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Home , Barred owl, Flying squirrel, Northern flying squirrel, Northern spotted owl

j. RaptorRes. 35(3):221-227 ¸ 2001 The Raptor ResearchFoundation, Inc.

DIETS OF NORTHERN BARRED AND NORTHERN SPOTTED OWLS IN AN AREA OF SYMPATRY

THOMAS E. HAMER HamerEnvironmental, 19997 Hwy. 9, Mt. Vernon,WA 98274 U.S.A.

DAVID L. HAYS WashingtonDepartment of Wildl!fe,600 N. CapitolWay, Olympia,WA 98501-1091 U.S.A.

CLYDE M. SENGER 1 Departmentof Biology,Western University, Bellingham, WA 98225 U.S.A.

EPdC D. FORSMAN USDAForest Service, Research Station, 3200 SWJeffersonWay, Corvallis, OR 97331 U.S.A.

ABSTRACT.--Wecompared diets of Northern Barred Owls ( varia varia) and Northern Spotted Owls (Strix occidentaliscaurina) in westernWashington during 1985-89. Diets of both specieswere dominated by nocturnal , but diets of Barred Owls included a more diverse and more even distribution of prey. Estimateddietary overlapbetween the two speciesbased on the Pianka Index was76%. Barred diets included more terrestrial mammals,more , more diurnal prey, and more prey that were associatedwith riparian areas,including , ,and .The snowshoehare (Lepusameri- canus)comprised 35% of prey biomassin the diet of Barred Owls. The diet of Spotted Owls wasdom- inated by the northern flying (Glauc0myssabrinus), which comprised51% of prey numbersand 57% of prey .We speculatethat Barred Owls and Spotted Owls compete for food becausetheir dietsoverlap considerably, their food appearsto be limiting in manyyears, and Barred Owlsare gradually invading territories historicallyoccupied by Spotted Owls. KEYWORDS: NorthernBarred Owl; Strix varia varia; NorthernSpotted Owl; Strix occidentaliscaurina; diet;, competition;; Washington.

Dietas de Strix varia varia y Strix occidentaliscaurina en un area simpattica. RES•MEN.--Comparamoslas dietasde Strixvaria variay Strixoccidentalis caurina en el oestede Washington durante 1985-89. En las diemsde arebasespecies predominaron los mamifkrosnocturnos sin embargo la diem de Strixvaria fue mas diversay con una distribuci6nmas uniforme de presas.Estimamos el traslape de las dietasentre las dos especiescon baseen el lndice de Pianka (76%). La diem de Strixvaria incluy6 mas mamiferos terrestres,mas aves,mas presasdiurnas y mas presasasociados con fireasriberefias inclu- yendo peces,antibios y caracoles.Lepus americanus constituy6 el 35% de la biomasade presasde la dieta de Strixvaria. En la diem Strixoccidentalis predomin6 Glaucomyssabrinus con un 51% del nfimero de presas y un 57% de la biomasade presas.Especulamos que Strixvaria y Strixoccidentalis compiten por cornida debido a que susdietas se traslapanconsiderablemente, su alimento parece limitarse pot aftosy debido a que Strixvaria gradualmenteesm invadicndolos terdtorios historicamenteocupados pot Strix occidentalis. [Traducci6n de C6sar Mfirquez]

During the last century, the Northern Barred man 1976, Boxalland Stepney1982, American Or- Owl (Strix varia varia) has gradually expanded its nithologists'Union 1983, Dunbar et al. 1991, Ham- range westward acrossCanada and south into the er 1988, Hamer et al. 1994, Dark et al. 1998, Wright Pacific Northwest and northern (Grant and Hayward 1998). As a result, the range of the 1966, Campbell 1973, Shea 1974, Taylor and Fors- Northern now almost completelyover- laps the range of the Northern (Stnx • Presentaddress: 1103 Yew St., Bellingham,WA 98225 occidentaliscaurina)(Dark et al. 1998, del Hoyo et U.S.A. al. 1999). In some areas in and

221 222 HAMER ET AL. VOL. 35, NO. 3

Washington state, Barred Owls are now so abun- varied from young stands on recent clearcuts to dant they outnumber Spotted Owls (Hamer 1988, forests >200 yr old. Dunbar et al. 1991). Diets were estimated primarily from prey re- Although Barred Owls are generallybelieved to mains in regurgitated pellets, but the sample also compete with Spotted Owls for space (Dunbar et included a few freshly-killedprey remains found in al. 1991, Leskiw and Guti6rrez 1998), the extent to roosts or . Pellets from Barred Owls were col- which they also compete for food is unknown. Di- lected in 1985-89 and pellets from Spotted Owls ets of Northern Spotted Owls have been described were collected in 1986-89. Most pellets were col- in many areas (Barrows1980, Forsmartet al. 1984, lected during the spring and summer (April-Au- Rachards 1989, Ward 1990), but no data have been gust), and were of recent origin as indicatedby the published on diets of Northern Barred Owls in the fact that they had not been washed apart by rain area of range overlap with the Northern Spotted or . Thus, our analysisprimarily reflects the Owl. During radio-telemetry studies of Barred diet during spring and summer. Some of the owls Owls and Spotted Owls in Washingtonin 1985-89, that we studied were breeding, but the breeding regurgitated pellets were collected in roosts and statusof many of the owlswas not known in each areas to determine composition of the diets year. of the owls. We analyzed the samples, and We estimated the number of prey in pellets by herein describeand compare the diets of these two counting skulls,jaws, or bones of the appendicular closelyrelated speciesin an area of sympatry. skeleton, whichever gave the highest count. Num- bers of insectswere estimated from fragments of STUDY AREA AND METHODS exoskeletons.Biomass was estimated by multiply- ing the estimated number of individuals of each We collected pellets from 12 Barred Owl terri- speciesby the estimated mean mass of each spe- tories and 28 Spotted Owl territories in western cies, or by individuallyestimating the massof each Washington.All data from Barred Owls came from prey based on comparisonsof skeletalremains with the 317 km'• studyarea surroundingBaker I•ake on specimens of known age and mass. Estimates of the west slope of the CascadeRange in northern mean masswere obtained from a varietyof sources, Washington (Fig. 1). Most data from Spotted Owls including Maser et al. (1981), Chapman and Feld- (87% of prey items) came from 14 territories in hamer (1982), Steenhof (1983), and Forsman et the Baker Lake study area and nine territories in al. (1984). All comparisonswere based on the com- adjacent areas within the Mt. Baker-Snoqualmie bined sample for all owls, because sampleswere National Forest (Fig. 1). Small numbers of prey too small to estimate average diets for individual from Spotted Owls were also included from four territories. territories on the Gifford Pinchot National Forest We used the modified Simpson'sIndex (Odum in the southern WashingtonCascades (N = 14), 1975, Simpson1949) to estimatedietary diversity, and one territory on the Olympic Peninsula (N = and the modified Hill Ratio (Hill 1973, Alatalo 9). Barred Owls and Spotted Owls are sympatric 1981) to estimate evennessof prey in the diet. throughout this entire region. These indices range from 0-1, with larger values The Baker Lake studyarea was characterizedby indicating greater diversityor evenness.If all prey steep-sidedvalleys with elevationsranging from 244 were taken in equal numbers, then dietary even- m on the valley floor to 1800 m at the upper limits ness would be 1. We used Pianka's Index (Pianka of the fbrested zone on the slopesof Mount Baker. 1973) to comparedietary overlap;this index yields Mean annual precipitation was 254 cm, most of values i?om 0-1 (no overlap to complete overlap, which was rain during winter (Franklin and Dyr- respectively). All estimates of dietary diversity, hess1973). The studyarea was largely forested, ex- evennessand overlapwere basedon prey numbers. cept for small areas of talus, meadow,marsh, and To evaluatedifferences in timing of foragingand recent clearcuts.The dominant vegetationwas for- habitats used for foraging, we grouped prey based ests of western hemlock (7}uga heterophylla)and on their primary period of activity (nocturnal, di- Douglas-fir (Pseudotsugamenziesii) at lower eleva- urnal, both), primary habitat association(forest, ri- uons, and forests of mountain hemlock (T. merten- parian, meadow, talus), and primary behavior type szana)and Pacificsilver fir (Abiesamabilis) at higher (arboreal, semiarboreal, terrestrial/aquatic). We elevations(Franklin and Dyrness1973). Forestage then used X2 teststo compare the relativepropor- SEPTEMBER 2001 DIETS OF B•mt) •'qO SPOTrED OW•S 223

Whatcorn Baker Lake County StudyArea Okanogan County

Skagit "'

Chelan Snohomish :::::::::::: Cou13ty County

King County

Kittitas County

ß Spotted Owl ß Barred Owl

50 0 50 Kilometers

Figure 1. Locations where pellets were collected from Northern Barred Owls and Northern Spotted Owls on the Mt. Baker-SnoqualmieNational Forest (stippled area) in northwesternWashington, 1985-89. 224 HAMER ET AL. VOL. 35, No. 3

Table 1. Percent numbers (Num) and biomass (Bio) of prey in samplesof pellets collected from Northern Barred Owls and Northern Spotted Owls in western Washington, 1985-89.

BAm•ED OWLS SPOTTED OWLS

PREY 2N• % Num % Bio b iN3 % Num % Bio b

Mammals 202 76.1 74.5 285 96.2 98.6 Sorexspp. 26 9.8 0.4 11 3.7 0.2 Neurotrichusgibbsii 20 7.6 0.6 1 0.3 tr• Scapanusspp. 17 6.4 2.9 ------Ochotonaprinceps ------9 3.0 4.8 Lepusamericanus 22 8.3 35.0 9 3.0 13.4 Tamias townsendii 2 0.7 0.5 ------Tamiasciurusdouglasii 22 8.3 14.1 5 1.7 3.6 Glaucomyssabrinus 53 20.0 18.4 150 50.7 58.1 Thomomystalpoides ------I 0.3 0.3 Peromyscusmaniculatus 18 6.8 1.2 61 20.6 4.5 Neotoma cinerea ------13 4.4 11.6 Clethrionomysgapperi 3 1.1 0.2 20 6.8 1.6 Microtusspp. 2 0.7 0.2 ------Microtusoregoni 16 6.0 0.9 2 0.7 0.1 Zapustrinotatus 1 0.4 0.1 1 0.3 0.1 Mustela erminea ------2 0.7 0.3

B•rds 29 11.0 19.4 8 2.8 1.4 Bonasa umbellus 6 2.3 11.3 ------Otus kennicottii 1 0.4 0.4 ------Corvusspp. 3 1.1 4.2 ------Unident. small 19 7.2 3.5 8 2.8 1.4

Miscellaneous 34 12.9 6.1 3 1.0 tr c Fish (small salmonids) 7 2.6 4.7 ------Amphibians (f?ogs) 15 5.7 1.4 ------Molluscs (snails) 2 0.8 tr .... 10 3.8 tr c 3 1.0 tr •

Totals 265 100.0 100.0 296 100.0 100.0

Indicatesnumber of individualprey identified in pellets. Total biomasswas 32 745 g for Barred Owlsand 29 154 g for SpottedOwls. tr -< 0.05%. tlon of prey biomassin different categories.Each ted Owls was dominated by mammals, which specieswas assignedto only one activity period, comprised96.2% of prey numbers,and 98.6% of habitat association,and behavior type, with the ex- prey biomass.In contrast,Barred Owl dietsinclud- ception of snowshoe hares (Lepus americanus) ed only 76.1% mammals,with the balance made which were split evenlybetween forest and riparian up of birds, fish, ,snails, and insects.In terms habitats.Although snowshoehares were sometimes of total biomass,the most important prey in diets activeduring the day,we labeled them asnocturnal of Spotted Owls and Barred Owls were northern becausethey are most active at night (Keith 1964). flying (Glaucomyssabrinus) and snowshoe hares,respectively. R•SU LTS Mean mass of individual prey captured by We identified 265 prey items from 13 Barred Barred Owls and Spotted Owls was 123.6 and 98.5 Owl territories and 296 prey items from 26 Spotted g, respectively.Both speciescaptured prey in a Owl territories (Table 1). Diets of Barred Owls in- broad range of sizecategories, up to and including cluded ->24 ,and diets of Spotted Owls in- snowshoehares weighing about 1000 g each. How- cluded 17 species.The sampleof prey from Spot- ever,diets of SpottedOwls were dominatedby prey SEPTEMBER 2001 DIFTS OF BARRED AND SPOTTED OWLS 225

60 • Both •, Diurnal [] BarredOwl *>' Nocturnal [] SpottedOwl :>_- • 40

Terrestdal Semi-arboreal o• 2o Arboreal

lO

1-10 11-40 41-80 81 - 160 161 - 300 >300 • Meadow Prey size classes based on body mass (g) ß• Eipa•an Figure 2. Distribution of prey by size classin diets of -r F•st I I Northern Spotted Owls and Northern Barred Owls in 10 2D 30 40 50 60 lO 80 90 100 western Washington, 1985-89. %of prey biomass

Figure 3. Diets (% of total biomass)of Northern Spot- ted Owls and Northern Barred Owls in westernWashing- in the 81-160 g range, whereas prey taken by ton, subdivided based on habitat associations, behavior Barred Owls were more evenly distributed across types,and primary activityperiods of prey. all prey-sizecategories (Fig. 2). The diet of Barred Owls was considerablymore DISCUSSION diversethan the diet of Spotted Owls (SimpsonIn- dex = 0.917 vs. 0.699). The distribution of prey in Our results indicated that Northern Spotted the diet of Barred Owls was also more even than Owls preyed on a fairly broad range of prey, but in the diet of Spotted Owls (Hill Index = 0.814 vs. primarily focused on a few speciesof mammals. 0.533). That is, Barred Owls captured many differ- That is, the distribution of prey in the diet wasvery ent kinds of prey in similar numbers,whereas Spot- uneven. In contrast, the Barred Owl was more of ted Owls tended to concentrate on a few kinds of a generalist,preying on a broader range of species prey. The Pianka Index of dietary overlap between at lower frequencies.This finding agreeswith pre- the two specieswas 0.76. vious studies in which diets of Northern Spotted Diets of both specieswere dominated by noctur- Owls were generally dominated by a few types of nal , but Barred Owl diets included more arboreal or semiarboreal forest mammals (Barrows diurnal prey (Fig. 3, X•2 = 64.7, P< 0.001). Barred 1980, Forsman et al. 1984, 2001, Ward 1990), Owl diets were also dominated by terrestrial spe- whereas diets of Barred Owls typically include a cies,whereas Spotted Owl dietswere dominated by diversemixture of prey (Bent 1938, Hodges 1947, arboreal or semiarborealspecies, especially north- Smith 1952, Sweeny 1959, Korschgen and Stuart ern flying squirrelsand bushy-tailedwoodrats (Neo- 1972, Rhodes 1974). toma cinerea)(Fig. 3, Table 1, X22 = 72.4, P < Although both specieshunted primarily in for- 0.001). Diets of both specieswere dominated by ests, the composition of the diet suggestedthat animals associated with forest habitats, but diets of Barred Owls made greater use of meadowsand ri- Barred Owls included more speciesassociated with parian areas than did Spotted Owls. During the riparian areas,, or meadows,and fewer spe- study,we often observedBarred Owls perched at cies associatedwith talus (Fig. 3, XS• = 62.3, P < the edge of marshesand ponds, or directly above 0.001). Diets of Spotted Owls included 16.4% bio- small streams (unpubl. data). A sample of pellets massfrom mammals associatedwith rock outcrops collected from two Barred Owl territories in western or talus, such as bushy-tailedwoodrats and pikas included mostlymicrotines associated with (Ochotonaprinceps), whereas none of the prey cap- meadowsand riparian areas (Marks et al. 1984). tured by Barred Owlswere typicallyassociated with Potential sources of variation in our data includ- rock outcropsor talus (Fig. 3). ed differencesamong years,territories, and breed- 226 HAMEP, ET AL. Vot.. 35, No. 3 ing status.We did not have enough data to stratify not particularly effective in this case,especially in the samplesand evaluate these effects. However, view of the fact that Barred Owls have recently in- Spotted Owl territories in the Baker Lake study vadedand taken up residencein areastraditionally area often overlapped two or more Barred Owl ter- occupied by Spotted Owls. ritories and the habitat compositionof Barred and ACKNOWLEDGMENTS Spotted Owl home ranges showedfew differences (unpubl. data), indicating both species of owls S. Seim with the USDA Forest Service, Pacific Northwest were likely feeding on similar prey populations. Research Station, helped collect pellets, trap owls, and track owls. This studywas funded by the USDA Forest Similarly,Herter and Hicks (2000) found consid- Service, Pacific Northwest Research Station. erable overlap in the kinds of forestsoccupied by Barred Owls and Spotted Owls in the central Cas- LITERATURE CITED cades of Washington. Therefore, we believe that ADAMCIK, R.S., A.W. TODD, AND L.B. KEITH. 1978. De- the differenceswe observedwere primarily the re- mographic and dietary responsesof Great Horned suit of differences in prey selectionas opposed to Owls during a snowshoehare fluctuation. Can. Field- differencesin prey availability. Nat. 92:156-166. The high proportion of diurnal animals in A•AT•LO, R.V. 1981. Problems in the measurement of Barred Owl pellets suggestedthat these owlswere evennessin ecology.Oihos 37:19-204. more active during the day than were Spotted AMERICAN ORNITHOLOGISTS' UNION. 1983. Check-list of Owls. This was confirmed from observations of ra- North American birds. Allen Press, Inc., Lawrence, KS dio-marked Barred Owls on the Baker Lake study U.S.A. area, which moved, on average, 131 m/hr during BAPmOWS,C.C. 1980. Feeding ecologyof the SpottedOwl the day and 260 m/hr at night (unpubl. data). in California. RaptorRes. 14:73-77. BENT, A.C. 1938. Life histories of North American birds Spotted Owls studied by Sovernet al. (1994) in the of prey. Part 2. SmithsonianInst., U.S. Natl. Mus. Bull Washington Cascadesonly moved 20.6 m/hr on 170. average during the day. BOX•L, P.C. AND P.R. STEPNE¾.1982. The distribution Although the distributionof prey speciesin pel- and status of the Barred Owl in . Can. Field- lets of Barred Owls was more diverse and more Nat. 6:46-50. eventhan in pelletsof SpottedOwls, the 76% over- CAMPBF.LL, R.W. 1973. Coastal records of the Barred Owl lap in the two samplessuggested that the two spe- for British Columbia. Murrelet 54:25. cies may compete for food, especiallyif prey be- CHAPMAN,J.A. ANDG.A. FELDm•MEP,.1982. Wild mammals comes limiting. That prey is limiting for Spotted of . John Hopkins Press, Baltimore, Owls and many other speciesof owls in the north- MD U.S.A. ern hemisphereseems likely, given that they do not DAPd•,S.J., RJ. GUTIERREZ,AND G.I. GOUI,D,JR. 1998. The Barred Owl (SO•x va•a) invasion in California. Auk breed in many years (Southern 1970, Adamcik et 115:50-56. al. 1978, Mikkola 1983, Forsman et al. 1984, 1996, DEL HOYO,J., A. ELLIOTTAND J. SARGATAL[EDs.], 1999 Hayward and Garton 1988). Handbook of the birds of the world, Vol. 5. Barn Owls Some previouscomparisons of dietary composi- to hummingbirds.Lynx Edicions,Barcelona, Spain. tion in owls have shown clear differences between DUNBAR, D.L., B.P. BOOTH, E.D. FOPSMAN, A.E. HETHER- species, but others have demonstrated consider- INGTON,AND DJ. WILSON.1991. Statusof the Spotted able overlap between some species(e.g., Korsch- Owl, So•x occidentalis,and Barred Owl, Starxva•a, •n gen and Stuart 1972, Marti 1974, Herrera and Her- southwestern British Columbia. Can. Field-Nat. 105 aldo 1976, Hayward and Garton 1988). However, 464-468. high dietary overlap does not necessarilyprove FOPSMAN,E.D., E.C. MESLOW,aND H.M. WIGHT. 1984. Dis- that speciescompete for food becausethey can still trihution and biology of the Spotted Owl in . avoid direct competition by foraging in different Wildl. Monog½87. --, S. DESTEFANO,M.G. RAPUAEL,AND RJ. GuTIglmEz areas or at different times. Our data suggested lEDS.I, 1996. Demography of the Northern Spotted some species-specificdifferences in timing and lo- Owl. Stud. Avian Biol. 17. cation of foraging, but for the most part, Barred --, I.A. OTto, S.G. SOVERN, n. TAYLOR, D.W. HAYS, Owls and Spotted Owls were similar in that they H. Ai. LEN, S.L. ROBERTS,AND D.E. SEAMAN. 2001. Re- were primarily nocturnal and foraged primarily in gional, local and temporal variation in the diet of the forests.Therefore, we suggestthat the above men- Spotted Owl in Washington.J. Raptorlies. 35:141-150. tioned approaches to avoiding competition were FRANKLIN,J.F. ANDC.T. DYRNESS.1973. Natural vegetation SEPTEMBER2001 D•ETS OF BARRED AND SPOTTED OWLS 227

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