OBSERVATIONS ON GYRFALCONS (FALCO RUSTICOLUS) BREEDING NEAR LAKE MYVATN, ICELAND, 1967 by Nick Woodin Star Route AusableForks, New York 12912

Introduction Thispaper is based on observations made between 2 Marchand 23 August1967 in the LakeMyvatn region of northeasternIceland and during a briefstay (24 to 28 August)on the islandof Hriseyin the Eyia•ordurof northcentral Iceland. I wassupported during my stayin Icelandby a FulbrightFellowship. Icelandis oneof the few placesin the worldwhere it is possibleto studyrelatively abundant Gyrfalcon populationsfrom accessible and permanent human habitations. One of the reasonsI have written up a single season'swork at suchlength is in the hopethat someother student will takeup the studywhere I left it. For advice and assistance I am indebted to Finnur Gudmundsson and Arnthor Gardarsson. Dr. Gudmunds- sonsuggested the proiect,and without his and Arnthor'shelp, it wouldhave come to muchless. I am gratefill to the staffof the Museumof NaturalHistory, Reykjavik, for their kindnessto a foreigner,and to Professor Tom Cade for inspirationand advice. The Region Lake Myvatnis locatedat approximately65% ø north latitudein the northernpart of the arid, young-vol- canic zone of Iceland. The lake, 277 m above sea level, is about 55 km inland from the north coast. Character- isticsof the young-volcaniczone are relativelylow precipitationand muchdry moorland,with a vegetation chieflyof low-growingheaths (Ericaceae), creeping willows (Salicaceae), dwarf birch (Betulanana), and crow- berries(Empetr.um nigrum agg.) (Gudmundsson 1960). The Myvatn basinalso contains extensive brushwoods of birch (Betulapubescens) and largewastes of poorlyvegetated sand, gravel, or lava. Southof the lake lie marsheswith clumpsof willow. The lake environsare excellenthabitat for waterfowl.Bengtson (1971) esti- matesthat betweenMay and Septemberthe areacontains rarely fewer than 25,000ducks. Shorebirds, includ- ingRedshank (Tringa totanus), Golden Plover (Charadrius apricarius), Snipe (Gallinago gallinago), Dunlin (Ca- lidrisalpina), and Whimbrel(Numenius phaeopus), also breed about the lake.There is at leastone colony of the BlackheadedGull (Larusridibundus). The regionis the centerof the distribt•tionof the IcelandicRock Ptarmigan(Lagopus mutus' islandorum), the onlygallinaceous bird in Iceland(Gudmundsson 1960). All these birdsare potentialprey for Cyrfalcons.Mammals are not commonbut includethe Arctic fox (Alopexla- gopus),long-tailed field mouse(Apodemius sylvaticus), probably the mink (Mustelarison), and the brown rat (R attus norvegicus ). Lake Myvatn lies in a basinmore or lessenclosed by slopes,low hills, and mountains.What I have called the Myvatneyries are locatedin the canyonsor on the exposedfaces of the nearerhills and slopes(fig. 1). They tend to be within 10 km of the lake and 100 to 200 m aboveit. By way of contrast,the highestmoun- tainsnear the lake rangefrom 500 to 900 m aboveit, at distancesof 12 to 15 km. The mostgeologically isolatedeyrie is Falkaklettur,which is locatedbeyond the easternhills on a high rock in the middle of an extensive,well-vegetated lava field. The farmersof the Myvatnbasin raise sheep and cattle.Livestock graze outside for part of the year.Hay is raisedfor winterfeed, for the mostpart within a few hundredmeters of the lakeshore.Sheep have exerted an enormousinfluence on the Icelandiclandscape, but otherthan that man'seffect on the wildlife of the Myvatn basinhas probably been relatively little. In 1967farmers still supplementedtheir incomesby nettingArctic char (Salvalinusalpina), and shootingRock Ptarmiganand Grey-lagGeese (Anser anser), but their harvests were not excessive.The numberof peoplein the basinis small.But as of this writing "modernlife" has arrivedat LakeMyvatn. The bottommud of the northern,shallower basin of the lakeis beingmined to pro- ducediatomaceous earth. This activitywill forcedevelopnient of reliableyear-round road communications and a year-roundport on the GreenlandSea. The lake is locatedwithin a geothermalarea that is a potential sourceof electricpower. The next decadesmay well involveflirther industrial disturbances of the Myvatn landscape,with unforeseeableeffects on its wildlifepopulations, including its populationof Gyrfalcons. Purposeand Methods I stayedwith the familyof SnaebjornPetersson in Reynihlid(fig. 2). The chiefpurpose of my stayat My- vatnwas to determinewhether the Gyrfalcons'food habits showed any changeover the breedingseason. F.

97 RaptorResearch 14(4):97-124 98 RAPTOR RESEARCH Vol. 14 No. 4

Figure1 ResidentGyrfalcon Pairs and Habitats at Lake Myvatn,1967

GaesadalurO •

.. ---'/x /x /', /x . . • '/• A '• - •'•

• o• A A o ' ' "z••- - ' •ø'-AZ- "•.•

oO A A • PimmuborgirA '

• Thismap is intended solely BirchBrushwood •__I•- ß-. ' ' ' /• togive ageneral ideaof the •] • -- w •/ /N /• LakeMyvatn area. The Bog)Swamp/Meadow--- ß "habitat"categories are • •..__ Seljahjallagil/X O/'• threecatch-alls.habitats Insupportgeneral, breedingthe first Moorland • . " . populationsof RockPtarmigan (Lagopus .{-75-] ' ' i/'Nmutus)and also (thoughof the variousofgreatly species differingof shorebirds.densities) Wasteland(unvegetated or ' ' /'N Themargins andislands ofthe lakes and [•] verysparsely vegetated)ß . •/• ponds,supportandthe the majority bog/swamp/meadow of the ducks,as well habitat as Hills,Mountains ,/X thegulls and other waterbirds. (Wasteland,unless otherwise noted) . /•

O Gyduhnjuksgil Winter 1980 Woodin-Gyrfalcons 99

Figtire2 EyrieAreas of Gyrfalcons at LakeMyvatn

O Gaesadalur

! ( -- 10 km )

O Seljadalur o Dalfiall

Reynihlid Kraeduborgi-•!

o•Vindbelgjarfjall Falkakletturi•O J;

Lake O Dimmuborgir

Threngslix ß? !

Seljahjallagil

Figure2 summarizesmy interpretationof eyrieareas at Myvatn. Occupiedareas are markedby a circle,former ones by a cross. Alternateareas are joined by a solidline. Areas which I suspect functionprimarily as alternate areas, but whichare morethan 3 km awayfrom each other, are joined by a dottedline.

O Gyduhnjuksgil 100 RAPTOR RESEARCH Vol. 14 No. 4

Gudmundssonand A. Gardarssonhad observedthat Gyrfalconpredation on the Rock Ptarmiganon Hrisey virtuallyceased the first weekin June.They wantedto know if a similarshift away from ptarmiganas food occnrredat the eyrie.At the sametime I wasto fit in whateverother observations on GyrfalconsI could. Transportationwas by ski, foot, and bicycle.Use of the bicyclebecame possible in mid-May. Until then I observedthe closestof the eyries:Seljadalur, Dimmuborgir, and Dalfjall (fig. 2). Seljadalurwas just a 45-min. ski froin the house.When the Seljadalurpair failed to lay eggs,I focusedon Dimmuborgir,with periodic visitsto the other eyries.A. Gardarssonhad shownme the locationsof most of the eyriesbefore I left Reykjavik. One of my chiefconcerns was the extentto whichmy presencemight influencethe Gyrfalcons'behavior. Their reactionto me varied as the seasonprogressed. They becamemore aggressiveafter egg-hatching.Be- fore egg-laying,adults were rather shy,and it wasdifficult to assessone's effect. I tried to remainas incon- spicuousas possible.My equipmentincluded 10 by 40 binocularsand a 30-powertelescope. I did not use a blind. The positionsfrom whichI watchedthe eyrieswere not alwaysgood. Rarely could I commanda view of both the eyrieand the approachesto it. At SeljadalurI generallypicked a spoton the canyonfloor, 250 m or more from the nearestperching place. This positionafforded a restrictedhorizon but a goodview of the interiorof the canyon.A few timesI observedfrom one of the ridgesthat borderedthe canyon,where I had an excellentlong view in all directionsbut wasunable to seemost of the perchingplaces in the canyonitself. When I beganregular observations at Dimmuborgir,the pair wouldtolerate me within 250-300m aslong as I remaineddown near the valleyfloor. When I took a morecommanding position on a slope,about 300 m from the nestledge, they beganperiodic fly-overs or otherwiseindicated their agitation.I finally adopteda positionon the valleyfloor about250 m from the nest ledge,which gaveme a goodview of the nestand of the surroundingsfor a few hundredmeters in eachdirection. My concernwas to tell the directionfrom which the mateswere arriving, bnt thiswas rarely possible. At SeljahjallagilI took a positionthat let me view the nestledge, the inner wallsof the canyon,and the sandywaste and distant lake beyond. Thus I wasable to observein somecases the directionof arrivalof the adultswith prey. Falkakletturwas probably the best eyrie to observe.It was situated on a lavarock in themiddle of a fairly flat lava field. I could keep both the rock and approachesto it in view. After egg-hatching,however, the femaleat Falkakletturwould not tolerateme within 500 m of the nestunless I remainedcompletely hidden. Early in the seasonI went out wheneverweather allowed, generally for shortperiods of 4 to 5 hours.In May and Junemy observationsfocused on Dimmuborgir,and I spent4 to 8 hoursa day watchingthe eyrie4 or 5 daysa week.In July and AugustI alternatedlonger trips to the distanteyries with shortertrips nearer home.Long trips involved2 to 3 hoursof travel to the eyrie, 4 to 8 hoursof observationthere and a similar amountof travel back. I spentabout 700 hoursin the field, 250 to 300 of them in direct observationof Gyrfalcons.

The Eyries Brownand Amadon(1968) define home range and nestingterritory as they apply to diurnalraptors. The homerange is the entirearea used by a breedingpair or solitaryindividual. The nestingterritory is the area aboutthe nestthat is defended.In somecases the two coincide,but often the "outer" partsof the home range(which usually comprise the huntingterritories) are not vigorouslydefended and may evenbe shared. Cade(1960) uses a modelof the latter sortto describethe homeranges of the Peregrine(Falco peregrinus) alongAlaskan rivers. His diagramof a typicalhome range consists of three concentriccircles of increasing diametercentered on the eyrie.The innercircle is alwaysdefended against other Peregrines. Inside the sec- ond circleintruders are sometimesattacked, and in the outercircle otherPeregrines are attackedonly over fooditems or favoriteperches. The diameterof the inner circle variesfrom 100 m to 1.6 kin, and that of the outercircle is a little morethan 3.2 km. The outercircles of adjoininghome ranges overlap. The notionof a largehome range, vigorously defended for a smallradius about the nestledge, and looselydefended or even "shared"at greaterdistances from the nest,fits my observationsof Gyrfalconbehavior at Myvatn. Beebe(1960) uses the term eyriearea to refer to the entire area in which,year after year, differentnest ledgesare occupiedby onepair of Peregrines.Gyrfalcons, like Peregrines,tend to usealternate nest ledges on a givencliff or in a givencanyon (Hagen 1952). However, at Myvatntwo of the homeranges contained two distincteyrie areas, about 3 km apart,which appeared to hinctionas alternate areas for the samepair. In this case,it seemsthat the spacebetween these areas should not be classifiedwith them. Ratherone mightrefer to themas alternate eyrie areas. At Dalfjall,on the otherhand, several nest ledges were scattered along ap- proximately3 km of a mountainfault. 1. Dimmuborgir.The eyriein 1967was in an oldnest of the CommonRaven (Corvus corax) on a ledgein a lavabutte. The ledgefaced west and was completely overhung. One couldwalk up to it. Anothernest report- Winter 1980 Woodin-Gyrfalcons 101 edlyexists in Dimmuborgir,but I did not searchfor it. Dimmuborgir(Dusky Castles) is a lava depression, roughlycircular, between 1 and2 km in diameter.A smallcentral plateau is stirroundedby a conillsingmaze of lavabuttes, pinnacles, and valleys. A pair eachof ravensand Merlins(Falco columbarius) nested near the part of the depressionoccupied by the Gyrfalcon. The pairhatched 4 youngfrom 4 eggs.Three of the youngdisappeared within a week,probably because of my, or others',interference. The fourthyoung died of unknowncauses when virtually hilly fledged.Dimmu- borgiris a nationalpark. One can drive to it, and after the first of Junehuman disturbance is common.Be- causeof itsaccessibility, it was also the eyrie I watchedmost frequently in May andJune. 2. Dalfl'all.The eyriearea at Dalfjallconsists of approximately3 km of a fault that cutsthrough a motln- tain ridge.Several inaccessible rocks and facesshowed apparent Gyrfalcon use. The largecliffs overlooking the plain to the eastshowed much use, held at leastone formernest ledge, and seemedto be the centerof activityfor the nonbreedingpair in 1967.The previousyear's nest had been an old ravennest flirther north alongthe fault.This nest was in a low facein a potholejust largeenough to holdit. About 1 m belowthe nest wasa grassyledge to whichone could walk. A pair of ravensnested on the northernextremity of the fault in 1967.Copulation and one attempted food transfer were observed in the Gyrfalcons. 3. Seljadalur.The eyrie area centerson the eastcliffs of a narrowcanyon which opensout into a little plainbelow. The cliffsare aboutmidway along the canyon,where it narrowsbefore bending abruptly 90 ø andending in a steepslope. The Gyrfalconsused several ledges and rockson the westernside of the canyon in late winterand early spring, including an outcropbelow the canyonproper. The onlysite that seemedto showsigns of previousnesting was an oldraven nest on the eastcliff. In Aprila pair of ravensbegan building a new nestnear the old one.After egg-layingby the ravens,a seriesof spectacularfights occurred between the ravensand the Gyrfalcons,which finally resulted in the ravensremaining where they were and the Gyr- falconsestablishing control over a sectionof the cliffswith what appearedto be a suitablenest ledge above andup the canyonfrom the ravens.However, the Gyrfalconsdid not lay eggs.Their ledgehad freshprey pluckingsand molted Gyrfalcon feathers on 20 July. Whetherthe Seljadalurarea is a separateeyrie area, or an alternateone (to eitherDalfjall or Gaesadular, mostlikely the latter)is not clear.Three factors make me questionits separateness:it is ratherclose (about 5.5 km) to both Gaesadalurand Dalfjall;it is betweenGaesadalur and Lake Myvatn;and vegetativesigns seemto indicatethat Gaesadalurand Dalfjall havebeen more consistently occupied. 4. Vindbelgjarfiall.The eyrie area centerson cliffsalong the southeastside of "WindbagMountain," a steephill risingat)out 250 m frowna •narshyshrubland dotted with ponds.A pair of ravensnested in 1967on the westernend of the cliffsand a pair of Merlinson the easternend. At leastone ledge in the centraland highestsection seemed to showsigns of formerGyrfalcon occupation, and thisarea also comprised the center of activityfor the nonnestingpair in 1967. Breedingactivity extended only throughcopulation. On 26 April 2 Gyrfalconsin juvenilepitimage were "escorted"away from the cliffsby the adults. Between4 and 5 km awayover the mooris a low, rockyupjut that formspart of a bowl (Holkotsgil)in the sideof the LaxaRiver Valley. There I sawGyrfalcons several times and foundprey remains and moltedfeath- ers.The upjutcontains a ravennest, but asfar asI couldtell Holkotsgilhas not beenused by Gyrfalconsfor breeding.I thinkthe GyrfalconsI saw there were from Vindbelgjarfjall. 5. Falkaklettur.The eyriearea centers on a lava conethat risesabout 6 •n abovea grassybase on the Bur- fellshraunlava field. The Burfellshrauniswell-vegetated and supports breeding populations of ptarmiganand shorebirds.The eyriewas in a north-facinghole, about 0.5 by 0.3 m, jnstbelow the top of the cone.A crevice on the westface for•ned a narrowledge through an accumulationof prey remainsand pellets.It apparently servedonly as a perch.A lavarock to the southshowed signs of occupationby Merlins,but nonenested there in 1967. About3 km to the eastis a groupof tipjuts(Kraeduborgir) with ledgesthat showeduse by Gyrfalcons. Accordingto localfarmers, this is an alternatesite for Falkaklettur.I alsofound a siteused by Gyrfalconsin Threngsli,between 6 and 7 km southof Falkaklettur.Here a smallpothole or cavenear the top of the cliff had a moundof pellets,another of excrement(mutes), some old prey remains,and severalrecently molted Gyrfalconfeathers. About 10 m aroundthe cliff in a similarpocket was a new ravennest and about 15 m fi•rtheron an olderone. The maleat Falkakletturwas very shy of hismate, and possibly he usedthe firstcave as a perch.However, because of its distancefrom Falkakletturit couldhave been a separateeyrie. If so, it wasnot successfidin 1967.I foundit on 7 August,3 weeksafter youngfledged at Falkaklettur.As I climbed aboutthe cliff, 4 Merlinsswooped and tumbledoverhead. 6. Seliah]allagil.The eyriearea consists of the tipperpart of a narrowcanyon (gil) that cutsinto the upland borderingthe lake basin on theeast. Two ledgesshowed use, but the onlyone I couldidentify as an eyriewas that usedin 1967.The slightlyoverhung ledge measured about 1 m by 0.3 m and waslocated on the south 102 RAPTOR RESEARCH Vol. 14 No. 4 sideof the canyon,with a grassyand stony slope below. It showedprevious use. Four young were fledged in 1967.The lowerpart of the gil containeda volcaniccrater, several of whoseinner ledges showed signs of use by Merlins.One held an eyrie,where 3 yot•ngwere fledged. Approximately3 km southof the Gyrfalconeyrie are two smallercanyons that opentogether into the plain towardthe lake, and immediatelysouth of them are two bowlsin the sideof the upland,which hashere becomea mountain(Blaf]all). The moresoutherly canyon had a possibleformer eyrie site,and the northern hollowat leasttwo. The furtherhollow had one very suspiciously green ledge. Because of the area'scloseness to SeljahiallagilI thought it an alternatearea. An old ravennest in the southernpart of Threngslaborgircould alsoserve as an alternatesite. Threngslaborgir is very confusing, and I couldnot find the ravennest again, but it wasalso about 3 km fromthe eyrieat Seliahiallagil. 7. Gaesadalur.I foundGaesadalur on 17 August.The eyriewas on one of the ledgeson the southside of the valley,near where it opensinto the plain Randir.The ledgewas about 8 m abovethe valleyfloor, in- accessiblewithout a rope,and was apparently a much-usedsite, iudging from the amountof orangelichen, the thicknessof the mosson the cliff, and the vigorof the Salixherbacea on the slopebelow. Cht•nks of moss hadfallen off the face,out of whichI pickedptarmigan bones dyed a dark red.Juvenile down on a lower ledgeand alongthe shoreof the pond,where there were alsothree recentlypicked-over duck carcasses, in- dicateda broodhad fledged. This face was the onlyeyrie site I foundin the canyon,but the uplandbetween Gaesadalurand Seljadalur contains many likely-looking places. 8. Gyduhniuksgil.I visited Gyduhnjuksgil on 12 August.The gil opensout into a dry washin a stonyplain that risesslowly between the two mountainsat the southernend of the lake. The area that containedthe formereyrie sites was the lower150 m of the gil. About1 km up the gil I founda rockon the westside that lookedlike a Merlin site.The Gyrfalconeyrie was in a ravennest on a rockface that stoodout into the gil itself.The nestwas inaccessible without rope. There was another raven nest on a ledgebelow. A facenear the entranceto the washhad two possibleformer eyrie ledges, overgrown with tall grass.Juvenile down on a perchat the entranceto the gil andfresh prey remains below the nestindicated that a broodhad fledged. 9. Hrossaborg.Hrossaborg is not a Myvatn eyrie. The eyrie area consistsof a crater whosewalls make a naturalshelter for the level groundinside: thus, perhaps, its name-the horses'castle. The crateris located justsouth of the roadacross the Burfellshraun,about 3 km fromthe Jokulsa-a-FiollumRiver, and 17 km from Falkaklettur.It hasfour main cliffs, all of themon the outside.On 19 June,I foundnothing on the northeast cliff, a pair of broodingravens on the northwestcliff, a partlycollapsed old ravennest high up on the south- westcliff, and a pair of Gyrfalconswith 4 downyyoung in a ravennest on the southeastcliff. The nestwas tuckedinto oneof a seriesof long,slanting shelves, about 3.5 m abovea gravelslope. It facedeast and was completelyoverhung. On 19 Junethe nest contained4 large downyyoung. However, several things bothered me aboutthe adults.During the 3 hoursI watchedthe nest,the malebrought prey once.The femaletook it, fed herself, anddid not feedthe young. I sawthis happen very few timesduring my stayat Myvatn.It mighthave been a responseto my presence.Second, the colorof the female'splumage was midway between that of an adultand a year-oldiuvenile. I wasnot sureher feet wereyellow. Finally, during my approachto the nestthe malewas muchmore aggressive than the female.Perhaps the femalewas a youngbird breedingfor the firsttime. Between19 and 24 Junea photographerset a blind about 50 m from the nest. I dismantledit when I visitedthe eyrieon 24 June.On thisdate, 1 younghad disappeared,1 wasapparently dying, and the other2 were somewhatlethargic. Neither adult appearedduring the hour I spentthere. On 26 Junethe remaining3 youngwere dead.Neither adult waspresent. Apparently the nestwas abandoned because of the blind, and the youngthen starved.

Behaviorduring the BreedingSeason Pre-laying.A centralquestion in the studyof wild Gyrfalconshas been whether they winter on the breedinggrounds. Platt (1976),working in the westernCanadian Arctic, foundGyrfalcons in the vicinity of their nestsin Januaryand February.Bengtson (un- publ. ms.)reports Gyrfalcons about Lake Myvatn in December.Platt (1976) presents datathat suggestthat, at leastin the higherlatitudes of North America,the adultmales may remainon the breedinggrounds over the winter while the juvenilesmove south. Platt foundpotential prey, Willow Ptarmigan(Lagopus lagopus), relatively abundant in Januaryand February.The maior difficulty for predatorsat high latitudesis probably the shortlength of day (3 hoursin mid-Januaryfor Platt'splot) duringwhich to obtain Winter 1980 Woodin-Gyrfalcons 103 prey. Casual observationsnear Reykjavik,southern Iceland, during the winter of 1966-1967 indicatethat at leastsome of the juvenileGyrfalcons remain on the island. They seemto suffera heavylate autumnand earlywinter mortalityfrom starvationand disease(probably aspergillosis). Platt (1976)suggests that juvenileGyrfalcons, lacking huntingskills, tend to pursueground-dwelling prey, suchas lemmings,and disperseto areaswhere such prey is moreabundant for their first winter. In fact, Gyrfalconsmay learnto hunt on essentiallyground-dwelling prey. The iuvenileGyrfalcons I watchedin late Augustin northernIceland were catchingjuvenile ptarmigan on the ground.The youngGyrfalcons would attempt to grab the youngptarmigan while flying over them and,if thismaneuver failed, would land and pursuethem on the ground.One oftensees juvenileGyrfalcons at the Reykjavikcity dump in the autumn,where they pursue brownrats (Rattus norvegicus). WhetherGyrfalcons spend the entireyear on their homeranges depends on the avail- ability of prey. The RockPtarmigan, which formsthe chief prey of at leastthe inland populationsof IcelandicGyrfalcons, spends the autumnand winter groupedin nomadic flocks,which perform a limitedseasonal migration. In the autumnafter acquiringwhite winter plumage,ptarmigan move from their breedinggrounds to higher elevations where snowhas alreadyfallen. They then follow the snowline as it movesdown the mountains.Thus, depending on the otherprey speciesavailable to a givenpopulation of Gyrfalcons,one might expect a generalautumn dispersal of Gyrfalcons(perhaps still in familygroups) in pursuitof ptarmigan,followed by a return,perhaps of the adultsonly, to the breedinggrounds in December,when snow cover is moregeneral and the ptarmi- ganmore spread out. The returnwould depend on suitablewinter coverfor ptarmigan beingavailable in the homeranges of the Gyrfalcons.Suitable winter coverfor ptarmi- ganin Icelandconsists primarily of brushwoodsof birch (Betulapubescens). Since Gyr- falconsbreed so early and presumablyneed some weeks of socializationprior to egg- laying,it may be that one of the prerequisitesof a homerange is a certainamount of birch brushwood. In March,when I arrivedat Myvatn, Gyrfalconswere paired.At Dimmuborgirthe nestwas being used for a perch.On 10 March a cup had beenscraped. Upon my ap- proachthis day, the singlebird did not leavethe areadirectly, but circledseveral times aroundthe lavabutte containing the nestbefore disappearing. Prior to egg-laying,all the falconsspent a gooddeal of time perching,singly or to- gether,in the immediatevicinity of the formernest sites.They alsoroosted near old nestsat night.The Seljadalurpair had perchesboth in the immediatevicinity and scat- tered aboutthe generalarea. The more distantperches were within 1.5 km of the can- yon, mostlyto the westand south.In April, after copulationbegan, they perchedto- getherfor hoursin the afternoonand evening, copulating every hour or two, andgoing from one perchto another.It wasnot clear if the birdsat Dimmuborgirand Dalfiall madeuse of suchdistant perches. Hunting at this time still appearedto be a solitary activity. Ptarmiganwere commonin March and the beginningof April both in the birch scrub on Dalfjalland at Sel]adalur.Judging by remainsfound then and later duringthe thaw, they hadbeen taken by the Gyrfalconsat both places.Thus it seemsthat the free or safe area for prey aboutthe eyrie, which someobservers have reported(Dementiev 1960), doesnot existprior to egg-laying. On 4 April I firstobserved copulation (at Dimmuborgir).It wasaccompanied by sev- 104 RAPTOR RESEARCH Vol. 14 No. 4 eral vocalizations.Since the callsI recordedagree well with the repertoiredescribed by Wrege and Cade (1977),and that summarizedby Cramp and Simmons(1980) for the variousevents in the breedingcycle, I will not enumeratethem here. I have mademy classificationof callsin thispaper fit theirs. On 4 April preeningmovements by the femaleand manysoft calls immediately pre- cededmounting by the male. During copulationthe femalebowed her head almostto the ground,while the malebalanced on her back,slowly moving his wings.After cop- ulationthe male left the area and had not reappearedby the time I left, an hour and 45 min later. Suchdisappearances at Dimmuborgirseemed the rule only for the first few days.They alsooccurred from time to time at Seljadalur,and there wassome indication that they were associatedwith huntingby the male.On 8 April copulationin Dimmu- borgirwas accompanied by a chitteringcall. Copulationat Seljadalursometimes, but not always,involved more elaboratepre- copulatorybehavior. The mostelaborate of suchbehaviors that I observedoccurred on 5 April and consistedmostly of preeningmovements. The matesfaced each other about 8 m apart on a snow-coveredslope. First onebird and then the other took the initiative at preening.The movementsincluded lifting andpreening under the wings;bending the headback while elevatingthe tail, asthough to touchthe beak to preengland; scratch- ing underthe chin;and loweringthe headwhile bobbingthe tail. One of the lastin the sequence,done only by the female,was a bendingof the tail sideways,while reaching back underit with the head. Suchpreening episodes occupied 20 min, when the male tookoff andmounted the female,as usual, from the wing.Billing was part of the cop- ulatorybehavior at Dimmuborgirbut not at Seljadalur.I alsoobserved mantling of the matesto each other duringan afternoonof perchingand copulationwhen the mates were perchedfacing each other, less than a meterapart. Flight displaydid not seemhighly developedamong these Gyrfalcons. The bulk of their courtshipbehavior consisted of perchingnear eachother. However, normal flight patternsseemed to becomemore elaborate, or were executedwith morespeed or grace duringcourtship. I twice sawmutual gliding during courtship. At Seljadalurthis behav- ior followedcopulation. The matessoared above the canyonand then descended in long glides,so that they crossedand recrossedeach other'spaths. One dovefrom a shortdis- tanceat the other,which I think turnedon its back to meet it, and finally one followed the otherin a longswoop (I hearda burstof callsat thispoint) that carriedthem out of sight. Later in the season,from time to time I sawone or both of the matesat Dimmuborgir performa flight displayabove the eyrie; the bird would either soarwith the eyrie as a centerbelow it or glide at a lower altitude in a seriesof circlesor arcsthat took it back and forth over an area of 200 to 300 m radius.Twice duringthe fourth week of in- cubationat Dimmuborgir,I saw one of the mates(once the female,once the male)go into a stoopin the eyrie area,once right beforethe eyrie ledgeand oncea little way off. In both casesthe stoopwas associated with a flight displayabove the eyrie. Finally, on 11 April, I sawa singleGyrfalcon make a morningflight at Seljadalur.That morningI had been watchingthe canyonsince 0445. Sunfirst shonein at 0700. At 0800 a gyr appearedon the wing, swoopeddown acrossthe valley, then up and back and dis- appearedfrom sight.In lessthan a minute,the falconcame around the face again,made a feint down,falling and twisting, then swoopedtwice way up, slidingdown the rising air currentsabove the top of the canyon.Finally it camemotionless into the wind,high Winter 1980 Woodin--Gyrfalcons 105 up, and thendisappeared in a long circle.A few minuteslater I spotteda gyr perched on a face.It preeneduntil 0830, and wasstill perchedin the sunwhen I left at 0900. Of the 4 pairsof gyrsI wasobserving in April only onepair (Dimmuborgir)laid eggs. I observedcopulation in 3 of the 4 and suspectit occurredin all 4 pairs.At Dimmubor- gir, egg-layingessentially marked the end of copulation.Egg-laying ended between 18 and 19 April. I observedonly two copulationsafter this, on 19 and 24 April. One oc- curredafter I had disturbedthe broodingbird (the male),and the other seemedto be a substitutefor the male'sfailure to bring food. The last copulationI observedat the othereyries was at Seljadaluron 20 April. On thisday the birdsspent most of my obser- vationperiod (1000 to 1900)perching side by sideon differentposts within 1-1.5 km of the canyon.Copulation occurred twice (oncein the canyon,once about a km away).All the pairsseemed to usetheir eyrie areasas a homebase at leastinto June. Egg-Layingand Incubation.Egg-laying occurred about the middleof April. As nearly as I couldjudge from the developmentof youngin othereyries, the Dimmuborgirpair laid eggsnear the middleof a two-weeklaying period for the region.Egg-laying thus coincidedwith the arrival of cockptarmigan on the breedinggrounds and also(but I think lesssignificantly) with the arrival of the first migrantwaterfowl. At Dimmuborgir egg-layingalso marked the startof fairly regularfood transfersfrom male to female.It wasnot clearwhen food transferring began or whetherit wasregular before egg-laying. An instanceof foodtransferring occurred near Dimmuborgiron 8 April, and I observed an apparentlyunsuccessful attempt at foodtransferring at Dalfjall on 20 March.If early foodtransfers occur on the huntinggrounds, I wouldnot havebeen likely to seethem. It was not clear whetherthe incubatingfemale got all her food from the male or whethershe also hunted. Platt (1975)states that femaleGyrfalcons do not hunt from the beginningof food transferringthrough one week after hatching.The male at Dimmu- borgir not infrequentlyappeared at the eyrie without prey and took over the in- cubation,whereupon the female left for periodsof up to 4 hours.The length of her absencessuggests she was hunting.However, she may have been retrievingprey the malehad cachedor just"relaxing." For severalweeks after hatching,the femaleceased any semblanceof hunting,and the male thenhunted for himself,her, and the young. To givesome flavor of the Gyrfalcons'behavior at thistime, I quotetwo entriesfrom my journal.Several explanations have been advancedto explainsexual size dimorphism in Gyrfalconsand other raptors (see Snyder and Wiley 1976).Of the severalideas Cade (1960) favorsthe notion that femalesare larger becauseof their need to dominatethe male duringthe breedingcycle. One presumedreason for suchdominance is to protect the youngfrom predationby the male. However,it may be ascrucial for her to be able to forcethe male to feedher duringpart of the breedingcycle. Gyrfalcons are active huntersand "jealous"of their prey. In a sense,and within limits,the female'slarger size letsher "force"the male to feed her (Cade 1960). Storer'sexplanation (1966) putsfor- wardthe ideathat the sizedifference allows the matesto exploitmore efficiently differ- ently sizedspecies of prey. There is someevidence that this is the caseamong other raptors,especially in winter, and sizedifferences might have someadvantage for a pop- ulationof Gyrfalconsthat had accessto differentlysized species of prey.At Myvatnthis advantagewould likely appearduring July and August.I prefer Cade'sexplanation be- causeof its generalityand because it seemsto explainmuch of my data. The first egg at Dimmuborgirwas probablylaid between11 and 12 April. About 1330 on 13 April the femaleflew in to perchsideways on the nestledge. The position 106 RAPTOR RESEARCH Vol. 14 No. 4 was somewhatunusual: her usualposition was perchingwith her tail hangingover the edge.About 5 min later the male flew by the ledgeand away, evokinga squawkcall from the female. Nothing then happenedfor about 2 hours. The female remained perchedon the nestledge, facing inward, sometimes looking over her shoulderinto the valley. At 1515 the male swoopedup and landedon the nestledge, to wailing calls, probablyfrom the female.Both birdsbowed to each other and seemedto scrabbleon the floor of the ledge with their beaks.While this went on, I heard a medley of squawks.The malethen moved by the femaleinto the nestcup, where he remainedfor a few moments,bowed forward. Just before leaving he half-squattedand madea motion asthough muting, and then flew to a nearbyledge where he lookedback overhis shoul- der towardthe eyrie.The femaleremained motionless for 3 or 4 min, bowedtoward the nest,then walkedto its near edgeand perchedon its inner slope,still partly bowed over. The male left about 1530. The female remained where she was until he returned at 1700.During the hour and a half shepreened for a few minutesand oncegave a low trechalmost inaudible to me, accompaniedby a ierk of the head.She held the samehalf- bowedposition. At 1700 the male swoopedup in front of the eyrie and landedabove it on the butte. The femalehesitated a moment,walked throughthe nest-cup,and flew to a nearby butte,where she perched, still tendingtoward the horizontal.I heardno calls.She let go with a tremendousmute, took off, circledonce, and swoopedup very swiftlyto perch besidethe male.I heardwailing calls,and the male pluckedat a partly eatenptarmigan carcassin his talons.The femaletook off again,circled, floated perhaps6 m over the top of the butte, then settledslowly beside the male. For 3 or 4 min there was much chitteringsuch as accompanied copulation. The femaleseemed to be pushingagainst the male'sbelly with her tail and flank.He finally movedaway, and shestarted to pluckat the carcass.Perched upright on a rock,he let her feed for about7 min. Normally,feed- ingslast 20-30 min. Then he approachedher from the front and after a few falsetries pickedup what was left of the carcassand flew away. The female remainedfor some minutes,picking up a few bits,and then took off, gainedheight, circled once very fast overthe little valley,and swooped up to the nestledge. After a few minutesshe walked to the nestand climbedover it to its inner edge,where she fluffed her feathers,spread her wings,and settled down in the samehalf-bowed position she had been in before.She remained so for the half hour until I left at 1800. What onemight note in connectionwith Cade'shypothesis is the changefrom wail- ing callsto copulativechittering which precededthe male'srelease of his prey to the female,as well as the moderationof her movementsin her secondapproach to him, in contrastboth to her first approachand her return afterwardsto the nest.She also al- lowedhim to removethe remainsfrom her possession.While sheappeared subservient, I thoughther behaviorpattern implied dominance. She walked a narrowline between frighteningthe male away and gettinghim to bring her food (I do not meanto imply shewas "conscious"of this). By 28 April, incubationhad been underway for about 10 days.At 1700 the male, who had beenabsent from the vicinity of the eyrie for at leastan hour and a half, reap- pearedon a perchnear the eyrie.The femaleapparently did not seehim approach:the perchis not visiblefrom the eyrie ledge.He had a fidl crop.After i hour and 45 min the male swoopedup in front of the eyrie ledgeto land on top of the butte. As he swoopedby, I heard the wailing call. The femaleleft the eggsand flew to a nearby Winter 1980 Woodin-Gyrfalcons 107 butte and beganrepeating the call. She then flew back to the butte and landedright besidethe male;there was a slightscuffle in which shemay have actuallypushed him from his perch,upon which he flew directlyto the nestledge, walked into the nest,and beganto incubate.The femalesettled down, feathers all fluffedout, as if to incubate,then hoppedback, perched, and began to preen.She preened steadily for about20 min, res- ted, then mantiedand preenedoff and on for another20 min, whereuponshe flew. Abouta half hour later the male left the nestwithout a sound;I then hearda wailing call, and he swoopedup as if to perch,out of my sight.I heard anotherwailing call endingin cluckings,and 2 or 3 min later the female flew aroundthe eyrie butte, swoopedup to the ledge,and began again to incubate. In the light of Cade'shypothesis, I would interpretthis sequenceof behaviorsas fol- lows:the male made a "mistake"by landingwithout first showinghimself to the in- cubatingfemale. Recognition between the Gyrfalconsseemed to be by sight;vocaliza- tion was secondary.The male usuallyflew by the nest ledge or otherwiseshowed himselfwhen appearingin the vicinity of the nest.He had fed, which implied, if they were sharingprey, that shewas hungry. The femalethen actedas if shehad expected him to bring food:this accountsfor the "scuffle."But the male had not broughtfood. The female'spreen and apparent"dust bath"-the soletime I saw sucha performance on the top of the butte-indicated her "anxiety" or "confusion"over his not having broughtprey. In thiscontext her mantlingwas quite remarkable. It hasbeen suggested to me that shewas "rousing,"but my journalrecords "mantling." Finally sheleft the area,perhaps to hunt, perhapsto obtainfood from a cache,perhaps to exercise.Her shortabsence makes it unlikely,though not impossible,that shewas hunting. While one canargue that the female'sbehavior was a resultof her being"tired" of incubatingand simplytaking advantage of the male'spresence to preenand "relax,"this explanation doesnot adequatelyaccount for the detailsof her behavior,while Cade'snotion does. The femaleperformed the main part of the incubationduty. Shewas not alwaysre- lieved by the male while she fed. At thosetimes the eggsremained unattended for 20-30 min. Both sexesmade many of the samemovements while on the eggs.They con- sistedof (1) an incompleterotation, which involveda rockingfrom sideto sideof the body,a backand forth shuffling of the feet,and later a downwardsbumping of the belly (thesemovements I assumewere to accomplisha turningand regroupingof the eggs); (2) a pickingup and droppingof the materialin the nestcup with the beak;(3) preen- ing; (4) snappingat flies.The time betweenegg turnings was variable and rangedfrom over an hour to a few minutes.The functionof her handlingof the nestcup material with her beak,if not merelya nervousrelease, might havebeen to keep the nestmate- rial, which tended to become compacted,broken up and soft and thus a better in- sulationagainst the cold. I did not observethe male handlingthe nestmaterial in this way. The mechanicsof nestexchange ceremony varied greatly.The female'sdeparture (if shewas goingto go) seemedto be stimulatedsimply by her awarenessof the male's arrivalin the vicinity.When shedeparted, the male would then either go or not go to the eggs:I couldnot distinguisha pattern.If he did not beginto incubate,she returned to the eggs,though sometimes with a delay of 20-30 min. Only occasionallydid he in- itiate the exchangeof incubationduties; this he did by flying to the eyrie ledge and callinggently until the femalesurrendered the eggs.His way of surrenderingthe eggsto her wasalso variable. Sometimes the mere sightof the returningfemale was enoughto 108 RAPTOR RESEARCH Vol. 14 No. 4 makehim depart.Other times he left the eggsonly reluctantly and with muchurging on the part of the female.I did not discoveran evolutionof the ceremonyover time: one of the leastceremonious of the exchangesoccurred on 18 May and one of the mostreluc- tant on 19 May (bothnear the endof the incubationperiod). From aboutthe secondweek of May onward(the last two weeksof incubation),the malebegan to makemore frequent and vocalvisits to the eyrie. Thisbehavior con- trastedwith his previoushabit of alternatinglong periodsin the nestarea with long periodsof absence.Perhaps this change was a preparationfor the morefrequent feeding visitsafter hatching.Sometimes the femaleleft the eggswhen he appearedbut would immediatelyreturn to them. The male'svisits were accompaniedby muchscreaming and clucking.They did not seemto be accompaniedby an increasein the frequency with whichthe malebrought prey. A summaryof datafor the incubationperiod at Dimmuborgirfollows: ß 11-12 April, 1steggs probably laid (13 April observation). ß 18 April, 1500,3 eggslaid. ß 19 April, 1530,4 eggslaid. ß 22 May, 1230,the youngin at leastone of the eggssqueaking. No chippingappar- ent. ß 23 May, 1130,2 eggscracked near the large end, the 3rd had a smallhole on one sideof the largeend. ß 24 May, 1100,2 dry young,i wet young,4th eggcracked. ß 25 May, 0830, 4 dry young. Incubationtime for thefourth egg: Maximtuntime--18 April, 1500, to 25 May, 0830, or 36 days,171A hrs. Minimumtime-- 19 April, 1530,to 24 May, 1100,or 34 days,19V2 hrs. Thusthe incubationperiod for this egg was 35-37 days.It is abouta week longer than the 28-29 daysreported by Brownand Amadon(1968). It is similarto the 35-day incubationperiod noted for captive Gyrfalconsby Cade and Weaver (1976). Platt (1976)noted 35 daysfor incubationof wild Gyrfalconswithout specifyingdetails. My estimateof the timingof the first eggdepends on Platt'sobservation of 8 daysfor laying. Aggressivenesstoward Man. At Dimmuborgir,eyrie defenseshowed a definite in- creaseup to hatching,leveled off for a few weeks,and then fell. As the time of hatching neared,the broodingfemale sat tighter upon my approach.Finally she let me come within about3 m beforeflying and wouldreturn immediately after I left the ledge.On 23 May the pair screamedat me while I was at the nest,and on 25 May one or both stoopedat me for the first time but did not comeclose. Thereafter their aggressiveness towardme slowlydeclined. On 5 Junethe femalestill stooped;on 16 Juneshe merely flew backand forth abovethe eyrie screaming. Femalesat Seljahjallagiland Falkakletturwere muchmore aggressive. The Falkaklet- tur femaleonce struck me, and the Seljahjallagilfemale would dive repeatedlywithin lessthan 1 m of me. The aggressivenessof the female at Falkakletturincreased as in- cubationprogressed and did not decreaseafter hatching.She sat much tighter than the Dimmuborgirfemale. Her aggressivenessseemed to be focusedeither on defenseof the rock or (moreprobably) of prey, ratherthan on defenseof the young.On 11 July,less than a week beforethe youngleft the vicinity of the eyrie, shedove repeatedlyat me when I climbedthe rock to collectprey remains,even thoughthe youngremained in the lava field below. Shethen ignoredme when I tried to catch the youngto weigh Winter 1980 Woodin-Gyrfalcons 109 them.The femaleat Selljahjallagilbehaved similarly. On 19 July sheattacked me while I was on the slopebelow the nest,something she had never donebefore. At this time, however,the slope,rather than the nest ledge, was the place to which prey was brought.At any time, the presenceof more than one personmade the attacksof both femalesless aggressive and, if I happenedto arrive when they were away, the attacks upontheir return seemed less vigorous. In descendingorder, the aggressivenesstoward me of the breedingfemales I wasob- servingwas: Falkaklettur, Seljahjallagil, Dimmuborgir, and Hrossaborg.The eyrieswith the moreaggressive females were alsomore successfulin fledgingyoung. This observa- tion couldbe interpretedas supportingthe casefor a relationshipbetween female do- minanceand successin fledgingyoung. One mustassume that the aggressivenessof fe- males toward a human intruder was a measureof their tendencyto dominate their matesas well. It is alsoclear that too muchfemale aggressiveness will createan impos- siblebreeding situatior,: what is required is the proper aggressivebalance for a pair. Althoughthe femalesthat were the mostaggressive toward me were alsothe mostsuc- cessfulin raisingyoung, the secondmost aggressive female raisedwhat appearedto be the mosthealthy and vigorous young. The male of that pair wasalso considerably more aggressivetoward me than the malesat other eyries.The Falkakletturmale seemedto visitthe nestvery infrequentlyand cautiously,which may havebeen a signof his fear of the female.The whole issueof successin fledgingyoung becomes confusing because of the different locationsof the nests,the implied differencesin prey available,and the differencesin the huntingabilities of males.These factors may alsobe relatedto aggres- siveness,but not solelyto it. NestlingPeriod. Most of my informationconcerning the nestlingperiod comes from the Dimmuborgireyrie. After hatching,the male did all the hunting.Brooding by the malewas rare. Twice I sawhim beginto broodthe youngwhile the femaleleft to carryoff prey remains.Jen- kins(1974) reports brooding of the youngby the male while the femalefed. I sawonly the femalefeed the young.She always took someof the prey for herself, and on at leastone occasion (27 May), shefed herselffrom a carcassbefore bringing it to the chicks.Usually she fed herselfand the youngconcurrently, concentrating first on the young,then for a while on herself,then on the youngagain. Such feedings usually took 20 to 30 min. Wayre and Jolly (1958) report feedingtimes at a Myvatn eyrie of about16 min and statethat feedingby the male wasnot uncommon.After feeding,the remainsof the prey were sometimesremoved, usually in the feet. Twice the female left carryingsuch remains in her beak. Becauseof this removalI couldnot calculatethe total amountfed. I couldnot discerna regulardaily patternin feeding.Some days had manyand other days few feedings. The ceremonysurrounding the exchangeof prey betweenmale and femalehad be- comequite regular by the time of hatching.In the first weeksafter hatching,while the femalewas remainingnear the youngor in the immediatevicinity of the eyrie, prey exchangeoccurred near the eyrie,as it had duringincubation. A relativeinnovation was the commencementof wailingcalls by the femaleafter receivingprey, or after arrival of the male withoutprey. Sometimesshe stopped such calls by herselfand sometimes onlyafter the departureof the male,who duringincubation often stayed in the areafor an houror soafter bringingprey. In the mostremarkable instance (in this casehe had broughtin prey)she called continuously for 16 rain until he left the area.Jenkins (1974) 110 RAPTOR RESEARCH Vol. 14 No. 4 reportsthe cachingof uneatenremains about the nestingcliff. ThisI did not observe. During the first week after hatching,the female spentmost of her time brooding, leavingthe nestfor only shortperiods. By the secondweek, the singleremaining young hadgrown sufficiently to moveabout the nestand disturb the broodingfemale. She still spentmost of her time brooding,leaving only for shortperiods. During the secondweek I heardthe youngcalling during feeding periods. Begging consisted of shakingthe raised headstiffly back and forth andcalling. By the middleof the third week the chickat Dimmuborgirwas too big to broodcom- fortably,but the femalestill spentmost of her time standingover or nearit. Broodingat Falkakletturstopped during the third week. This eyrie had three young,however, and broodingprobably became physically impossible as well as unnecessaryearlier. Wayre andJolly (1958) report that broodingwas over by the fourthweek, but that the female remainedalmost constantly in the vicinity of the eyrie. During the third week at Dim- muborgirthe youngbegan to take a more active part in the feedingprocess and often, when the female held a bit of food before it, would move to take the food from her beak. By the middle of the fourth week the young at Falkakletturwere flapping their wings.Wayre and Jolly (1958)report the sameobservation. At Dimmuborgir,from my observationpost, quills became visible through the down duringthe fourth week, and the chick alsobegan preening and flappingits wings.Cade (1960) assumedyoung gyrs are 3 weeksold whenquills showed. The youngnow movedvigorously about the nest and called.The femalewas still spendingmost of her time in the nestnear the young but would leavethe nestfor periodsof more than an hour. Duringthe fifth week the Dimmuborgirfemale abandoned her positionin the nestfor one on the eyrie ledge,and by the end of the fifth week sheabandoned the ledgebut still remainedin the area.Her wailingcalls were becominghoarser, more like thoseof early March.On 27 June(nestling about 33 daysold), sheleft the eyrie areafor an hour and a half. Duringthe fifth weekthe chickbent towardthe prey asthe femalefed herself.Every now and then it managedto steala few bits from the carcassor from her beak.On 27 Juneit movedto the edgeof the ledgeto mute. The mute shotclear of the ledge.Jen- kins (1974) connectsthis practiceand the removalof prey remainswith the needfor nestsanitation-to minimize infection of youngby parasitesor disease.But removalof prey remainsseems to be practicedirregularly. Bengtson (1971) reports"heaps of re- mains"under some nests. I sawa similarcondition at Gyduhnjuksgil,where rotting re- mainswere scatteredon ledgesall over the cliff. It may be that aslong asthe remains are off the nestshelf the dangersof parasitebuildup are minimized;such removal tends to occurnaturally with older young,who knockthem off. Furthermore,adults presum- ably suffersome risk to themselvesin trying to removeremains from underthe feet of well-grownyoung. The removalof remainsby the adultsmight thenbe restrictedto the earlyweeks of the nestlingperiod. The accumulationsfound under nests would occurin later weeks.Removal of remainsfrom the eyrie areaby the gyrsearly in the nestling periodwould tend to biaslate-season prey collectionsagainst prey takenpreferentially early in the season. During the sixthweek the Dimmuborgiryoung began to feed itself, and the female apparentlybegan to hunt again. If the Dimmuborgirfemale beganto hunt between27 June and 2 July, shebegan Winter 1980 Woodin-Gyrfalcons 111 comparativelyearly. The gyrsat Falkakletturand Seljahjallagilbred slightlyearlier. At Seljahjallagil,the femalecould have beenhunting on both 13 and 19 July and almost certainlywas by the 25th. I do not think the femaleat Falkakletturwas hunting on 11 July,though I assumeshe had begunby 13 July, when the eyrie was deserted.Thus, allowingfor the differencein the breedingcycle, the femalesin the last two eyries startedhunting between 3 and4 weeks.Cade (1960)states that in the latter daysof the nestlingperiod both the male and femalehunt; Wayre andJolly (1958)found a Myvatn femalehunting during the fourthweek of the nestlingperiod. FledglingPeriod. Jenkins (1974) reports45-47 daysfor fledging.On 9 July the 8- week-oldyoung at Falkakletturwere flying. On 12 JulyI foundthe 7-week-oldyoung at Dimmuborgirdead on the slopebelow the eyrie. It had beenalive on 9 July, with the femalepresent. It was fully fledgedand weighed660 gm. The youngat Seljahjallagil, whichwere less than a weekolder, all weighedmore than 1000g on 13 July. The Falkaklettureyrie was empty when I visitedit on 13 July but showedsigns of someflirther use (juvenile down, prey remains)when I visitedit againon 7 August.The adultsbrought food to the youngat Seljahjallagilat least until 3 August(their 11th week).I did not visit this eyrie again until 19 August(the 13th week), when it was desertedexcept for a pair of scavengingravens. Bringing the prey to a centralfeeding place(first the eyrie ledge,then the slopebelow) ceased between 19 and21 July.From 21 Julyon, remainswere scatteredall overthe canyon.Cade (1960) mentions a broodof younggyrs that were flying on 10 July and still near the eyrie the secondweek of Au- gust.Bengtson (1971) states that Gyrfalconeyries at Myvatn are "abandoned"the last weekin Juneor first weekin July. I first sawyoung Gyrfalcons near the lake on 28 July (the 10th or 11th week for juve- niles)and after that foundsigns of themalong the eastshore. I did not seeadults in their company,but I had them under observationtoo briefly and too infrequentlyto know if they were still beingfed by adults.Eyries at Gaesadalurand Gyduhnjuksgil,which had both raisedyoung, were desertedwhen I foundthem on 17 and 12 August,respectively (the 12th or 13th week for the otherbroods). Generally,then, it seemsthat within 2 or 3 weeksof learningto fly the youngstart to followtheir parentsto the huntinggrounds. At thistime thesegrounds are the shoresof the lake,which from mid-Julyto mid-Augustare homefor thousandsof youngand vul- nerableducks, shorebirds, and gulls.The eyrie areasare now eitherabandoned or used only sporadically.Platt (1976) reportsobservations by Wiseley and Koslerof apparent "familygroups" of juvenileGyrfalcons still togetheron the "huntinggrounds" in Sep- tember.Whether the youngwere stillbeing fed by the adultswas not clear. Adultsappeared to ceasefeeding young toward the end of August.I spent24-27 Au- guston Hrisey,an islandoff northernIceland with a largebreeding population of Rock Ptarmigan.This would be the 14th week for the Myvatn juveniles.At this time, two juvenileGyrfalcons were apparentlysuccessfully feeding themselves on juvenileptarmi- ganthat they caughton the ground.The presenceof the younggyrs on thisisland in the middleof the fjord, 4 km from the mainlandand probablyat leasttwice that from their eyrie,strongly suggests that the youngfollowed the adultsthere. The productivityof 1967is shownin table 1. 112 RAPTOR RESEARCH Vol. 14 No. 4

TableI. Productivityof GyrfalconEyries near Myvatn in 1967.

Eyrie Eggs Eggshatched Oneweek 4-5 wks Fledged& flying

Dimmuborgir 4 4 4 i 0 Falkaklettur 4 ? ? 3 3 Seljadalur 0 Seljahjallagil ? ? ? 4 4 Dalfjall 0 Vindbelgjarfjall 0 Gaesadalur ? ? ? ? ?* Gyduhnjuksgil ? ? ? ? ?* Hrossaborg ? ? ? 3 0•

*Unknownnumber of youngraised to flyingage in eachcase. •Fouryoung were present on 19 June,three on 24 June(when the wingquills were poking through). All weredead in thenest on 26 June.The causeof deathwas probably starvation. Between 19 and 24 Junea photographersetup a blindabout 50 m fromthe nest; I dismantledthe blind on 24 June.By then one young was apparently dying, and the other two were quite lethargic. Neither adult appeared during the hour we spentthere. Thethree dead young are presently in thecollection of the Museumof NaturalHistory, Reykjavik. AgonisticBehavior Interspecific.Three predatorybirds-the CommonRaven, the Gyrfalcon,and the Merlin--areregular summer residents of the LakeMyvatn basin. The ravenand Gyrfal- con remainin the regionfor muchof the year; the Merlin is migratory.Except for a smallover-wintering population of ducks-primarilyBarrow's Goldeneye (Bucephala is- landica),Goosanders (Mergus merganser), and Mallards (Anas platyrhynchos)-the only preyavailable to Gyrfalconsduring the winterand early spring is ptarmigan.Ravens are bothpredators and scavengers. They probably scavenge prey remainsleft by the gyrs, but the importanceof suchremains in their diet is unknown.Judging from signsabout the remains,the ravens,at leastin winter,face considerable competition for themfrom Arctic foxes. The three predatorsoften nestnear each other. The situationin 1967 is detailedin the eyrie descriptionunder The Eyries.I did not make an exhaustivesearch for ravenor Merlinnests, but nestingassociations seemed to be the rule ratherthan the exception. The only nestsof either Merlinsor ravensof which I was aware,other than thosenear Gyrfalconeyries, were two suspectedeyries of Merlins.The frequentuse of ravennests by gyrsmight help explainthe gyr-ravenassociation,' but the Merlin-gyrassociation re- mainssomething of a mystery.There doesnot seemto be aiack of suitablegeological structuresfor nestsites for any of thesespecies in the Myvatn basin.However, various writershave reported finding Gyrfalcons nesting near other raptors.Hagen (1952) re- portseyries of the Gyrfalconand the Rough-leggedHawk (Buteolagopus) within 60 m of eachother. White andCade (1971) report nests of the Gyrfalconand Peregrine with- in 40 ydsand those of the Gyrfalconand ravenwithin 20 yds.In no casedid I find nests closerthan 200 m. White andCade (1971) suggest ravens use Gyrfalcons' cliffs in order to scavengethe gyrs'prey remains.I sawno scavengingnear the nestcliff itselfas long asthe Gyrfalconswere in residence. I sawtwo aggressiveencounters between Merlins and gyrs. On 17 May in Dimmubor- gir, themale gyr returned about a minuteafter leaving the eyriewith a Merlinstooping at him but not touchinghim. After two or threestoops the Merlin departed.It did not followthe gyr into the immediatevicinity of its eyrie.The gyr ignoredthe attackand soaredover the eyrieupon his return. Winter 1980 Woodin-Gyrfalcons 113

The other incidentoccurred at Vindbelgjarfjall,where the Merlinswere nestingand the gyrsnot. In thiscase, the male Merlin attackedone of the gyrsthat flew in right over its nest.The Merlin chasedthe gyr into the gyrs'usual perching area, then dis- lodgedeither it or anotherfrom a perch,and finally dove again and again at onewhile the othercircled nearby. Eventually the Merlin departed,and the gyrsreturned to the cliff. One of the gyrswas screamingat the beginningof the attack,but neitherat- temptedto attackthe Merlin. The one caseof clearcutcompetition for nest sitesoccurred at Seljadalur.During March and April a pair of ravensand a pair of gyrsshared the canyonin apparent peace.The gyrsused the valley(mostly, however, the westcliffs which had not been usedfor nestingby eitherspecies) for roostingand perching. The ravensstarted building a neston the eastcliffs (the traditional nesting cliffs) about I April. Asfar asI couldtell, thegyrs did not pick a nestsite, though I foundsome evidence of scrapingon a ledgeon the westcliffs on 27 April. A nesthere would have been right acrossthe narrowcanyon from and in full view of the raven nest. A nest in the former gyr site (old raven nest) wouldhave been level with, in sightof, and perhaps30 m from the new ravennest. The ravensbegan to broodabout 27 April. On 30 April, alertedby excitedcroakings from the ravens,I sawa gyr (probablya male)make a feint at a ravenperched on a ledgeof the eastcliffs. It did not actuallystrike the raven,both birds flew up, the raven got abovethe gyr and lungeddown, and both birds fell almostto the valleyfloor, turning roundand roundand apparentlygrappling. Then the gyr flew straightaway screaming with the ravenin pursuit.A few minuteslater the ravenreturned to the canyon. On 8 May I arrivedat 0500at Seljadalur.As on 11 April, the ravenswere then active. The gyrsbecame active about 0800. The gyrswere roosting in the canyon,and a ledge of the eastcliff aboveand upvalley of the ravennest showed much new useby gyrs.The ravensdid not follow me to this area of the cliff. No interactionoccurred this morning thoughboth species were present. I heardwhat soundedlike a quarrelon 13 May, per- hapsstimulated by a low-flyingairplane (a rareoccurrence which usually brought the gyrsinto the valley). The gyrscontinued to usethat ledgeat leastinto July. Interspecificcompetition may have prevented the Gyrfalconsin Seljadalurfrom nest- ing,but I doubtit. April 27 is at leasta weeklater thanany of the otherGyrfalcons in thearea began laying. Moreover, two otherpairs of gyrsnear the lakealso failed to lay. The Seljadalurgyrs were successfulin establishingthemselves on the east cliffs and seemedcontent there. Althoughcompetition with the ravensprovided yet another stress,I suspect the primarystress causing nonbreeding at Myvatnin 1967was a relative lack of prey. I sawone agonisticencounter between ravens and gyrsthat was apparentlyover huntinggrounds. On 19 MarchI wascoming across the lakeon the icetoward a shrubby hill whereI frequentlyfound either gyrs or ravens,when I sawa ravenand a gyr above the hill, eachof themcalling and circling. At first,the raven,who had the heightadvan- tage,dived at the gyr,then it ceaseddiving but kept its advantage.The gyr eventually brokeoff andflew downto perchon a rock,after whichthe ravendescended, but did not land.It wasjoined in the air by anotherraven. The ravensflew off together,and the gyr remainedperched. Dt•ringthe summer,gyrs that hunted from perches along the lakeshorewere struck at by passingArctic Terns (Sterna paradiseae). It seemedan almostcasual display of antag- onism.The Gyrfalconwould flinch but not fly, and the ternsnever seemed to gangup to drive it away. 114 RAPTOR RESEARCH Vol. 14 No. 4

lntraspecific.I observedno intraspecificquarrels over perchingposts or hunting grounds.In fact, therewas some indication that, at leastin Marchand April, hunting groundswere sharedby gyrsof differentpairs. The only spaceI saw defendedfrom othergyrs was that immediatelyabout the eyrie. On 16 Junethe male returnedto the Dimmuborgireyrie from a 2-hourabsence in an unusualfashion-slowly circling, both gliding and flapping, toward the eyriebutte. Both performingsuch a displayupon his returnand the form of the flight movementsthem- selveswere unusual.He wasnot high,passed over the eyrie andbeyond, then returned. The femalesaw him but did not respond.Suddenly she left the eyrie,and I sawa third gyr flyingoverhead on a paththat would take it rightover the eyrie(toward Seliahjalla- gil). Bothgyrs were on the interloperimmediately. First the femalegrappled at it from below,and then the male droppedon it from lessthan a meter,so that it had to flip overand present its talonsupwards. It wasattacked once more, then merelyclosely fol- lowed,until they were 200-300 m southof the eyrie, when the pair broke off and re- turned,gliding close together, then soaredseparately over the eyrie for about 10 min, keepingwithin a radiusof perhaps250 m. At thistime (2 weeksafter hatching) the pair wasat the peakof their aggressivenesstoward me. Earlierin the seasonthe incubating femalehad shownsome reaction (once even leaving the nest)to GreylagGeese (Anser anser)that passeddirectly overhead. Finally,at Vindbelgjarfjallon 26 April I watchedthe adultschase away (perhaps, "es- cort away")two gyrsin iuvenileplumage. I had been watchingthe cliff for 2 hours whenthe adultsappeared. One landedin view, and the seconddisappeared into a cut in the cliffsand began to screamexcitedly. At this point a iuvenileflew from behindme, headingtoward the cliff, and the adult reappearedfrom the cut, followedby another juvenile.The threegyrs on the wingflew screamingalong the face,the otheradult then ioinedthem, and all four flew out of sightaround the mountain.An adultsoon returned, circled,and disappearedinto the cut. The other adult appearedand perchedon the openface. In about2 min the iuvenilesreturned from the directionin whichthey had gone,one flying straight for that invisibleledge in the cut.Both adults then gave chase, sometimesdropping down toward a juvenilefrom just above it but for the mostpart just stayingnear the youngbirds. Once a youngbird flippedon itsback as though to grapple with an adult.An adultcircled back and began to land,when a iuvenileflew right over its head,not makingany movedown toward it. The adult flinched,I heardscreams, and the two flew away alongthe cliff. I heardmore screams after abouta minuteand sawa gyr circling,but then all was quiet.A searchof the part of the cut that had seemedso intriguingrevealed the freshcarcass of a ptarmiganon an inaccessibleledge. Thisoccasion was my only certainsighting of iuvenilesfrom the previousyear at My- vatn. Sharingof HuntingGrounds Severaltimes in March and early April, up until cock ptarmiganbegan settingup territories,I saw what were apparentlyGyrfalcons from differenteyries hunting the birchscrub northwest of Dimmuborgir,at distancesof from 2 to 5 km from the Dimmu- borgireyrie. Cade's (1960) schematic of the breedingterritories of Peregrinesalong the YukonRiver impliesthat the outerparts of the territoriesmay be sharedin thisspecies. WhetherGyrfalcons usually share, or do not defend,their home ranges,except for a limited area aboutthe eyrie, is unclear.The only territory I saw defendedfrom other gyrswas that immediatelyabout the eyrie. I did seea Gyrfalcondrive a pair of ravens Winter 1980 Woodin-Gyrfalcons 115 from what was a favoritehunting area for both species.The spotwas 4 km from the Dimmuborgireyrie (the closest). In late winter and early springptarmigan are virtually the only foodavailable to the MyvatnGyrfalcons. Since at thistime the ptarmiganfeed heavily on birch catkins,birch shrublandis the favoredhunting ground for the gyrs.Such shrubland is limited in extent, and the ptarmiganpopulation of a given piece of it seemsto fluctuatedaily. Thus a certainamount of pressureexists for sharinghunting areas. When aboutthe middleof April the cockptarmigan set up territoriesand spreadout over larger areas,this pres- surewould appear to be reduced.Such pressure might rise again in late June,over areas favoredfor the takingof ducks,gulls, or shorebirds.

Nest-Site Selection Cade(1960) states that 18 of the 21 Gyrfalconeyries he foundalong the ColvilleRiv- er were overhung.He thoughtthat the extensiveuse of old ravennests by gyrsin part explainsthe overhangs.Ravens, like gyrs,nest before melt-off and choosean overhung sitefor its protectionfrom the snow. To testthis hypothesis I noted the snowcover on severaloverhung and non-overhung formereyrie sitesduring March and April. The overhangsdid not seemto make much differenceto snowcover. In fact, the high windsfrequent to the area sometimesblew the exposedledges clear while driftingover the overhungsites. My chiefnon-overhung site,an old ravennest in Seljadalur,did not seemto accumulatemore than a few inches of snowat any time. Unfortunatelythese observations cannot be taken to mean that snowcover is not a factor in nest-siteselection by ravensand gyrs,nor that overhangsmay not, in many situations,prevent snow and ice from buildingup on a potentialnest site. However, especiallyamong ravens, the choiceof an overhungsite may have to do with something other than snow cover-concealment of the nest, for instance.

Hunting On 4 March I watchedGyrfalcons attempt two kills. The first resembledthe "tail chase"described by Cade (1960),White and Weeden (1966),and Bengtson(1971). It wassnowing, and I had pickeda spotto sit whereI had seenptarmigan the daybefore. After I hadbeen there about 15 min, a ptarmiganflushed, about 40 m away. Ten min later a ptarmiganflew overmy headwith a Gyrfalconabout a meterand a half behind it and gainingaltitude. The ptarmigandived into the scrub,whereupon the falcon turnedabruptly and plungeddown into the scrubalso. They were then about 60 m away.Five min later the gyr reappearedfrom the samespot without prey. Later, I saw a gyr flying over the scrubturn abruptly and dive down, seeminglyto the ground. Twenty seclater the gyr reappearedand flew away, whetherwith or without prey I could not tell. Theseincidents differed from what I sawwhen the ptarmiganwere displaying in that there is somesuggestion that the Gyrfalconsighted its prey while flying. In March I occasionallysaw gyrs flying low over the scrub.The ptarmiganalong their flight path either froze or flushed,in the latter caseexposing themselves to pursuit.However, I never sawa gyr strikein this situation.Similarly, ducks often flew rather than swam awayif theywere too closeto perchedfalcons. Flying is faster,but I wouldthink swim- mingis safer.However I neversaw a Gyrfalconstrike at flying ducksin this situation either. 116 RAPTOR RESEARCH Vol. 14 No. 4

The mostcommon method of huntingat Myvatn seemedto be the "sneakattack" from a perch.The gyr watchedfrom someadvantageous (not necessarilyvery spec- tacular)lookout, and then flew, droppingdown almost to the groundand moving with startlingrapidity iust over the groundor the scrubtoward an unsuspecting,nonflying prey.I sawthis method used once in Marchagainst ptarmigan and againin August againstshorebirds, but it was mostapparent during the displayperiod of the cock ptarmigan.A searchof a busymoor often revealeda Gyrfalcon,perched motionless, watchingthe activityof the noisycocks around it. By Juneit seemedas if everyother ptarmigandisplay post had a few feathersor a wingfrom a previousoccupant. Bengtson (1971)reports similar observations and states that the mostcommon hunting technique wasthe snatchingfrom the ground,or morerarely water, of an unsuspectingprey. BothWhite andWeeden (1966) and Cade (1960) mention a highflight by Gyrfalcons, whichthey connectwith huntingbehavior. Bengtson (1971) saw soaring frequently.. I observedsoaring in severaldifferent situations. Two hadto do with display.Soaring pre- cededmutual gliding at Seliadalurand followed an attackon an intrudinggyr at Dim- muborgir.On 13 Junethe femaleat Falkaklettursoared tmtil almostout of eyesight (about350 m up) andthen traced a greatcircle about the eyriein whatwas apparently eithera signalto or a searchfor the male,which had not comewith preyfor the 8 hours I had beenwatching. She returned in abouthalf an hour, overheated,without the male and withoutprey. The Seliahiallagilpair oftensoared at 200-300 m whenleaving the gil for the lakeshoreand vice versa,when returning with prey.It wasundoubtedly an easierway thandirect flight to crossthe 7-8 km of sandywaste that intervened.White and Cade(1971) suggest that Gyrfalconsmay go up to 10 milesfrom the nestto obtain prey. The restrictedlocation of availableducks, gulls, and shorebirdsin midsummerat Myvatnmakes commutes of somewhatlesser distances necessary for severalof the My- vatn eyries.I sawadults at Seliahiallagilreturning with prey from the directionof the lakeand leaving the lakeshorewith prey in the directionof the eyrie. Finally,the maleat Dimmuborgirsometimes soared, but not sohigh, and then went into a longglide when leaving the area.In short,I did not seesoaring, or soaringand glidingused by Gyrfalconsfor hunting,but onlyfor travellingand display. Treatmentof Prey FinnurGudmtmdsson (pers. comm.) suggested that one differencebetween the treat- mentof preyby Gyrfalconsand ravens is that,while both species pluck their prey, only Gyrfalconspluck the primaries.Hagen (1952) reported that in 31 Gyrfalconkills all had hadsome primaries plucked. Of the wingsof the approximately250 Gyrfalconkills that I examined,only onehad not had at leastone primaryplucked. The numberof pri- mariesplucked varied from I or 2 to virtuallyall. Mostof the carcassesI inspected were from eyrieswith young,and casual observation indicates that suchremains are handled muchmore extensively than those killed and eaten by adultselsewhere. But carcassesof Gyrfalconkills from the prehatchingperiod and from the earlyflying stage (when the amountof handlingseems to decrease)also had plucked primaries. I notedthree other characteristics of Gyrfalconkills. In smaller-bonedprey, such as ptarmigan,the headis almostalways eaten. It is apparentlybitten off from the back. The mandiblescan often be found,sometimes with enoughof the frontalskull attached to determinethe sexof the bird.Hagen (1952) reports not findingheads among the re- mains,the implicationbeing that theyare discardedor eaten.Secondly, a lengthof in- testineand sometimes the gizzardare removed and discarded. I found such lengths near Winter 1980 Woodin-Gyrfalcons 117 kills"in the field"and at eyriesafter the younghad fledged.Hagen (1952) reports that the"intestines (gizzards)" tend to be eaten,but he maybe simplysaying that theywere missingfrom the kills. In April after snowthaw I foundmany matsof discardedin- testinesalong the baseof the Dalfiall ridge,with both bird mutesand fox scatsnext to them.The mutescould have been from either Gyrfalcons or ravens.The intestinesap- pearedto be all that remainedof Gyrfalconkills that had been scavenged by ravensand foxes.The scavengers apparently were not interested in theintestines either. Finally, the Gyrfalcondoes a neatiob. The breastis cleanlystripped of flesh,as may also be the legs andwings. With smallerprey, the keel mayhave large bites taken out of it or be eaten completelydown. The raven'swork tends to be cruderin appearance. Preyof the Gyrfalcon In compilingprey remains (tables 2, 3, 4), I tookCade's (1960) approach to numerical estimation.I matchedright and left wingsby sexand speciesand then with sternaor

Table 2. Prey RemainsCollected from the FalkakletturEyrie in 1967.

6/2 ø 6/13' 7/3 7/9 7/11' 7/1•

Ptarmigan(Lagopus mutus) 34a 6 45b 15c 0A 3-4 Mallard(Anas platyrhynchos) Gadwall(Anas strepera) 1 Wigeon(A nas penelope) 3 i 1 Pintail (Anasacuta) Anas spp. 2 TuftedDuck (Aythya fuligula) 2 1 Scaup(Aythya marila) 1 1 Aythya spp. 1 Old Squaw(Clangula hyemalis) Barrow'sGoldeneye (Bucephalaislandica) 1 Red-breastedMerganser (Mergusserratot) CommonScoter (Melanitta nigra) Eh•ckspp. 2 4 1 Totalducks (%) 0 (0) 0 (0) 9 (16) 9 (35) 1 (10) 3 (38) Black-headed Gull (Larusridibundus) ih 3e Ig GoldenPlover (Charadrius apricari us ) 11• Whimbrel(Numenius phaeopus) 1 Redwing(Turdus iliacus) 11• Unknown 1 Totalother than ptarmigan (%) 0 (0) 0 (0) 12(21) 11 (42) 4 (40) 4 (50)

Estimateddate of hatchingMay 20-25. *Remainsin the eyrieand immediately about it not collected,or lost. •12old, 22 new. c33old, 4 new,8 uncertain. 10 old, 5 new. d6old. e f2 juvenile,1 of unknown age. •Eyriedeserted atleast 2 daysbefore. All the prey remains were fresh. X_Ageunknown. Juvenile. 118 RAPTOR RESEARCH Vol. 14 No. 4 otherbody parts present. Backbones, while usedas a roughcheck on the total numbers of prey collected,were, becauseof the difficultiesin identifyingthem, not associated with the restof a skeleton.It was oftenpossible to checksuch skeletal reconstructions by tryingthe fit of bonesinto correspondingsockets. Duckswere difficultto identify.A. Gardarssonassisted me in their identification.Pri- marieshad usuallybeen pluckedentirely exceptfor two or three; and we had to work with what few feathersremained and the generalsize and shapeof the skeleton.Unfor- tunately,the softsternal edges and keel, whose outlines are usefulin separatingthe duck species,were habituallybitten away. Featherscommonly left includedone or more of the following:upper and lowerleading wing edges;scattered primaries and secondaries; scapulars;coverts; or a tuft from the sideof the breast. The tablesdo not providea reliable chronologicalestimate of prey broughtto the eyriesbecause of the removalof remainsfrom the nestvicinity by adults.Such removal was rather irregular.At Dimmuborgirit becamemore frequentafter hatching.Some

Table 3. PreyRemains Collected from the DimmuborgirEyrie in 1967.

6/3 6/5 6/16 6/27 7/3 7/1•

Ptarmigan(Lagopus mutus) 19 3 6 4 5 4 Mallard(Anas platyrhynchos) Gadwall(Anas strepera) Wigcon(Anas penelope) 2 9 3 3 Pintail (Anasacuta) Anas spp. 1 Tufted Duck (Aythyafuligula) 2 Scaup(Aythya marila) Aythyaspp. 1 Old Squaw(Clangula hyemalis) Barrow'sGoldeneye (Bucephalaislandica) Red-breastedMerganser (Mergusserrator) CommonScoter (Melanitta nigra) Duck spp. Total ducks(%) 0 (0) 0 (0) 2 (25) 11 (73) 4 (44) 8 (67) Black-headed Gull (Larusridibundus) GoldenPlover (Charadrius apricarius) Whimbrel(Numenius phaeopus) Redwing( Turdus iliacus ) Unknown i 1c Total otherthan ptarmigan(%) i (5) I (5) 2 (25) 11 (73) 4 (44) 8 (67)

Date of hatching,May 25. Thenest had been abandoned atleast one day before. One of the duckswas a duckling. CA•mall, blue, maggot-infested duckfoot. The unknown on3 Juneconsisted ofa matched radius and ulna. The foot appears toantedate this collection,and the two remainsmay be fromthe samebird. Winter 1980 Woodin-Gyrfalcons 119 remainswere droppedwithin 100 m of the eyrie, otherscarried further off (seeWeir 1967).The moredistant remains were probablyquickly scavenged by ravensor Arctic foxes. Collectionproblems at the Falkakletturand Seljahjallagileyries included the appear- anceof old ptarmiganremains along with the new onesin my weeklyor fortnightly collections-apparentlybecause I neverfound all the remainsat any given time. The aggressivenessof the femalesat theseeyries made collectiondifficult. Both siteshad beenused in the recentpast, and the slopesbeneath were speckledwith bits of remains. Remainsof smallerprey get broken badly by the youngif theyremain in the eyrieover any periodof time. They dry out rapidlyand blow about:some undoubtedly get lost. Old remainsmay get dug out of the mud of the eyrie shelf.Since I foundit difficult to be absolutelysure about what was old andwhat new,I composedthe tablesaccording to whatwas picked up with the "old" and "new" designationsin footnotes. With thesequalifications, the data in the tablessuggest that there was a shift in the prey taken, from ptarmiganto other species,that occurredfrom the secondweek of Juneonward. Prior to 16 Junethe only remainsI foundat Dimmuborgir,with two ex- ceptions,were of ptarmigan.On 16 JuneI foundremains of two Widgeon(Anas pene- lope)along with sixptarmigan, and on 27 June,nine Widgeon,two unidentifiedducks, andfour ptarmigan.At FalkakletturI foundthe first remainsother than ptarmiganon 3 July.Thus at Dimmuborgirthe shift beganbetween 5 and 16 Juneand at Falkaklettur between13 Juneand 3 July.The Falkaklettureyrie wassituated in an areawhere prey otherthan ptarmigan was either scarce or a considerabledistance away. The shiftin prey can probablybe correlatedwith changesin the behaviorof the cock ptarmigan.When I arrived at Myvatn in early March, the chief prey of the Gyrfalcons appearedto be ptarmigan.At the sametime, duckswere presentin somenumbers. A counton 3 March along4 km of the lake (includingmost of the lake that remainsopen duringwinter) gave 170 Barrow'sGoldeneye (117 males,53 females),about 100 Goosan- ders(Mergus merganser) (sexes equally divided), about 45 Mallards(Anas platyrhynchos) (30%females), along with 37 Whooper Swans(Cygnus cygnus). The ducksdid not ap- pear to be takenin any numbers.Migratory waterfowl begin arriving in mid-April. By May they are presentin force. About the middleof April ptarmigancocks begin to set up territories.For the next 2 monthspredation seemed to fall heavilyon them. The cockskeep their white plumage and preoccupationwith territorialdefense into early June.Thus they remainvulnerable to Gyrfalconpredation through the beginningof that month. Signsof Gyrfalconpre- dationon ptarmigancocks in the form of remainson (reoccupiedor unoccupied)display postsare commonduring the displayperiod. In June,when the ptarmiganegg clutchesare complete,the cocksacquire summer plumageand becomemuch lessterritorial and more skulkingin behavior.There is still someerratic display,especially at twilight, and sinceGyrfalcons hunt at night in sum- mer, the cocksremain still somewhatvulnerable. In the Myvatn area this midnightdis- play activity continuedinto July. For the Gyrfalcons,the changein the behaviorof the cockscomes at a pivotal time- a week or two after the younghave hatchedand need a large and regularsupply of food.The male, which ordinarilydoes all the huntingduring the first part of the nest- ling stage,must now huntfor the equivalentof three adultsor more.Thus he hasa pow- erful stimulusto break whatever"fix" he hason ptarmiganand to turn to other species. 120 RAPTOR RESEARCH Vol. 14 No. 4

Table 4. PreyRemains Collected from the SeljahjallagilEyrie in 1967.

6/30 7/7 7/13 7/19

Ptarmigan(Lagopus mutus) 11 4 17a 17• Mallard(Anas platyrhynchos) 1 3 Gadwall(Anas strepera) 1 Wigcon(Anas penelope) 3 2 4 2 Pintail (Anasacuta) 1 Anas spp. 6 1 TuftedDuck (Aythyafuligula) 7 1 3 Scaup(Aythya marila) 4 3 1 2 A ythya spp. 5 2 2 Old Squaw(Clangula hyemalis) 2 1 1 Barrow'sGoldeneye (Bucephalaislandica) Red-breastedMerganser (Mergusserrator) 1 CommonScoter (Melanitta nigra) 1 Duck spp. 7 11 7 1e Total ducks(%) 29 (73) 22 (81) 24 (53) 13 (34) Black-headed Gull (Larusridibundus) 1 (juv) 4e 8 GoldenPlover (Charadrius apricarius) Whimbrel(Numenius phaeopus) Redwing(Turdus iliacus) Unknown Total otherthan ptarmigan(%) 29 (73) 9.3(s5) 9.s(69.) 9.1(55)

Estimateddate of hatchingMay 20-25.

a b10old, 7 new. c15old, 2 new. d 1 juvenile' 3 unknownage ' Ageunknown. eDncklingofdiving duck.

The completenessof the shift probablydepends on the availabilityof thesespecies to individualGyrfalcons. Since habit entersin, and chancefalcon-ptarmigan encounters occur,one would not expecta completeshift in mostcases. As mentionedabove, Gud- mundssonand Gardarssonfound that predationby Gyrfalconson the nestingptarmigan of Hrisey virtually ceasesin early June.Hrisey is about4 km from the nearestpoint on the mainlandand likely at leasttwice that from the eyrie from which they surmisedthe Gyrfalconsare coming.In sucha situation,where Gyrfalconscome from a considerable distanceto hunt an islandalmost exclusively for ptarmigan,and where "reservoirspe- cies"exist elsewhere, the shiftmight appearmuch more complete.An investigatorcol- lectingremains from the eyrie of thosesame Gyrfalcons might continuefinding ptarmi- ganthat werepicked up closerto the eyrie. Winter 1980 Woodin--Gyrfalcons 121

A corollaryof the shiftin prey seemsto be that the presenceof one or more "reser- voir species"is helpfulfor the successfulraising of youngby Gyrfalconswhose primary preyis ptarmigan,especially in yearsof marginalptarmigan populations, where perhaps ptarmiganare abundantenough to inducethe Gyrfalconsto breed,but not abundant enoughto ensuresurvival of the young.Cade (1960) and White and Cade (1971) de- scribesituations somewhat like this along the Colville River in Alaska.The Colville Gyrfalconsprey almostexclusively on ptarmigan;and their breedingsuccess, or whether they will breed at all, fluctuateswith the ptarmiganpopulation. In 1959 Cade (1960) observeda shift to rodentsand rodent-eatingbirds about the middle of June. He ex- plainedthe shift by the concurrentdeparture of a high late-winterptarmigan popu- lation,which had induceda recordnumber of Gyrfalconsto breed,and the snowmelt- off that exposeda peak rodentpopulation. The rodentpopulation attracted many non- breeding,rodent-eating birds that were highly vulnerableto Gyrfalcon predation. Cade'sexplanation is ratherspeculative, and it is difficult to assesswhether the shift in prey wasmotivated by a specialsituation or whetherpressure exists for it annually. Hagen (1952) describesan apparentlymarginal breeding situationfor ptarmigan- dependentgyrs in the Norwegianmountains, in which successfulbreeding in a certain valleycoincided with peak rodentyears. Rodents were not importantin the diet of the NorwegianGyrfalcons. Hagen speculates that the coincidencehas three causes:(1) Less pressurefrom other predatorsmakes Willow Ptarmigan(Lagopus lagopus) live more openlyin peak rodentyears (an observedfact); (2) competitionbetween carnivores is reducedin high microtine-rodentyears; and (3) the two Lagopusspecies that form the bulk of the Gyrfalcons'prey tend to fluctuatewith the microtine-rodentcycles and are moreabundant in yearsof highmicrotine population. Thus, in a situationwhere a satis- factoryreservoir species for the gyrsapparently does not exist,other factorstend to cre- ate the sameeffect in certainyears. Bengtson(1971) reports data concerninga prey shift in Myvatn Gyrfalcons,in the form of preybeing transported by flyingGyrfalcons. For ducksbeing transported in the monthsMay throughAugust, the numbersare 15, 10, 20, 20; for RockPtarmigan in the samemonths, the numbersare 9, 8, 3, 1. ThusBengtson observed an increasein ducks being carried in July and August,and a decreasein Rock Ptarmiganduring those months.Bengtson's studies focused on the waterfowlpopulation, and there is no way of knowinghow randomhis samplewas. It may be biasedtoward ducks.He certainlyre- cordsrelatively more ducks being taken in May andJune than I observed.However, this situationmay vary from eyrie to eyrie and year to year. For instance,the Vindbelgjarf- jail eyrie,unproductive in 1967,is locatedamidst some of the bestwaterfowl habitat at Myvatn.Parts of their huntinggrounds for ptarmiganare alsogood waterfowl habitat. It wouldbe surprisingif thispair did not take waterfowlin substantialnumbers early in the season. My data suggestthe tendencytoward specialization at the three eyrieson Widgeon, Tufted Duck (Aythyafuligula), and Scaup(Aythya marila). Much has been writ.•en aboutthe Gyrfalcon'stendency to developa "fix" on prey, and thesedata perhapssup- port the notion.There is somequestion in my mindabout whether such "fixes" are the restiltof prey behavioror prey presence.The three ducksin questionare alsothe three mostcommon species at Myvatn,according to Bengtson(1971). Even moreto the point, as I have tried to show,a Gyrfalconhunting ptarmigan in May facesquite a different situationfrom onehunting them in March. 122 RAPTOR RESEARCH Vol. 14 No. 4

Distributionof Nests If oneignores the eyrieat Seljadalur,all the eyriesto the eastof the lake were spaced between9 and 11 km apart.This distribution suggested that perhapsthe gyrsat Seljada- lur andDalfiall failedto breedbecause their potentialnests were "psychologically"too closetogether. If the gyrsat Holkotsgilwere not the sameas thoseat Vindbelgiarf•all, thishypothesis would alsoexplain the nonbreedingat Vindbelgiarfiall.However, I was not ableto confirmthe statusof the Holkotsgilgyrs. At any rate, thishypothesis did not explainwhy the Gaesadalurbirds bred. Sel•adaluris about 5.5 km from both Dalfiall and Gaesadalur.If this distancewere below the minimumterritorial limit for IcelandicGyrfalcons, one would expectto see at least someagonistic behavior, and I did not. In Alaska,Gyrfalcons have nestedas closeas 3.2 km, thoughthe averageis about 17 km (White & Cade 1971). In western Greenlandthe mean minimumdistance between 7 siteswas 7.8 km (Burnham1975). ErnestVesey found three casesof pairsof Gyrfalconsnesting within 4 or 5 km of each otherin the westernfiords of Icelandin 1936.All thesesets of pairslaid eggs,one of the setsof pairsraised young, and at leastone pair in the other two setsraised young. The fact that noneof Vesey'snests were closerthan 4 or 5 km, combinedwith (a) my finding of pairsresiding at minimumdistances of about5.5 km, plus (b) the discoveryof what appearedto be alternatesites or areaswithin 3 or fewer km of currentlyoccupied sites, leadsme to concludethat the minimum"psychological" distance between nesting pairs of Gyrfalconsin Icelandis between3 and 5 km. The mostlikely explanationfor the nonbreedingat Myvatn in 1967 is probablythe classicalone. Ptarmiganpopulations over the whole of Iceland follow a well-defined ten-yearpopulation cycle (Gudmundsson1960). Bengtson(1971) reporteddensities of breedingptarmigan for the years1961-1966 increasing from 1.7 to 7.4 per km vatn. Accordingto A. Gardarsson(pers. comm.) and Bengtson(1971), 1966 was a peak ptarmiganyear. Presumablythe populationof survivingGyrfalcons in the springof 1967was at a peak,and the occupationof potentialbreeding sites at Myvatnwas also at a peak. (Thisis one of the considerationsthat led me to defineunoccupied, previously usednests 3 km or lessfrom occupiednests as "alternativeeyrie areas."If they were not "too close"to occupiednests, some should be occupiedin a peak Gyrfalconyear.) How- ever,the ptarmiganpopulation had fallen in the springof 1967 from the springof 1966. Thusa relativelack of prey was probablythe primary stresscausing nonbreeding by Gyrfalconsat Myvatnin 1967.The tendencyof the closerresident pairs, with (oneas- sumes)smaller home ranges, not to breedwould appear compatible with thishypothesis. At the sametime the pair at Sel•adalurhad the additionalstress of competingwith a pair of ravens,and the pair at Vindbelgiarfiallmay have been disturbed by the presence of immatures. My interpretationof eyrie areasat Myvatn is rather speculative.Observation over an entire 10-yearpopulation cycle of the ptarmiganwould be the only way of clarifying the situation.The statusof the Seljadalureyrie especiallyis problematic.Only observa- tion over a periodof yearswould determinewhether it shouldbe consideredan inde- pendentarea or an alternateto Gaesadalur.I would attach Seliadalurto Gaesadalur ratherthan to Dalfiall becausethe Dalfjall ridge containedseveral alternate sites while I onlyfound one each in Gaesadalurand Seliadalur. Winter 1980 Woodin-Gyrfalcons 123

Literature Cited Beebe, F. L. 1960. The marine peregrinesof the northwest Pacific coast. Condor 62(3):145-189. Bengtson,S.-A. 1971. Huntingmethods and choiceof prey of Gyrfalcons(Falco rustic- olus)at Myvatn in northeastIceland. Ibis 113:468-476. Brown,L., and Amadon,D. 1968.Eagles, hawks and falconsof the world, vols. 1, 2. New York: McGraw-Hill. Burnham,W. A. 1975.Breeding biology and ecology of the PeregrineFalcon (Falco per- egrinus)in West Greenland.Master's thesis, Brigham Young University, Provo, Utah. Cade,T. J. 1960.Ecology of the peregrineand Gyrfalconpopulations in Alaska.Univ. Calif. Publ.Zool. 63:151-290. Cade,T. J., andWeaver, J. D. 1976.Gyrfalcon-peregrine hybrids produced by artificial insemination. ]. N. Amer. Falconers' Assoc. 15:42-47. Cramp, S., and K. E. L. Simmons(eds.). 1980. The birds of the westernPalearctic. Ox- ford UniversityPress, vol. 2. Dementiew,G. P. 1960.Der Gerfalke.Franckh'sche Verlagshandlung--Kosmos Verlag, Stuttgart. Gudmundsson,F. 1960.Some reflections on Ptarmigancycles in Iceland.Proc. Int. ¸rn. Congr.12:259-265. Hagen,Y. 1952.The Gyr-falconin Dovre, Norway. Skr.Norske Vidensk Akad. I. Math.- Nat. K1. No. 4, 1952:1-37. Jenkins,M. A. 1974.Behavior of the Gyrfalcon(Falco rusticolus) from hatchingto fledg- ing: a time-lapsephotographic study. Master's thesis. Brigham Young University, Provo,Utah. 83pp. Lewis,E. (pseudonymof E. Vesey).1936. In searchof the Gyrfalcon.London. Platt, J. B. 1976. Gyrfalconnest site selectionand winter activity in the westernCana- dianarctic. Can. Field-Naturalist90(3):338-345. Snyder,N. F. R., and J. W. Wiley. 1976. Sexualsize dimorphism in Hawksand Owls of North America.A.¸.U. Monograph#20. Storer,R. W. 1966.Sexual dimorphism and foodhabits in threeNorth Americanaccipi- ters. Auk 83:423-436. Sutton,G. M. 1961.Iceland summer: adventures of a bird painter.University of Okla- homa Press,Norman. Wayre, P., and Jolly,G. F. 1958.Notes on the breedingof the IcelandGyr-falcon. Brit. Birds 51:285-290. Weir, D. N. 1967.Possible source of error in raptorfood analysis. Bird Study14:194. White, C. M., and Cade,C. J. 1971. Cliff-nestingraptors and ravensalong the Colville Riverin arcticAlaska. Living Bird 10:107-150. White, C. M., andWeeden, R. B. 1966.Hunting methods of Gyrfalconsand behavior of their prey (Ptarmigan).Condor 68(5):517-519. 124 RAPTOR RESEARCH Vol. 14 No. 4

Figure3. Viewfrom the lip ofDimmuborgir into the depression. The photo does not give an adequate idea of thedepth or the extensiveness of Dimmuborgir. In the distance some of thehills that form the eastern bound- aryof the Myvatn basin. In theforeground low bushes of birch (Betula pubescens). May 1967.

Figure4. Viewof Hrossaborg from the road across the Burfellshraun. The Gyrfalcon eyrie is on the opposite side.Note the sparsely vegetated ".sandy waste." In thedistance are the monntains of theCentral Highlands of Iceland.June 1967.