DECEMBER 1992 LETTERS 269

J. Raptor Res. 26(4):269 ¸ 1992 The Raptor ResearchFoundation, Inc.

AERIAL MOBBING OF A GYRFALCON BY GLAUCOUS GULLS

On 1-2 June 1990, while campedat a small island in Alexandra Fiord on EllesmereIsland (78ø54'N 75ø45'W) near a lowland oasis(B. Freedman and J. Svoboda1982, Can. Field.-Nat. 96:56-60), I observeda male Gyrfalcon (Falcorusticolus) quartering back and forth in front of a cliff where the female was nesting.The orographiclift of the Gyrfalconwas interruptedby a group of three GlaucousGulls (Larushyperboreus) on two separateoccasions, each lastingseveral minutes. During the first encounter,the gulls, which initiated the engagement,acted in unison, each gull takingturns divingat the Gyrfalcon,while the otherscircled close by callingloudly. The Gyrfalcon,in an attempt to fight the gulls,swiveled about, talons extended, occasionally even doing so while completelyinverted. After returning to the nest, the Gyrfalcon ignoredthe gulls until after 10 min it tried to make a kill of a single passingbird. The panickedavoidance by the GlaucousGull consistedof droppingvertically to severalmeters above the pack ice and calhngloudly. Within a few moments,two gulls arrived and the three dissuadedthe Gyrfalcon from hunting. The falconthen returnedto the nestsite where it stayedfor the next hour of observation,disregarding nearby gulls. Gyrfalconscommonly take prey in the air (e.g., C.M. White and R.B. Weiden 1966, Condor68:517-519; S.A. Bengtson1971, Ibis 113:468-476) and customarilyprey uponseabirds (e.g., G.P. Dementievand N.N. Gortchakovskaya 1945, Ibis 87:559-565; K.G. Pooleand G.A. Boag 1988, Can. J. Zool. 66:334-344) includingGlaucous Gulls (C.M. White and T.J. Cade 1971,Living Bird 10:107-150).Similar mobbingof Gyrfalconsby CommonRaven (Corvus corax) has been noted previously(M.A. Jenkins 1978, Auk 95:122-127). N. Wooden (1980, Raptor Res. 14:97-124) also observedperched Gyrfalcons struck by passingArctic Terns (Sternaparadisaea) which, however,never groupedto drivethe raptoraway as in the presentencounter with the larger GlaucousGulls. This seeminglyparadoxical behavior of self-endangermentby mobbersmay be necessaryto "convince"the predatorthat their threat is real (S.A. Sordahl 1990, WilsonBull. 102:349-353). The result is that both ravens(Jenkins 1978) and GlaucousGulls (this study) can be ignoredeven when they fly directlybeneath or over an occupiedGyrfalcon eyrie.--R.L. France, Department of Biology, McGill University, 1205 Ave. Dr. Penfield, Montreal, PQ.• Canada H3A lB1.

J Raptor Res. 26(4):269-270 ¸ 1992 The Raptor ResearchFoundation, Inc.

AN AGGRESSIVE INTERACTION BETWEEN A NORTHERN GOSHAWK AND A RED-TAILED HAWK

During September1987, D. Crannell observedan aggressiveencounter between a Northern Goshawk(Accipiter gentilis) and a Red-tailed Hawk (Buteojamaicensis) on the Alsea Ranger District (Township 13 South, Range 09 West) of the SiuslawNational Forestin the CoastRange of westernOregon. An adult Red-tailedHawk was observed flyingerratically, apparently grappling with anotherbird. The birdstumbled to the grounda shortdistance away and whenthis locationwas approached,the Red-tailedHawk was seenhanging upside down in the talonsof a mature goshawkabout 3 m up in a smalltree. The pair of birdswere about5 m from the observer.The goshawkdropped the Red-tailedHawk, possiblybecause of the closeproximity of a human,and after about60 secthe Red-tailedHawk hoppedto an adjacentbush. The two hawksthen watchedeach other for a few seconds,and the Red-tailedHawk flew southacross a pastureand landedin a tree.Within seconds,the goshawkpursued the redtail,struck it, and both birdswent to the ground.The outcomeof this last encounterwas not observed. Vegetationin the areawas dominatedby second-growthDouglas-fir (Pseudotsuga menziesii), vine maple(Acer czrcinatum)and red alder (Alnusrubra) growingwhere recenttimber harvesthad taken place. This habitat was not typical of mostnesting sites for Northern Goshawksin Oregon,and goshawksare not known to nest in the Coast Range of Oregon (R.T. Reynoldset al. 1982, J. WildlifeManage. 46:124-138), althoughsightings of goshawksare often recordedthere. Red-tailed Hawks do nestin the CoastRange. No vocalizationswere heard, so it was not known if either bird emittedalarm or defensivecalls. It is possiblethat this encounterwas an act of predationby the goshawk on the redtail rather than aggressiveterritoriality. We do not know if the goshawkconsumed any of the redtail. Encountersbetween Northern Goshawksand Red-tailed Hawks are of interest becauseof the possibilitythat the two speciesare beingdrawn into closerproximity during nesting because of wide-spreadalteration of forestedhabitat due to timber harvest(D.C. Crocker-Bedford1990, Wildl. Soc.Bull. 18:262-269). Physicallyaggressive encounters between 270 LV.TTV. RS VOL. 26, NO. 4 two speciesof similar sizeare seldomobserved, and may be rare in nature,but suchdirect sources of mortalityor competitionundoubtedly occur in the wild. We thank E.C. Meslow,D.K. Rosenberg,and B. Woodbridgefor reviewingthis note.--Dan Crannell, Bureau of Land Management,P.O. Box 10226,Eugene, OR 97440-2226;Stephen DeStefano, Oregon Cooperative Wildlife ResearchUnit, Nash 104, Oregon State University, Corvallis, OR 97331-3803.

J. Raptor Res. 26(4):270 ¸ 1992 The Raptor ResearchFoundation, Inc.

THESIS ABSTRACT

HABITAT USE, MOVEMENTS, MIGRATION PATTERNS,AND SURVIVALRATES OF SUBADULT BALD F•AGLES IN NORTHERN FLORIDA

The stateof Florida supportsover half of the breedingpopulation of Bald Eagles(Haliaeetus leucocephalus leucocepha- lus) in the southeasternUnited States;this representsa significantresource for the Southeastand for Florida. Currently, primary managementemphasis and protectionis focusedon bald eaglenest sites. No habitatprotection or management activitiesare aimedat foraging,roosting or loafingareas for subadulteagles. In fact, habitatsand habitatcharacteristics importantto subadults have not beenquantified. In this study,I examinedvarious aspects of eaglebiology that might be pertinentto survivalor managementof the Florida subadulteagle population. Specifically, using radiotelemetry, I examinedpost-fledging habitat needs, factors affecting timing of migration,local movements, habitat use, and survival in north-centralFlorida from spring 1987 through spring 1991. Fledgling eagles(birds prior to their initial migration) remaineddependent on adults and remainedwithin 4 km of the natal nestuntil theyinitiated migration (approximately 7 wk post-fiedging).It wasdetermined that habitatprotection within the 229 m primary protectionzone usedin Florida was not sufficientto meet the habitat needsof fledgling eagles and that the protectionperiod should extend until fledglingsinitiate migration in the summer. Timing of migration for fledglingand 1- to 4-year-old eaglesappeared to be correlatedmore with prey availability than with temperature,although both factorsappear to affect migration. Locationsof radio-taggedeagles outside of Florida rangedfrom SouthCarolina to Prince Edward Island, Canada. Data suggestthat eaglesare philopatricto summeringareas, which emphasizesthe needfor protectionof significant summeringareas. Known and assumedmortality occurred primarily duringmigration in northernstates. The 1V2 year age classhad the lowestsurvival. Survival was significantlylower for eaglesfledged from 1-chicknests and for the youngerchick in 2-chicknests. The minimumsurvival rate through4« yearsof agewas 50% and did not vary by sex. After subadultsreturned to the north-centralFlorida studyarea in the fall, individualscontinued to use the same generalareas each year. Temporallyand locallyabundant food sources resulted in temporarysmall concentrations of eagles.Certain portions of the studyarea were used consistently each year by largenumbers of eagles.Subadult eagles werenot distributedrandomly over the studyarea. Logistic regression analyses revealed that eaglestended to be located closeto largewater bodies,and eaglenests were frequentlyin cypressand marshhabitats, and avoidedmain roads and developedareas. Immature eagles (1-year olds) tended to be locatedcloser to eaglenests than 2- to 4-year-olds Thus,management for subadultpopulations must include these heavily used concentration areas that supplythe habxtat featurespreferred by subadults.Survival of subadultsmay be affectedif a highly usedarea becomesunsuitable.- Petra Bohall Wood. 1992. Ph.D. thesis,Department of Wildlife and Range Sciences,University of Florida, Gainesville, FL 32611. Presentaddress: West Virginia Coop. Fish and Wildlife ResearchUnit, West Virginia University, P.O. Box 6125, Morgantown, WV 26506-6125.