversoscasosde predaci6nde
nidosde alimoche(Neophronpercnopterus)
y azor (Accip•ter gentilis) por parte del bfiho real (Bubobubo)en el
norestede la Peninsula Ib•rica. Estas predacioneshan
aumentadoa partir de 1989 coincidiendocon un importante descenso
en las poblaciones
de su presaprincipal, elconejo(Oryctolaguscuniculus),pop lo que se sugiereun
importante papel del bfiho real en la dinfimica de las
poblacionesde rapacesen perlodosde escasezdel lago-VOL. 27, NO. 2

and Red-tailed

Field-Nat.

Hawk

in CentralAlberta.

Can88:1-20.

MIKKOLA,H. 1983. Owls of Europe. T. and A.D. Poyser, Calton, U.K.

POLIS,G. A. ANDC. A. MYERS. 1989. The ecologyand
evolutionof intraguild predation:potentialcompetitorsthat eat eachother. Annu. Rev. Ecol. Syst.20:297-330.
REAL,J. ANDS. MAi•OSA.1990. EagleOwl (Bubobubo)
predationon juvenile Bonelli'sEagles(Hieraaetusfasciatus).J. RaptorRes.24:69-71.
ROHNER,C. AND F.I. DOYLE. 1992. Food-stressedGreat

Horned Owl kills adult Goshawk:exceptionalobservation or communityprocess?
J. RaptorRes.26:261-morfo.

263.

ACKNOWLEDGMENTS

We are grateful to J.A. Donfizar for his advicewhen
writing the paper, and to J.L. Lagares for helping in thefield. C. Rohner, S. De Stefano,J. Reynolds,N. Aebischer,and F. Hiraldo made very useful commentsof an earlyversionof the manuscript and R. Sageimprovedthe English of the draft. A F.P.I. grant from the Ministerio deEducaci6ny Cienciafinancedthe work on goshawk,andfield work on Egyptian Vulture was partially financedby
Diputaci6n General de Arag6n, Secci6nde Conservaci6ndel Medio Natural, Plan 533-1 de Protecci6ny Conservaci6n de la Fauna Silvestre,projectR-297-91, administrative

director

M.

Cabrera.

TELL& J.L. 1991. Dinfimica poblacionaldel alimoche(Neophronpercnopterus)
en el valle medio del Ebro.
Valoraci6ndela NHV del conejosobrelas poblaciones
de alimoche.Unpubl. reportDirecci6nGeneraldeArag6n,Secci6nde Conservaci6ndel Medio Natural, Zaragoza, Spain.
--AND I. TORRE.

1990.

Observaciones

sobre re-lacionescleptoparasitariasinterespecificasen el alimocheNeophronpercnopterus.
Butll. G.C.A. 7:33-35.
Received14 September1992; accepted14 March 1993