Spatial Relationships of Nesting Golden Eagles in Central Utah
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White, C. M. 1974. The 1974 raptor surveyof the AlaskaPipeline between Franklin Bluffsand Big Delta. Unpubl.interim rep. to U.S.Fish and Wildl. Serv.,Anchorage, AK. 10 pp. White, C. M., T. D. Ray,and L. W. Sowl.1977. The 1970-1972-1974raptor surveys alongthe Trans-AlaskaOil Pipeline.pp. 199-221.In.' Chancellor,R.D. (ed.).World Conferenceon Birdsof Prey, Reportof the Proceedings.International Council for BirdPreservation. 440 pp. + appendices. SPATIAL RELATIONSHIPS OF NESTING GOLDEN EAGLES IN CENTRAL UTAH by DwightG. Smith• BiologyDepartment SouthernConnecticut State University New Haven, Connecticut 06515 and JosephR. Murphy Departmentof Zoology BrighamYoung University Provo, Utah 84602 Abstract We examinedthe distributionof GoldenEagle (Aquila chrysaetos)nest sites in the easternGreat Basinusing the Clark-Evansnearest-neighbor method of analysis.Distri- butionvalues indicated. uniform spacing of nestsites within the hills and ridges,which providedall of the potentialnesting sites in the studyarea. Analysis of GoldenEagle nestsite distributionwith three other large raptorssuggested a tendencytoward uni- form spacingbetween diurnal raptor speciesand aggregationwith the Great Horned Owl (Bubovirginianus). Introduction Odum(1971) suggested that populationsexhibit one of threedistribution patterns: (1) random,where habitat is homogenousand individualsdo not normallyinteract; (2) uni- form,where habitat is homogenousand individualscompete for one or moreresources; (3) aggregated,that mayresult from either heterogenous habitat or a tendencyto aggre- gatesocially. Newton (1979) noted that manyspecies exhibit uniform distribution of ter- ritoriesand nest sites except where available sites are limitedand concentrated,such as in the SnakeRiver Birds of PreyNatural Area. Our observationsof GoldenEagles in the easternGreat BasinDesert revealed a concentrationof nestsites in high, north-south orientedridges and hills but nonein interveningbroad, flat valleys.Herein we examine thisdistribution using the Clark-Evansnearest-neighbor model (1954). 'Addressall correspondenceto: Dwight G. Smith 128 RaptorResearch 16(4):127-132 Winter1982 Smithand Murphy--Spatial Relationships 129 StudyArea We beganlong term raptorstudies on a 7700km • portionof the easternGreat BasinDesert in 1966.Data usedherein are from a 120km • segmentof the areawhich includes portions of Utah and TooeleCounties. We havedescribed the climate,topography and major vegitation of the studyarea previously (Smith and Murphy 1973). Methods Our studieswere from 1966-1970.Details of methodsare providedin Smithand Murphy(1973). The Clark-Evansmodel is a measureof the ratio (R) of the distancebetween nearest-neighbor sites (fa), in thiscase active eagle nests, to thetheoretical nearest-neighbor distance (œs) expected if all nestswere in fact uniformlydistributed: wherera isthe average of the observed distances, ia = E distances/Nand N is the totalnumber of distances sampled.The theoretical value, ifs, is determined, i s = 1/2•/p wherep isthe population density expressed as the numberof activenests per unit of area.The calculatedR indicatesthe spatialpattern of distribution.If random,R equalsor approaches1; ff aggregatedR equalsor approachesO; if uniformR equalsor approaches 2.15.The significanceof the calculatedR is testedby the following: C = ra - rE/ofE whereC is the standardvariate of the normalcurve and oœs is the standarderror of the averagedistance to the nearest-neighborin a randomlydistributed population of densityp. This maybe determinedas follows: œs= 0.26136/Np The 5% and 1%significance levels are representedby C valuesof 1.96and 2.58 respectivelyfor a two-tailed test. The Clark-Evansmodel requires knowledge of the breedingdensity of the species.Golden Eagles and other largeraptors in centralUtah nested primarily in foothillsand ridges and hunted in the valleys.Large tracts of thesevalleys were not usedby the collectiveraptor population during the nestingseason. In 1969and 1970, we determinedminimum home ranges of GoldenEagles and otherraptors on a smaller120 km z of the origi- nal studyarea by observationof movementsof eachnesting pair fromblinds. We observedeach pair a mini- mum of 10 hrs overseveral days. The area of the combinedGolden Eagle home ranges was consideredthe ecologicaldensity and includedin the calculations.Miller and Stephen(1966) used this methodto examine spatialrelationships of individualsin feedingflocks of SandhillCranes (Grus canadensis) with the Clark-Evans model. Results and Discussion Distributionof activenests of the GoldenEagles, Great Horned Owl, Ferruginous Hawk (Buteoregalis), and Red-tailedHawk (Buteoiamicensis) on the 120 km2 area are shownin Figure1. In 1969 and 1970, nearest-neighbordistances between the 34 active GoldenEagle nestsaveraged 4.3 4-1.33 km andranged from 1.7 km to 5.1 km. Comparatively,nearest-neighbor distances between the 34 activeGolden Eagle nests locatedon the entire1170 km 2 areafrom 1967-1970,were slightly,but not significantly less,averaging 3.714-1.1 km (range1.12 km to 10.3 km). Nearestactive nests were usu- ally locatedon oppositesides of high ridgesand pairshunted primarily in oppositeval- leys.For example,in 1968,nests of 3 pairs in the foothillssouth of Lake Mountain formeda triangle1.12 km, 2.4 km and 2.4 km on the three sides.Caminzand (1969) notedthat the 2 nearestnests of this trianglewere separatedby a ridge 38.1 m higher than either nest site. Adults at these3 nestsreportedly occupied separate hunting ranges.These observations coupled with the distributionpatterns illustrated in Figure I 130 RAPTORRESEARCH Vol.16, No. 4 1969 1970 ,kmSCALE Figure1.Distribution ofactive nests ofGolden Eagles (GE), Great Horned Owls (GHO), Ferruginous Hawks (FH)and Red-tailed Hawks (RTH) on a 120km 2portion ofthe central Utah study area in 1969 and 1970. Winter 1982 Smithand Murphy--SpatialRelationships 131 suggestan aggregatedpattern of nestplacement resulting from the limited potential nestsite habitat. Results of nearest-neighboranalysis of GoldenEagle nest distribution do nothowever indicate an aggregatedpattern (Table 1). For comparison,we havein- eludedanalysis of GoldenEagles with 3 otherlarge raptors nesting on the studyarea. Table1. Analysis of nestsite distribution of GoldenEagles and other large raptors in centralUtah. GoldenEagle Goldeneagle X GoldenEagle X GoldenEagle X GreatHorned Owl FerruginousHawk Red-tailedHawk 1969 1970 1969 1970 1969 1970 1969 1970 N 4 3 8 6 12 7 6 4 /'n- 3.28 2.57 0.82 1.26 1.98 2.29 2.32 2.28 rv. 2.20 3.15 1.43 1.58 2.34 1.41 1.86 1.81 R 1.49 0.82 0.57** 0.79* 0.84 1.62' 1.24' 1.15 *Significantat P < 0.05 **Significantat P < 0.01 Distributionvalues of GoldenEagle active nestsin 1969-1970, were R = 1.49 and R = 0.82 respectively.The 1969 valueindicates a trend towardsuniform spacing of nests whilethe 1970value shows random spacing. The analysisdoes not suggestaggregation, evenwith the restrictedarea of potentialnest sites. Distributionof active GoldenEagle and Great Horned Owl nestshad valuesof R = 0.57 in 1969and R = 0.79 in 1970.Both values are statisticallysignificant (p<0.05) and indicatea trendtowards aggregation, undoubtedly a functionof similarityin choiceof nestsites by the two speciesplus lack of interactionbecause of differingdaily activity patterns.Distribution of activeGolden Eagle and Ferruginous Hawk nestshad valuesof R -- 0.84 in 1969 and R = 1.62 in 1970. These values indicate random nest location in 1969and a trendtowards uniform spacing in 1970.Comparative values of GoldenEagle and Red-tailed Hawk nest distribution were R = 1.24 in 1969 and R = 1.15 in 1970. Bothvalues suggest a trendtowards uniform spacing. GoldenEagles nested in cliffsor rock outcropsat elevationsranging from 1460-1910 m, althoughmost were locatedat elevationgreater than 1680 m. Great Horned Owls nestedin cliffsand junipers at elevationsranging from 1485-1890m. Distancesbetween activeGolden Eagle and Great HornedOwl nestsaveraged 1.12 km. The nearestactive nestswere 0.08 km apart in the samequarry cliff face while the mostdistant was 2.45 km. Red-tailedHawks selected nest sites in severalcliff typesand treesat elevations rangingfrom 1508-1847m. Distancesbetween active nests of GoldenEagles and Red- tailed Hawksranged from 1.34 km to 4.26 km and averaged2.45 km, considerably greaterthan those between Great Horned Owls and Golden Eagles. Nestsof FerruginousHawks were locatedon rock outcropsand in a varietyof tree and groundsites at middleand lower elevations.A total of 85.1%were foundbetween 1511-1660m and nonewere at elevationshigher than 1710 m. Distancesbetween ac- tive nestsof GoldenEagles and Ferruginous Hawks averaged 2.45 km andranged from 0.88 km to 4.04 km. Thereis an apparentcorrelation between daily activitypatterns of the four raptor speciesand distribution of active nests. Thus the two raptors with the least similar daily activitypatterns, Golden Eagles and Great Horned Owls show an aggregateddistribu- tion patternwhen considered together. The two raptorspecies with the greatestsim- ilarityin dailyactivity patterns, Golden Eagles and Red-tailed Hawks, show a trendto- wardsuniformity of distributionof activenests, suggesting interaction to at leastsome degreeduring nest site selection. The spacingof GoldenEagle and Ferruginous Hawk activenests is morerandom and may reflectthe somewhatcrepuscular activity of the FerruginousHawk. Literature Cited Caminzand,F. J. 1969.Nesting ecology and behavior of the GoldenEagle (Aquila chry- saetos).Brigham Young Univ. Biol. Ser.10:4-15. Clark,P. J.,and