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A MODEL of GOLDEN EAGLE &Lpar;<I>AQUILA CHRYSAETOS</I

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A MODEL of GOLDEN EAGLE &Lpar;<I>AQUILA CHRYSAETOS</I J RaptorRes. 36(1 Supplement):62-69 ¸ 2002 The Raptor ResearchFoundation, Inc. A MODEL OF GOLDEN EAGLE (AQUILA CHRYSAETOS) RANGING BEHAVIOR Michael J. MCGRADY Natural Research,Ltd., Am Rosenh•2'g.el59, A-3500 Krems,Austria Justin R. GRANT Druminault,Tullich, Invergvrdon, Easter Ross, 11/18 OMW, U.K. Ian P. BAINBRIDGE RoyalSociety for theProtection of Birds,Scottish Headquarters, Dunedin House, 25 RayalstonTerrace, Edinburgh,EH4 3TP, U.K. David R.A. MCLEOD 14 CrailinghallCottages, Jedburgh, TD8 6LU, U.K. )mSTRACT.--Eightterritory-holding adult Golden Eagles (Aquila chrysaetos)were radiotrackedin all sea, sonsfrom 1991-96 in western Scotland.Mean territory size was 6827 ha (range = 2604-12 835 ha). Core areas (50% of locations)used by tracked eaglesaveraged 498 ha. Tracked eaglesmoved up to 9 km from the center of their territories,but over 98% of observationswere <6 km of the center.Log- linear modelsshowed no significantpreference for land cover,although relative use suggestedthe order of preference by eaglesto be: montane > heather > coarsegrassland > bracken > smoothgrassland > bog > broad-leavedforests > pre-thicket fbrest > post-thicketforest > pasture > other habitats. Elevationsranged from sea level to 900 m but eaglesappeared to prefer elevationsbetween 150-549 m. Based on data from these eagles, we constructed a simple model to define likely boundaries of territoriesand to identify areaswithin thoseboundaries that are likely to be important to eagles.Features of the model included range centersidentified from nest locationsand nest-usedata, boundarieswith near-neighborshalfway between respective nest centers,and a 6-km cutoff in directionswhere neighbors were distant. The model designatedcore areas 2-3 km in radii using information on local eagle nesting density.Outside core areas,low elevationswere avoided.We discussthe advantagesand shortcomings of the model and its robustnesswhen exported to other parts of Scotland. KEYWORDS: GoldenEagle; Aquila chrysaetos;range; movements; territoriality; Scotland. Un modelo del rango de comportamiento del f•guila real (Aquila chrysaetos) RESOMEN.--Nueveaguilas reales (Aquila chrysaetos)adultas poseedoras de territorios fueron rastreadas con radios en todas las estaciones desde 1991-96 en el oeste de Escocia. E1 tamafio medio de los territoriosfue 6827 ha (rango = 2604-12 835 ha). Lasareas centrales (50% de laslocalizaciones) usadas por las f•guilasrastreadas promediaron 498 ha. Las aguilasrastreadas se movieron pot eucimade 9 km desdeel centro de susterritorios, pero masde198% de lasobservaciones estuvieron a <6 km del centro. Dos modelos de Logaritmo linear no mostraron preferenciassignificativas para la cobertura terrestre, aunque el uso relativo sugiereel orden de preferencia para aguilasasi: montano > brezal > pastizales toscos> helechos > pastizalesfinos > pantanos > bosquesde hoja ancha > bosque pre-matorral > Bosquespost-matorral > pasturas> otros habitats.Las elevacionesvariaron desdeel nivel del mar hasta 900 m pero pareci6 que las aguilasprefieren elevacionesentre 150-549 m. Con base en los datos de estasf•guilas, construimos un modelo simple para definir los limites probables de los territorios y para identificar areas dentro de aquellos limites que probablementeson importantes para las f•guilas.Las caracteristicasdel modelo incluian los centros de los rangos identificadosa partit de la localizaci6nde los nidos, los datos del uso de los nidos, los limites con los vecinos mas cercanos a mitad del camino entre los respectivoscentros de susnidos, y un corte de 6 km en las direcciones donde los vecinosno fueron claramenteconocidos. E1 modelo design6f•reas nilcleo de 2-3 km de radio usandoinformaci6n sobre la densidad local de anidaci6n de la f•guilas.Afuera de las f•reas nilcleo, las bajas elevaciones 62 MARCH 2002 GOLDEN EAGLE RANGING MODEL 63 fueron evitadas.Nosotros discutimoslas ventajas y deficiencias del modelo y su robustez cuando es extrapoladoa otras partes de Escocia. [Traducci6n de Ctsar Marquez y Victor Vanegas] In the British Isles,the breeding range of Gold- per presentsthe mapping model and discussesits en Eagles (Aquila chrysaetos)has been much re- advantagesand shortcomings. duced. Today, Golden Eaglesare found primarily STUDY AREA AND METHODS in the Highlands of Scotland,although a few pairs breed in southwesternScotland and northern Eng- The study area covered about 500 km2 in mid- and land (Watson and Dennis ]992, Gibbons et al. south Argyll, Scotland (Fig. 1). Both fresh and saltwater lochsare present,and the topographyis hilly, with some ]993, Green ]996). In recent years, numbers of peaks over 950 m. In general, the agricultural potenual breeding Golden Eaglesin Britain have been stable of the area is limited, with most land being capableof at about 425 pairs, although regional fluctuations supporting only rough grazing and plantation forestry have occurred (Dennis et al. ]984, Green 1996). Some agricultural improvement has occurred such as drainage and fertilizing but this is generally limited to Afforestafon of parts of upland Scoffand has areas at lower altitudes. had a large effect on the habitat of the country. Purple moor grass (Molinia caerulia) and white bent Preliminary studieshave associatedlarge-scale af- (Nardusmicta) dominate the areasgrazed by sheep,and forestation with the decline in the numbers of there are areas of poor condition dwarf-shrub heath. In some areas, bracken (Pteridiumaquilinum) cover is exten- breeding eaglesin western Scotland (Watson et al. sive. Large and small blocks of commercial conifkrous ]987), and breeding successin western and south- plantation (mostlyPicea sitchensis) are quite common. western Scotland (Marquiss et al. ]985, Watson In terms of land use, sheeprearing and forest planta- 1992) has been shown to be dependent on affor- tions are the predominant forms of land use. Some deer estation. Watson et al. (1987) predicted that for- stalking is pursued but there is relatively little manage- ment of moors for Red Grouse (Lagopuslagopus scoticu 0 estry would have a negative effect on eaglesif it Watsonet al. (1987) reporteda generaldecline in grouse exceeded 40% of the area within 4 km of the cen- stocksin the area due to overgrazing. ter of an eagle'sterritory. Land cover, land use, terrain, and precipitation vary The Golden Eagle is an "amber list" speciesof from region to region within the breeding range of Gold- en Eagles in Scotland. In general, the east mainland in- medium conservationconcern in the United King- cluding the Cairngorms is drier, with higher elevation, dom (Gibbons et al. 1996), because it has an un- and supportsrelatively more grouse moor and relatively favorable conservationstatus in Europe due to its lesssheep rearing than in the westmainland. The islands of the Hebrides are also variable. Their climate is oceanic rarity (Tucker and Heath 1994). The European and few grouse are present. population amounts to 5000-7200 pairs of which Eagleswere trapped in funnel trapsand usinga power 5.8-8.4% are in the United Kingdom. snare (McGrady and Grant 1996). Transmitters were fit- Between 199]-96, the Royal Societyfor the Pro- ted as backpackswith a degradable link. They weighed tection of Birds (RSPB),working with the Research 45 g and had a potential life of up to 4 yr. There wasno evidence that the tagging of eagles,even both members Division of the ForestryCommission, conducted a of a pair, affected breeding or any other activities,and study of the ranging behavior of Golden Eaglesin some eaglesfitted with tagshave bred in yearsafter being Argyll, Scotland.The primary aim of this studywas instrumented. to capture and fit backpack-mounted radiotrans- Normally, tracking was done by at least two people m mitters to free-flying eagles and to follow their radio communication with one another. Immediately af- ter fitting an eagle with a transmitter, it was followed in- movements, and then to relate these data to both tensivelyto make sure that it was able to fly properly land cover and land use. As a product of this re- After some days,birds newlyfitted with transmitterswere search,a simple model mapping eagle ranging be- worked into the rotation of radio monitoring; thereafter havior was constructed. The advantagesof this they were visited as often as possibleon a regular basis Because of logistical constraints, it was unusual to track mapping model are that it is easyto use, requires eaglesin more than one territory per day. that the user has little a priori knowledgeof eagles Birds were tracked using a directional (yagi) antenna in general or of particular pairs, and is robust even and a compassto generate a bearing of the eagle's d•- when information on eagle pairs is limited. The rection. From this, we could triangulate and estimate the location of the eagle. Our minimum aim was to get one mapping model was published as a Research In- high qualitylocation (< 100 m accuracy)per dayof track- formation Note by the ForestryCommission in the ing. We often exceeded this aim. Because the most ac- United Kingdom (McGrady et al. ]997). This pa- curate locations were not from triangulation but f•om 64 McGv, z•¾ ET AL. VOL. 36, No. 1 SUPPLEMENT Figure 1. Study area in western Scotland. direct observations,we aimed to visually locate eagles. to ensure independence; we separated successiveloca- Once in view, we would observe eaglesthroughout the tions by at least 1 hr. day, mapping their movements. Our observationloca- We used MacAulay Land Use Research Institute (MLU- tionswere chosenso that we were
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