j RaptorRes. 31 (2):160-166 ¸ 1997 The Raptor ResearchFoundation, Inc.

GREAT GRAY ( NEBULOSA NEBULOSA) AND FOREST MANAGEMENT IN NORTH AMERICA: A REVIEW AND RECOMMENDATIONS

JAMESR. DUNCAN Box 253, Balmoral, Manitoba, Canada ROC OHO

AgSTP,ACT.--Great Gray (Strix nebulosanebulosa) populations in North America have likely been stable over the past 10-100+ yr. Local populationsfluctuate in responseto food supplyand/or nest- site availability.Breeding Great Gray Owls require preexistingnest structuresin forest standstiNat are adjacent to open foraging habitat, preferablywitIN hunting perches.Current forestry practiceshave the potential to affect about 75% of the Great Gray Owls' breeding range in North America.Intensive timber managementtypically removes large diameter and deformed nest trees, leaning trees usedby juveniles for roostingbefore they can fly and standswith dense canopy closure used by juveniles and adultsfor coverand protection.Modified forestmanagement can, however,create new foraginghabitat by openingup large, denseforest stands. Specific recommended guidelines include restriction of harvest unit size (-<5-10 ha), but within a mosaic of multi-sized units, retention of forest stands within 300 m of known or potential nest trees/sites,provision of hunting perchesin cut-overareas, ensuring irregu- larly shapedharvest units and maintenanceof forestedtravel corridors between cut-over areas. Because Great Gray Owls can breed on home rangesup to 800 km apart in successiveyears, integration of local managementregimes at a landscapescale is recommended.Ideally, spatio-temporalpatterns of natural disturbance(e.g., fire) shouldbe emulated in a managementplan to sustainthe region'snatural bio- logicaldiversity, including Great Gray Owlswhen appropriate. KEyWORDS: Strix nebulosa;North America; forest management; habitat us< GreatGray Owl.

E1 Gran Bfiho Gris Strix nebulosanebulosa y administraci6nforestal RESUMEN.-•Laspoblaciones del bfiho Strix nebulosanebulosa en norte amtrica han estado estable con seguridadlos pasados10 a 100+ aftos.Poblaciones locales fluctfian en reaccitn a suministrosde comida y/o la disponibilidada sitiosde nido. Bfihos en cria necesitanestructuras de nidos hechosen bosque que estin pegadosal hibitat de forraje libre preferible con perchasde cazar.Costumbres actualmente de forestalestienen la potenciapara afectarcasi 75% de el campode cria de el bfiho en norte amtrica. La administracitnde •naderacon intensidadtipicamente quita iirbolescon diimetros grandesy irboles deformadoscon nido, irboles inclinadosusados porjuveniles para percha antesque puedenvolar, ireas con densidadcerrada usada porjuveniles y adultospara cubrirsey proteccitn. Administraci6nde bosque modificadospueden, sin embargo, inventar nuevo hibitat de ibrraje con abriendo grandesydensas ireas de bosque. Recomendacionesespecificos de las reglas incluyen: (1) restriccionesde cosecha de cierta altura (<5-10 ha), pero dentro de un mosaicode varios tamafios, (2) retenci6n de ireas de bosqueconocido o en potencia de irboles/sitios con nido dentro de 300m, (3) provisi6nde perchas de cazar en ireas cortadas,sugiriendo conjuntos de cosechacon formas irreguladas,y (4) manteni- miento de corredoresde viaje en ireas de bosqueen parcelascortadas. Porque bfihos se pueden criar en campos naturaleshasta 800 km aparte en aftos seguidos,integraci6n de administracitn local con una escala de paisaje es recomendado. ldealmente, que muestrasde spacio-temporalde disturbios naturales(e.g., lumbre) debe ser emulado en el proyectode la administracitn para sostenerlas regiones, diversidadnatural y bioltgica, incluyendobfihos cuandoes oportunado. [Traducci6n de Rafil De La Garza,Jr.]

In November 1995, a symposium on Holarctic provided and in response to specific questions. raptor responsesto forest managementwas held in This paper is a review of such information for the Duluth, MN U.S.A. Presenters were asked to review Great Gray Owl (Strix nebulosa)in North America. information on a speciesaccording to guidelines The Great Gray Owl is unevenly distributed

160 JUNE1997 GRF•ATGRAY OWLS AND FOREST MANAGEMENT 161 across the Holarctic over 30 million km 2 of boreal erence and use. To best evaluate the relationship forests in Eurasia and North America (Clark et al. between Great Gray Owl populations and forest 1987). Throughout its rangeit occupiesforest hab- management,one would ideally have knowledgeof itat; however, it also successfullybreeds north to the 'habitat requirementsand/or prefer- within the transition zone between the boreal for- ence. Since thesedo not currently exist,except for est and the treeless tundra (Lang et al. 1991). In a few habitat preference studies (e.g., Servos western North America, it extends its range south 1986), the following reviewof Great Gray Owls and by occupyingmontane forestsin the RockyMoun- forest management is based primarily on nesting tains, the Cascade Range and the Sierra Nevada and foraging habitat use data. Range (Duncan and Hayward 1994). It is the larg- est northern forest owl, although not the heaviest. POPULATION TRENDS Its yellow eyes, set in a facial disk with nearly con- Typically considered rare, the Great Gray Owl centric gray and white rings, are framed by a large occursat low densitieswithin its range (Nero 1980, round head that lacksear tufts.Two subspeciesare Bull and Duncan 1993). Nero estimated a conti- currently recognized: S.n. nebulosain North Amer- nent-wide population of 5000-50 000 owlsin North ica and S.n. lapponicain Eurasia (Bull and Duncan America; up to 25 000 breeding pairs have been 1993). estimated for Canada (Kirk and Hyslop pers. Great Gray Owls in the boreal forest region of comm.). However, Great Gray Owls are easilyover- North America have greater diet similarity(% sim- looked and are probably more common. I estimate darity) to populationsin Eurasia (i = 95.3%, SD the current North American population of Great = 1.46, N = 3, range = 94-96.9%) than to those in the southwestern U.S. that occur in montane GrayOwls to be 20 000-70 000 breedingpairs. The range of this estimate reflects the dependence of forests(i = 49.8%, SD = 13.9, N = 9, range = 39- this specieson prey, primarily microtines (e.g., Ms- 69.4, Duncan 1992). Great Gray Owls from both o'otusspp.), that exhibit unstablepopulation fluc- continentsexhibit similar plasticityin selectionof tuations over 3-5-yr periods (Duncan 1992). nest sites, although ground nesting is more fre- The degree to which local prey populationfluc- quently reported from northern Europe (Mikkola 1983). They appear to use similar foraging habitat tuations are synchronousover the Great Gray m Eurasia and North America (Mikkola 1983, Dun- Owl's North American range is unknown. Extrinsic can and Hayward 1994). Therefore, one would ex- factors such as severe temperatureswith little or pect S.n. nebulosaand S.n. lapponicapopulations to no snow cover, occasionallysynchronize (reduce) respond similarly to forest changesthat alter the microtine populationsover large geographicareas. availability of nest sites and/or foraging habitat. More commonly, other factors (both intrinsic and EurasianGreat Gray Owls are paler with more ver- extrinsic) appear to disrupt such synchronizingef- tical barring, perhaps relating to habitat differ- fects (Pruitt 1968, Lidicker 1988). ences (Oeming 1955, Mikkola 1983); they also ap- Great Gray Owls are most often seen in winter, pear to be more aggressivetoward humans at nest and in more heavily settled areas along the south- sitesthan North American Great Gray Owls (Nero ern portions of their breeding range. Irregular, 1980, Mikkola 1983). large-scale,continental movements of popula- Considerableattention has recently been direct- tions are called irruptions (Collins 1980). The ed toward evaluating the statusof North American Great Gray Owl is one of severalirruptive species Great Gray Owl populations (Winter 1986, Hay- of owlsin North America. When large numbersof ward 1994). As with any species,this invariablyre- appear locally, it is commonly referred to as quires an assessmentof the habitats that are an influx or invasion. The fluctuations in winter thought or known to sustainpopulations. Current occurrenceof Great Gray Owls in these areas (Fig. forestry practices have the potential of affecting 1) suggestthat large-scaleirruptions occur lessfre- about three-quarters of the Great Gray Owls' quentlythan local invasions.Large-scale irruptions breeding range in North America (Bull and Dun- are thought to be the product of one or more years can 1993). McCallum (1994) providesa critical re- of high Great Gray Owl reproductive successfol- view of the complexities involved in determining lowed by a widespreaddecrease in prey availability owl-habitat relationships and the importance in on the breeding range. The recent apparent in- distinguishingbetween habitat requirements, pref- crease in Great Gray Owl winter occurrences is 162 DUNCA• VOL. 31, No. 2

lOO

117 8o

o 60

• 40

E "• 20 z

1890 1900 1910 1920 1930 1940 1950 1960 1970

Year

Figure1. NorthAmerican Great Gray Owl winter irruptions based on specimensand sight records, 1890-1976. Data from southern Canada and northern U.S.A. Modified from Collins (1980). likely due to increased observer effort and in- local human-causedmortality (Nero and Copland creased accessto winter habitat (Collins 1980). 1981). The availabilityof nest sitesand suitablefor- The overall North American Great Gray Owl aging habitat are considered the most important long-term (•10 yr) population trend is unknown. factors limiting Great Gray Owl populations (Dun- There are no long-term, rigorous or standardized can and Hayward 1994). I will addressthese first, Great Gray Owl breedingpopulation trend data on but some discussion is warranted on the short- a range-wide, regional or local scale. There is cir- term (3-5 yr) influence of prey availability,which cumstantial evidence that some local and/or re- can profoundly affect conclusions drawn from gional populationshave either remained stable,in- short-term (•5 yr) local Great Gray Owl surveys. creasedor decreasedover periods of •10 yr (Fyfe Diet. Local Great Gray Owl breeding densities 1976, Collins 1980, Nero 1980, Nero et al. 1984, fluctuate considerably,primarily due to the insta- Winter 1986, Bryan and Forsman1987, Franklin bility of microfine prey populations (Henttonen 1988, Collins and Wendt 1989, Bull and Henjum 1986, Duncan 1992). Individual radio-marked owls 1990, Duncan 1992). Ongoing local surveysare un- in Manitoba and northern Minnesota have dis- derwayat a number of locationsin Canadaand the persed up to 684 km between breeding home U.S. Nonetheless,it is useful to speculate on the rangesin responseto prey populationcrashes (• = relationship between forestry and local Great Gray 328.8, SD -- 184.9, N = 27, range = 41-684 km); Owl population trends. those marked birds that did not disperse(N-- 11) Reliable population distribution data may only did not survive (Duncan 1992). be available after widespread and standardized In contrast, breeding Great Gray Owl popula- monitoring programshave been operating for sev- tions in montane regions of the western U.S. are eral years. While classic"playback" survey tech- thought to be relatively stable (Winter 1986, Bull niques (Smith et al. 1987) can increasethe num- ber of Great Gray Owlsdetected by up to 40%, it and Henjum 1990, Bull and Duncan 1993). In is unlikely that they can yield data that identifies northeastern Oregon, the maximum distancetrav- statisticallysignificant population trends. elled from nest sitesby adult radio-marked Great Gray Owlsaveraged 13.4 km (N = 23, range = 2.4- PP.IM)mV F^CTOe. S AssocI^TV, t) W•H Tm•t)s 43.2 km; Bull and Henjum 1990). Bull and Hen- North American Great Gray Owl populations are jum (1990) speculatethat in yearswhen prey pop- relativelyunaffected by human persecutionor di- ulations are low, Great Gray Owls in their study rect chemical effects (Nero 1980, Winter 1986, area remain as nonbreeding residents. In Califor- Hayward 1994), notwithstandingoccasionally high nia, Winter (1986) suggestedthat under similar cir- JUNE1997 GREATGRAY OWLS AND FOREST MANAGEMENT 163 cumstances,Great Gray Owls possiblysustain them- thobiumspp.) causesexaggerated branch configu- selveson pocket gophers (Thomomysspp.), as well rations which are conduciveto nesting and/or that as other prey species.Pocket gopher populations promote nest-buildingactivity by raptors and corv- are relatively stable and noncyclical (Chase et al. ids (Bull and Henjum 1990). Nest-siteavailability 1982, Teipner et al. 1983). generally increaseswith forest stand age (Duncan Because breeding populations of Great Gray and Hayward 1994). Owls in different parts of their North American Tree pathogen outbreaksand other nest-sitecre- range exhibit variablebreeding dispersal strategies, ating disturbancestend to have a clumped spatial owl surveysundertaken to determine distribution and temporal distribution. The territories of the or local population trend data need to be con- Northern Goshawkand Broad-wingedHawk (Buteo ducted for at least as long as one prey population platypterus)often hold several stick nests (Palmer cycle to ensure reliable results.Therefore, ideally, 1988, Goodrich et al. 1996). If nest sites are limit- one should concurrently monitor prey populations ing and have a clumped distribution, perhaps it is and Great Gray Owl diet. Great Gray Owls may or no coincidence that Great Gray Owls have nested may not be present during yearsof low prey pop- in what has been described as loose colonies ulations;if presentat suchtimes, they are lesslikely (Wahlstedt 1976, Bull and Duncan 1993), a trait to respond to conspecificcall playbackused in sur- that undoubtedly also relates to their specialized veys(Smith et al. 1987). diet (Mikkola 1983). Nest-site and Foraging Habitat Availability. Nest-site availability appears to be important These factors are significandyaffected by natural enough to Great Gray Owls that they have evolved forest disturbances such as disease outbreaks, suc- the ability to relocate and use nest sites hundreds cession and the effects of fire and wind (Larsen of kilometers apart over 2 or more yr (Duncan 1980, Habeck 1994). The temporal and spatial 1992). Therefore, forest management activities scale of the impact of these ecologicalprocesses, that reduce the number of nest sites (e.g., fire sup- and the relative stabilityof prey biomassproductiv- pression,disease control and shorter rotation pe- ity, have had a strong influence on the riods) have the potential to reduce Great Gray Owl of Great Gray Owl life history traits,such as breed- breeding densities. Mitigation by installing artifi- ing dispersal and post-fledging nest-site fidelity cial nest structuresis impractical at larger spatial (Duncan 1992). Consequendy,forest management scales,but works well locally (Bull and Henjum doesaffect Great Gray Owl populationsby altering 1990). natural disturbanceregimes and by the application Foraginghabitat. Voles and/or pocket gophers of managementprotocols. Anecdotal observations dominate the diet of Great Gray Owls (Duncan and current knowledgeof Great Gray Owl ecology and Hayward 1994). Microtine voles generally oc- permit some speculation about how forest man- cupy moist grass/sedgeopenings and open forests agement likely impacts Great Gray Owl popula- with herbaceous ground cover. Meadows consid- tions. ered in good ecological condition for voles, and Nestsites. Great Gray Owls use preexistingstruc- hence Great Gray Owls, are dominated by a variety turesfor nesting,including desertedor vacantstick of climax perennial grasses,sedges, and forbes. nests of some Buteohawks, Northern Goshawks (Ac- Factors that reduce vole abundance (e.g., moder- cipitergentilis) and larger corvids(Duncan and Hay- ate to heavygrazing) decreasethe suitabilityof for- ward 1994). They will also nest on a variety of ar- aging areasfor Great Gray Owls (Winter 1986). tificial structures,in natural depressionsin broken- Great Gray Owl foraging habitat includes bogs, topped snags or stumps, and, rarely, on the fens, muskeg, peatland, natural meadows, open ground, on rock cliffs, or on top of haystacks(Mik- forests and selective and clear-cut logged areas kola 1983, Duncan and Hayward 1994). Nest-struc- (Nero 1980, Mikkola 1983, Servos 1986, Winter ture type or nest-treespecies appears to be lessim- 1986). Dense coniferous stands (e.g., jack pine, P•- portant than nest-site habitat characteristicsand nus banksianaand black spruce, Picea mariana), the availability of foraging habitat (Duncan and open areas with few or no trees and habitats with Hayward 1994). Forest tree pathogens (e.g., ants dense shrub layers are avoided by hunting Great and fungi) and fire can weaken trees, possiblyre- Gray Owls (Servos1986). suiting in tree death and subsequentsnag or stump Great Gray Owls hunt primarily from perches, formation. More directly, dwarf mistletoe (Arceu- listening for prey and watching the ground intent- 164 DUNCAN VOL. 31, NO. 2 ly. When prey is detectedthe owl usuallystoops eas provide suitable nest structures. Therefore, only a short distance,generally no more than 50 Great Gray Owl populationscan likely persistwith m. Bull and Henjum (1990) recorded an average some amount of forest cutting. The following rec- perch to prey distanceof 10.5 m. ommendations are based on what is generally Bull and Duncan (1993) reported that Great known about Great Gray Owl ecology and not on Gray Owlsalso forage in open forests.In northeast specific responsesof owls to measured habitat Oregon,males foraged in standswith 11-59% can- changes.New information should alter these spe- opyclosure (Bull and Henjum 1990). Thesestands cific conservationstrategies through an adaptive had meadowlike grass-dominatedground cover. management approach. The large lifetime home Open tamarack(Larix laricina)stands with dense rangesof Great Gray Owls, e.g., in Manitoba and sphagnum/sedge/grassunderstory are often used Minnesota (Duncan 1992), suggestthat a coordi- by foragingGreat Gray Owlsin Manitoba. nated landscape-levelperspective to management While hunting, Great Gray Owlsperch at varying is needed to maintain viable populations.With this heights, but usually3-5 m above the ground, in in mind, I suggestthe following managementrec- both live trees and snags adjacent to or within ommendations. open grassyareas (Duncan and Hayward 1994). OccurrenceData. The occurrenceof Great Gray Perch heightsfor male Great Gray Owls averaged Owls is poorly documented in many parts of its 5.5 m in Oregon (Bull and Henjum 1990). In Cal- range (Duncan and Hayward 1994). A review of ifornia, perch heightsvaried from 0-12.2 m above historic site-specificoccurrence information (e.g., the ground (i = 3.3, SD = 2.3, N = 143; Winter literature, specimendata and personal communi- 1986). Great Gray Owlsrarely hunt while perched cations) is an appropriate first step. It should be on the ground or while flying (Bull and Duncan determined if Great Gray Owls currently occur in 1993). the managementarea becausepre- and post-har- SuccessfulGreat Gray Owl reproduction de- vest occurrence information can be used to adapt pendson the availabilityof suitableforaging hab- harvestguidelines accordingly. Secondly, the pres- itat within 1-3 km of nest sites (Bull and Henjum ence or absenceof Great Gray Owls may influence 1990, Duncan and Hayward 1994). Such habitat the degree to which forest resourcesare managed. can be ephemeral over shorter periods (e.g., post- Assumingthat forest resourcesare to be managed fire or post-cuttingearly successionhabitat) or rel- for this species(e.g., the managementarea is with- ativelypermanent (e.g., sedgemeadows, peatland in suitable habitat and is within or adjacent to its and muskeg). Burned or cut-over areas can pro- expectedNorth American range) then one must vide foragingopportunities for Great Gray Owlsfor decideto implementa landscape-levelor a specific 20 yr or more, dependingon the rate of succes- managementregime only at siteswhere Great Gray sion or on post-harvestmanagement practices. Owls are known to occur. The efficiency of various Kirkland (1977) and Parker (1989) reported that meadow vole populations increase 3-18 yr after surveytechniques have not been rigorouslytested clear-cuttingforests. and survey methods may not be practical over Great Gray Owl population declinesfrom ances- large expansesand in remote areas.Therefore, in tral levelshave been reported in California (Win- the absenceof Great Gray Owl occurrencedata, ter 1986). These were attributed to habitat an appropriate landscape-levelmanagement rec- ommendation would be the retention of a 300 m changes,e.g., fire suppressionand overharvesting of forests.Paradoxically, clear-cuts can create Great buffer area around natural openingssuch as mead- Gray Owl foraging habitat in denseforests in pre- ows or fkns (Winter 1986, Bouchart 1991). viouslyunoccupied areas. Clear-cut Size. Clear-cutsup to 10 ha in size are probably ideal tbr Great Gray Owls, but these FOREST MANAGEMENT RECOMMENDATIONS should occur within a mosaic of multi-sized units Throughout its North American range, the acrossa landscape.Great Gray Owlswill use larger Great Gray Owl thrives in a variety of habitats clear-cuts,but typicallythey catchprey within 50 m (Duncan and Hayward 1994). It is adapted to cap- of hunting perches.While they hunt from isolated turing prey in permanentopen habitatsand in ear- perchesin open areasfarther than 50 m from edg- ly forest successionalstages (Nero 1980). Older es, in thesesituations they are more vulnerableto and mature forest habitats adjacent to foraging ar- avian predatorssuch as Northern Goshawksand JUNE1997 GREATGP•¾ OWLS AND FOREST MANAGEMENT 165

Great Horned Owls (Bubo virginianus) (Duncan recommended as a minimum guideline (see also 1987). Winter 1986, Bouchart 1991). The provisionor re- Clear-cut Shape. Because Great Gray Owls fre- tention of leaning trees used byjuveniles for roost- quently hunt from forest edges, irregular cut ing before they can fly and standswith dense can- shapeswith convoluted or scalloped-shapededges opy closure (>60%) for cover and protection would reduce mortality from avian predators.This (from heat stressand predators) of adults and ju- designwill therefore also increaseaccess to newly veniles is also thought to be critical (Duncan and created open foraging habitat. Becausethey catch Hayward 1994). Maintenance of forested travel cor- prey within 50 m of hunting perches, larger cuts ridors between nesting habitat is considered nec- should therefore be elongated so that the maxi- essaryto minimize predation of dispersingadults mum distance across the cut is <100 m. and juveniles. Nest-site Availability. Timber management has BecauseGreat Gray Owl breeding dispersalcan reduced nesting opportunities for all forest rap- be significant (e.g., up to 800 km, Duncan 1992), tors, including Great Gray Owls (Habeck 1994). a coordination of local management regimes at a Therefore, the impact of managementpractices on landscape scale is recommended. Ideally, spatio- nest-siteavailability needs to be assessed. temporal patterns of natural disturbance (e.g., Some types of nest structures (e.g., mistletoe fire) should be emulated in managementplans to brooms and snags) used by Great Gray Owls are sustaina region's naturally occurring biological di- either directly or indirectly created by tree patho- versity,including Great Gray Owls when appropri- gens (e.g., insectsand fungi). These pathogenscan ate. cause significant financial losses on commercial ACKNOWLEDGMENTS forest land. Likewise, fire-killed trees can provide Great Gray Owl nest sites (e.g., snags),but fire also I thank Gerald Niemi and others involvedwith orga- nizing and hosting the November 1995 Symposiumon destroysvaluable timber. Great Gray Owls often Raptor Responsesto Forest Management: A Holarctic nest in stick nests built by large corvids and diur- Perspective,for the opportunity to write this paper. The nal raptors. These sitesoccur more frequently in Symposiumwas an event that I will fondly remember I older forest standswith larger trees. Shorter rota- also thank Evelyn Bull, Patricia Duncan, James Habeck, Seppo Sulkavaand Daniel Varland for their review of ear- uon periods or selectiveremoval of large-diameter lier drafts of this manuscript. trees has reduced nest-site availability. Forest pathogen control, fire suppressionand shorter ro- LITERATURE CITED tation periods are economically important forest BOHM,R.T. 1985. Use of artificial nestsby Great Gray management practicesthat impact Great Gray Owl Owls, Great Horned Owls, and Red-tailed Hawks in nesting opportunities. The provision of artificial northeastern Minnesota. Loon 57:150-151. nest structures,while locally effective, is labor in- BOUGHART,M.L. 1991. Great Gray Owl habitat use in tensiveand costly(Bohm 1985). Their use may still southeastern Manitoba and the effects of forest re- be justified in certain situations. source management. M.S. thesis, Univ. Manitoba, Winnipeg, Canada. Influence of Residualsand Optimal Mix, Includ- BRYAN,T. AND E.D. FORSMAN. 1987. Distribution, abun- ing Dispersal Corridors. Leaving residuals in cut- dance, and habitat of Great Gray Owlsin southcentral overs (e.g., live trees and dead snags)provide im- Oregon. Murrelet68:45-49. portant hunting perches.In Manitoba, the smallest BULL,E.L. ANDM.G. HENJUM.1990. Ecologyof the Great Great Gray Owl nest stand was 4 ha (N = 18, me- Gray Owl. USDA For. Ser. Gen. Tech. Rep. PNW-GTR- dian = 232 ha) and there were at least 69 ha (N 265, Portland OR U.S.A. = 15, median = 136 ha)of foraging habitat within BULL,E.L. ANDJ.R.DUNCAN. 1993. Great Gray Owl (Stnx 1 km of nest sites. Bull and Henjum (1990) re- nebulosa).Pages 1-16 in A. Poole and F.B. Gill [EDs.], ported that 52-99% of the area within 500 m of The birds of North America. Academy of Nat. Sc•., nest sitesin Oregon wasforested. While Great Gray Philadelphia, PA and Am. Ornithol. Union, Washing- ton, DC U.S.A. Owls have successfullynested on the edge of for- CHASE,J.D., W.E. HOWARDAND J.T. ROSEBERRY.1982. aging habitat, the distanceto the nearest opening Pocket gophers, Geomyidae. Pages 239-255 in J.A. averaged 256 m in Manitoba (Bouchart 1991) and Chapman and G.A. Feldhamner lEDs.I, Wild 143 m in Idaho and Wyoming (Franklin 1988). of North America: biology,management and econom- Therefore, retention of forest stands within 300 m ics.John Hopkins Univ. Press,Baltimore, MD U.S.A. of known or potential Great Gray Owl nest sitesis CLARK,R.J., D.G. SMITHAND L. KELSO. 1987. Distribu- 166 DUNCAN VOL. 31, No. 2

tional status and literature of northern forest owls. 1991. Great Gray Owl and Northern Hawk Owl nests Pages47-55 in R.W. Nero, RJ. Clark, RJ. Knapton at Churchill, Manitoba. BlueJay49:208-214. and R.H. Hamre [EDS.],Biology and conservationof LARSEN,J.A. 1980. The boreal ecosystem.Academic northern forest owls. USDA For. Serv. Gen. Tech. Press, NY U.S.A. Rep. RM-142, Fort Collins,CO U.S.A. LIDICKER,W.Z., JR. 1988. Solving the enigma of micro- COLLINS,B.T. aNDJ.S. WEN•)T. 1989. The breeding bird tine "cycles".J. .69:225-235. survey in Canada 1966-1983. Analysisof trends in MCCALLUM,D.A. 1994. Methods and terminology used breedingbird populations.Tech. Rep. Ser. No. 75, with studiesof habitat associations.Pages 5-8 in G.D. Canadian Wildlife Service, Ottawa, Canada. Hayward and J. Verner [T•.CH. EDS.], Flammulated, COLLINS,ICM. 1980. Aspectsof the biologyof the Great Boreal, and Great Gray Owls in the United States:a Gray Owl, Strix nebulosaForster. M.S. thesis,Univ. technical conservation assessment. USDA For. Ser. Manitoba, Winnipeg, Canada. Gen. Tech. Rep. RM-253. Fort Collins,CO U.S.A. DUNCaN,J.R. 1987. Movementstrategies, mortality, and MIKKOLA,H. 1983. Owls of Europe. Buteo Books,Ver- behavior of radio-marked Great Gray Owls in south- million, SD U.S.A. eastern Manitoba and northern Minnesota. Pages NERO,R.W. 1980. The Great Gray Owl--phantom of the 101-107 in R.W. Nero, RJ. Clark, RJ. Knapton and northern forest. SmithsonianInst. Press,Washington, R.H. Hamre [EDS.], Biology and conservation of DC U.S.A. northern forest owls. USDA For. Serv. Gen. Tech. --aNt) H.W.R. CO?LAND.1981. High mortality of Rep. RM-142, Fort Collins,CO U.S.A. Great Gray Owls in Manitoba--winter 1980-81. Blue 1992. Influence of prey abundance and snow Jay 39:158-165. cover on Great Gray Owl breeding dispersal.Ph.D. NERO,R.W., H.W.R. COPLANDAND J. MEZIBROSKI.1984 dissertation,Univ. Manitoba, Winnipeg, Canada. The Great Gray Owl in Manitoba, 1963-1983. 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