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Home , Great horned owl, Northern goshawk

J. Raptor Res. 26(4):261-263 ¸ 1992 The Raptor ResearchFoundation, Inc.

FOOD-STRESSED GREAT HORNED KILLS ADULT GOSHAWK: EXCEPTIONAL OBSERVATION OR COMMUNITY PROCESS?

CHRISTOPH ROHNER Departmentof Zoology,University of ,Vancouver, BC, CanadaVGT 1Zd

FRANK I. DOYLE Kluane Lake ResearchBase, Mile 1054 AlaskaHighway, YukonTerritory, Y1A 3V4

Great Horned (Bubo virginianus)can prey on locationsonce every secondnight. Food stressduring the other owl and diurnal raptor speciesup to the size of broodaddition experiment was suggestedby a decreasein Northern Goshawks( gentilis), and the question the amountof foodbrought to the platform, decliningowlet has been raised why this behavior occursand whether it weights,and increasedhunting distancesfrom its nest by affectsthe structureof raptorcommunities (Craighead and the femaleowl. The goshawknest was within the territory Craighead 1956, Mikkola 1983, Voous 1988). Most re- of the owl pair, but the telemetry locationsdid not reveal ports originate from analysesof pellets of prey remains any relation to the goshawknest. On 27 June, we found collectedat owl nestsand roostsites. There is little specific the remains of the right leg of an adult female goshawk informationon how the owls kill potential harmful prey, besidethe owl tethering platform. The remains were sev- nor about the ecologicalconditions that facilitate suchpre- eral daysold, and presumablywere droppedby the owls

dation. During our studyof avian predationin the boreal DISCUSSION forestecosystem near Kluane Lake in the southwestern (Krebs et al. 1992), we encounteredcircumstantial Why Publish a Single Observation? Becauseof the evidencefor an owl predationof an adult femalegoshawk, nature of rare events,a sufficientsample size for testing hypothesescan only be achievedas a collaborativeeffort which led us to a revised assessmentof such interspecific of different researcherswho publish few or even single killings among raptors. observationson this topic (Schmutz 1992). The fact that On 18 June 1991, we founda goshawknest on the flat Great Horned Owls kill other of prey has beenwell top of a dead White (Piceaglauca) about 10 m documented(reviews in Craighead and Craighead 1956, high. The nestwas unusuallyexposed above canopy height Mikkola 1983, Voous 1988), and no further publications of the surroundingtrees (all other 28 goshawknests found are neededto simply report this behavior.We agreewith in our studyareas were 4-8 m belowcanopy height). Fresh Bortolotti(1992) that the publicationof singleobservations streaksof whitewash and two plucking sites with fresh should allow links to the analysisor interpretation in a higher-level context. As a consequence,we suggestnot prey remainsindicated that the nest was active,and we publishingshort notesthat simply report the interspecific were attacked by both parents. Becausewe heard loud killing amongraptors--instead we shouldask the question beggingcalls, but the chickswere not yet visible at the when and why it occurs,and focuson the contextof these nest edge, we estimatedtheir agesto be 2-3 wk. observations.In our case,we presenta singleobservation On 25 June 1991, the nest area was quiet and there with additional information that showspotential links to were no freshwhitewash or new prey remains.We found causesand implicationsof this behavior:we will 1) try to numerous breast feathers and the left wing of an adult estimatehow rare suchevents were during our study, and goshawk2 m from the baseof the nest , togetherwith 2) discusshow the documenteddetails of the ecological four Great feathers.More goshawkfeathers, contextof both predatorand prey relate to hypotheseson including a goshawk'sright wing, were found under a 1 the evolutionof interspecifickilling among raptors. How Frequently do Great Horned Owls Kill Other m high log perchabout 12 m from the nesttree. The wings Birds of Prey? We monitored17 goshawknests during measured 356 mm, indicating they were from a female 1989-91 and found a maximum of two possiblecases of goshawk(Mueller and Berger1968). Becausethe goshawk on goshawks.The second remainswere severaldays old on 25 June, we estimated casewas a brood that disappearedfor unknown reasons. that the predationoccurred between 18 and 22 June. The describedgoshawk nest was exposedabove canopy During the sameperiod, we monitoreda Great Horned height, which is an unusual situation in our study area Owl family with a nest 1.0 km from the goshawknest. and elsewhere(Shuster 1980, Hall 1984). Owl predation The two owl fledglingswere tetheredto an elevatedar- may rather affectthe nestsite selection than the population tificial platform for diet study (Petersenand Keir 1987). dynamicsand densityof other raptors.Predation by Great Horned Owls, however,has been reported to accountfor We moved two additional young Great Horned Owls to higher mortalitiesin other species:up to 30% of juvenile the platform for a brood size manipulation experiment SpottedOwls (Strixoccidentalis; Forsman et al. 1984, Gu- from 10-20 June. The adult female owl was equipped tierrez et al. 1985, Miller and Meslow 1986), 65% of with a backpackradiotransmitter, and we recordedher juvenileGreat Gray Owls (S. nebulosa;Duncan 1987), 0-

261 262 SHORTCOMMUNICATIONS VOL. 26, NO. 4

44% of young Red-tailed (Buteo jamaicensis; vary with prey abundance,however, and may be more McInvaille and Keith 1974, Houston 1975), up to 27% pronouncedwhen other prey is scarce.Based on the de- (Walton and Thelander 1988) or locallyeven more (Steidl tailed knowledgeof the ecologicalsituation of our case, et al. 1991a) of fledged or released Peregrine we questionthe current perspectivethat raptors killing (Falcoperegrinus), up to 21% of hatchedOspreys (Pandion raptorsare anomaliesthat involvea high risk and require haliaetus;Steidl et al. 1991b), 25 predationson young competition as an explanation. More observationsin a Harris' Hawks from 64 nests(Parabuteo unicinctus; Daw- known contextare neededto test hypotheseson why this son and Mannan 1991). It is possiblethat the literature phenomenonoccurs. is biased toward high predation, becausesurprising re- suitsmay be morelikely to be published.We encourage RESUMEN.--Hemos estudiado los nidos del Gavilln Azor also the reporting of low predation rates in areas where (Accipitergentilis) y del TecoloteCornudo (Bubo virgima- the populationsof severalraptor speciesare known. nus) que estuvieronubicados a 1 km de distanciael uno Killing Other Birds of Prey: a Responseto Food del otro. Los residuos encontrados en ambos nidos son Stress?We found it interestingthat the goshawkwas killed evidenciade que uno de los bfihosde la especieB. virg•- by an owl under food stress,which we had induced ex- nianusmat6 a un A. gentilishembra cercade su nido. Los perimentally. During our study,the overallprey basewas bfihosestuvieron sometidos a escasezde comida,la que high becauseSnowshoe (Lepusamericanus) were at fue inducidapor nosotrosal aumentarel nfimerode pollos the peak of their populationcycle (Krebs et al. 1992), and en el nido. E1 nido del A. gentilisestuvo extremadamente the overall predationby owls on goshawkswas low. It is expuesto.Durante nuestroestudio, esta depredaci6nfue intriguing to hypothesizethat top-predators kill lower- un eventoraro, con pocoimpacto en la poblaci6nde A. level predatorsmore often when other prey is scarce.In gentilis. La frecuencia de tales depredacionespuede ser support of this hypothesis,McInvaille and Keith (1974) mils numerosacuando la presa es escasa.Basados en el found a lower predation rate by Great Horned Owls on conocimientodetaliado de la situaci6neco16gica de nuestro Red-tailed Hawks when SnowshoeHares were at peak estudio,nosotros dudamos de la creenciade que la muerte densities.More predation rates on raptors should be re- de una ave raptora causadapor otra, esuna anomallaque ported in conjunctionwith estimatesof other prey species. 11evaun gran riesgo,y que s61ose explica por la compe- Raptors Killing Raptors:Predatlon or Competition? tencia entre raptoras. Observationsof raptors killing raptors have been consid- [Traducci6n de Eudoxio Paredes-Ruiz] ered anomalies.As a consequencemany short notesand specificlists in handbookshave beenpublished (review in ACKNOWLEDGMENTS Voous 1988). This perspectiveis basedon the assumption We thank C. Kullberg and B. Zimmermann for their that raptors are a costlyprey becauseof the high risk of help with field work on owls. We receivedhelpful sug- injury to an attacking predator. Why raptors kill other gestionsfrom U. Breitenmoser, R.J. Cannings, J.W. raptorsdespite the high costsinvolved, has beenexplained Dawson, J.R. and P. A. Duncan, C.L. Esser,C.J. Krebs, by the additional benefitsof removing a potential com- R.N. Rosenfield,A.R.E. Sinclair, and J.N.M. Smith. We petitor (review in Mikkola 1983). Benefitsother than re- were supportedby the Natural Sciencesand Engineering duced competitionfor food may be reducedcompetition ResearchCouncil of Canada (grant to C.J. Krebs), and for nestsites, increased protection of youngfrom predation, an R.J. Thompson Wildlife Fellowship to C. Rohner. and increasedprotection from harassment(Klem et al. This is contribution 15 of the Kluane Boreal Forest Eco- 1985). systemProject. When raptors kill other raptors, do they really suffer a LITERATURE CITED higher risk of injury? We are not aware of analysesof risks involvedin capturingdifferent prey. Our caseof an BORTOLOTTI,G.R. 1992. Evaluating the merit of single owl possibly attacking a brooding or sleeping goshawk observations--responseto Schmutz.J. RaptorRes. 26. suggeststhat there may be no more risk involved than 100. when attacking any other prey. The most parsimonious CRAIGHEAD,J.J. AND F.G. GRAIGHEAD.1956. Hawks, explanation is that raptors kill raptors simply to obtain food, or in other words, to obtain direct and immediate owls and wildlife. StackpoleGo., Harrisburg, PA. benefits.At the presentstate of our knowledge,we should DUNCAN,J.R. 1987. Movementstrategies,mortality, and take this simpleexplanation as a null-hypothesis,and our behaviorof radio-markedGreat Gray Owls in south- scientificeffort should be directed toward testing it. We easternManitoba and northern . Pages101- can rejectthis null-hypothesisonly if field data do not meet 108 in R.W. Nero, R.J. Glark, R.J. Knapton, R.H the predictionsderived from it. For example, the null- Hamre [EDS.],Biology and conservationof northern hypothesispredicts that killed raptors are as likely to be forest owls: symposiumproceedings. General Tech- consumedas any other prey, or that the proportion of nical Report RM-142, U.S. Department of Agricul- raptorsin the diet shouldreflect their availabilityas much ture, Forest Service, Ft. Gollins, GO. as any other prey. FORSMAN,M.D., E.G. MESLOWAND H.M. WIGHT. 1984. CONCLUSIONS Distributionand biologyof the SpottedOwl in A Great Horned Owl killing an adult goshawkwas a Wildl. Monogr. 87:1-64. rare event with little impact on the goshawk population GUTIERREZ,R.J., A.B. FRANKLIN,W. LAHAYE,V.J. during our study. The frequencyof suchpredation may MERETSKYAND J.P. WARD. 1985. Juvenile Spotted DECEMBER 1992 SHORT COMMUNICATIONS 263

Owl dispersalin northwesternCalifornia: preliminary MUELLER, H.C. AND D.D. BERGER. 1968. Sex ratios results.Pages 60-65 in R.J. Gutierrez and A.D. Carey and measurementsof migrant goshawks. 85:431- lEDS.I, Ecologyand managementof the SpottedOwl 436. in the . General Technical Report PETERSEN,L.R. AND J.R. KEIR. 1976. Tether plat- PNW-185, U.S. Department of Agriculture, Forest forms--an improvedtechnique for raptor food habits Service, Portland, OR. study. Raptor Research10:21-28. HALL, P.A. 1984. Characterizationof nestinghabitat of SCHMUTZ,J.K. 1992. Shouldsingle observations be pub- goshawks(Accipiter gentilis) in northwesternCalifor- lished?J. Raptor Res. 26:99. nia. M.Sc. thesis,Humboldt State University. SHUSTER,W.C. 1980. nest site re- HOUSTON,C.S. 1975. Close proximity of Red-tailed quirementsin the ColoradoRockies. Western Birds 11: and Great Horned Owl nests. Auk 92:612-614. 89-96. KLEM, D., B.S. HILLEGASSAND D.A. PETERS. 1985. STEIDL,R.J., C.R. GRIFFIN,L.J. NILESAND K.E. CLARK. Raptors killing raptors. WilsonBull. 97:230-231. 1991a. Reproductivesuccess and eggshellthinning of KREBS,C.J., R. BOONSTRA,S. BOUTIN, M. DALE, S. a re-establishedPeregrine population.jr. Wildl HANNON, K. MARTIN, A.R.E. SINCLAIR, R. TUR- Manage. 55:294-299. KINGTON,ANt) J.N.M. SMITH. 1992. What drivesthe , --, AND -- 1991b. Differential re- SnowshoeHare cyclein Canada'sYukon? Pages 886- productivesuccess of Ospreysin .J. Wildl 896 in D. McCullough and R. Barrett lEDS.],Wildlife Manage. 55:266-272. 2001: populations,Elsevier, London, U.K. Voous, K.H. 1988. Owls of the MCINVAILLE, W.B. AND L.B. KEITH 1974. Predator- Collins Sons and Co., London, U.K. preyrelations and breeding biology of the GreatHorned WALTON,B.J. AND C.G. THELANDER. 1988. Peregrine Owl and Red-tailed Hawk in Central . Can. Falcon managementefforts in , Oregon, Field-Nat. 88:1-20. ,and . Pages587-599 in T.J. Cade, MIKKOLA,H. 1983. Owls of . Buteo Books,Ver- J.H. Enderson, C.G. Thelander, and C.M. White, million, SD. [EDS.], PeregrineFalcon populations:their manage- MILLER, G.S. AND E.C. MESLOW. 1986. Dispersal of ment and recovery.The PeregrineFund Inc., Boise, juvenile Northern SpottedOwls in the PacificNorth- ID. west Douglas- Region, ProgressReport, Prelimi- nary Analysis1982-1986. General TechnicalReport PNW-82-322, U.S. Departmentof Agriculture,Forest Service, Portland, OR. Received11 May 1992; accepted10 September1992

] RaptorRes. 26(4):263-265 ¸ 1992 The Raptor ResearchFoundation, Inc.

NESTING ASSOCIATION BETWEEN THE WOODPIGEON (Colurnbapalurnbus) AND THE HOBBY (Falco subbuteo)

GIUSEPPE BOGLIANI Dipartirnentodi Biologia Anirnale, University of Pavia, Piazza Botta 9, 27700Pavia,

EUGENIO TISO Via Carena 706, 27050 CaseiGerola (PV), Italy

FRANCESCO BARBIERI Dipartimentodi BiologiaAnitaale, University of Pavia, Piazza Botta 9, 27100Pavia, Italy

Nest predationis the main causeof breedingfailure in amongothers, species which use "protectivenesting as- birds (Ricklefs 1969). Various mechanismsfor defending sociationwith formidablespecies"; the formidable species nestsagainst predators have evolved. In their classification can be large birdsof prey, wasps,bees or termitesand of nest defenses,Collias and Collias (1984) recognized, their nests,or humansand their habitations.It is presumed