34 SHORT COMMUNICATIONS VOL. 27, NO. I

TACHA, T.C., P.A. VOHS AND G.C. IVERSON. 1985. A TRELEAVEN,R.B. 1977. Peregrine.The private life of comparisonof interval and continuoussampling meth- the Peregrine Falcon. Headland Publications, Pen- ods for behavioral observations.J. Field Ornithol. 36: zance, Cornwall, U.K. 258-264. Received4 February 1992; accepted28 October 1992

j Raptor Res. 27(1):34-36 ¸ 1993 The Raptor ResearchFoundation, Inc.

ARE BALD EAGLES IMPORTANT PREDATORS OF EMPEROR GEESE?

ROBERTE. GILL, JR. AND KARENL. KINCHELOE• U.S. Fish and Wildlife Service,Alaska Fish and Wildlife ResearchCenter, lOII E. Tudor Road, Anchorage,AK 99503 U.S.A.

Bald Eagles (Halmeetusleucocephalus) and geeseoften trationsof stagingwaterfowl, primarily eiders(Somateria occur together, especiallyat sites used by geesefor mi- mollissirnaand Polystictastelleri) and geese(Cackling Can- grationalstaging and wintering. Althoughnumerous stud- ada, Emperor, and geeseBranta bernicla).Indeed, les have been directedat these taxa, there are only anec- betweenSeptember and Novembereach year mostof the dotal accounts(Parris et al. 1980, Bennett and Klaas 1986, entire populationof Emperor Geese(about 90 000 ) stageson these estuaries(Petersen and Gill 1982, King Bartley 1988) of Bald Eagleskilling healthy geeseat any and Brackney 1991). About 15000 of theseuse Cinder- ume of the year (but see Raveling and Zezulak 1991). Hook Lagoon (R. Gill unpubl.). Most speciesof geesemay be too large, as suggestedby Shetrodet al. (1976) and Palmer (1988), or they may not OBSERVATIONS regularly allow eagles an advantageousattack position Our first observation of an attack occurred on 12 Oc- (J.M. Gerrard in litt.). tober 1987, when we saw an adult in aerial Here we report observationsof attacks on Emperor pursuit of a flock of 12 Emperor Geese.The eagle sepa- Geese(Chen canagica) by Bald Eagleson the Alaska Pen- rated a juvenile goosefrom the flock, grabbedit by the insula in autumn. We discuss these and other observations back and neck with its talons, and then flew with it for of eagle-gooseinteractions vis-a-vis the role of Bald Eagles about 400 m before landing and eating the goose. as predators of Emperor Geese. Our second observation, on 27 October 1987, involved an attackon a gooseby six eagles.One adult eagleflushed STUDY AREA AND METHODS a flockof 40 Emperor Geeseand attackedan adult, knock- We recorded observations on the occurrence and be- ing it from the air. The eagle landed about 200 m away havior of Bald Eagles and Emperor Geeseduring a three- but did not try again to kill the goose.Over the next 20 year study (1986-88) of CacklingCanada Geese(Branta min, five different eagles(two adult, one subadult, and canadensisminima) staging at Cinder and Hook lagoons two hatching-yearbirds) stooped individually on the goose (57ø15'N 158ø15'W), two large, adjacentestuaries on the a total of 16 times. The goosewas able to evade each northeasternAlaska Peninsula (Gill et al. in press). Ob- attack, and none of the eagleshit the gooseagain. About servationswere made daily from blinds and on foot be- 30 minutes after the initial attack, the goosetook flight tween the last week of September and the last week of from the mudflats surroundedby sevenperched eagles, October each year. We also interviewedseveral long-time none of which made any attempt to attack the gooseonce residentsof the Alaska Peninsula and most biologistsin- it was in the air. volved in on-ground studiesthere during the past 25 yr. DISCUSSION Several hundred Bald Eaglesare year-round residents of the Alaska Peninsula,but probablyfewer than 40 pairs From interviews we learned of only sevenother suc- nestalong the north sideof the peninsula(Wright in press, cessfulattacks observedwhen the two speciesoccur to- Dewhurst in press). Each year, however, between July gether on the Alaska Peninsula (September-April); five and November,many eaglesgather among five or six of theseentailed juvenile geese(G.V. Byrd, P.J. Kust, prominentestuaries along the north sideof the peninsula. P.E. Gundersen,and J.A. Schmutzpers. comm.,R. Gill They are presumably attracted to these sites becauseof pers. observation).The solepublished account, from the the large runs of anadromousfish and the large concen- (Sherrod et al. 1976), documenteda Bald Eagle catchingan adult Emperor Gooseand carrying it to a sea stack. • Present address: 5421 E. 131st Ave., Anchorage, AK Anotherpoint of evidenceis that Bald Eagleshave fre- 99516 U.S.A. quently been seenfeeding on carcassesof recentlykilled MARCH 1993 SHORT COMMUNICATIONS 35

or injuredEmperor Geese (R.D. Jones,Jr. andJ.A. Pratt For example, by early May, when most eaglesin south- in litt., P.j. Kust, C.P. Dau, and P.E. Gundersenpers. westernAlaska hatch and begin feedingtheir young, Em- comm., G.V. Byrd, J.A. Schmutz,J.D. Mason and R. peror Geesehave already migrated to the breedinggrounds Gill pers.observations). Although some of thesebirds were (Sherrod et al. 1976, Petersen and Gill 1982, Gill et al. known to havejust been shot by hunters,the others may 1981, Byrd and Williams in press). have been injured or killed by the eaglesbefore the ob- Given the recent decline in the Emperor Goose popu- serversencountered them. Residentsof Nelson Lagoon, a lation (Petersen and Gill 1982, U.S. Department of In- village alongthe northcentralAlaska Peninsula,also have terior 1989, King and Brackney1991) and the relatively reported that in years in which the onset of winter was large and stablepopulation of Bald Eaglesin southwestern suddenand severe(e.g., 1970 and 1971), Bald Eagles Alaska, any assessmentof winter mortality in Emperor concentratedthere and frequently killed and fed on Em- Geeseneeds to considerpredation by Eagles. This infor- perorGeese. Residents have observed eagles attacking geese mation is especiallyneeded for areas such as the eastern physicallytrapped in ice, geesewhose flight was encum- Aleutian Islands, where geeseremain for long periods, beredby freezingspray, and geeseweakened to the extent eaglesare abundant,and frequentand harshwinter storms that they were unable to fly. Such situationsat Nelson may directly kill geeseor lead to increasedpredation by Lagoon often entailed a few hundred geese(P.E. Gun- eagles. dersenin litt., P.j. Kust and S. Johnsonpers. comm.). Amongall speciesof waterfowlpresent during our study, RESUMEN.--Lapresencia tanto de •tguilasHaliaeetus leu- Emperor Geeseexhibited the mostconsistent and strongest cocephaluscomo de gansosChen canagica ocurre al mismo flight responseto the presenceof Bald Eagles.They often tiempoen el sudoestede Alaska y las IslasAleutinas, entre took flight when an eaglewas severalkilometers away and septiembrey abril. Durante los estudiosen la penlsulade well beforemost other species (e.g., Cackling Canada Geese) Alaska entre 1986 y 1988, hemosobservado dos ataques reactedto the eagles'presence. Since Bald Eagles do not de ,/•guilaCabeciblanca contra los gansos. Presentamos occuron the breedinggrounds of the Emperor Goose,the tanto estasobservaciones como otras que documentan la flight responseby geesemust have been reinforced through depredaci6ncausada por estaespecie de figuilaen el ganso frequentand successfulattacks on the goose'sstaging and C. canagica;tambi•n se discuteel rol de esta ave raptora wintering grounds. comodepredadora de ellos,especialmente cuando las vlc- Furthermore, observationssuggest that juvenile Em- timas est•tnen su etapa juvenil. La recientedeclinaci6n peror Geesemay be more susceptibleto eagle predation poblacionalde gansos,y la grandey establepoblaci6n de than adults. Healthy adult Emperor Geeseare relatively H. leucocephalus,justifican adicionalesestudios sobre esta large (• = 2680 g + 55 SE, range = 2230-3100 g, N = •tguila comofactor en la mortalidad queen invierno ocurre 22; C.P. Dau unpubl. data) comparedwith most prey en los gansos. taken by Bald Eagles (Shetrod et al. 1976, Gerrard and [Traducci6n de Eudoxio Paredes-Ruiz] Bortolotti 1988). However, juvenile Emperor Geese are smaller(• = 2230 g _+98 SE, range = 1900-2680 g, N ACKNOWLEDGMENTS = 8; C.P. Dau unpubl. data). Becausejuvenile geeseare also lessexperienced and have overall greater nutritional We thank C.P. Dau for sharinghis unpublisheddata, demandsthan adults (Wypkema and Ankney 1979, Gi- and G.V. Byrd, P.J. Kust, P.E. Gundersen,J.A. Schmutz, roux and B•dard 1988, Ravelingand Zezulak 1991), the R.D. Jones,Jr., J.D. Mason, S. Johnson,and C.P. Dau harshconditions throughout the winter rangemay weaken for sharingtheir unpublishedobservations. R.G. Anthony, them and render them more vulnerable than adults to G.V. Byrd, C.P. Dau, C.M. Handel, W.G. Hunt, J.A. predation. Gerrard, R.D. Jones,Jr., M.R. Petersen,J.A. Schmutz, Another measureof the effect eagleshave on geeseis and R. Thurken commented on earlier drafts of the manu- the numberof disruptionsto feedingflocks of geesecaused script. by eagles.One or more Bald Eagleswere presenton the area on 70 of 81 d, representing192 eagle-daysof sight- LITERATURE CITED rags. Mass disruptionsof feedinggeese by Bald Eagles occurred71 times during the study, or on averageabout BARTLEY,J. 1988. Bald Eagle kills and carries Giant once a day. Disruptions were most prevalent (• = 2.7/ Canada Goose.Blue Jay 46:87. day) towardsthe end of the stagingperiod in mid- to late- BENNETT,R.S., JR. ANDE.E. KLAAS. 1986. Bald Eagle Octobereach year, when numbersof geeseand eagles pursuesand injuresSnow Goosein flight. RaptorRes reachedtheir peak. On a few days during this period as 20:75-76. many as 8-10 disruptions were noted. Numerous other BYRD,G.V. ANDJ.C. WILLIAMS. In press. Distribution disruptionsof geesewere observed,many were too distant and statusof Bald Eaglesin the Aleutian Islands,Alas- to identify the cause,but behaviorof the flockssuggested ka. In P. Schempfand M. Hermans [EDS.],Bald Eagles that eagleswere involved. in Alaska. American Bald Eagle ResearchInstitute and The evidencewe have presentedsuggests that Bald Ea- gles frequently attack live Emperor Geese,particularly University of Alaska Southeast,Juneau, AK U.S.A. juveniles.That this speciesis rarely recordedas eagle prey DEWHURST,D.A. In press. History and status of Bald (Murie 1940, Krog 1953, White et al. 1971, 1977, Sherrod Eagle population and productivity studieson the Alas- et al. 1976) is probably an artifact of the relatively short ka Peninsula.In P. Schempfand M. Hermans [EDS.], periodswhen eagles,geese, and biologistsoccur together. Bald Eaglesin Alaska.American Bald Eagle Research 36 SHORTCOMMUNICATIONS VOL.27, NO. 1

Institute and University of Alaska Southeast,Juneau, PETERSEN,M.R. AND R.E. GILL, JR. 1982. Population AK U.S.A. and statusof Emperor Geese along the north side of GERARD, J.M. ,XNDG. BORTOLOTrI. 1988. The Bald the Alaska Peninsula. Wildfowl 33:31-38. Eagle, the haunts and habits of a wildernessmonarch. RAVELING,D.C. AND D.S. ZEZULAK. 1991. Autumn diet SmithsonianInstitution Press,Washington, DC U.S.A. of Cackling Canada Geesein relation to age and nu- GILL, R.E., JR., M.R. PETERSENAND P.D. JORGENSEN. trient demand.Calif. Fish Game77:1-9. 1981. Birds of the northcentral Alaska Peninsula, SHERROD,s.m., C.M. WHITE AND F.S.L. WILLIAMSON. 1976-1980. Arctic 34:286-306. 1976. Biologyof the Bald Eagle on Amchitka Island, , C.A. BABCOCK,W.I. BUTLER,JR., D.G. RAVE- Alaska. Living 15:143-182. LINGAND C. M. HANDEL. In press. Site fidelity and U.S. DEPARTMENTOF INTERIOR. 1989. Comprehensive useof fall staginggrounds in Alaska by Cackling Can- managementplans for arctic nestinggeese in Alaska. ada Geese.In D. Rusch, D. Humberg, and M. Samuel Unpublishedreport U.S. Department of the Interior, [EDs.], The Canada Goose.The Wildlife Societyand Fish and Wildlife Service,Anchorage, AK U.S.A. International Canada GooseSymposium, Milwaukee, WHITE, C.M., W.B. EMISON AND F.S.L. WILLIAMSON WI U.S.A. 1971. Dynamicsof raptor populationson Amchitka GIROUX,J-F. ANDJ. Bft.DARD. 1988. Age differencesin Island, Alaska. BioScience21:623-627. the fall diet of Greater SnowGeese in Quebec.Condor , F.S.L. WILLIAMSON AND W.B. EMISON. 1977 90:731-734. Avifaunal investigations.Pages 227-260 in M.L. Mer- KING, R.J. ANDA.W. BR•CKNV.¾.1991. Fall population ritt and R.G. Fuller [EDS.], The environment of Am- surveyof Emperor Geese(Chen canagica)on coastal chitka Island, Alaska. National Technical Information southwestAlaska. Unpublished report. Fish and Wild- Service,Springfield, VA U.S.A. life Service.U.S. Department of the Interior, Anchor- WRIGHT, J. In press. Distribution and statusof Bald age, AK. Eaglesin westernAlaska. In P. Schempfand M. Her- KROG,J. 1953. Notes on the birds of Amchitka Island, mans [EDs.], Bald Eagles in Alaska. American Bald Alaska. Condor 55:299-304. Eagle ResearchInstitute and University of Alaska MUmE, O.J. 1940. Food habits of the northern Bald Southeast,Juneau, AK U.S.A. Eagle in the Aleutian Islands,Alaska. Condor42:198- WYPKEMA, R.C. AND C.D. ANKNEY. 1979. Nutrient re- 202. servedynamics of LesserSnow Geesestaging at James PALMER,R.S. 1988. Handbook of North American birds. Bay, Ontario. Can.J. Zool. 57:213-219. Vol 4. Diurnal raptors(Part 1). Yale UniversityPress, New London, CT U.S.A. PARRIS,S.D., E.E. KLAASANDR.A. WILSON. 1980. Coy- ote steals from Bald Eagles. Raptor Re- search 14:88-89. Received31 March 1992; accepted28 October 1992