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Boreal Owl Responses to Forest Management: a Review

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Boreal Owl Responses to Forest Management: a Review j. RaptorRes. 31 (2):125-128 ¸ 1997 The Raptor ResearchFoundation, Inc. BOREAL OWL RESPONSES TO FOREST MANAGEMENT: A REVIEW H•,ai HAKKARAINEN,Em4• KORPIM 'AKI,VESA KOIVUNEN AND SAMI KUPd•I Laboratoryof EcologicalZoology, Department of Biology,University of 7•trku,FIN-20014 7•trku,Finland ABSTRACT.--Modernforestry during the last decadeshas stronglyincreased fragmentation of forest habitats.This may resultin harmful effectson raptor specieswhich are strictlydependent on boreal forests,such as the vole-eatingBoreal Owl (Aegoliusfunereus). The long-term data from Finland shows that in extensiveforest areas,fledgling production of Boreal Owls is higher on intensivelyclear-cut territories than on lessclear-cut territories. Breeding frequency,clutch size and laying date, however, havenot been shownto be relatedto the proportionof clear-cutareas within a territory.Snap-trapping data suggeststhat large clear-cutareas sustainmore Microtusvoles than small clear-cut areas. The in- creasednumber of saplingsand clear-cutareas during the last two or three decadeshas created new suitablegrass habitats for Microtusvoles, and simultaneouslynew hunting habitatsfor BorealOwls. There is someexperimental evidence that the presenceof the Ural Owl (Strixuralensis) decreases the breeding densityof Boreal Owlswithin 2 km of Ural Owl nests.Therefore, forestfragmentation does not seem to harm BorealOwls at the presentday scale,but a lack of nest holeshas to be compensatedfor by settingnest boxesfar (>2 km) from medium-sizedand large raptorsthat can prey upon the Boreal Owl. In the long-term,however, establishment of snagsand patchesof mature forestswith large trees, denseenough to satisfythe ecologyof the hole-nestingBlack Woodpecker (Dryocopus martius), will provide a natural way to establishnew nestingcavities for Boreal Owls. KEYWOP, DS: Aegoliusfunereus; Strix uralensis;clear-cuttings; modern forestry; vole density. Respuestadel Bfiho Boreal a la Administraci6nForestal: Un Reviso RESUMEN.--E1forestal moderno durante los filtimosd•cadas ha aumentadocon frecuenciala fragmen- taci0nde hfbitat de bosque.Esto puede resultar en efectosdafiosos en especiede rapacesque estfn estrictamentedependiente en bosquesboreal, como el Bfiho Boreal (Aegoliusfunereus) que comerato- nes.La informaci0nde Finlandiaensefia que larga duraci0nen fireasde bosquesenormes, la produc- ci6n de pajaritosde bfihoses mrs alto en territorioscortados-completo con intensidad queen territorios menoscortados-completo. La frecuenciade crla, tamafiode nidada,y la fechade poner,no han ensefi- ado estarrelacionado a la proporci0nde freas cortadas-completoentre el territorio.Informaci0n de trampassugiere que fireasgrandes que estfn cortadas-completosostienen mas ratones,y simultfnea- mentehfbitat nuevopara cazar para los bfihos. Hay un pocode pruebasexperimental que la presencia de Bfiho Ural (Strix uralensis)reduce la densidad de cria del Bfiho Boreal dentro de 2 km del nido del Bfiho Ural. Por lo tanto, la fragmentaci0ndel bosqueno pareceha cerle dafio al Bfiho Boreal en la escalapresente, pero la falta de nidosde agujeronecesita que estarcompensado con poniendonidos de agujerolejos (>2 km) de rapacesmedianos y grandesque puedencazar a los bfihosboreal. En la larga duraci0nel establecimientode toconesy parcelasde bosquemaduros con frboles grandes,de suficientedensidad para satisfacer la ecologfade losnidos de agujerode el CarpinteroNegro (Dryocopus martius),va proporcionaruna maneranatural para establecercavidades de nidosnuevos para el Bfiho Boreal. [Traducci6n de R•afilDe La Garza,Jr.] During the last decades, modern forestry has from larger forestcomplexes (Hansson 1992). Rap- had a strong and perceivableimpact on boreal for- tors living in forest habitats are generally consid- est ecosystems,both in Palearctic and Nearctic ered to be one of the most sensitivegroups of ver- regions. At the landscapelevel, there is a lack of tebratesto forest management and habitat change large pristine forests (Ohmann et al. 1988), while (Newton 1979, Forsmanet al. 1984, Carey et al. remaining mature forest patches have become in- 1992). This is at least in part becauseraptors in- ternally more homogeneous and more isolated habit large territories(Newton 1979) where astop 125 126 HAKKARAINEN ET AL. VOL. 31, NO. 2 Table 1. Annual breeding percentageof nest boxes,laying date (1 = 1 April), clutch sizeand fledglingproduction in sparselyand widely clear-cutterritories of Boreal Owls in the Kauhavaregion, westernFinland (ca 63øN,23øE). Statisticaltests were performed by Student'st, test and Mann-Whimey •test (two-tailed).N = number of territories. PROPORTION OF CLEAR-CUT AREAS •ITHIN TERRITORY LOW • HIGH b i (_+ SD) N i (-+ SD) N TEST VALUE P Breedingpercentage 15 (9) 17 14 (15) 13 U = 139.0 0.22 Layingdate 1.41 (19.44) 14 1.10 (21.98) 10 T = 0.04 0.97 Clutch size 5.43 (0.88) 14 5.20 (1.26) 11 T = 0.54 0.59 No. of fledglings 2.45 (1.26) 14 3.55 (1.39) 11 T = 2.06 0.05 18% (SD = 7%, range = 10-30%) of total areawithin 1.5 km of nestwas clear-cut. 49% (SD = 11%, range = 35-70%) of total area within 1.5 km of nestwas clear-cut. carnivorescapture prey which is scarceand diffi- THE EFFECTS OF CLEAR-CUT AREAS ON BOREAL OWLS cult to catch (Temeles 1985). Therefore, they ex- The long-term study (1981-95) conductedin the pend considerableenergy in each feeding event, Kauhavaregion of western Finland made it possi- especiallyif prey is sparselyand patchilydistributed ble to evaluate the effects of clear-cut areas on the within the territory. In addition, due to forest har- Boreal Owl. These areas comprise clear-cut areas vesting, there often is a lack of suitable nesting with 0.2-1.5 m high saplings(<10-yr old) covering places, such as natural cavitiesand large nesting about one-third of the forests in our study area. trees for many raptor species. Boreal Owls breeding in areas that are primarily The Boreal Owl (Aegoliusfunereus)is a small noc- forestedwith a mean of 18% (SD = 7%, range 10- turnal hole-nestingraptor which commonlybreeds 30%) (herein referred to as sparselyclear-cut) of in coniferousforests in northern Europe (Mikkola the total forest area clear-cut within 1.5 km of nests 1983). Microtusvoles (field vole, Microtus agrestis; produced about one fledgling less than those in sibling vole, M. rossiaemeridionalis;and bank vole, areaswith a mean of 49% (SD = 11%, range 35- Clethrionomysglareolus) are the main prey of this 70%) of the area clear-cut (herein referred to as species(Korpim/iki 1988). Field and sibling voles widely clear-cut) (Table 1). Most of the territories inhabit fields as well as clear-cut areas, whereas the and areas sampledwithin sparselyclear-cut areas bank vole inhabits mainly forest habitats (Hansson were small cuts of <10 ha with most areas between 1978). In poor vole yearsalternative food sources 1-5 ha. In contrast,in the territories sampledwith- have to be used, such as shrews (Sorexspp.) and in the widely clear-cut areas, most were relatively small passerinebirds (Korpim/iki 1988). Males are large cuts of up to 200 ha. In addition, territories resident after the first breeding attempt, while fe- within the widelyclear-cut areas exhibited relatively malesdisperse widely (up to 500 km) betweensuc- high fledgling production (i = 3.6) for Boreal cessivebreeding attempts (Korpim/iki et al. 1987). Owls (Korpim/iki and Hakkarainen 1991). Terri- In this review, we focus on how clear-cut areas in tories in both clear-cut areas were occupied with Boreal Owl territories affect reproductive output equal frequencyin different vole years (Table 2), and breeding frequencyof this species.We alsodis- indicating that Boreal Owls breed successfullyin cusshow clear-cutareas affect the main prey den- the neighborhood of large clear-cuts also in low sitiesof Boreal Owls. Finally,we identify how inter- vole years.Clutch size,breeding frequency and lay- specific interactionshave to be consideredwhen ing date, however,were not affected by the pro- setting new nest boxes for owl speciesthat suffer portion of clear-cutareas within a territory (Table from the lack of natural cavities. This review is 1). Therefore, forest management does not seem based on recent investigations(Hakkarainen and to harm Boreal Owls at present day scales,if no Korpim/iki 1996) and on snap-trappingdata which more than half of the total forest area is clear-cut are now examined especiallyfrom the perspective at long intervalsenough (>60 yr). In contrast,the of forest management. positive effects of clear-cutareas on fledgling pro- JvNv.1997 BOREALOWL AND FOREST M•d•AGEMENT 127 Table 2. The number of Boreal Owl nestsin proportion forests and clear-cutsmay increase the amount of of landscapewith clear-cutsof low and high percentages alternativeprey of Boreal Owls in poor vole years. (see Table 1), in different phasesof the vole cyclein the Prey abundance and fledgling production ap- Kauhavaregion, westernFinland (ca. 63øN, 23øE). pear to increase with forest fragmentation. How- ever, clear-cutting also decreases the number of PROPORTION OF CLEAR-CUT suitable natural cavities for Boreal Owls. Large AREAS WITHIN TERRITORY treesand aspengroves with suitablenesting cavities PHASE OF VOLE CYCLE LOW HIGH for the Black Woodpeckers(Dryocopus martius) are Low 1 2 decreasingdue to logging. There is a need to pro- Increase 7 4 tect thesesuitable nesting sites in forest landscapes. Peak 13 12 Alternatively,nest boxes can be provided for Bo- Total 21 18 real Owls to compensatefor the lack of natural cavities. ESTABLISHING NEST-BOX LOCATIONS
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