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J. Ornithol., 70(4):535-542

NEST SITE SELECTION BY URBAN AND RURAL GREAT HORNED IN THE NORTHEAST

DWIGHT G. SMITH BiologyDepartment SouthernConnecticut State University New Haven, Connecticut 06515 USA

THOMAS BOSAKOWSKI Consultants Inc. 12931 NE 126th Place Kirkland, Washington98034 USA

ARNOLD DEVINE BiologyDepartment SouthernConnecticut State University New Haven, Connecticut 06515 USA

Abstract.--We studied nest site and habitat characteristics associated with 75 Great Horned (Bubo virginianus) nestsin Connecticut, northern New Jersey,and southeasternNew York. Nest siteswere categorizedas either urban (30) or rural (45) and were comparedto data from available habitat (24 random sites for microhabitat; 70 random sites for macro- habitat). Urban nest treeswere significantlylarger in diameter and taller than rural nest , and accordingly,nests were higher in urban nest trees as well. Urban nest siteswere significantlydifferent than random sitesfor all eight habitat variables,but rural nestswere significantlydifferent for only five variables.Urban nestswere significantlydifferent than rural nestsfor five of eight habitat variables.Only urban owl nestshad significantlylower site basalarea, higher composition,and lower shrub cover.Both urban and rural owl nestsshowed lower canopycover and closerproximity to edge,paved roads, habitation,and water than random sites.Although both urban and rural Great Horned Owls demonstratedhabitat selection (use different from availability),urban owls showed a stronger degreeof selection,probably because of the greatercomplexity of habitatsavailable in the urban .

SELECCION DEL LUGAR DE ANIDAMIENTO POR PARTE DE BUBO VIRGINIANUS EN ZONAS URBANAS Y RURALES DEL NORDESTE DE LOS ESTADOS UNIDOS

Sinopsis.--Estudiamosel lugar de anidamiento y caracteristicasdel habitat asociadoal nido de 75 individuosde Bubovir•nianus en el estede los EstadosUnidos. Los lugaresde ani- damiento fueron categorizadoscomo urbanos (30) o rurales (45) y fueron comparadosa datosdisponibles sobre utilizaci6n de habitats(24 localidadesal azarpara microhabitat,y 70 tambi•n al azar para macrohabitat).Los Jrbol½surbanos utilizados para anidar resultaron set significafivamentemJs grandesen diJmetro y mJs altos que los Jrbolesrurales e igual- mente, los nidos fueron localizadosa mJs altura en las localidadesurbanas. Los lugaresde anidamientosurbanos fueron significafivamentediferentes a los lugaresestudiados al azar para ocho variablesen el habitat, pero los nidos rurales tan solo difirieron en cinco de las variables.Los nidos urbanos resultaron significafivamente diferentes de losrurales para cinco de las ocho variablesde habitat estudiadas.Los nidos urbanostuvieron significativamente menor area basal, mayor composici6nde coniferosy menor cobertura de arbustos.Tanto nidos urbanos como rurales mostraton menor covertufade docel, y mayor proximidad a horde de bosque,carreteras pavimentadas, construcciones de humanosy aguaque loslugares examinadosal azar.Aunque tanto buhos rurales como urbanosdemostraron selectividad de habitat (usodiscriminativo a la disponibilidad),los buhos urbanos mostraton un gradomayor de selecci6n,debido, probablemente, a la mayorcomplejidad del habitaturbano. E1 presente

535 536] D. G. Smithet al. J.Field Ornithol. Autumn 1999

trabajo confirma que el muy adaptableBubo virginianus selecciona habitats boscosos frag- mentadospor el desarrollourbano.

The Great (Bubovirginianus) is the mostwidespread owl of the Western Hemisphere and also one of the most studied (references in Clark et al. 1978). Its nestingecology has been describedfor many regions of the ,including (Bent 1938), (Hagar 1957), NewJersey and Connecticut(Bosakowski et al. 1989), (Misztal 1974, Siminski1976), (Craigheadand Craighead 1956), (Orians and Kuhlman 1956, Petersen 1979), (Baumgartner 1939), (Fitch 1940), (Gilmer et al. 1983), (Sidenstickerand Reynolds 1971), (Fitch 1947), and central (Smith and Murphy 1973, 1979, 1982). Acrosstheir range, nesting Great Horned Owls occupya wide range of habitat typesand qualities.Minor et al. (1993) found that productivityof Great Horned Owls was normal in the urban-suburban environments of Syracuse,New York, but nesting densitywas about three-fold lower than in rural areas. Based on a macrohabitat analysisaround owl locations, Bosakowskiand Smith (1997) suggestedthat Great Horned Owls were habitat generalistsin northern New Jerseyforests and had a tendencyto occupyareas impacted by forest fragmentationfrom suburbandevelop- ments.In this paper,we providea quantitativeevaluation of habitat char- acteristics at 75 nests in Connecticut, northern New Jersey,and southeasternNew York. In addition, we comparerural and urban nest sites to available habitat to determine if there are differences in habitat selectionby Great Horned Owls occurring in urban and rural environments.

STUDY AREA The studyarea included Orange and westernRockland counties in New York; Sussex,Morris, Middlesex,and Passaiccounties in New Jersey;and Litchfield, Fairfield, and New Haven counties in Connecticut. Physio- graphic sectionsincluded within this area include the Lowlandand High- land Piedmont and New England provinces (Braun 1950). Forestsare predominately deciduousor mixed standsdominated by (Quercus spp.), maple (Acerspp.), and hickory (Carya spp.); smaller tractsof east- ern hemlock (Tsuga canadensis)or white (Pinus strobus)are on cooler slopesor in ravinesor as plantations.Across the studyarea, mean annual temperaturesrange from 7-11 C and mean annual precipitation varies from 112-127 cm/yr, with lesser amounts at lower elevations (Brumbach 1965). Human populationdensities within this area varyfrom >1370 people/km2 in urban centersto suburbanand exurban popula- tions of widelyvarying densities to rural areaswith lessthan 39 people/ km 9 in the rural districts (Lewis and Harmon 1986).

METHODS Great Horned Owl territories were found by a combinationof foot and vehicle searches,checks of areasin which Great Horned Owls responded Vol.70, No. 4 GreatHorned Owl Nest Sites [537 to playbackof tape-recordedsong or vocal imitations, and by monitoring previouslylocated nestsof other raptors during the courseof long-term raptor studiesin Connecticut and New Jersey-NewYork area (Smith and Gilbert 1984; Lynch and Smith 1984; Bosakowskiet al. 1987, 1989; Bo- sakowskiand Smith 1992, 1997). Prior to foliage development,known territories were searched for active nests. Nest- and nest-site variables were measured after fiedging in late summer or early fall. At 75 Great Horned Owl nests, we measured four microhabitat variables (stand level) and four macrohabitat features (landscapelevel). We also measured the same microhabitat variables at 24 random sites and the same macrohabitat variables at 70 random sites. All random sites were obtained from computer-generatedcoordinates, which were plotted on 7.5-minUSGS quadrangle maps and then locatedin the field. Macrohab- itat variables measured to the nearest 1.0 m included distance from the nest or random site to the nearest road, nearest forest opening greater than 1 ha, nearest water source, and nearest human habitation. At the microhabitat level, tree basal area at nest and random sites was measured using a "Cruz-all" 10-factor forestry angle gauge (English scale), and data were convertedto metric equivalents.Basal area samples were tallied at the nest tree and at four sites 50 m distant from the nest tree in each of the four directions.Canopy cover and shrub cover were recorded simultaneouslyat 10-m intervals along 50-m transectsthat extended from the nest tree into each of the four cardinal directions. Canopycover was recorded as positive or negativeusing an ocularsighting tube (Jamesand Shugart 1970). Shrub coverwas recorded if shrubsor saplingsgreater than 1 m in height but lessthan 10 cm in diameterwere within a 1.7-m diameter circle centered on the observer (Collins et al. 1982). Conifer compositionwas calculatedas a percent of all treesfrom basal area tree tallies. Statisticalanalysis.--In order to prevent pseudoreplication,we only re- port data from one nest per territory so that each nest representsan independent sample. For analysis,nest siteswere categorizedas either urban or rural. Urban areaswere defined as habitatslocated in cities,city open space, and suburban developments.Rural areaswere defined as predominantly undeveloped landscapeswith naturally occurring fields, ,, or wetland habitats.Means were compared by the parametric equal variance t-testprocedure, or unequal variance t-testif varianceswere significantlydifferent at P < 0.05 (Zar 1974).

RESULTS AND DISCUSSION Nestdescriptions.--Nest type was determined at 61 of the 75 nests.Of these,29 (38.7%) were in old nestsconstructed by Red-tailedHawks (Bu- teojamaicensis),11 (18.0%) were in old nestsof the AmericanCrow (Cor- vus brachyrhynchos),8 (13.1%) were in gray (Sciuruscarolensis) nests, 6 (9.8%) were in old nests of Red-shouldered (Buteo linea- ms), 5 (8.2%) were in old Cooper'sHawk (Accipitercooperil) nests, and one each (1.6%) in an old (A. gentills)nest and Red 538] D. G. Smithet al. J.Field Ornithol. Autumn 1999

TABLE 1. Nest tree characteristics of Great Horned Owl nest sites in urban and rural areas in Connecticut,southern New York, and northern New Jersey.Data representthe mean (_+1 SD).

Urban nests Rural nests Variables n = 30 n = 45 p a

Nest tree DBH (cm) 56.9 (11.6) 48.5 (10.8) 0.0020 Nest height (m) 15.0 (2.00) 12.4 (2.77) 0.0001 Nest tree height (m) 21.50 (3.71) 17.4 (4.25) 0.0001

a t-test. squirrel ( Tamiasciurushudsonicus) nest, respectively. The remainingnests were of undetermined origin. Rural-nestingGreat Horned Owls used rap- tor nestssignificantly more frequentlythan urban pairs, they most com- monly chosecrow or squirrelnests (X 2 = 4.01, df = 1, P < 0.05). Nest treedescriptions and metrics.--Nesttree specieswas determined at 67 nest sites. Of these, 45 (59.7%) Great Horned Owl nestswere in live deciduoustrees, mostly and red maple (Acerrubrum), three (4.5%) were in dead trees (snags),and 19 (28.3%) were in ,primarily white pine, easternhemlock, and Norway spruce (Piceaabies). A signifi- cantly higher number of urban nestswere in coniferscompared to rural nests (X2 = 5.22, df = 1, P < 0.05). Urban nest trees were significantly larger in diameter (DBH) and taller than rural nest trees,and nestswere higher in urban nest trees (Table 1). This differenceis probablydue to the fact that manyestablished urban open spaceareas and cityparks had large trees that were retained for ornamentation,shelter, or shade.Fur- thermore, smallgroves, and remnant patchesof treesin urban open spac- es are in lesscompetition for sunlight,and therefore, growth rates are greatly enhanced over the naturally regenerating second-growthforests of rural nest sites.However, none of the nestsin this studywere located in isolated trees. Nest-sitehabitat.--Urban nestswere significantlydifferent from random sitesfor all eight habitat variables(Table 2), but rural nestswere signifi- cantly different from random sitesfor only five variables.In comparing urban and rural nests,five habitat variableswere significantlydifferent. Urban nests had a lower basal area than random sites, but rural nests were not significantlydifferent from random. This differenceis probably due to the more open, park-like conditionsfound in urban open-space environments,whereas rural nest siteswere typicallyin denser,naturally regenerating, second-growthforest. Conifer composition of urban nest siteswas significantlyhigher than at random sites,but not at rural nest sites. Conifer plantations and large overgrown ornamental conifers are more numerous in the urban environment than in most of the oak-hard- wood dominatedforests that has regeneratednaturally. Canopy cover was relativelyhigh at both urban and rural nest sites(70%), but both were significantlylower than random sites.This consistentfeature of both ur- ban and rural owl nests indicates that small openings in the canopy Vol.7o, No. 4 GreatHorned Owl Nest Sites [539

T•mLE 2. Habitat characteristics of Great Horned Owl nest sites in urban and rural areas in Connecticut,southern New York, and northern NewJersey and comparisonto avail- able habitat (random sites).Data represent the mean (-1 SD).

Microhabitat Urban nests Rural nests Random sites variables n = 30 n = 45 n = 24

Basal area (m2/ha) 18.4ab 22.8 23.7 (4.87) (3.81) (4.83) Conifer composition% 40.2'•b 11.7 8.2 (32.4) (15.2) (17.2) Canopycover % 70.8a 71.8• 82.5 (14.9) (21.0) (8.76) Shrub cover % 52.3 • 60.5 69.0 (20.5) (26.6) (27.5) Macrohabitat Urban nests Rural nests Random sites variables n = 30 n = 45 n = 70

Distance to water source (m) 168.2a 143.9• 250.8 (153.8) (98.3) (210.9) Distanceto forest edge (m) 70.7•b 164.3a 238.1 (41.8) (98.9) (210.0) Distance to paved road (m) 106.2•b 329.3a 501.9 (70.8) (209.5) (452.7) Distance to human habitation (m) 144.8•b 575.84 730.1 (80.0) (302.1) (516.5)

Indicatesa significantdifference from random sites(t-test, P < 0.05). Indicatesa significantdifference from rural sites (t-test,P < 0.05). around the nest tree are probably desirable.A more open canopy may make for easier accessto nests,more sunlight for developing chicks,and easiervisibility for spotting potential nest predators. Shrub cover was significantly lower in urban nest sites compared to random sites, but rural nest siteswere not significantlydifferent from random. This difference was probably due to the fact that urban Great Horned Owls often selectedmature open park-like forest stands(artifi- cially grown and maintained by intensivesilviculture and horticulture) as opposedto random sites,which coverthe whole spectrumof foresttypes, ages,and densitieswith a correspondingdiversity of shrub cover. Distance to a water sourceat urban and rural nest siteswas significantly closerthan random sites.This situationseems to infer a degree of habitat selection,because both urban and rural owlsresponded similarly to this variable in the nest selectionprocess. Diets of Great Horned Owls in our studiesrevealed that riparian and wetland prey specieswere often taken (Bosakowskiand Smith 1992), although their capture may have been for- tuitousduring other water usessuch as drinking and bathing. Distance to forest edge for urban and rural nestswas significantlyless than random sites,although urban nestswere closestto edge. Urban nests 540] D. G. Smithet al. J.Field Ornithol. Autumn 1999 were situatedin a highly fragmented, mosaicof small tree stands,open- ings, roads, and buildingswhereas rural nestswere more often situated in areasof contiguousforest with lessavailability of large forestopenings and edge. Other investigationsin the Northeasthave alsofound the Great Horned Owl to favor edge habitats(Bosakowski et al. 1987, Laidig and Dobkin 1995, Bosakowskiand Smith 1997). In addition to edge, both urban and rural nestswere found significantlycloser to pavedroads than random sites,although rural nestswere not as closeas urban nests.Great Horned Owls are commonlyobserved hunting along interstatehighways in New York (Bosakowskiand Speiser1984) and, in this study,several of the nestswere lessthan 50 m from major highways,including one nest in a cloverleafisland at a junction of three major highways. As would be expectedfrom our selectioncriteria for urban versusrural, urban nestswere significantlycloser to human habitation than random sites or rural nests. Rural nests were also closer to human habitation than random sites,but not as closeas urban nests.Apparently, Great Horned Owlsare benefitingfrom the associatedalterations in habitat (edge,frag- mentation, openings) and food supply (rats, mice, , , ,opossum, racoon, farm ) that result from human settle- ments. In conclusion, both urban and rural Great Horned Owls demonstrated habitat selection,but urban owls showeda stronger degree of selection (deviance from random). However, the results also demonstrate that even rural owlswill tend to selectnest sitesin areaswith more edge and forest fragmentationcreated by the impact of suburban/urban sprawl.Bosa- kowskiet al. (1987) and Bosakowskiand Smith (1997) analyzedhabitat around Great Horned Owl locationsand concludedthat theywere habitat generalistsand were more tolerant of suburban habitat fragmentation and disturbancethan other sympatricforest raptors, except for the Red- tailed . Laidig and Dobkin (1995) analyzedowl locationsin south- ern New Jerseyand also concludedthat they were habitat generalists,but did not mention suburban or urban influences. Previous studies of nest siteshave alsoreported that the Great Horned Owl is a habitat generalist (Petersen1979, Bosakowski et al. 1989), althoughHagar (1957) reported that great horned owlspreferred nestingin densewoodlots, generally 8 ha or larger. The present report confirms that the adaptable Great Horned Owl purposelyselects forest habitat that is fragmentedand al- tered by urbanizationand showsmore selectivityin complexhabitats (ur- ban mosaic) than in monotonous ones (i.e., contiguoussecond-growth forest) where there are probablyless choices to make during the nestsite selectionprocess.

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