j. RaptorRes. 31 (2):151-159 ¸ 1997 The Raptor ResearchFoundation, Inc.

THE GREAT GRAY ( NEBULOSA) IN THE CHANGING FOREST ENVIRONMENT OF NORTHERN EUROPE

SEPPO SULKAVA Planeetankatu2 D 24, FIN-02210 Espoo,Finland

KAUKO HUHTALA Tehtaanpuistokatu3, FIN-85310 Sievias., Finland

ABSTRACT.--TheGreat Gray Owl (Strixnebulosa) breeds in northern Europe mostlyin older coniferous forests.Nest sitesare usuallyin twig nests of large hawks,sometimes on stumps,and occasionallyon the ground. The availabilityand the quality of tree nest sites is generally lower in managed forests; however,use of artificial nestshas partly compensatedfor declinesin natural nest sites.The Great Gray Owl has increasedin abundancein northern Europe over the last 30 yr. It was absentalmost entirely from Finland from 1940-60, havingbeen numerousthere, especiallyfrom 1880-1910. It feeds mainly on field voles (Microtusagrestis), which are abundant in fields and grassyareas following forest clear- cuts. The area of clear-cutshas increasedsince 1950, providing more open hunting habitat and vole resources.Also, protection of all owlsand increasingpositive attitude toward of prey hascoincided with the Great Gray Owl increasessince the late 1960s.Forest managementpractices that may benefit the Great Gray Owl include shapeof cutswhich should be irregular and not broader than 400 m. Perch trees left in cut areaswould expand the hunting area from the forest edge. KEYWORDS: Strix nebulosa;Great Gray Owl; trendsin distribution;breeding;, diet;, effects of forestry.

E1 Gran Bfiho Gris Strix nebulosaen cambios del ambiente en los bosquesde Norte Europa RESUMEN.--E1Gran Bfiho Gris Strixnebulosa se cria en el norte de Europa masfrecuente en bosquesconiferos maduros.mas viejos. Sitios de nidosestrin muchas veces en nidosde ramita de halconesgrandes, a vecesen tocones,y de vezen cuandoen el terreno.La disponibilidady la calidadde fibolescon nido esgeneralmente mas bajo en bosquesmanejados. Sin embargo, uso de nidos artificiales ha compensadoun poco en la reducci6nde nidosnatural. El Gran Bfffto Gris ha aumentadoen abundanciaen el norte de Europa sobrelos filtimos30 aftos.Estuvo ausente casi completamente en Finlandiade 1940-1960,viendo sido nu- merosoalii, especialmentede 1880-1910. Se alimenta principalmentecon ratonesde labor Miovtusagrestis, que sonabundante en loslabores y fieas pastosasdespu•s de cortes-completosde bosques.E1 fiea de cortes- completosha aumentadodesde 1950, proporcionando mas h/tbitat abierto para cazary recursosde Microtus agrestis.Tambi•n, protecci6nde todoslos bffftosy aumentandoel actitudpositivo para los avede rapifia ha coincidido con el aumento del Gran Bfiho Gris desde el fin de los 1960s. Costumbres del administraci6n de bosquesque puede dar beneficioal Gran Bfiho Gris incluyela forma de los cortesque debenser irregular y no masamplio que 400 m./•trboles de perchadejados pueden aumentar fireas de cazarde la orilladel bosque. [Traducci6n de Rafil De La Garza, Jr.]

A number of raptor populationshave declined is now rare in the northern countries of Europe. during the past 50 yr (e.g., the Peregrine Falcon, In addition, ornithologists have prepared many Falcoperegrinus, and the Osprey,Pandion haliaetus), nest boxes and other nesting structuresfor several largely due to pesticides.Many forest ,such speciesof .In Finland during 1994, 22 691 owl as nonmigratory owls,have not experiencedsimi- nest boxes were checked for occupancy (Saurola lar declines, but have been affected by rapid 1995). changesin forest structurecaused by forestryprac- The purpose of this paper is to review existing tices. knowledge on the distribution and ecology of the Public attitudestoward raptors, especiallyowls, Great Gray Owl in northern Europe and to discuss have improved in recent times. Killing by hunters possibleeffects of forestry. The main topics cov-

151 152 SVLI•VA AND HUHTALA VOL. 31, NO. 2 ered are: changesin distribution,population size, Table 1. Nest-sitedistribution (%) of the Great Gray causesof mortality, nest sites,diet and the possible Owl in three areas: all of Finland, western Finland and effects of forestry. northeast of the northern Bothnian Bay (Kemi-Tornio area). NESTING AND FEEDING ECOLOGY Nesting Habitats. The nesting-habitatrequire- WEST- •kLL OF ERN KEMI- ments of the Great Gray Owl are fairly flexible, but FIN- FIN- TORNIO most nests are found in older spruce-dominated LANDa LANDb AREAc coniferous or mixed forests. Great Gray Owls do NEST SITES % % % not build their own nest structures. Nest location is, therefore, determined by nest-siterequirements Hawk twig nest 72.7 88.9 42.6 Corvid nest 4.8 1.0 3.8 of large hawks,which build twig nests.Older for- estsmay be preferred becausethe Northern Gos- Other twig nest 4.4 -- 0.8 Man-made twig nest 3.6 2.0 37.2 hawk (Accipitergentilis), the main nest-siteproduc- Man-made platform or open box -- 3.0 9.3 er, prefersthis nestinghabitat. The Great GrayOwl Stump 10.8 2.0 5.4 alsonests in pure deciduousand pine forests.Nests Ground 2.4 2.0 0.8 in structuresother than twig nests have in a few Cliff, stone, ant hill 0.8 1.0 -- cases been in open habitats (e.g., almost open Barn roof 0.4 -- -- clear-cut area or open field). Number of nestings 249 99 129 Nest location is often near an opening such as a a Hildtn and Solonen 1987. natural open bog, a clear-cutarea or a small field b I• Huhtala unpubl. data, not included in "All of Finland." (Pulliainen' and Loisa 1977, Mikkola 1983, Hildtn c Liehu et al. 1995. and Solonen 1987). When nests are 50-100 m from forest edges,the Great Gray Owl often perch- es at the forest edge. The most commonly used perch trees are often near the edge. Nestslocated nations. In addition, becauseowls avoid long pe- in more exposedenvironments have shelter avail- riods of exposure to sunlight (Osborne 1987), able nearby and youngleave the nestrelatively ear- nestson low stumpsmay offer more shade,which ly to avoid direct sunlight (Helo 1984). is likely more important in southern areas. Many Nest Sites. Forest-managementpractices have stump nestsin Finland have been found in rela- decreased natural nest sites available for many tively warm springs (Mikkola 1983), but correla- large forest owls,including large stumpsand holes, tions with temperature need further investigation. the former of which are used by the Great Gray Ground nests, which have been recorded several Owl (Table 1). Natural twig nest sitesused by the times in Finland (Table 1), may be a responseto Great Gray Owl are mainly old goshawkor buzzard forestswith no twig or stump nest sitesor they may (Buteospp.) nests,both occupied and abandoned. be a responseto microclimatic factors. Nests on top of stumps (1-5 m in height) have Nesting Density. Nests were usually some dis- been found in Finland, Sweden (Stefansson1979) tance apart. Saurola (1985) estimateda mean of 1 and Alaska (Osborne 1987). The Great Gray Owl pair/100 km2 for Finland. In severalcases, howev- also occupiesartificial nest structures,either open er, two or more nestshave been reported only 100- boxesor platforms (Table 1). Use of artificial nests 400 m apart (Mikkola 1981, 1983). Some group indicates a lack of natural nest sites, as in the case nestingsare likely due to local abundanceof field of the Kemi-Tornio area, but also indicates an abil- voles. In some other cases,old "goshawkforests" ity to use a wide variety of nest sitesand structures. with several good alternative twig nests were the Stump nests are more common in southern ar- reason why two nestswere situated only 100-200 eas (Hildtn and Solonen 1987, Osborne 1987). m from each other. In some cases,male Great Gray This is observed to some extent in Finland, but Owls have been polygamouswith the secondfe- more clearly in the U.S., where only stump nests male laying about 1 wk later than the first. Also, were usedin most southerlyareas (Osborne 1987). two pairs have nested only 100-300 m apart (Htg- Imprinting on nest structuresby young (Hildtn lund and Lansgren1968, Mikkola 1983) to usegos- and Solonen 1987) and lack of large stumps in hawk nestswhich were situatedin groupsin older northern areas (Osborne 1987) are possibleexpla- forests. Ju•. 1997 GRF&TGP,•¾ OWL IN NORT•.RN EUROPE 153

Table 2. Compositionof Great Gray Owl diet in northern Europe (%).

NORTHERN WESTERN WESTERN SWEDEN EUROPE FINLAND FINLAND KOL• 1955--643 1955-74 b 1966--89 c 1973 d PE•INSUI• e

F•eld vole, Microtusagrestis 54.3 66.2 73.3 42.6 1.5 M. oeconomusand M. spp. 17.6 7.3 ------Bank vole, Clethrionomysglareolus 7.7 10.3 9.6 11.0 0.8 Grey-sidedvole, C. rufocanus 1.5 2.9 -- -- 93.8 Clethrionomysspp. 8.4 3.2 ------Water vole, Arvicola terrestris 1.5 1.7 4.3 1.0 -- Wood letoming, Myopusschisticolor 2.8 1.8 1.3 -- -- Common shrew, Sorexaraneus 2.3 2.8 7.4 36.3 -- Soricidaespp. 2.5 1.7 2.2 2.3 0.8 Birds, Aves 0.8 1.0 0.6 0.8 -- Frogs,Amphibia 0.2 0.5 0.1 1.0 -- Other -- 0.4 2.1 5.0 3.1 No. items identified 1977 5177 4858 830 130

Htglund and Lansgren 1968. Mikkola 1983. K• Huhtala unpubl. data. K. Huhtala unpubl. data. From Mikkola 1983.

Hunting Habitats and Diet in Northern Europe. land (Table 3) indicates that other small There are numerous observations of Great Gray as well as field voles are available in the same lo- Owls hunting in open habitats in Finland, but cality.Tengmalm's Owl hunts mainly in forests,but quantitative data are lacking. They may fly over also at forest edges and in grasslands(Korpim/iki open terrain like Short-earedOwls (Asioflammeus), 1981, 1988). However, it preys on field voles much but most observationshave been of perching birds less and feeds more on bank voles (37-46%) and on trees, bushesor telephone poles at or near the shrews (15-24%). This confirms that the Great edges of forests (Wahlstedt 1969, Mikkola 1981, Gray Owl hunts mosdy in open habitats and pre- 1983). In winter, hunting dive pits in the snoware fers Microtus. usually •50 m from the forest edge. The preference for field voles in open habitats The diet of the Great Gray Owl in northern Eu- may be partly due to the large size of the Great rope is mainly Microtusvoles (M. agrestis,M. oecono- Gray Owl, which may make it difficult for the spe- musin Lapland; Table 2). This specializationis sur- cies to hunt in the dense forests of central Finland. prisingbecause of the owl'ssize and becausethere This notion is supported by results of Pulliainen are also other numerous small mammals (Cleth- and Loisa (1977) in northeasternFinnish Lapland, nonomysvoles and Sorexshrews) available in its en- where most old forests are rather open. There, Ms- vironment. Field voles (Microtusspp.) comprise an crotusand Clethrionomysvoles are representedin the averageof 72-74% of the diet, while 8-10% consists diet of the Great Gray Owl in the same percentages of bank voles (Clethrionomysspp.) and 5-10% of as are found from small captures. shrews.In mostyears other prey,such as birds, frogs High abundance of Microtusvoles in the species' and larger voles (Arvicolaspp.) are only occasionally diet may be overemphasized,because data are pri- found in the diet. The diet of the Great Gray Owl marily based on its nesting diet and the species suggeststhat it hunts mainly in grassyareas (fields, usually nests only in good Microtus years. Three meadows,open bogsand clear-cutareas), where Mi- samples of food from poorer Microtusyears when crotusspecies are found. Although there are numer- the Great Gray Owl nested (Table 2) indicate that ous shrewsavailable in north European grasslands, it is also capable of capturing other prey. Field they seem to be avoided. voles, however, were still the main prey even in A comparison with the diet of Tengmalm's Owl theseexceptional samples from 1973-77. All three (Aegoliusfunereus)in the same area in western Fin- pairs nesting in central Ostrobothniain 1973 fed 154 SULI•VA AND HUHTALA VOL. 31, NO. 2

Table 3. Compositionof Great Gray Owl and Tengmalm'sOwl diet (%) in centralOstrobothnia (western Finland) in 1966, 1977 and 1989 (K. Huhtala unpubl. data).

GREAT GRAY OWL TENGMALM'S OWL

1966 1974 1989 1966 1974 1989

Field vole, Microtusagrestis 83.0 76.7 76.7 25.8 28.7 39.2 Bank vole, Clethrionomysglareolus 9.2 12.2 7.6 45.8 40.4 37.2 Wood lemming, Myopusschisticolor -- 1.2 2.3 -- -- 6.7 Shrews, Soricidae 6.0 5.4 5.0 23.7 23.3 15.3 Birds, Aves 1.1 0.9 0.4 2.8 7.2 1.5 Other animals -- 1.4 0.4 1.9 0.4 0.2 No. items identified 283 1064 931 528 460 406

largely on shrews(33-41% of the diet compared breeding localities in Europe are in Belorus and to 1-6% of the diet in other years).Whereas the Poland (Mikkola 1983). diet of great graysusually consists of only 0-2% The nesting population in northern Europe has frogs, 12% of the diet of a pair nestingon the is- changed considerablyin the past 100 yr (Fig. 1). land of Hailuoto in the BothnianBay in 1977 con- One hundred years ago (1880-1910), there were s•stedof frogs. Similarly,the great gray diet consists severalgood nesting years and Finnish egg collec- of only 1-3% water volesbut, in westernFinland tions alone contain more than 10 clutches from in 1977 it contained 8-15% water voles. More wa- severalyears. At that time, the population bred in ter voleswere available,because Eagle Owls (Bubo the northernmost coniferous forests, mainly in bubo)also consumed more water volesin 1977 than northern Finland and in northeastern Norway normal. (Fig. 2).

DISTRIBUTION AND POPULATION TRENDS IN THIS CENTURY In this century, the Great Gray Owl has nested in most of Finland, northern Sweden and occa- sionallyin the far north of Norway (Mikkola 1983). It breeds rarely throughout RussianKarelia and is included in the Karelian Red Book on endangered animals (Shehter 1985). The most southwesterly

225 ,-[ o.../ 200• 175 • 150 = 125

• 75, Z 25,so, N N 1 5 8 10 1880 1900' 19'20 i940 1960 1980 2000 Number of nests Years (by decade) Figure 2. Distribution and relative densityof Great Gray Figure 1. Number of Great Gray Owl nests found in Owl nests in Finland and northern Sweden in 1955-1974 F•nland at 10-yr intervalsin 1880-1960 and at 5-yr inter- (adapted from Mikkola 1983). Squares show the main vals in 1960-1995. Before 1940 data are from clutches in breeding areasin northern Finland and Norway in 1880- F•nnish egg collections. 1910. JUNF.1997 GP,•T G•.•¾Ow• IN NORTHFRNEUROPF. 155

200 120

, 175 100. 150 80' 125

100

75 50 20 25

-25 50 55 60 65 70 75 80 85 90 95 5'5'6b-d5 7'0 7'5-8b-8•-90 Years Years

Figure 3. Number of Great Gray Owl nests found in Figure 4. Number of Great Gray Owl nests recorded Finland in 1955-1994,yearly (lower line) and at 4-yrpe- yearly in Sweden in 1955-1989, according to Mikkola riods (upper line). (1983), Stefansson (1983) and Niemi (1989).

From 1910-1930, few nestswere reported in Fin- found in northern Europe in 1960-1994 was par- land. Some nestswere again seen during the 1930s, tially due to the increasein nest searchingfor all especiallyin the far north of northeasternFinnish owls for ringing (banding) and monitoring (Sau- Lapland in 1938 (Haartman et al. 1963-72, Fig. 1). rola 1986) and for building of artificial nests (Ta- Breeding Great Gray Owlswere then almostabsent ble 1). No increasein the Finnish Great Gray Owl from Finland for about 10 yr from 1940-1950. A population was determined from 1966-1984 by few nests were found in the 1950s farther south, in Saurola (1985), but since then the numbers have central Finland (Merikallio 1958, Mikkola and Sul- been higher (Fig. 3). An increasein the Great Gray kava 1969, Hild(•n and Helo 1981). Few nests were Owl population is also mentioned in other studies found during this period in Sweden (Curry-Lin- (Mikkola 1983, Helo 1984, Solonen 1986). dahl 1961). The breeding area of the Great Gray Owl after There have been several winter invasions of 1960 has been concentrated in central and south- Great Gray Owls in south and central Finland, eastern Finland, 300-500 km south of the main south of the normal breeding area. However,only area of breeding before 1940 (Mikkola and Sulkava occasionalnestings occurred in spring following 1969, Hild(•n and Helo 1981, Fig. 2). The main invasions.Invasion years were 1895-1896, 1907- breeding area now is in the Oulu district and since 1908, 1911-1913, 1935, 1949 and 1955 (Merikallio 1980 also in the Kemi-Tornio area (Solonen 1986). 1958, Haartman et al. 1963-1972). Becauseextensive cutting in Lapland began 20-40 The number of nests found has increased in Fin- yr later, this move from Lapland to central Finland land sincethe 1960s(Fig. 3). Regularbreeding was was not caused by forestry. After this range shift, reestablished in Finland in 1966-1967, and since clutch size was smaller (• = 4.30, SE = 0.10, N-- then Great Gray Owls have been nestingin the two 70) than in 1880-1910 (• = 4.63, SE = 0.12, N = best yearsof every 4-yr vole cycle (Fig. 3). In Swe- 80; t = 2.16, P < 0.05). However, this potential den, nestingwas rare until 1973 and has increased decreasein offspring production has not affected since then (Fig. 4). an increasein the population in recent times. There has been a steadyincrease in number of nestsfound in Finland over the last25 yr, especially EFFECTS OF FORESTRY PRACTICES in the last 10 yr (Fig. 3). There wasa slight reduc- Effects of forest-managementpractices on owls tion in number of nestsin the 1980scompared to vary, depending on the practicesand the needsof the number found afterward. The 4-yr vole cycle, the owl. Forest owls may lose nesting habitats and regular since the 1950s, became irregular in the nest sites, but hunting for prey may be easier in early 1980s(Henttonen et al. 1987). Consequently, cut areas. No numerical data are available on the the field vole peakswere not high enough to allow effectsof forestryon Great GrayOwls or their pop- all Great Gray Owls to breed. ulation in Finland. However, possibleinfluences The reported increase in the number of nests may be determined from indirect data on food 156 SULKAVAAND HUHTALA VOL. 31, No. 2

1000 ha 160

, Y¾'(.•:'.': :"?'..?.'.-:'.•. ::?'.:.'-'.'-::.•: (•:•.-"Y:.?: i.'•.::. •:.?('.: :,:.:.. --'.¾• ¾.•;:;Co....( ::.,.'C-•,.';.? (,.':.?• '.¾'.'•: ;,.';-'.'.:.: .:Y.'.(• 120 Private & other ...... •;.:`.•`..•;?•...•;:•;•.•`:i•i4.:•?•-i•.•?•?:•:....•f....;•:•...::•!•.i ...... •:•.?.•:•?:•?:•:..:....•:•.?..•:•e;`•.•...•.•?•.•6....•-•:..:•:•::::.•.•.•..•!•.• ..... •',•, Forest•n.dustnes. . •:•i•i':.".:':.':.?C <•-':.%?•':." V-:'.-:.::'..'-.:', :'.:•' .".:::'<' :.Y..': •'•:. ::z•::•..':..':.• ?.'.:-.:_'-..-_'-::(, (c...? :.:•i?y: ? ?.-?-.-c-de:: :'.-': :::-'Y :,".::-". Y:.'?'- '-'•:?:: ::-?:-'.' %"--'." '.:.'-:C'- •".-':< -•".-'.":' ?.: 80

40

...... ' ':-"-""' :':-':'-" ' ' '-'-' ::-?::"-" '- '--"'":'":"-::: :'::'?:'-' :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: 5O 55 60 65 70 75 80 85 90 93

Year

Figure 5. Area of clear-cutforest seededor planted in Finland in 1950-1993 (total and by forest ownershipcategory) (from Aarne 1994). habits, changesin the environment, and from mis- ture. The availabilityof nesting habitat may not be cellaneous direct observations. a limiting factor, although it is not known how Forestry Practices in Finland, 1950-1990. The large a forest a nesting pair of Great Gray Owls "modern and efficient" logging of forests began needs between cut areas. in northern Europe about 1950. Until that time, Decline in Availability and Quality of Nesting timber was harvested mostly by thinning the for- Sites. The Great Gray Owl nests in a variety of ests. Since then, forests have changed rapidly in structures (Table 1), but large twig nests and many ways that may affect the Great Gray Owl. stumpsare the main natural sitesused. The num- First, the area of old forestshas rapidly decreased bers of these nest sites have decreased because of resulting in a decreasein large trees, in which di- forestrypractices for severalreasons. First, the area urnal birds of prey build their twig nests.Second, of older forests that contain hawk nests has de- dead and broken trees have been removed result- creasedbecause of clear-cutting.Second, the num- ing in a reduction in stump nest sites.Third, dear- ber of stump nest sitesin forestshas decreasedbe- cut areas have increased (Fig. 5) resulting in an cause dead and broken trees have been removed increase in field vole production and an increase in the course of forest management. Third, the in food. Fourth, the area of young, dense forests number of alternative nestsper hawk pair is prob- has increased, decreasing the area available for ably smaller in young forests. Young trees have hunting or nesting. Fifth, the area of drained wet weaker branchesand, consequently,nests collapse bogs has increased, leading to a decreasein the more often. In addition, poor quality of twig nests area available for field vole production. Sixth, in young forests may lead to nest failures, but the roads are being built in remote forest areasleading extent of such losses is unknown. to increaseddisturbance (Fig. 7). The number of twig nest sitescurrently available Nesting Habitat Availability.Large areasof clear- seems to be sufficient in Finland. Solonen (1986) cutting (severalkm • at a time) indicate that the estimated that there were about 50 000 twig nests Great Gray Owl has lost considerable breeding available in Finland in 1986, and even if 50% of habitat, which may reduce local breeding popula- thesewere outside the normal breeding area of the tions. It may be argued that, becauseof the species' Great Gray Owl, the number would still probably flexibility in nest habitat use, it will have adequate sufficefor the estimated500-1500 Great Gray Owl forestsfor breeding in northern Europe in the fu- pairs in the country. In addition, populations of JUNE1997 GRFA•rGRAY OWL IN NORTHERNEUROPE 157

Clear-felled area large hawks,goshawks, Honey-buzzards (Pernis api- Great Gray Owl nests vorus)and buzzards (Buteospp.) have been stable • 200' in Finland in recent times (Saurola 1985, Haapala , et al. 1995). These species will likely produce • •o. enough new twig nests for Great Gray Owl use in the future, too, despite the fact that hawk nestsare sometimesdestroyed by the Great Gray Owl when it digsout the nest bowl. In the first diggingphase • •2o in early spring, the owl often digs a 10-15 cm deep bowl. This perhapsdries the nestmaterial; the final bowl for the eggs is only about 6 cm deep. Rapid • •o wear of hawk nests used by Great Gray Owls has also been observedby Stefansson(1979) in Swe- o 60 7b ' -73-' -sb-' -8•' ' '9b'' -95 den. Years The readinessof the Great Gray Owl to use var- Figure 6. Total clear-cut area (in 1000 ha) in Finland ious nest structurespoints to a lack of suitablenest- from 1970-1993 (from Aarne 1994) and Great Gray Owl ing sites.Man-made nestsare often used in North nestsrecorded at 4-yr intervalsfrom 1970-1994. America (Nero 1980, 1982, Bohm 1985, Bull et al. 1987) and, in Finland, open box nestshave been used in Kainuu (Helo 1984) and central Ostro- including dead and young trees, and the land was bothnia. Almost half the Great Gray Owls reported often burned, ditched or plowed before it was in the Kemi-Tornio area in the 1990soccupied ar- seeded or planted (Fig. 5). tificial nestscomposed of twigs (Liehu et al. 1995, During the expansion of the Great Gray Owl Table 1). population in Finland since the 1960s, clear-cutar- Increase in Hunting Habitat Availability.To find eas have provided more field volesfor raptors than enough Microtusvoles, the Great Gray Owl needs in earlier periods (Teivainen1979). In stateforests open habitatswith grasses,herbs and sedges.These in northern Finland and in forests of timber com- habitatsinclude hay fields (cultivated,temporarily paniesin central Finland, the clearingshave often unused or abandoned), open wet bogs, bog mar- been too large (more than 20 ha) and too open gins or clear-cutforest areas.The area in hay fields to be used as hunting grounds.The owls probably has increased, especially those unused or aban- hunted only at the edges of these large openings. doned. Only a small proportion of abandoned The future may be better because new rules for fields have been planted with trees. The number forest management and cutting practices have of clear-cut areas of different ages has increased been prepared and partly introduced in 1994- substantiallysince the 1950s (Fig. 5). The vegeta- 1995. Cut areas will be smaller in size (mostly not tion on most of these areas will support field vole more than 5-10 ha) and groups of both live and populations.The common practice of plowing cut dead treeswill be left in cut areas.Also, forest strips areas has increased the growth of grasses.The will be left along lake shoresand stream valleys. number of wet sedge bogs has decreased due to Prey Availability.The Great Gray Owl requiresa extensivedraining for forestry or peat production high density of voles (mostly field voles) to breed, (Aarne 1994). Drained areas are only partly suit- and therefore generally breeds only in increasing able for field voles. and peak years of the northern vole population Clear-cutting is obviouslythe main factor which cycleswhich are more pronounced than in more has increased hunting habitat availability to the southern areas of Europe (Hansson and Hentto- Great Gray Owl in forest areas since the 1950s. Be- nen 1985). In the northernmost breeding areas of fore 1950, timber wasremoved by selectiveharvest, northern Europe (Fig. 2), M. oeconomusand even but between 1950-1994, clear-cuttingwas the pri- C. rufocanusmay produce enough food for breed- mary logging practice. Only in the last few years ing (Table 1) becausethese speciesare often the has "continuous cultivation" (selective harvest) of most abundant voles in open habitats. the forest been allowed again; clear-cuttingis still The increased vole resources due to clear-cuts the main practice. Before the early 1990s,clear-cut (Teivainen1979) haveobviously been important to areas were mostly carefully cleared of all timber, support the increasein Great Gray Owls in north- 158 SULI•AV^ AND HUHTALA VOL. 31, No. 2

Kilometres 5000

tal

"----- ......

2OOO

100050 55 60 65 70 75 80 85 90 93

Year

Figure 7. Permanent forest roads (in kin) completed in Finland from ]950-1993 (total and by forest ownership category,from Aarne 1994). ern Europe. In Germany,the number of breeding some larger forest areas, 5-10 ha in size, between Tengmalm's Owls increased after extensive clear- the cut areas to provide nesting habitat. cutting and decreased again with reforestation. Most detrimental for the Great Gray Owl would This was due to the greater abundance of rodents be large-scaleclear-cuts of more than 100 ha that in recently cut areas (Mebs 1987). The breeding are circular or square in form, especiallyif they are population of Great Gray Owls seems to have totally treelessand with only small patchesof forest grown more than what would be supportedby only remaimng betweenthem. In this situation,the spe- an increase in area of recent clear-cuts in Finland cies would not have appropriate food resources, (Fig. 6). Since about 1980, the total area of clear- nesting habitats or resting and perching places. cutshas decreasedslightly, but the number of nests In practice,fairly diversecut sizesare acceptable of the Great Gray Owl identified has increased. by the Great Gray Owl as long as small groups of Size, Shape and Distribution of Harvested Areas. live and dead trees are left and forest corridors Our knowledge of the breeding and hunting re- are left along shores and streams, and between quirements of the Great Gray Owl is still inade- large cuts. quate for precise recommendations on forestry ACKNOWLEDGMENTS practices.Its hunting habits in large openingsand the amount of forest necessaryfor breeding arc The authors are most grateful to the reviewers,T. Dun- can, G. Niemi, D. Varland and an anonymous referee, not sufficientlywell known. The following descrip- who patiently gave numerous constructivecomments and tions of beneficial and detrimental forestry practic- advice for revising and improving the content and the es arc therefore only approximate: (1) Most cut- language of the manuscript. tings should bc restricted to areas of 20 ha in size; LITERATURE CITED cutsof 2-5 ha arc probablythe best size.Cut areas larger than 20 ha should bc irregular in shape,not AAR•E, M. [ED.] 1994. Mets/itilastollinen vuosikirja 1993-94 (Yearbook of Forest Statistics 1993-94). The broader than 400-500 m, and with convolutcd Finnish Forest Research Institute, Helsinki, Finland. edgesto give shelterwhen hunting at the edge. All BOHM, R.T. 1985. Use of artificial nests by Great Gray cuts should have groups of trees left for perching. Owls, Great Horned Owls, and Red-tailed Hawks in (2) Forest strips (corridors), 50-100 m wide, northeastern Minnesota. Loon 57:150-151. should bc left between the cut areas for moving BULL,E., M. HENJUMAND R. ANDERSON.1987. Nest plat- and sheltered resting places,and there should bc forms for Great Gray Owls.Pages 87-90 in R.W. Nero, JuNw1997 Ge•V•TG•¾ OwL IN NORTHWRNEURO?W 159

RJ. Clark, RJ. Knapton and R.H. Hamre [EDS.], Bi- MIKKOLA, H. 1981. Der Bartkauz Strix nebulosa. Die Neue ologyand conservationof northern forestowls, USDA Brehm-Biicherei 538:1-124. For. Ser. Gen. Tech. Rep. RM-142, Fort Collins, CO 1983. Owls of Europe. T. & A.D. Poyser,Calton, U.S.A. U.K. CURRY-LINDAHL,K. [ED.] 1961. V•tra Ffiglar I Norden. --AND S. SULKAVA. 1969. On the occurrence of the Stockholm, Sweden. Great Gray Owl (Strix nebulosa)in Finland 1955-68. •L•, J., J. KORHONWNAND P. S^UROL•.1995. Peto- Ornis Fenn. 46:126-131. lintuvuosi 1994: Nousuvuoden toiveikkuutta (Sum- NwRo,R.W. 1980. The Great Gray Owl--phantom of the mary: Breeding and population trends of common northern forest. SmithsonianInstit. Press,Washing- raptorsand owlsin Finland in 1982-94). Linnut 30: ton, DC U.S.A. 20-25. 1982. Building nestsfor Great Gray Owls. Siaha HAARTMAN, L., O. HILDI•N, P. LINKOLA, P. SUOMALAINEN 4:43-48. ANDR. TWNOVUO.1963-72. Pohjolanlinnutv/irikuvin NIEMI, T. 1989. Lappuggla, Strix nebulosa.Fauna Flora 84: (Birds of Finland). Otava, Helsinki, Finland. 185-191. HANSSON, L. AND H. HENTTONEN. 1985. Gradients in OSBO•E, T.O. 1987. Biologyof the Great Gray Owl •n densityvariations of small rodents:the importance of interior Alaska.Pages 91-95 in R.W. Nero, RJ. Clark, latitude and snow cover. Oecologia67:394-402. RJ. Knapton and R.H. Hamre [EDs.], Biology and HWLO,P. 1984. Y6n linnut, kirja Suomen p6116istfi(A conservation of northern forest owls. USDA For. Ser. book on Finnish owls,in Finnish). Kajaani,Finland. Gen. Tech. Rep. RM-142, Fort Collins, CO U.S.A. HENTTONEN,H., T. OKSANEN,A. JORTIKKAAND V. HAUKIS- PULLIAINEN,E. ANDI• LOISA.1977. Breedingbiology and ALMI. 1987. How much do weaselsshape microfine food of the Great Gray Owl, Strix nebulosa,in north- cyclesin the northern Fennoscandiantaiga? Oikos 50: easternFinnish Forest Lapland. Aquilo S• Zool.17:23- 33. 353-356. HILDI•N,O. ANDP. HELO. 1981. The Great Gray Owl Strix S^U•OL•, P. 1985. Finnish birds of prey: statusand pop- nebulosa--abird of the northern taiga. OrnisFenn. 58: ulation changes.Ornis Fenn. 62:64-72. 159-166. 1986. The raptor grid: an attempt to monitor HILDI•N, O. AND W. SOLONEN. 1987. Status of the Great Finnish raptors and owls. Vdr Fdgelvdrld,Suppl. 11: 187-190. Gray Owl in Finland. Pages115-120 in R.W. Nero, RJ. Clark, RJ. Knapton and R.H. Hamre [EDS.], Biology 1995. Suomenp6116t (Finnish owls,in Finnish) and conservation of northern forest owls. USDA For. KirjayhtymS,Helsinki, Finland. SHEHTER,D. [ED.]. 1985. The red book of Karelia. Pet- Ser. Gen. Tech. Rep. RM-142, Fort Collins,CO U.S.A. rozavodsk, Russia (in Russian). HOGLUND,N. AND E. LANSGREN.1968. The Great Gray SOLONW•,T. 1986. Lapinp6116npesint5 Suomessa(Sum- Owl and its prey in Sweden. Viltrevy5:363-421. mary: Breeding of the Great Gray Owl Strix nebulosa KORPIM&KI,E. 1981. On the ecologyand biologyof Teng- in Finland). Lintumies21:11-18. malm's Owl (Aegoliusfunereus). Acta Univ. Ouluensis, STW•ANSSON,O. 1979. Lappugglan,Strix nebulosa, i Norr- S• A Biol. 13:1-84. botten 1975-78. VdrFdgelvdrld 38:11-22. 1988. Diet of breeding Tengmalm's Owls Aego- 1983. LappugglanStrix nebulosa i Sverige 1979- liusfunereus:. long-term changesand year-to-yearvari- 82. VdrFdgelvdrld 42:245-250. ation under cyclicfood conditions.OrnisFenn. 65:21- TEIV•,INEN,T. 1979. Mets/ipuidentaimien myyrStuhot 30. metsSnuudistusaloillaja metsitetyil15pelloilla Suomes- LIWHU,H., P. Suo?^J'A•tvIAND J. YLI?WKI•XL•.1995. Peto- savuosina 1973-76 (Summary:Vole damageto forest lintujen pesintfiTornion ja Kemin seuduilla1974-95. tree seedlingsin reforestedareas and fields in Finland Sirri 20:36-49. in the years1973-76). FoliaF• 387:1-23. MEBS, T. 1987. Eulen und Kfiuze. W. Keller & Co., Stutt- WAHLSTEDT,J. 1969. Jakt, matning och l•ten hos lap- gart, Germany. puggla Strixnebulosa. Vdr Fc•gelv[irld 28:89-101. MWmK•LUO, E. 1958. Finnish birds, their disribution and numbers. Fauna Fenn. 5:1-181. Received2 November 1995; accepted6 March 1997