Department of Fishery and Wildlife Biology, Colorado State ...

SHORT COMMUNICATIONS

j. RaptorRes.30(3):157-159 ? 1996 The Raptor ResearchFoundation,Inc.

GOLDEN

EAGLE PREDATION

ON PRONGHORNS

GREAT DIVIDE BASIN

IN WYOMING'S

R.D. DEBLINGER 1 AND A. WILLIAM ALLDREDGE

DepartmenotfFisheryand WildlifeBiologyC, oloradSotateUniversityF,t. Collins,Colorado80523 U.S.A.

I?? Wo?t)s: pronghorng; oldeneagleA;,ntilocapraamericana;Aquila chrysaetosp;redationb;ehaviwW;, yomingG;reat

Dzvzde Basin

and 23 goldeneaglesfor a densityof 17.7pronghorn and 0.13 goldeneaglesper km9.Datafrom our twostrip-transectsprovidedan estimateof 0.94 lagomorphsper ha.

We considered these estimates to be accurate indicators

From 1980-85, while studyingpronghornantelopein of the relative abundanceof predatorsand prey in our

Wyoming'sGreat Divide Basin,we recorded seveninci- studyarea.

dentsof goldeneagles(Aquilachrysaetoast)tackingprong- All but one of our observationsof golden eaglesat-

horns (Antilocapraamericana).Most reported observa- tackingpronghornsweremadefrom Novemberthrough

uonsof golden eagle predationon pronghornsinvolve February 1981-84. Six observationswere made from a

newborn fawns during spring and summer (Beale and vehicle and one from the air. We do not know if our

Smith 1973, Barrett 1978, Beale 1978, Bodie 1978, Von aircraft influencedpronghorn or eaglebehaviorduring

Gunten 1978, Autenrieth 1980), but a few attackshave this attack,but during all five winter observationsmade

been recordedin winter (Lehti 1947,Thompson1949, from the vehicle,we were parked,watchinggroupsof

Bruns 1970, Goodwin 1978) when two adult males, one pronghornsandtheybeganto run from eaglesn, ot from

adult female and four fawnswere killed. Herein, we pro- us. We were travelingalong a two-trackroad when our

vide further evidence for golden eagle predation on summerobservationwasmade and, becausepronghorns

pronghorns,particularlyduring the winter.

were running when we first observedthem, we do not

The studywasconductedin the Great Divide Basin,an know if our vehicleor the eaglefirst causedpronghorns

areaof shrubsteppehabitatlocatednorthwestof Rawlins, to flee. Eagleswere not marked, thus we do not know

Wyoming. Golden eagles utilize the area preying on how manyindividualeagleswereinvolvedin attacksor if

pronghorns,white-tailedjackrabbits(Lepustownsendii), the sameeagle may have been observedin more than

desertcottontails(Sylvilagusaudubonii)and sagegrouse one attack.

(Centrocercuursophasianu(sU) .S. Dept. of Interior 1978). Eachwinterattackinvolveda singlegoldeneagleand

We observedeaglesin the studyarea throughoutthe groupsof 120 to 350 pronghorns.The pronghornsbe-

year.Rabbitsand sagegrousewereobservedalmostdaily. came alarmed when the eagle vocalizedwhile circling a

In late summer1982,we conductedtwostrip-transecstur- herd at an estimatedelevationof 60 m. Pronghornsfled

veysto estimateIagomorphdensitiesT. hesesurveyswere only when the eagle flew low (10 m or lessabovethe

conductedby driving a vehicleat night and countingall ground) and directly toward them. In one instance,

lagomorphsin a transectdefined by the width of the pronghornsfled whenthe eaglewasseenflyingat them

headlight beam. In March 1983, we attempted to count from 300 m away.When running from an eagle, prong-

all pronghornsand eaglesin the 182km? areawherewe hornsappearedto groupmore closelythan whenwe ob-

observedeagle attacks.Counts coveredthe entire area servedthem escapingfrom coyotes(Canislatrans)or hu-

and were made usinga Piper Supercubaircraftflown at mans.

an elevationof 60 m along 400-m-widenorth-southtran- When circling pronghorns, eaglesdid not alwaysat-

sects.Surveysbegan 30 min after sunriseon dayswhen tack.When theydid attack,therewasa consistenpt attern

ws?bilitywasexcellentand usedone observerwho always of circling,vocalizing,and flyingawayfrom the groupof

looked awayfrom the sun.

pronghornsjust prior to the initiation of a chase.During

During our 1983 survey,we counted 3230 pronghorn all fivewinter observationsmadefrom the ground,eagles

vocalized.Pronghornswatchedeaglesfly awayand con-

tinued to watch the area where they had disappeared.

? Present Address: Massachusetts Division of Fisheries When an eaglereappearedflyinglowto the groundand

and Wildlife, Field Headquarters,Westboro,MA 01581 direcfiy toward them, the pronghorns bunched more

U.S.A.

closely together and then ran. When the pronghorns

157

158

SHORT COMMUNICATIONS

VOL. 30, NO. 3

fled, the eagletargetedthe lastanimalin the group,and pronghornsvulnerableto eaglepredation.Snowdepth

all subsequenatttacksweredirectedtowardthatisolated rangedfrom 0-10 cm and did not impedepronghorn

individual.

movementnor did it appear to influence eagleswhen

All attackswere similar and, after the eagle caughtup they attackedpronghorns.

to the pronghorn,it landedon itsbackat pointsvarying It wasinterestingthat eaglespreyon adultpronghorns from just posteriorto the withersto slightlyanteriorto nearlyasfrequentlyason immaturesN. early50%of our

thewhiterump patch.Onceaneaglelandedon a fleeing observationsinvolvedattacksby singleeagleson adults

pronghorn,the pronghornfell or continuedto run ap- andfawnsfrom the previousspringwhentheyapproach

parentlytryingto dislodgethe bird.The longestimean adult bodysize.Bruns (1970) observeda similartrend

eaglerode a pronghornwasapproximately20 secfor an towardwinter predationon adultsby singleeagleshunt-

estimateddistanceof 200 m. Eaglesdid not balancewell ing herdsof pronghorns.Tandemhuntinghasbeenre-

on fleeingpronghornsand all attacksendedwhenthe portedin breedingareas(Collopy1983),but thisbehav-

eagleandpronghornfell to the ground.Prior to hitting ior isapparentlyrare in winter(Tjernberg1986).Collopy

the ground,the eaglefoldeditswingsandafterthe fall, (1983) reported two golden eaglespreyingon black-

it remained standingon the ground for approximately tailedjackrabbits(Lepuscalifornicusa)nd smallermam-

30 secbefore attackingagain.During eachencounterwe mals.Hatch (1968) observeda pair of goldeneaglessuc-

observed,the eagleattackedthe samepronghornfrom cessfullykilling a red fox (Vulpesfulva). In both reports,

one to four times and, of the three pronghornskilled, one eagleeither flushedthe preyor diverteditsattention

one, two, and three attacks were involved before the while the secondeagle attacked.Thompson(1949) ob-

pronghornwaskilled.Killswewitnesseidnvolvedonefe- servedtwogoldeneaglessimultaneouslychasingtwosep-

male and one male fawn (estimatedage 8-9 mo) and aratepronghornherdsbut the sizeand relativeconspic-

one male (estimatedage 20-21 mo). Pronghorns that uousnesosf pronghornsmayeliminatethe needfor tan-

escapeddid sobyrunningbackto the fleeingherd.We dem hunting of this species.

do not know the fate of these animals, but we did observe

Bruns(1970) reportedan eagleattackingtwodifferent

otherwisehealthylookingpronghornswith dried blood pronghornsbeforesuccessfulklyillinga femalefawn.Ea-

on their backs.

gleswe observedattackingpronghornsdirectedtheir at-

Eaglesbeganfeedingon pronghornsafter theyfell. tention to a singleanimal and actuallyrode on their vicTwo pronghornsappearedparalyzedp, ossiblyfrom spi- timsfor aslongas20 sec.Bruns(1970)observedaneagle

nal injuries,but remainedalivefor at least10 min after riding a pronghorn for nearly 5 min. eaglesbeganfeeding.Eaglesalwaysbeganfeedingdor- We observeda 50% successratefor goldeneaglesprey-

sallyon the carcassat the pointwheretheir talonshad ing on pronghornsin winter.Thisestimateishigherthan puncturedthe skin.Killswerequicklydetectedbyother the 23-30.5% succesrsatereportedbyCollopy(1983)for goldeneaglesand coyotesO. n one occasiona, second eagleshuntingsmallmammalsin Idaho,and the 21%

eaglearrivedwithin8 min aftera kill wasmadeand,after successrate for golden eagleshunting smallanimals?n

27 min, five goldeneagleswerefeedingon the kill. We Sweden(Tjernberg 1986). Smallmammalsmaybe better

observedcoyotesapproachinggoldeneaglesfeedingon able to find coverfor their escape.Pronghornsare more

deadpronghorns,but eaglesdid not attackcoyotesn, or conspicuoutshan smallmammalsand theylivein open

did coyotesdisplaceeaglesfrom the carcasses.

habitatswhereopportunityto usecoverfor escapeislim-

We observeda fawn attackedby an eaglein July 1985. ited.

In thisinstance,approximately10 pronghornswererun- Pronghornsseemto recognizeeaglehuntingbehavior

ning in front of our vehiclewhena fawn (?1 mo. old, andbunchtightlytogetherwhentheyfleefrom attacking estimatedweight 4 to 5 kg) in the rear of the groupwas eagles(Bruns 1970). Eagleswe observedelicitedthisreattackedby a pursuinggoldeneagle.The eaglegrasped sponseby either vocalizingwhile circlingimmediately the fawn in the back,lifted it approximately10 m verti- abovepronghorns,or by flying closeto the groundd?cally then releasedits grip dropping the fawn. It rose, rectlyat herds.Thisbehaviorisolatesan animalfrom the begantorun withanawkwardgaitandtheeagleinitiated groupthat then becomesthe focusof ensuingattack

a secondstoop,but appearedto shyfrom our vehicle.

The t?twnran out of sight,and the eagleflew in the op- RESuMEN.--EstudiamoasAntilocapramericanean la gran

posite direction.

cuencadivisoradeWyoming,en un periodode 57meses,

During winter, pronghornsbecameconcentratedin entre 1980y 1985.Durante estetiempo,registramoss?ete

our studyareawhichmay,in part, explainwhywe ob- ataquesde Aquila chrysaetossobre A. americanaS. e?s

servedmore eagle predation in winter than in summer. ataquesen inviernoinvolucraronun machoadulto,uno

Basedon our field observationsof jackrabbit,cottontail del afio, una hembra adultay trescervatillos.En verano

rabbitsand sagegrouse,thesepreyappearedto be avail- s61oseobserv6un ataquesobreun cervatillo.En inv?er-

able. Thus, we doubt that eagleswere attackingprong- no, las/tguilasmataronel 50%de lasvecesqueatacaron,

hornsbecausealternatepreywasunavailableM. orelikely, matandoun machoy una hembra (ochoa nuevemeses

theywere related to the winter conditionswhich made de edad) y un macho de 20 a 21 mesesde edad. E1

SEPTEMBER ]996

SHORT COMMUNICATIONS

159

ataquerealizadoen veranosobreel cervatillo,no rue exi-

pronghorn antelope in westernUtah. Proc.Pronghorn

toso, posiblementedebido a nuestrapresencia.En in-

AntelopeWorksho8p:445-448.

vlerno, las figuilasatacarona A. americanadesdela parte --AND A.D. SMITH. 1973. Mortality of pronghorn

posteriorde gruposque huian, aterrizabansobresuses- antelopefawnsin westernUtah.J. Wildl.Manage.37:

paidas,derrivabanel grupo o liberabansusgarrasantes

343-352.

que A. americancaayera.E1mayortiempode persecusi6n BODIE,W.L. 1978. Pronghorn fawn mortality in the up-

rue de 20 segundos.Cuando A. americana,se sentia

per PahsimeroiRiverdrainagein centralIdaho. Proc.

amenazadapor A. chrysaetocso,rdan asociadosen reba-

PronghorAnntelopWe orksho8p:417-428.

rios, una conductaque difiere de respuestasde escape BRUNSE, .H. 1970. Winter predation of golden eagles

suscitadaspor otto tipo de peligros.Sugerimosque esta

and coyoteson pronghorn antelope. Can.Field-Nat.

84:301-304.

respuestaconductualfinica de A. americanapuede indicar una interacci6nhist6ricadepredador-presa.

[Traducci6n de Ivan Lazo]

COLLOP?M, .W. 1983. Foraging behavior and successof eagles.Auk 100:747-749.

GOODWING, .A. 1978. Golden eaglepredation on prong-

ACKNOWLEDGMENTS

horn antelope.Auk94:789-790. HATCH,D.R.M. 1968. Golden eagle hunting tactics.Blue

We thank C.E. Braun, A.R. Harmata, P.L. Kennedy,

Jay 26:78-80.

L.C. McEwen, S.K. Skagen,K.J.Sejkora,R.A. Ryder and LEHTI,R.W. 1947. The goldeneagleattackingantelope.

twoanonymousreviewersfor constructivecriticismof this

J. Wildl.Manage.11:348-349.

manuscript.Fundingwassuppliedby MineralsExplora- THOMPSONW, .K. 1949. Predation on antelope.J. Wildl.

non Companyof Rawlins,Wyomingand U.S. Bureau of

Manage.13:313-314.

Land Management and the WyomingGame and FishDe- TJERNBERGM,. 1986. Hunting behavior, prey selection

partment.

and strike successof wintering golden eaglesin south

LITERATURE CITED

Sweden.VdrFdgelvdir4ld5:337-338. U.S. DEPT.OFINTERIOR.1978. SevenLakesgrazing en-

AUTENRIETH,R.E. 1980. Vulnerability of pronghorn fawnsto predation. Proc.PronghornAntelopeWorkshop

vironmental impact statement.U.S. Bureau of Land Management, Washington,DC U.S.A. VON GUNTEN,B.L. 1978. Pronghorn fawn mortality on

9:77-79.

the National Bison Range. Proc. PronghornAntelope

B^RRETTM, .W. 1978. Pronghorn fawn mortality in Al-

Worksho8p:394-416.

berta. Proc.PronghorAnntelopWe orksho8p:429-444.

B?LE, D.M. 1978. Birth rate and fawn mortality among Received8 December 1995; accepted26 April 1996

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download