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j. RaptorRes. 38(3):231-237 ¸ 2004 The Raptor ResearchFoundation, Inc.

A COMPARISON OF BARRED AND SPOTTED -SITE CHARACTERISTICS IN THE EASTERN CASCADE MOUNTAINS,

JOSEPHB. BUCHANAN1 WashingtonDepartment of and Wildlife,600 CapitolWay North, Olympia,WA 98501 U.S.A.

TRACY L. FLEMING National Councilfor Air and StreamImprovement, 23308 N.E. 148th, BrushPrairie, WA 98606 U.S.A.

LARRY L. IRWIN National Councilfor Air and StreamImprovement, P.O. Box 68, Stevensville,MT 59870 U.S.A.

ABSTRACT.--We describe 10 nest sites of the northern ( varia varia) in mixed-coniferous forestsof the easternCascades in Washington,a region where the speciesis syrupatticwith the northern (S. occidentaliscaurina). Our goal was to determine whether Barred and Spotted owlsused similar habitatsfor nesting.In contrastto Spotted , Barred Owl nest siteswere situated on gentle slopes or fiat ground, closer to water, and included more hardwoods and a greater richness of .Barred Owl nestswere usuallyin cavitiesor platforms created at the broken top of the tree bole. Only two Barred Owl nestswere in abandoned hawk or clumps of branches infected by dwarfmistletoe(Arceuthobium douglasii), which were the two most common nest structuresused by Spot- ted Owls in our studyarea. Barred Owls used a greater range of tree speciesfor nesting,including three nestsin black cottonwoods( trichoca,pa), a speciesrarely usedfor nestingby SpottedOwls in the . Although differences in these attributes suggestthat the two speciesused somewhat different habitats, Barred Owls have become more abundant in our study area and the region, and further assessmentsof habitat use may indicate greater niche overlap.

KEYWORDS: northernBarred Owl; Strix varia; northernSpotted Owl; Strix occidentalis;habitat; nest sites; Washington.

UNA COMPARACI6N LAS CARACTER•STICAS DEL NIDO STRIX VARIA VARIA Y STRIX OCC•DEN- TALIS CAURINA EN LAS MONTAI•AS DE EASTERNCASCADE, WASHINGTON RESUMEN.--Describimosdiez sitiosde nido de Strixvaria en bosquesmixtos de confierasde las montafias Eastern Cascadeen Washington,una regi6n donde la especiees simpfitricacon S. occidentaliscaurina. Nuestro objetivo era el de determinar si Strix varia varia y Strix occidentaliscaurina, utilizan habitats similarespara anidar. En contrastea S. occidentaliscaurina los sitiosde los nidos de S. varia varia estaban ubicadosen pendientes suaveso en pastizalespianos, cerca del agua e incluian mas maderas duras y una mayor riqueza de especiesde •trboles.Los nidos de S. varia varia estabanubicados en cavidadeso en plataformascreadas por firbolesquebrados en su parte superior del tronco. Tan solo dos nidos de S. varia varia se encontrabanen nidos abandonadosde gavilaneso en el follaje de ramasinfectadas por Arceuthobiumdouglasii, las cualesfueron las estructurasmas comunesutilizadas por S. occidentaliscaurina en nuestra area de estudio. S. varia varia utilizo un mayor rango de especiesde firbolespara anidar, incluyendoa Populustrichoca,pa una especieraramente utilizada para anidar por S. occidentaliscaurina en el Noreste Pacifico.Aunque las difcrenciasen estosatributos sugieren que de algfin modo las dos especiesutilizan difcrenteshabitats, S. varia varia se ha vuelto masabundante en nuestraarea de estudio yen la regi6n. Una evaluaci6nposterior del uso de habitat puede indicarnosun mayor traslapede nicho. [Traducci6n de Casar Mfirquez]

E-mail address:buchajbb@dfw. wa.gov

231 232 BUCHANAN ET AL. VOL. 38, No. 3

Until recently,the range of the northern Barred METHODS Owl (Strix varia varia) was limited to the hardwood Between 1988 and 1994, we visited all known Barred and mixed /hardwood forests of eastern Owl nestsin our study area, except for one nest that was (Mazur andJames 2000). In the last reported to us but located in an extremelyremote area. Biologistsfound nestsof both speciesduring timber-sale century, Barred Owls expanded their range west- evaluations,Spotted Owl surveys,or in the caseof some ward across the continent to Barred Owl sites,during specificsearches for their nests. (Grant 1966) and then south to central At each nest, we described the nest and nest tree. This (e.g.,Taylor and Forsman1976, Leder andWalters information included the species,age (based on an in- crement core extracted at breast height), diameter at 1980, Dark et al. 1998). Barred Owls are now sym- breastheight (DBH), height, canopyposition, condition patric with northern Spotted Owls (S. occidentalis of the nest tree (i.e., alive or dead; top intact or broken), caurina) over nearly the entire range of the latter and the nest type (i.e., cavity,broken-top platform, aban- species (Dark et al. 1998). The continuing range doned hawk nest, cluster of mistletoe-infected branches), height, and orientation relative to the tree trunk. expansion and increase in population density of Vegetation structure at nest siteswas quantified in a Barred Owls in the Pacific Northwest has raised single 0.10-ha circular plot (18.0-m radius) centeredon concernsbecause they may compete with Spotted the nest tree and five 0.04-ha plots (11.4-m radius). The Owls (USDI 1992, Kelly et al. 2003), and the two middle 0.04-ha plot was centered on the nest tree and the others were immediately adjacent in the di- specieshybridize (Hamer et al. 1994, Kelly 2001). rections (Buchanan et al. 1995). Characteristicsof snags The effects of competition and hybridization on and three dominant or codominant and three interme- the Spotted Owl, a (USDI diate (tree height, canopy height, age) were as- 1990), are unknown but potentially deleterious sessedin the 0.10-ha plot. Snagswere measured (DBH, (see Pearsonand Livezey2003). height) and identified to speciesand decayclass (Cline et al. 1980). In the 0.04-ha plots we identified and mea- Stand- and landscape-levelhabitat relationships sured all trees -->10 cm DBH, estimated the volume of of Barred Owls have been documented in other coarsewoody debris accordingto four decayclasses (Soi- parts of their range (e.g., Nicholls and Warner lins 1982), and counted the number of saplings<10 cm 1972, Apfelbaum and Seelbach 1983, McGarigal DBH. We measuredcanopy closureat Barred Owl nests using a sphericaldensiometer (Lemmon 1956). We were and Fraser 1984, Bosakowskiet al. 1987). In Wash- unable to make comparisonsof canopyclosure to the •ngton, landscapecomposition of stand types or paired sampleof SpottedOwl sites;instead, we compared forest age classesassociated with nesting Barred our sample to a previously-documentedsample of owl Owls have been described in the northwestern and nests that characterized the region (Buchanan et al. 1993, 1995). southwestern Cascades (Hamer 1988, Pearson and At each nest site, we described site disturbance and L•vezey 2003) and the central eastern Cascades topographicfeatures such as elevation,slope, topograph- (Herter and Hicks 2000), areas where both Barred ic position, and aspect.Site disturbancewas indicated by the presenceof fire scarson logs or trees,or by evidence Owls and Spotted Owls occur.In this paper, we de- of timber harvest. Because Barred Owls often use lowland scribe attributes of sitesused for nesting by syru- forestsnear water in other parts of their range (Mazur pattic Barred and Spotted owls in the eastern Cas- and James 2000), we estimated the distance to nearest cade Mountains. water channels ->5-m wide or ponds ->2 ha and evaluated whether Barred Owl nest siteshad different geophysical STUDY AREA attributes than sites in which Spotted Owls nested. A complete description of data collection proceduresis Our studyarea wasthe eastslope of the CascadeMoun- found in Buchanan et al. (1993, 1995). tains in Washington.The 875 000-ha area extendsca. 225 We compared site and vegetationattributes at Barred km in a north-south direction, t?om to the Owl nestswith a sample of Spotted Owl nestsfrom a border, up to 45 km eastwardfrom the Cascade previousinvestigation (Buchanan et al. 1993, 1995). The crest, and includes the Wenatchee and Okanogan Na- forest-standcharacteristics of Spotted Owl nest sites in tional Forests,portions of the Gifford Pinchot National the Eastern Cascades province vary intra-regionally Forest, tribal lands of the Yakama Nation, and adjacent (Buchanan and Irwin 1998). Much of this variation is state and private lands. The study area contains forest related to the moisture gradient and concomitantchang- associationsranging from the moist Douglas-fir (Pseudot- es in forest associationacross the region, and has been sugamenziesii) /western hemlock (Tsugaheterophylla) zone described in the context of Fire Management Analysis in the west to the xeric ponderosapine (Pinuspondero- Zones (FMAZ), a geographicanalysis and management sa)/artemesia zone in the east (Franklin and Dyrness systemthe U.S. Forest Service usesfor fire management 1973), and containsconiferous and mixed-coniferousfor- on the Wenatchee National Forest (Buchanan and Irwin ests(Cobb 1988). Fire suppressionin the lastcentury has 1998). Consequently,each Barred Owl nest was paired changedthe structureand compositionof forestsin parts with a randomly-selectedSpotted Owl nestfrom the same of the region (Agee 1993). FMAZ, except for a nest in Klickitat County that was SEPTEMBER 2004 BARRED AND SPOTTED OWl, NESTS 233 paired with a Spotted Owl nest from that region. The Table 1. Mean age and sizeattributes of Barred Owl (N pool of availableSpotted Owl nestsincluded 80 siteswell = 10) and Spotted Owl (N = 10) nest trees in the eastern distributed throughout the Wenatchee National Forest CascadeMountains, WA. Analysisresults based on paired and vicinity (FMAZ 1 = 14 sites, FMAZ 2 = 31 sites, t-tests with df = 9. FMAZ 3 = 19 sites, FMAZ 4 = 13 sites, FMAZ 5 = 3 sites; Buchanan and Irwin 1998) and 31 sitesin Klickitat Coun- ty and vicinity (J. Buchananunpubl. data). SPO'ITED We used a two-stepapproach to analyzeour data. First, BAPmED OWL OWL we used paired t-tests(Zar 1984) to determine if habitat NESTS NESTS features at Barred Owl nest sites differed from those at Spotted Owl sites. Percent-slope values were arc-sine ATTPdBUTE • SE • SE t P transformed prior to analysis.The Wilcoxon testwas used DBH (cm) 106.0 15.4 64.9 8.1 2.36 0.03 to compare the number of tree speciespresent at nests of the two species.Means +SE are presentedunless oth- Age (yr) 216.8 58.0 181.9 60.6 0.42 0.68 erwise indicated; df = 9 in all comparisons.We consid- Tree height (m) 25.1 2.6 30.5 3.3 1.30 0.21 ered statisticaltests significant when a <--0.1. Nest height (m) 16.4 1.2 16.6 1.9 0.12 0.91 Second,because of the possibilitythat someof our sig- nificant findings were the product of chance becauseof the large number of paired comparisons,we usedlogistic been disturbed by these activitiesdecades prior to regression(Hosmer and Lemeshow1989) to determine known owl use. Evidence of fire (in the form of whether our significantvariables would be included in multivariate models. For this analysiswe included all vat- fire scars on trunks) was present at nine of 10 robles with P•values <--0.25in the univariate analyses(see Barred Owl sitesand six of 10 Spotted Owl sites.A Mickey and Greenland 1989) and produced a set of all slashfire burned at one Barred Owl site the year possiblemodels using those variables.We evaluatedthe prior to nest use. Five of 10 Barred Owl sites had resulting models to identify variablesthat contributed to high rates of correct sample classification.We did not been lightly harvested;one site was logged several intend the models to be used as predictive tools because yearsprior to use and the others apparentlyseveral of our concern that habitat use by Barred Owlshas likely decades earlier. Four of 10 Spotted Owl sites ex- changed, thus necessitatingthe development of new hibited evidence of selective timber harvest several models (see below). decadesprior to use. Nests and Nest Trees. There were a number of RESULTS differences between the two owl species in nest General Site Characteristics.We quantified hab- tree attributes. Barred Owl nests were located in itat at 10 Barred Owl nest sites in the eastern Cas- five tree species,including three in black cotton- cadesprovince. These siteswere 0.4-12.5 km (• = (Populustrichocarpa), three in Douglas-firs, 4.8 _+ 1.4 km) from known Spotted Owl nest lo- two in grand firs (Abiesgrandis), one in a western cations. Nine sites occurred in four of the five fire hemlock, and one in a western (Larix occiden- managementanalysis zones (zone 1 = three sites, talis). In contrast, nine of 10 Spotted Owl nests zone 2 = two sites, zone 3 = three sites, zone 5 = were in Douglas-firs.Of the 10 Barred Owl nests, one site) and one site was in Klickitat County. two were in living and intact trees, and eight were Barred Owl nestswere generally in areas with in trees that had broken boles (six alive, two dead). low relief (• slope = 10.6 -+ 4.1%) in bottomlands In comparison,seven Spotted Owl nest trees were (7), on a mid-slopebench (1), a ridge-top (1), or alive and intact, and three were either dead or had at the base of a gradual slope (1). Spotted Owls broken tops. Barred Owl nest trees were signifi- nested on significantly steeper terrain (• slope = cantly larger in diameter (at breast height) than 41.9 + 4.5%; t = 5.19, P < 0.0001). Because the Spotted Owl nest trees, but there were no differ- terrain at several Barred Owl nests was level, we ences in tree age, tree height, or height of nest did not calculate site aspect.Elevation of Barred above the ground (Table 1). Owl sitesranged from 320-1180 m (• = 781 _+84.6 The two owl speciesexhibited differencesin the m) and did not differ from Spotted Owl nest sites typesof nestsused. Eight Barred Owl nestswere in (• = 913 -+ 54.8 m; t = 1.31, P= 0.21). Barred cavitiesor on chimney-likeplatforms created at the Owl nests were closer to water (• = 448 _+ 183.2 point where the tree bole had broken; one was in m) than those of Spotted Owls (• -- 1993 _+534.1 a clump of branches infected by dwarfmistletoe m; t = 2.74, P = 0.01). (Arceuthobiumdouglasii), and one was an aban- There was little evidence of recent fire or log- doned (Accipitergentilis) nest ging activityat Barred Owl nests,but most siteshad Only two Spotted Owl nestswere in cavitiesor on 234 BUCHANANET AL. VOL. 38, NO. 3

Table 2. Mean structuraland age attributesof vegetationat Barred Owl (N = 10) and SpottedOwl (N = 10) nest s•tesin the EasternGascade Mountains, WA. Analysisresults based on paired t-testswith df = 9.

BARRED OWL NESTS SPOTTED OWL NESTS

ATTRIBUTE ;• SE ;• SE t P

Age of dominant/codominant trees (yr) 164.4 41.0 191.9 62.1 0.37 0.71 Age of intermediate trees (yr) 88.7 18.2 108.0 17.4 0.77 0.45 Basal area of (m2/ha) 44.2 9.0 42.5 6.2 0.16 0.87 Basalarea of Douglas-firs(m2/ha) 15.1 4.1 22.8 3.5 1.43 0.17 Basal area of hardwoods (m2/ha) 7.8 4.0 0.21 0.1 1.9 0.07 Basal area of all trees (m2/ha) 52.1 8.1 42.7 6.1 0.93 0.37 Basalarea of snags(m2/ha) 6.3 1.8 14.6 4.1 1.84 0.08 Hmght (m) to base of canopy (dominants/ codominants) 12.8 1.8 16.6 1.0 1.84 0.08 Hmght (m) to base of canopy (intermediate trees) 8.5 1.3 8.7 1.3 0.09 0.93 Hmght (m) of dominant/codominant trees 31.2 1.6 33.8 0.9 1.42 0.17 Hmght (m) of intermediate trees 20.3 1.5 21.1 1.2 0.40 0.69 Sapling abundance (0.04 ha) 20.7 3.6 13.3 2.9 1.63 0.12

broken tops.The otherswere in clumpsof branch- of tree speciespresent at Barred Owl nest sites(N es infected by dwarfmistletoeor abandoned gos- = 5) was greater than at Spotted Owl nests (N = hawk nests. 3; Wilcoxon Z = 3.53, P = 0.0004). VegetationStructure. With few exceptions,there Logistic-regressionanalyses identified a number were no differencesbetween speciesin the vege- of modelsthat classifiednest siteswith a high de- tation structure we measured at owl nests. Barred gree of accuracy.Fourteen modelscorrectly classi- Owl sites had a lower height to the base of the fied -->18of 20 nest sites,including six modelsthat canopy of dominant/codominant trees, a greater correctlyclassified ->19 sites.Five of the six param- basal area of hardwood trees, and a lower basal eters that were significantin our paired analyses area of snags (Table 2). Other measures of tree were identifiedin the latter sixmodels: percent slope height, canopyheight, basalarea, or saplingabun- and numberof treespecies (included in four models); dance did not differ between the two owl species distanceto water(three models), basalarea of hard- (Table 2). Canopy closureat five Barred Owl sites woods,and basal area of snags(two models); and averaged70.6%, which is within the range report- heightof dominant/co-dominanttrees (one model). ed from the eastern Cascade Mountains (Buchan- an and Irwin 1998). DISCUSSION We recorded 16 tree speciesat the Barred Owl Many studieshave documented that Barred Owls nest sites,although sevenof thesewere present at inhabit or associatewith forestscontaining stand- two sites only. At the Spotted Owl nest sites, we ing water or (e.g., Nicholls and Warner observed8 tree speciesincluding three speciespre- 1972, Devereaux and Mosher 1984, Bosakowski et sent at --<2sites. Four specieswere present at -->5 al. 1987). Falk (1990) reported no associationof Barred Owl sites: Douglas-fir, grand fir, western Barred Owl nest sites in with water, redcedar (Thuja plicata), and red (Alnus rub- although there wassignificantly more water in the ra); and Douglas-firand grand fir were present at vicinity of nestscompared to two diurnal raptors ->8 sites.The latter two specieswere equally prev- included in that study. alent at Spotted Owl sites (N = 10 and N = 7, In the eastern CascadeMountains of Washing- respectively);ponderosa was the only other ton, Barred Owl nests were much closer to water speciespresent at -->5Spotted Owl sites (N = 6). and were in valleys or other areas with less relief Hardwoodswere present at sevenBarred Owl sites than were nestsof SpottedOwls. An apparentpref- and three Spotted Owl sites.The median number erencefor areasnear water by Barred Owlsmay be SEPTEMBER 2004 BARRED AND SPOTTED OWL NESTS 235 related to the greater diversity of small prey may occur in the northern CascadesRegion (Peffer 2001) and aquatic prey speciesassociated in western Washington. with rip arian zones within the eastern Cascades.In Despite the differenceswe found in habitat use, addition, the prevalenceof Barred Owl sitesin bot- a number of habitat attributes were common to the tomlands likely influenced the greater richnessof two speciesand suggesta certain amount of niche tree speciesand the greater abundance of hard- overlap.All Barred Owl nestswere within the geo- woodsthat we observedcompared to Spotted Owl graphic range of the Spotted Owl and occurred nests. within the elevation range and forest associations We found two substantial differences between Spotted Owls use on the east slope of the Cascade theseowl speciesin the type of nest structuresand Mountainsin Washington.There were no interspe- the size of nest trees used. First, Barred Owls most cific differences in total heights of dominant or often used cavitiesor platforms atop broken tree intermediate trees, or in the basal area of trees; boles and infrequently used old goshawknests or also,there did not appear to be differencesin can- clustersof branches infected by mistletoe. In con- opy closure.In short, both speciesprimarily nested trast, most Spotted Owl nestswere associatedwith in closed-canopy,mixed-coniferous forests at mid- mistletoe or old goshawknests (Buchanan et al. elevations.Although the shared use of forestswith 1993), a use pattern that appearsto be unique to these features placed the two speciesin close con- the eastern Cascades and the Klamath Mountains tact with one another, differencesin geophysical in Oregon (Forsmanet al. 1984). Barred Owlsmay position and habitat use may have minimized com- use nest structuresopportunistically; however, all petition when the two speciesfirst becamesympat- tic. six nests Yannielli (1991) found in Connecticut were in cavities. On the other hand, 18 of 38 Our data indicating use of different types of (47%) nestsreported by Bent (1938) were aban- nestsand nest tree speciesby the two owlssuggests the absence of significant competition for nest doned hawk nests.This pattern of nest use is con- structuresand perhapsnest sites.This perspective sistentwith the generalist nature of this speciesin was based on data collected between 1988 and western North America (Mazur and James 2000). 1994, a period when Barred Owls appeared to be Second, Barred Owl nest trees were larger than lesscommon than at present. Indeed, Barred Owls those Spotted Owls used in our study area. This now occur in many areas that severalyears previ- difference can be explained by: (1) Barred Owl use ouslysupported only Spotted Owls (T. Fleming un- of cottonwoods,which rapidly attain large size, (2) publ. data). Additionally, severalSpotted Owl nests the generally larger size of trees on gentle terrain in our study area have been used by Barred Owls and in bottomlands,where growing conditionsare since1994 (T. Fleming unpubl. data). Consequent- better than on the sloping terrain where Spotted ly, given the more generalist use of habitat by Owls nested (Buchanan et al. 1995), and (3) the Barred Owls (Mazur andJames 2000) comparedto comparativelygreater Barred Owl useof cavityand Spotted Owls, a future assessmentof Barred Owl broken-top platform structures, which occur in habitat use may indicate a different relationship larger trees than the goshawkor mistletoe nests between the two species. that SpottedOwls used (Buchananet al. 1993). Al- though Barred Owls are larger than Spotted Owls ACKNOWLEDGMENTS (Snyderand Wiley 1976), it is unknownif the for- We thank Dale Herter, Michael Kane, Heather Murphy, mer requires a larger cavityfor nesting. The com- Brian Ostwald, Stan Sovern, and Margy Taylor for direct- parativelylower basalarea of snagsat Barred Owl ing us to Barred Owl nest sitesor pair locations;many other biologists,acknowledged in earlier publications,for nests might be explained by this species' use of directingus to SpottedOwl nestsites; Leonard Brennan, varied habitatsand prey. for providing feedback on the analysis;and James Bel- The availabilityof nest sitesand prey are thought thoff, Rob Bierregaard,Eric Forsman,and DavidJohnson to be primary factors that limit raptor populations for making constructive comments that improved the manuscript. (Newton 1979, Nelson 1983). 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