Hunting Range and Strategies in a Tundra Breeding Peregrine and Gyrfalcon Observed from a Helicopter

THE JOURNAL OF RAPTOR RESEARCH A QUARTERLY PUBLICATION OF THE RAPTOR RESEARCH FOUNDATION, INC.

VOL. 25 FALL 1991 NO. 3

J. Raptor Res. 25(3):49-62 ¸ 1991 The Raptor ResearchFoundation, Inc.

HUNTING RANGE AND STRATEGIES IN A TUNDRA BREEDING PEREGRINE AND GYRFALCON OBSERVED FROM A HELICOPTER

CLAYTON M. WHITE Departmentof Zoology,Brigham YoungUniversity, Provo, UT 84602

R. WAYNE NELSON 4218-65rd Street, Camrose,AB, Canada T4V 2W2

ABSTRACT.--Between6-15 June 1974 an adult male PeregrineFalcon (Falcoperegrinus) from a breeding site on Alaska's North Slope was followed by helicopter and mapped during 21 hunting sorties.His huntingrange was about319 km2 and the greateststraight line distancefrom the eyrie where he stooped at prey was 14.6 km. All parts of the hunting range were not usedequally. An area with sedge-grass marshesand many small lakes was hunted most frequently with 51% of the prey capture attempts occurringthere. A secondarea of similar but drier habitat was usedsecondly with 36% of prey capture attemptsoccurring there. Three different hunting methodswere used:1) low, "ground-effect"flight, 2) high direct/maneuveringflight, and 3) soaring. One flight of long duration and one of short duration from start to finish are described.Seventeen hunts timed from leaving to returning to the eyrie averaged 57 min. The shortesthunting sortie followed by helicopterwas 22 min; when not followed, the shortest sortiewas 23 min. From 2-24 stoopswere required during a hunting foray beforea kill was made. Between27 June and 4 July 1978 near the DempsterHighway, Yukon Territory, Canada, a nesting femaleGyrfalcon (F. rusticotus)was followedby helicopter(on 9 flights)for a total of 4 hr 1 min and a male (on 5 flights) for 3 hr 10 min. The female hunted and patrolledwithin 3.2 km of the eyrie. The male went up to 24 km from the eyrie while hunting. On 1 sortiehe "patrolled" a three-quartercircle, travelinghigh approximatelymid-way betweenhis eyrie and thoseof four neighboringpairs. Hunting methodsare describedand includedsoaring, ridge-sailing, and "forest slalom." We also discussedcautions and advantagesof using the helicopterto study home ranges,habitat use and behavior far from the nest.

Areay estrategiasde caza de un Halc6nPeregrino y Gerifaltesde la tundraobservadas desde un helic6ptero EXT•½TO.--Entre el 6 y el 15 de junio de 1974, seguimosen helic6pteroa un ejemplarmacho adulto de Halc6n Peregrino(Falco peregrinus) desde su nido en la vertientenorte de Alaska, y describimossu recorridodurante 21 excursionesde caza. Su firea de acci6ncubri6 319 kms2. La mayor distanciaentre su nido y el puntoen que selanz6 en picadapara atrapar a la presaalcanz6 14.6 kms.E1 halc6nno us6 indistintamentetodas las seccionesdel firea,sino que mostr6preferencia por un fireapantanosa y lacustre. Un 51% de suslances ocurrieron en el fireamencionada. La segundapreferencia result6 ser un firea con un hfibitatsimilar, aunquemils seco,escenario de un 36% de los lances.E1 halc6nutiliz6 tres mgtodos diferentesde caza:1) vuelorasante; 2) vuelo,maniobras y suspensi6nen altura; y 3) planeode altura. Se describendos ruelos completos, uno de larga y uno de corta duraci6n.E1 tiempode duraci6nde 17 excursiones,desde la salidahasta la 11egada,promedi6 los 57 minutos.Las salidasque seguimoscon el helic6pteroduraron un mlnimo de 22 minutos;las salidasque hizo solo, duraron un minimo de 23 minutos.Durante las excursiones,el halc6nse lanz6 en picadaentre 2 y 24 veceshasta matar una presa. Entre el 27 dejunio y el 4 dejulio de 1978, en las proximidadesde la carreteraDempster (Territorio de Yuk6n) seguimosen helic6pteroa una hembra de Gerifalte (F. rusticolus)que estabaincubando. La seguimosen 9 ruelos por un total de 4 horas 1 minuto, y a un macholo seguimosen 5 ruelos por

49 50 CLAYTON M. WHITE AND R. WAYNE NELSON VOL. 25, No. 3

3 horas10 minutos.La hembracaz6 y patrul16en un radio de 3.2 kms.desde el nido. E1 machose alej6, en suscacerlas, hasta 24 kms.del nido.En una excursi6nrealiz6 un "patrullajede altura" recorriendo trescuartas partes de un clrculoentre su propionido y losde 4 vecinos.Se induyeny sedescriben tres m•todosde caza:planeo de altura,aprovechamiento decorriente de aire ascendente, y vuelo ziõzaõueante entre los rirboles('slalom'). Tambi•n secomentan las ventajas y desventajasde usarun helic6pteropara estudiarlos territorios, el usode hribitaty el comportamientode los halconescuando estrin lejos del nido. [Traducci6nde Hugo N. Olaiz]

There are few publisheddata on huntingranges Gyrfalconstudy (by R.W.N.) soughtinformation on of the PeregrineFalcon (Falco peregrinus), other than huntingmethods and rangesrelative to neighboring someinformative radio tracking studies such as those eyries,while gatheringpopulation, reproductive, and of Endersonand Kirven (1983), and Mearns (1985). behavioraldata for a subsequentlarger study by N. For the Gyrfalcon (Falco rusticolus)there are no Barichello.White was at first skepticalthat a heli- comparablepublished data. Much of the information coptercould be usedto follow huntingperegrine but on hunting techniquesused by wild falconscomes the techniquesthat evolvedproved to be perhapsthe from observationson the outcomeof specificstoops mostsuitable available for this type of studyat the fortuitouslyobserved from the ground(e.g., Rude- time. White's successwith peregrinesstimulated beck 1951, Campbell 1975, and Dekker 1987) or Nelsonto attemptsimilar observationson Gyrfal- from observationof nestingfalcons that huntedwith- conswhen that opportunityarose. in sight of nestingcliffs (e.g., Monneret 1973, Bird STUDY AREA AND METHODS and Aubry 1982). Becauseour studysites were se- In thispaper a hunt, forayor sortieis definedas a flight lectedon the basisof eyrie locationrelative to in- in which prey captureappeared to be the main objective. tensityof human activities(peregrine) and relative A huntingforay may includeany numberof stoopsor to neighboringeyries (Gyrfalcons)and becauseit capture attemptsof prey. A stoopis any dive during a was necessaryto know the travelsand spaceuse by huntingforay that appearsto be directedtoward a prey thesewide-ranging falcons, the logisticsand prob- item. A chaseis the following,in directflapping flight, of a prey item with apparent intent to capture. lems of using radiotelemetryor ground-basedob- Peregrine Falcons. Peregrineobservations were made servationsprecluded those methods. This paper re- on the SagavanirktokRiver on Alaska'sNorth Slopefrom portson the first successfulattempts to studyhunting 6-15 June 1974, variouslybetween 0600-2300 H, at one behaviorand use of homerange of two raptorsby eyrie ca. 3.2 km from an Alaskan pipeline construction followingthem in a helicopterat closequarters from campof about200 persons.A Bell 206B Jet Rangerhel- icopterwas usedto followthe falcon.Originally the hel- the birds' leavingtheir eyrie'sto their return. icopterwas parked below the eyrieand some180 m distartt. We recognizethat the singlepair of both species The male falcon,doing all the huntingfor the femaleand watchedmay not berepresentative of their respective recentlyhatched young, was watched with binocularsfrom speciesin tundra habitat. We have tried not to draw the waiting helicopteruntil he left the cliff to hunt. The helicopterwas then started.Because of a warm-up time too many generalconclusions from our observations. for the aircraft, the falconusually was out of sight and Becauseemotion regarding raptors frequently runs unlocatableby the time we were airborne.We then tried high, a good deal of prudenceand commonsense waitingon top of the cliff and some180 m away hoping to be ableto keeptrack of him as he flew away.This was must be used when studyingthem (cf. Fyfe and also unsuccessful. Olendorff1976). Althoughhelicopters are routinely Upon arrival at the cliff on 7 June after a refueling usedto locateraptor eyries,use of helicoptersnear stop,we interceptedthe falconas he left to hunt. It became eyriesis still controversial,especially around rarer evidentthat onceboth helicopterand falconwere in the species.Our studiesprovide further information on air it was relativelyeasy to follow him at a distanceof the responsesof 2 raptor speciesto "non-threaten- 150 m as long as he was silhouettedabove the horizon. At that distance, however, he was lost to view as he made ing" helicoptersnearby. his secondstoop. In general,once he was lostto visionand The peregrinestudy (by C.M.W.) was accom- if his directionof flight was unknown,he was impossible plished during the constructionof the Alaska oil to relocateexcept by chance.Because of this problemwe pipeline on Alaska'sNorth Slope.The study was adjusted our distance from the airborne falcon until it became dear that he could be followed from as dose as designedby the U.S. Fish and Wildlife Serviceand 45 m. oil companypersonnel to determine effectsof oil While waitingbelow the eyriewe timedthe male'srest developmenton huntingranges of the peregrine.The intervalsbetween hunting sorties,which were about 1.5 FALL 1991 FOLLOWING FALCONS WITH HELICOPTERS 51 hr. By waiting for this time periodand then flying slowly went unrecorded.Flying the helicopterupward 200-300 past the roostingmale, we found we couldflush him into m from the falcon,and then slowly approachingit hori- the air, veer away to let him start a hunting trip, and then zontally, causedneither attack nor avoidance. closethe distanceand follow him through the entire hunt. Usually we followed 30-50 m behind with the falcon It requirespractice and skill on the part of the helicopter at the 1000-1030 H position.If we were directlybehind, pilot to "stoop"along with the falcon,keep it in view and the falconoften glanced back over its wing, indicatingsome at the sametime not put the helicopterinto the ground; concern.A positionslightly below the falcon allowed for we were successfulin following the stoop from start to easier viewing of the bird above the horizon. When the finish and the hunt from start to finish in most cases. bird soared,we often followed it on the oppositeside of The sizeof the homerange was determinedby plotting the circle or hoveredjust outsideits soaringcircle. Flight movements of the male on a 1:63 360 scale (contour in- routesof the falconswere plotted on a 1:250 000 National terval 15 m) U.S. GeologicalSurvey map. Stoopshe ex- TopographicSeries map (150 m contours)and detailed ecutedwere marked on the map. Drawing lines around notesmade during the flights. the hunting range is at best approximate,and we chose to use the boundarylines of the 1 mi2 (2.5 km2) sections RESULTS AND DISCUSSION or half sectionsdiagonally on the map. Locationsof the dotsindicate the nature and extent of the hunting range. Peregrine Hunting Range: Size. The hunting Events were timed with the helicopter clock and notes range was approximately 319.8 km2 basedon 19 hr were madeas events occurred. Air speedsgiven were those 20 min that the male was followed (Fig. 1). The registeredon the helicopterair speedindicator. greateststraight line distancefrom the eyrie where Gyrfalcons. One pair, caringfor well-featherednest- lingsand thenjust-flying fledglings, was studiedabout 10 a stoopwas made was 14.6 km to the SE. Actual km from the DempsterHighway, Yukon Territory, Can- size of the hunting or home range has not been ada. A Bell 206A Jet Ranger helicopterwas used. Ob- determinedfor other pairs of tundra inhabitingper- servations were made between 0900-1800 H, on 6 d from egrines,so far as we are aware. Even in relatively 27 June to 4 July 1978. Helicopter-sharingand logistics precludedobservations (a) early in the day, (b) for long uniform tundra habitat, size of the hunting range durationsduring any oneday, and (c) earlierin the nestling may vary considerablyfrom individual to individual phase.We followedthe falconsfor just over7 hr but spent as shown for the Prairie Falcon (F. mexicanus;U.S. 29.6 hr on the ground observingthe nest and environs, Dept. Interior 1979) in a relatively homogeneous awaiting the movementof the adult female off the cliff or sagebrushhabitat. the arrival backof eitheradult. There wasno predictability to the timing of the hunts of thesefalcons. Presence of the Informationon homerange size of peregrineselse- male usually was fleeting.He quickly deliveredprey to where suggestssimilar extreme distancestraveled the female or nestlingsand immediatelyleft the vicinity from the eyrie and somehave rangesa fraction of of the eyrie. Sometimeswe waited up to 5 hr before an the size of the Alaskan male. A male and female in adult becamevisible and available for "shadowing."We California were radio-tracked to about 8 km from were aware of White's successwith waiting 1.5 hr before "bumping" the male peregrineinto flight, and then fol- their eyrie while a female in Coloradowas tracked lowing on a hunt. This proceduredid not work with Gyr- asfar as 19 km from her eyrie (Endersonand Kirven falcons.The male usually did not perch near the cliff nor 1983). Of two telemetered females in Scotland fol- beginhis huntsfrom there, and the female,if "bumped," lowedlate in the breedingseason, one ranged as far usually simply flew to anotherperch. When we arrivednear the nestcIiff we flew slowly as 18 km from her eyrie, and the two femaleshad along a ridge running west from the cliff, attempting to maximum hunting rangesof 23 km2 and 117 km2, find a falcon "ridge sailing," whereuponwe followed the respectively(Mearns 1985). Also, in Scotland,Weir bird; or after finding oneperched on a prominentrock. If (1978, notusing telemetry) found no evidence of prey no bird was located,we parked the helicopteron a "bench" slightlybelow the nestelevation and about 300 m in front beingtaken more than 6 km from an eyrie. In that of the cliff, to await an opportunityto follow a bird. After studyabout 60%, by weight, of the foodwas grouse someearly frustrationsat losingsight of adult birdswhile (Lagopussp.). In Utah, peregrineshave traveled 19- the helicopterwarmed up, we usedevery opportunityto 24 km from the eyrie to favored feeding localities follow the female as she flew and soaredin the general (White 1963, Porter and White 1973). vicinity of the nest cliff. Severaltimes this allowed us to find the male when the female broke off soaringand flew PeregrineHunting Range:Use. Different regions 1 km or more from the nest cliff to receive an aerial food and habitatsaround the eyrie were hunted with dif- transferof prey from the male. ferent intensity.This was probablyrelated in some Twice, while flying steeplyupward toward a high-soar- fashionto prey density and vulnerability (see also ing Gyrfalcon,the helicopterstimulated a stoopfrom the Schoener1971). Wet tussock-heathtundra areaswith falcondirectly toward the aircraft. Oncethe falconpulled out only 10-15 m over the rotors. The other time the numerous lakes and sedge-grassmarshes in the helicopter turned aside and fled and the falcon's actions quarter of the hunting range SE of the eyrie received 52 CLAYTONM. WHITE AND R. WAYNE NELSON VOL. 25, No. 3 FALL 1991 FOLLOWING FALCONS WITH HELICOPTERS

Figure 2. View from near eyrie 2 (seeE2, Fig. 1) on 12 June showingthe river valley in the midgroundand wet tussock-heathtundra with numerouslakes in the background. greatestuse (Fig. 2). Fifty-one percent (counting The least used areas were the river corridor and the those where we started to follow him but then sub- quarter of the hunting range E, overlandfrom the sequentlylost him) of the hunting foraysoccurred eyrie. That overlandhabitat was an elevatedregion overthis area. Likewise, the greatestamount of time of extremelydry tussock-heathtundra with several was spent by the male in that area and he went willow fringed streamsdraining S into the wet tun- farthestfrom the eyrie in that direction.West across dra. Only 13% of prey capture attemptsoccurred the river an area of similar habitat but much drier there and mostwere alonga streamrather than over and better drained receivedthe next greatestuse. the tussock-tundraitself, although the falcon did About 36% of the hunting sortiesoccurred there. crisscross the tundra.

Figure 1. Approximatehunting range (within blacklines, see text for discussion)of a breedingmale PeregrineFalcon in tundra environment.Top of figure is north. Squareson map = 1 mi2. Dots representlocations of stoopsor chases after prey, selectedto avoid clutter and to indicatenature and area of hunting range. E2 outsidethe hunting range is a secondperegrine eyrie and showsone locationwhere the female from that eyrie was followedand stoopedat prey. Within the hunting rangefrom top to bottomare a clusterof raptor nests;RL = Rough-leggedHawk (Buteolagopus) nest,E1 = peregrineeyrie from where male was followed,R -- CommonRaven (Corvuscorax), RL = a secondRough- leggedHawk, and P -- territory held by a lone femaleperegrine. 54 CLAYTON M. WHITE AND R. WAYNE NELSON VOL. 25, NO. 3

Althoughexperience elsewhere on the North Slope to go in excessof 16 km from the eyrie and Kistchin- indicated that riparian habitat along the rivers re- skii (1958) determined that on the Kola Peninsula ceivedconsiderable hunting pressure,this was not Gyrfalconsmay travel approximately19 km from the caseon the SagavanirktokRiver. On the Colville the nest to hunt. River, for example, about 17% of 433 bird-prey Gyrfalcon Hunting Range: Patrolling. On one items (12 of 41 prey species)consisted of willow- flight the male flew high N toward the mountains, inhabiting passerinestypical of riparian habitat passing8 km E of a neighboringoccupied eyrie, then (White and Cade 1971). Unlike the Colville, willows 7.5 km E of another eyrie. At that point he had alongthe SagavanirktokRiver were generallysmall curvedNE and eventuallyarrived over a bank of and lessdense and appearedto supportconsiderably low cloudwhich blanketedthe mountainrange. He fewer birds. Only three stoopswere seenover the then flew E for about 5 km over the edge of the main river. mountainsand clouduntil turning S when 6.5 km Gyrfalcon: Hunting Range: Size. We flew with W of anothereyrie. He flappedand glided almost the female on nine occasions,totalling 4 hr 1 min, directly south for about 14 km, turning SW when with the longestsingle flight lasting 63 min and 4.5 km from yet anothereyrie. After flying about consistingof much high soaring.Six attacksat prey 10 km SW over the tundra plain, he wandered 5 were observed, and one Arctic Ground Squirrel km NW and began a seriesof low level hunts. After (Spermophilusparryi)was captured. When at a perch, about 10 km of low-levelhunting he landedduring soaring at 600 m, or drifting on the wind up and a rain shower,and eventuallywe lost him there, in downthe high ridgesextending away from the eyrie, the generalvicinity of where we had losthim on the she appearedto be hunting. Once she attackedan two stoopingflights. The first part of this flight gave intruding immature Golden Eagle (Aquila chrysae- everyappearance of being a boundarypatrol of the tos). Her activities were all confined to within 3.2 line midwaybetween his eyrie and that of four neigh- km of the eyrie, possiblybecause of her reluctance boringGyrfalcon pairs to the WNW, NW, NE, and to leavewith nestingand non-nestingeagles nearby. ESE. (No activeeyrie was found in the mountain We followedthe male on five occasions,totalling range to the N of this eyrie.) This flight path de- 5 hr 9.5 min, with the two longestflights lasting 67 scribeda slightlyflattened circle and enclosed almost and 67.5 min beforehe was lost to view. One flight 200 km2. His total observedtravels in just over 3 hr was short,to a commandingridge-top perch, and as of our flight time encompassedabout 280 km2. we landednearby, he disappearedoff the far sideof Peregrine Hunting Sorties:Methods. The pre- the rock. One flight went rather directly high and dominantmode of hunting by peregrineshas gen- SW over a large tundra plain, another went high erally beenportrayed as a steepangle stoop from a and S about 9 km, then W for 8 km to near the "waiting-on"position at prey that is alreadyon the samelocation on the tundra plain. On both flights, wing. Watching peregrineshunt in an array of hab- by following at too great a distance,we lost him as itats (tundra,montane, taiga, tropical forest, marine he suddenlydived steeplyand went below the ho- and urban) suggeststhat certainmodes of hunting rizon, once at about 600 m and once at about 900 are used more in one habitat than in another and m aboveground level. The fourth flight went high that several types are used with considerablefre- and N for 10 km to a mountain range where he quency.Some of thetechniques used by theperegrine hunted the ridge tops and near the peaks at over weresimilar to thosebriefly described for Gyrfalcons 1525 m; at one point he was 24 km from the eyrie. (White and Weeden 1966, Bengston1971). We lost him as he crossedover a mountain ridge Three distinct hunting strategieswere observed and disappeareddown into a hugetundra-clad bowl. in this study:1) low, maneuvering"ground-effect" The fifth flight was long, appearedto not involve flight, 2) high, direct/maneuveringflight, and 3) hunting until the final minutes,again in the tundra soaring"waiting-on" flight. These typeswere used plain about 9 km SW of the eyrie; it is described in combinationand most frequently the latter two below. were usedalternatingly. However, soaringwas used Other studieshave shownthat Gyrfalconsrange almostexclusively in the caseof one hunting sortie widely to obtainfood, especially in yearswhen ptar- oncethe male was overthe preferredhunting area. migan (Lagopussp.) numbersare low (White and The low, ground-effectflight consistedof contin- Cade 1971). On Alaska'sNorth Slopethey appear uous, rapid wing beats while traveling on average FALL 1991 FOLLOWING FALCONS WITH HELICOIrrERg 55

60-80 kmph (air speed)about 12-25 cm abovethe The manner in which he followed the contour of the ground.In direct (unladen)flight to hunting areas ground at those speedswas similar to behavior we the male averagedabout 55 kmph which is nearly haveseen in peregrinesfollowing the contoursof the 20% faster than normal cruising air speeds(2 44 ocean'sswells as they hunt small alcidsand storm- kmph) of migrating peregrines(Cochran and Ap- petrels (Oceanodromasp.; cf. Sherrod 1988 for a plegate1986). On severaloccasions during the low discussion).Apparently by flying so low, falconsin ground-effectflights the tips of the wingsseemed to both environmentsare gaining the advantageof re- actuallytouch the tops of tussockmounds or the duced drag producedby the so-called"ground-ef- water over which the falcon flew. On one occasion fect" describedby pilots as lift producedby com- (flight startedat 1809 H) he continuedthis strategy pressionof air below the wing when within one-half overa zig-zaggingcourse covering about 7 km during the wing span of the ground (cf. Jenkins 1974, which time threepasserines flushed in front of him. Welch 1987) and therebyobtain higher speeds from He unsuccessfullygrabbed at each of them. He the sameenergy expenditure. Such rapid, low-level achievedat least 112 kmph on three occasions.Fi- flight also must be important in approachingand nally at 1830 H a male Willow Ptarmigan(L. la- surprisingpotential prey. gopus)flushed from the dwarf willowsalong a stream The courseof two high, direct/maneuveringhunt- the falconbegan to follow.The falconwas traveling ing sorties,one of long and one of short duration, so quickly that he had to make a rather wide arc are givenin Tables 1 and 2. One of the more salient before he could start chasingthe ptarmigan, which features of these sorties was the "all business" at- had by that time gainedabout 60 m on the falcon. titude of the male when hunting, similar to the high The falcon took a direct course at the ptarmigan intensityhunting of Treleaven(1980). Immediately until it was evidentthat the ptarmiganwas turning after a stoop he climbed directly back to a high to fly downhill at which time the falcon took a positionand begansearching for anotherprey item. diagonalcourse to interceptit. The falcon reached Note in Table 1 how consistentlyand quickly he the ptarmigan after about 215 m. Just as he was rose to near 150 m after each capture attempt. A aboutto grabthe ptarmiganfrom behind(both birds consistentheight was alsoregained after eachstoop being about 2 m abovethe tundra) the latter rose by the peregrinesmonitored by Alerstam (1987). verticallyinto the air and the falconpassed under Even when the falcon struck a potential prey but him, after which the ptarmigan droppedstraight to did not kill it he promptly beganhunting again. the ground (a typical ptarmigan evasivetactic to The peregrinedid not usethe cliff as a perchfrom avoidGyrfalcons as described by White and Weeden which to launch directly at prey nor did he make 1966). The peregrinemade 2 shallowstoops at the kills near the cliff. Hunting from a cliff perch was ptarmiganon the ground.An immatureGolden Ea- seen in the tundras of Greenland (W. Burnham, gle that was summeringin the immediatearea and pers. comm., Harris and Clement 1975) to catch perhapshad been sitting on the tundra, suddenly Lapland Longspur (Calcarius lapponicus) and flew in at the ptarmigan. The falcon immediately Wheatear (Oenantheoenanthe), in the Yukon taiga beganto soar(1834 H) and reached80 m where he to chasepasserines and shorebirds,and in the marine remained until 1836 H when he stoopedat a prey settingto catch small seabirds(R.W.N., pers. ob- item on the ground,missed, and beganthe low flight ser.). This method has also been observedin western again. At 1840 H he was momentarilylost to view montane regions (C.M.W., pers. obser.). We like- and as we sat the helicopterdown we saw him rise wise did not observeperegrines chase prey high into abovethe horizon (perhaps5 m in the air) in the the air where they are finally caught and killed fashionof a falcon that had missedits prey after a (Campbell 1975, Cade 1982, Clunie 1976). Both stoop,and he then resumedthe low-to-the-ground methods,however, probably are usedon occasionin flight. We were unableto relocatehim. He returned the tundra. to the eyrie with prey at 1936 H. More prey items were killed on the ground or During the time we were able to follow him he grabbedwhile only a few meters in the air than coveredabout 12-16 km of circuitousflying. He those "struck" in the air. Careful observations from spentat least22 min of the time travelingless than otherregions are similar. Monneret (1973) watched 30 cm off the groundat about72 kmph, occasionally about400 attacksby a breedingperegrine in moun- exceeding112 kmph, with a continuouswing beat. tainouscountry in France and foundthat the falcon 56 CLAYTON M. WHITE AND R. WAYNE NELSON VOL. 25, No. 3

Table 1. A long hunting sortieof male peregrineemploying both soaringand high flight on Alaska'sNorth Slope, followedon 12 July 1974.

TIME F•VENT COMMENTS

1125 Started hunt Left cliff goingto SE at 90 m and 56 kmph (helicopterair speed),climbing to 240 m, steadywing beat until arriving over ice-coveredlake about 12.8 km SE of eyrie. 1134 Arrive at lake Continuous,steady wing beat. 1136 Stoop Missed intendedprey, climbedimmediately again to 90 m, continuoussteady wing beat.

1137 Stoop Hit sandpiper(Calidris sp.) on ground but could not hold it, chasedsandpiper up to about 90 m where he begansoaring for first time trying to get abovethe sandpiper eventuallygoing to 120 m. 1141 Steadywing Altered wing beatingwith soaringfor 2 min, reaching130 m. beat

1142 Soaring Alternated wing beatingwith soaringmaintaining about 130 m. 1143 Stoop Low acrosslake, striking water surfacetwice in attempt at a Red-neckedPhalarope. 1145 Climbing Climbedimmediately back up to 150 m with steadywing beat.

1146 Soaring -- 1147 Stoop Began stoopat 200 m, leveledoff horizontal by ca. 10 cm aboveground while 180 m from prey and exceeded160 kmph (air speed)along ground, missed prey, climbed immediatelyback up to ! 10 m. 1148 Flight Began alternating soaringand steadywing beating. 1148.5 Stoop Hit an AmericanGolden Plover and draggedplover a few feet acrossground but lost hold.

1149 Climbing Climbed back to 90 m. 1149.5 Soaring Soaredup to 220 m. 1150 Alternate fly- Between1150 and 1203 H there was alternatingof a steadywing beat followedby a ing modes glide and then soaringfollowed by steadywing beat with a repeatof the sequence; reached 226 m.

1203 Stoop Missed sandpiper(Calidris sp.) on groundon first try, momentumcarried him ca. 5 m in the air where he tried twice more at sandpiperas it flew acrosstundra. 1203.5 Interspecific Chasedby ParasiticJeager (Stercorarius parasiticus) briefly but thenjeager began encounter chasingthe sandpiper. 1204 Climbing-soar- Climbed back up to 215 m where he soaredfor 6 min reaching330 m; defecated ing twice. 1210 Stoop Increasedspeed to 144 kmph, leveledoff about 60 cm aboveground while yet 30 m from sandpiper(Calidris sp.); missed. 1210.5 Climbing-soar- Climbed immediatelyback up to 120 m again, beganto alternatesoaring, steady ing wing beatingand gliding,rising to 135 m where he remainedfor about7 min. 1217 Stoop Missedprey, then he continuedacross tundra at 5-6 m as he grabbedat but missed2 passerinesand 1 sandpiperthat flushedfrom tundra in front of him as he flew past, then climbed. 1222 Stoop Pulled out after 125 m, leveledoff, discontinuedstoop and soared. 1229 Stoop A seriesof loopsand nearly vertical stoopsat a passefinebut missed. FALL 1991 FOLLOWING FALCONS WITH HELICOPTERS 57

Table 1. Continued.

TIME EVENT COMMENTS

1229.5 Climbing- Climbed backup to 120 m, steadywing beat for about 3 min. steadyflap= ping 1232 Stoop Hit prey on ground,continued to fly with prey backtoward eyrie. 1245 Arrived at ey= Returnedto eyrie with a PectoralSandpiper after 80 min hunt. tie

seemedto preferhunting from a highcliff and"bind- alter the durationor successof the falcon'shunting ing" to the prey rather than "waiting on" high in effort. The averagelength of 17 successfulhunting the air and striking the prey. Also, he found that sortiesfor whichwe haveprecise times on departure attackswere more often initiated when prey were from the eyrie to return was 57 min (on some of on the groundor perchedthan when prey were on thesethe falconwas lost so we returned directlyto the wing (seeMonneret 1987). This latter tendency the eyrie to await his arrival). When followed in wasclearly evident with the peregrineson the North helicopter(N = 10), the shortesttime betweenleav- Slope. ing the eyrie, making a kill and returning to the Most stoopswere executedat about a 60o-45ø eyrie was 22 min. When not followed (N = 7) the angle(but seeAlerstam 1987) reachingspeeds of at shortesttime was 23 min. The longesttime for a least 160 kmph with a levelingoff at about 60-150 sortie when followed was 80 min and 93 min when cmin the air while yet 30 m or sofrom what proved not followed. to be his intendedprey. Most frequentlywe could We followed the male falcon on 21 sorties al- not discern what he was after until we saw it flush thoughin 4 caseshe was lost in as few as 5 min and or saw him actually strike at it. Severalstoops were in 2 caseshe wasfound again after beinglost to view startedover 1.6 km from the point wherehe actually for as much as 5 min. We have a fair degreeof struckat the prey. The majority of the stoopswere confidencethat on 13 sortieshe averagedabout 10 made from an altitude of 215-320 m. stoops(range 2-24) beforemaking a kill. The 2 Peregrine Hunting Sorties:Duration and Suc- stoopswere directedat a singleprey item and the cess.The presenceof the helicopterseemed not to 24 involvedattacks at ninedifferent prey items. Dur-

Table 2. A shorthunting sortie of a male peregrineas followedon 12 July 1974.

TIME EVENT COMMENTS

1642 Started Left eyrie goingback overland over dry tussockhunt tundra. 1644 Steady flapping Turned south in direction of lakes. 1647 Stoop Start stoopat 125 m, missed,climbed back to 122 m.

1647.5 Soaring -- 1648 Stoop Grabbedsandpiper (Calidris sp.) about3 m abovethe groundwhere he nearly stalled out in the air but regainedflight (about 10.4 km SE of eyrie) and headedin the directionof the eyrie but a bit southof it. 1649 Flying Reached60-90 m in a steadycourse flying 44-50 kmph until he reachededge of river bluff at which time he was only 9 m abovebluff; droppedlow along edgeof cliff flying straightto eyrie but 1.6 km north alongthe bluff. 1705 Exchange Food exchangein air, female flew off to eat and male went to nest;from leavingcliff to kill 6 min elapsed.From departureto return to cliff 23 min elapsed. 58 CLAYTON M. WHITE AND R. WAYNE NELSON VOL. 25, No. 3 ing the courseof a hunt involving14 stoops,two he was attemptingto ambushprey already sighted sandpiperswere grabbedbut releasedor escaped. or whetherhe expectedunseen or seenprey to flush This male was not as efficientas that describedby in front of him as he suddenlyapproached. Cade (1982; New Jerseymarsh) or L. Hays (pers. The male used a very strongflapping pattern comm.;Zion Natl. Park) where frequentlyprey was whenclimbing in a straightline or circlingup quick- caught on the first attempt within a few minutes ly, and when travelinghigh (airspeed48-56 kmph). after the falconleft his perch.In more recentyears, Presumablyhe was hunting mostof the time when two differentmales along the ColvilleRiver, Alaska, he was traveling to an area where he was seento have beenseen to make a kill on the first stoopof a hunt. When aboutto initiate an attackhe might flap hunt (C.M.W., pers. obser.). with a more leisurelybeat, glide and flap, soar, or The prey itemsthat were either killed and iden- continue the strong flapping pattern--we had no tified or identified from the helicopterafter a stoop warningthat a seriousattack was to begin.He twice was made were: Willow Ptarmigan, Semipalmated startedsteep dives from great heights,but the rest Sandpiper(Calidris pusilia), LesserGolden Plover of those two hunts went unseen. The female often (Pluvialis dominica),Pectoral Sandpiper (Calidris soaredhigh, especiallyover the edgeof the tundra melanotos),Red-necked Phalarope (Phalaropus loba- plain. Sometimesshe was followedon a long,very tus), Lapland Longspur,Savannah Sparrow (Pas- shallowstoop from soaring,ending with a rapid glide serculussandwichensis), and American Tree Sparrow just abovethe vegetationfor 50 m or more before (Spizellaarborea). pulling out of unsuccessfulattacks. Gyrfal½on Hunting Sorties: Methods. Gyrfal- Gyrfalcon Hunting Sorties: Problems in Fol- conshunted (a) from perches,and while (b) in a lowing. It was extremelyeasy to lose sight of the harrier-like flight, (c) ridge-sailing,(d) ridge-hop- falcon.If we laggedtoo far behind,the falconcould ping, (e) swervingamong trees in forest(forest sla- disappearagainst dark talus or vegetation,and if the lom), and (f) travelinghigh with soaring.Both sexes falcon stoopedit becameinvisible when it passed hunted from commandingperches on rockson high below the horizon. Once the falcon crossed a moun- ridgesor hills from which a downhill attack could tain ridge, and we were unableto relocatehim when be initiated. We saw no attacks from the immediate we reachedthe ridge just secondslater. Not oncedid vicinity of the eyrie cliff. Severaltimes the female it appearthat the falconwas attemptingto losethe huntedfor manyminutes almost like a harrier, flap- helicopter;this would have beenrelatively easy for ping and sailingslowly along an erratic path 2-20 the falconto do, especiallywhen flying at a consid- m overthe tundra and fine gravelridges, apparently erable height. in searchof ground squirrels. If we had flown with the Gyrfalconsearly in the Ridge-sailing,on dayswith sufficientwind, was nestlingphase, before the female had seriouslyre- usedespecially by the female.With few wing beats sumedhunting, we probablywould have had more she sailed at 3-8 m along and back over the wind- opportunitiesto follow the male on hunts because ward sideof a long, high ridge. Once she suddenly he was much more conspicuousnear the cliff at that landed on a patch of dirt and caughta ground squir- time. rel. She sailed a short distance farther along the Observed Responsesto Helicopters. We have ridge, then swungback along the ridge and divedto accumulatednearly 1700 hr (asof 1990) of combined the eyrie with the freshlycaught prey. time in helicoptersobserving or searchingfor rap- In the mountains the male traveled close to some tors. From such flights we believethat peregrines low peaksand crossedhigh overintervening valleys. are particularly tolerant of helicopterseven when Severaltimes he crossedvery low overridges (ridge- not on nests (see White and Shetrod 1973, D.H. hopping) and suddenlyhad below him expansesof Ellis, pets. comm.). Gyrfalconsappear to be less tundra in which unsuspectingprey might be spotted. tolerant. It is possiblethat the falconswe observed Especiallythe male useda forest-slalomhunting for this study becamehabituated to the helicopters method in which, from various heights,he made a quickly as a result of our early and unsuccessful rapid, shallow swoop ending with a twisting race attemptsto follow. Oncewe masteredthe techniques, for 50 m or much more between the middles and the birdswere not visiblyintimidated by the rather tops of the scatteredspruce trees on the slopesof slow and unthreateningflight characteristicsof our draws in the tundra plain. It was not clear whether flying patterns.Although it is possiblethat many of FALL 1991 FOLLOWING FALCONS WITH HELICOPTERS 59 thesebirds will readilytolerate a helicoptertraveling as shedrifted southalong the cliff line. We remained with them, it must be notedthat much of the phi- about 24 m from her. We maintained the front of losophyand methodologywhich brought us these the helicopterfacing the falconin an effort to keep resultsare not detailedhere; the potentialthreat of it in sight forcingus to continuallyturn in a rather the helicopter-shadowingtechnique to the lives of tight circleas thoughsoaring. At 1135 shewas im- the raptors and their broods,and to the lives of the mediatelyabove the area occupiedby a lone non- humanobservers, may be high and mustnot be taken breedingfemale peregrine. The lonefemale prompt- lightly. ly beganto soar and soonreached the samealtitude Severaltimes we passedrelatively slowly within as the helicopter.A few stoopswere made by each 20 m over or besideperegrines in both Jet Rangers falcon at the other falcon. Both soared to within and Alouette helicopters.On thoseoccasions the fal- about 15 m of the helicopterand neither seemedto cons'response was to turn their headsand watch us be concerned with it until at about that distance. A pass.On one occasionin the Aleutian Islands, as we brief dodgingmovement was then made by each were landingthe helicopterat an archeologicalsite falconaway from the helicopterand then soaring near the beach,we noted an immature female per- wasresumed. An adult male peregrinesuddenly ap- egrine about 6 monthsold flying along the beach pearedwith the other two soaringfalcons, and he ridge toward us. She landed on a mound at the too beganto soar. For about 2 min there were three archeologicalsite (20 m away) just as we were set- peregrinesand one helicopterall "soaring"within tling the helicopterto the ground. She watchedthe a 60 m area. At 1145 H the male started in a direct helicopteras the main rotor stoppedturning, and flight toward some presumablydistant downriver she did not fly until being frightenedas one of us locality (the next eyrie site was about 8.6 km down- steppedinto full view from the aircraft. Similarly, river) and both femalesreturned to their respective the peregrineswe observedon the North Slopewere cliffs. unusually tolerant of the helicopter as long as a In a similar manner three of the Rough-legged human was not in view outsideof it. Although we Hawks nestingnear the peregrineeyrie (seeFig. 1) havenever witnessed a peregrineattack a helicopter, attemptedto "soar" with the helicopter.In that in- R.W. Fyfe (pers. comm.) observedthis on the Mac- stancewe had lost sight of the hunting male pere- kenzie River, Canada. In the samegeneral area J. grine to view within a coupleof minutesof leaving Campbell,Jr. (pers. comm.) reporteda peregrine the eyrie and we were slowlyturning the helicopter attack upon a fixed-wing aircraft. in tight circlesscanning the horizon for him. Sud- Gyrfalconsseem somewhat less predictable, per- denly we notedtwo, and a few minutes later a third hapsmore aggressive. Both sexes calmly soared within Rough-leggedHawk level with the helicopter(ca. 12-15 m of our hoveringhelicopter (Fig. 3). How- 300 m altitude), all trying to soar. There was no ever,Gyrfalcon on the North Slopeand in this study rising thermal in spite of the "soaring" helicopter flew rapidly from helicopterstraveling at high speeds, and the hawksslowly sank only to flap againto gain while the aircraft is still several hundred m distant altitudeopposite the helicopter.They were all within (Platt 1977, J.B. Platt, pers.comm. and pers.obser.). 30 m of the helicopter.We do not know how long In somesituations, Gyrfalcons vigorously attacked they would havemaintained this behavior,as we left fixed-wing aircraft (D. Roseneau,pers. comm.). As to return to the falcon eyrie to await the arrival of notedabove, Gyrfalcons have attackeda helicopter the male. We havenot followedin helicopterPrairie climbing toward them, and D. Mossop (pers. Falcons,Golden Eagles, Bald Eagles(Haliaeetus leu- comm.), while surveying nests during inclement cocephalus),or Osprey (Pandionhaliaetus). Individ- weather,had a numberof Gyrfalconsattack a mostly uals of the latter three specieshave attacked heli- white helicopter.The normal responseof the study coptersand Golden Eaglesand Ospreyshave hit the peregrineto the helicopteris demonstratedby the aircraft and killed themselves(R.W.N., unpubl.). following account.On 8 June at 1050 H the male Severaltimes while the male peregrinewas in a relievedthe female at her broodingduties, and she hunting soar he drifted to within about 18 m of the flew to a nearby gully to perch.At 1130 H shetook helicopterbefore he appearedto noticeit. He usually flightand began to soaras though she might be going made a quick turn and flew some 3-5 m further to hunt. She was in a thermal and slowly startedto fromthe helicopterwhere he resumedhunting, look- rise, eventually going from 156 m to about 190 m ing at the ground again. 60 CLAYTON M. WHITE AND R. WAYNE NELSON VOL. 25, No. 3

Figure 3. A femaleGyrfalcon, being "shadowed" by a helicopter,calmly soaredtoward and then past within 12-15 m in from of the helicopter.The 2 m antennaon the helicopter'snose shows because the helicopter'sslow forward motion was being stopped.

On 11 July the helicopter hoveredabout 14 m with data collectedby the methodwe usedand thus away from the cliff while the femalecompleted an comparisonsmay be difficult. entire sequenceof feedingyoung and then settledto Data gatheredfrom our studieswere unlike two brood without showing apparent distressfrom the other studiesof radio-telemeterednesting peregrines presenceof the helicopter.(See White and Sherrod in several aspects.Mearns (198S) in Scotlandde- 1973, for a photo from this feeding sequenceand termined the hunting rangesof two femalesnear the further discussion.) endof the breedingseason. An analysisof foodstaken Value of Helicopter Following. Most early also gave someindication of the differential use of studieson homeranges of raptorsemployed the tech- the habitat they hunted.Enderson and Kirven (1983) nique of mapping locationswhere individual birds in California studied a male and female and while were seenon severaldays and then drawing mini- obtaininggood data within 2 km of the eyrie they mum polygonsthat encompassall sightings(e.g., were uncertain about the extent of use of the main Craigheadand Craighead 1956). Many later studies hunting areas. In such radio-telemetry studies the usedradio telemetry (e.g., Cochran et al. 1965, Forbes subjectbird is out of sightof the investigatoron most and Warner 1974, Enderson and Kirven 1983). For of the hunting flight and, apart from its approximate long-distancetravelers, such as the falconswe stud- location on a map, only the most rudimentary be- ied, the latter method would have been preferable havioral information were obtained from the radio but has seriouslogistical constraints. Data using ei- signal. (Using satellitetracking will of courseelim- ther techniquemay not, however,correlate directly inate the aboveproblems.) While data from our per- FALL 1991 FOLLOWING FALCONS WITH HELICOPTERS 61 egrinestudy were obtainedover a much shortertime Gyrfalcons.We thank D.H. Ellis, D.E. Anderson,J.R. than the abovetwo, a very goodindication of the Parrishand an anonymousreviewer for helpfulcomments. differentialuse and extentof the huntingrange was LITERATURE CITED obtained.The falcon'sbehavior was continuously ALERSTAM,T. 1987. Radar observationsof the stoopof observedduring hunting flights from start through the PeregrineFalcon Falcoperegrinus and the Goshawk prey capture and return, and the prey species,lo- Accipitergentilis. Ibis 129:267-273. cations, and habitats hunted were recorded. BENaSTON,S.A. 1971. Hunting methodsand choiceof Becauseboth PeregrineFalcons and Gyrfalcons prey of Gyrfalcons(Falco rusticolus) at Myvatn in tendto be tolerantof helicoptersduring certainparts northeast Iceland. Ibis 113:468-476. of the breedingseason and when they encounterthe BIRD, D.M. aND Y. AUBRY. 1982. Reproductiveand helicopterunder certain conditions, following of these huntingbehavior of peregrinefalcons, Falco peregrinus, speciescan producefive kinds of information that in southernQuebec. Can. Field-Nat. 96:167-171. are difficult or impossibleto obtain through more C^DE, T.J. 1982. The falconsof the world. Comstock, conventionalmethods: 1) Hunting techniques,suc- Cornell UniversityPress, Ithaca, NY. cess, and duration, from the start of a hunt to the CAMPBELL,a.w. 1975. Hunting tacticsof a Peregrine Falcon on Black Turnstones. Condor 77:485. endcan be recorded.2) The habitatsactually hunted CLUNIE,F. 1976. A Fiji peregrine(Falco peregrinus) in and certainsites that are usedfor huntingmore than an urban-marine environment. Notornis 23:8-28. other areas are identifiable. 5) Precise routes .and COCHRAN,W.W. ANDR.D. APPLEGATE.1986. Speedof elevationsof hunting falconscan be determined.4) flappingflight of Merlins and PeregrineFalcons. Con- Prey speciesactually attacked and different tech- dor 88:397-398. niques used in various situations or with certain •, D.W. W^RNER,J.R. TESTER^ND V.B. KUECHLE. speciescan be observed.5) Behaviorspreviously un- 1965. Automaticradio-tracking system for monitoring recognizedin wide-rangingfalcons may be detected. animal movements. BioScience 15:98-100. Although the rental of a helicoptermay be very CRAIGHEAD,J.J. AND F.C. CRAIGHEAD,JR. 1956. Hawks, owls and wildlife. The StackpoleCo., Har- expensive,in situationswhere the logisticsare ac- risburg, PA. ceptableand the birds will tolerate the aircraft this DEKKER,D. 1987. PeregrineFalcon predation on ducks techniqueis very competitivewith other methods. in Alberta and British Columbia.J. Wildl. Manage.51' In a relativelyshort time it canproduce large amounts 156-159. of detailed, unusual information about the travels, ENDERSON,J.H. AND M.N. KIRVEN. 1983. Flights of habitatuse, and behaviorof extremelymobile birds. nestingPeregrine Falcons recorded by telemetry.Rap- As a postscript,the young peregrinesfrom the tor Res. 17:33-37. eyrie studiedwere &predated about 3 wk after the FORBES,J.E. ANDD.W. WARNER. 1974. Behaviorof a study ended(L.W. Sowl, pers. comm.) but either radio-taggedSaw-whet Owl. Auk 91:783-795. that pair or anotherused the sameeyrie the following FYFE, R.W. ^ND R.R. OLENDORFF.1976. Minimizing the dangersof nesting studiesto raptors and other year. sensitivespecies. Occas. Paper No. 23, Canadian Wild- life Service, Ottawa, Canada. ACKNOWLEDGMENTS HARRIS,J.T. AND D.M. CLEMENT. 1975. Greenland The peregrinestudy was funded by the AlyeskaPipeline Peregrinesat their eyries: a behavioralstudy of the Consortium(Service Company) under the supervisionof Peregrine Falcon. Mealdel.om Gron. 205(3):1-28. U.S. Fish and Wildlife Service,Alaska area. LeRoy W. JENKINS,M.A. 1974. Behaviorof the Gyrfalcon(Falco Sowl, USFWS, was helpful in securingfunds. Hunter rusticolus)from hatchingto fledgling:a time-lapsepho- Ridenour,Evergreen Helicopters, piloted the aircraft. Per- tographicstudy. M.S. thesis,Brigham Young Univer- sonnelof the Franklin Bluff pipeline camp were helpful in providinglogistics. sity, Provo, UT. The Gyrfalcon study was funded by World Wildlife KISTCHINSKII,A.A. 1958. Zur Biologie des Jagdfalken Fund (Canada) throughFred L. Bunnell. David Mossop auf der Kola-Halbinsel. Ornitologija1:61-75. and the Yukon Wildlife Branchprovided major logistical MEARNS,R. 1985. The hunting ranges of two female and other support.Dean Cameron, Denise Harwood, Don peregrinestoward the end of a breedingseason. Raptor Russell, and Jennifer A. Nelson assistedwith aerial ob- Res. 19:20-26. servations.Alora L. Nelson providedaerial and ground assistancein all phasesof this and the parentstudy. Kerry MONNERET,R.J. 1973. Techniquesde chassedu faucon Guenter, Trans North Turbo Air, flew the helicopterand pelerindans ane region de MoyenneMontagen. Alauda playeda majorrole in developingmethods of "shadowing" 41:403-412. 62 CLAYTONM. WHITE AND R. WAYNE NELSON VOL. 25, NO. 3

1987. Le faucon pelerin. Editions du Point U.S. DEPT.INTERIOR. 1979. SnakeRiver birdsof prey Veterinaire, Maisons-Alfort, France. specialresearch report. Bureau Land Manage., Boise PLATT,J.B. 1977. The breedingbehavior of wild and District, Boise, ID. captiveGyrfalcons in relationto their environmentand WEIR, D. 1978. Wild Peregrinesand grouse.The Fal- human disturbance.Ph.D. thesis,Cornell University, coner 7:98-102. Ithaca, New York. WELCH,B. 1987. Hawks at my wingtip.North Country PORTER,R.D. ANDCI.M. WHITE. 1973. The Peregrine Press, Thorndike, ME. Falconin Utah, emphasizingecology and competition WHITE, CI.M. 1963. Botulismand myiasisas mortality with the Prairie Falcon.Brigham Young Univ. Sci.Bull. factors in falcons. Condor 65:442-443. Biol. Series 18(1):1-74. -- tu'4DR.B. WEEDEN. 1966. Hunting methodsof RUDEBECK,G. 1951. The choiceof prey and modesof Gyrfalconsand behavior of their prey (ptarmigan). hunting of predatorybirds with specialreference to Condor 68:517-519. their selective effect. Oikos 3:200-231. • ^ND T.J. C^DE. 1971. Cliff-nestingraptors and SCHOENER,T.W. 1971. Theory of feeding strategies. ravensalong the ColvilleRiver in ArcticAlaska. Living Ann. ReviewEcol. and Syst.2:369-404. Bird 10:107-150. SHERROD, S.K. 1988. Behavioral differences in Pere- • ^ND S.K. SHERROD.1973. Advantagesand dis- grine Falcons.Pages 741-748 in T.J. Cade, J.H. En- advantagesof the useof rotor-wingedaircraft in raptor derson,C.G. Thelander and C.M. White [EDs.], Per- surveys.Raptor Res. 7:97-104. egrine Falcon populations: their management and recovery.The PeregrineFund, Inc., Boise,ID. TRELEAVEN,R.B. 1980. High and low intensityhunting in raptors. Z. Tierpsychol.54:339-345. Received16 June 1989; accepted26 January 1991