93

Ontario's Cavity-Nesting Birdsby

Christy MacDonald

IntroductionStanding dead trees (snags) play anessential role in the provision ofnesting, roosting, denning, perching,and feeding sites for a variety ofOntario birds and mammals.Approximately 85 species of birds inNorth America either nest or feed insnags, and these birds often represent30-45% of a forest bird community(Scott et al. 1977). Thirty-eightspecies of Ontario breeding birds areto some degree dependent upon snagsfor nesting (see Table 1).

The Role of Cavity-Nesting Birdsin Ontario Forests

Cavity-nesting birds can beseparated into two categories:primary excavators and secondarycavity-nesters. Primary excavatorsare those species which excavate anesting or roosting cavity in a live ordead tree. The species belonging tothis group are largely non-migratory,except Common Flicker (CoIaptesauratus), Yellow-bellied Sapsucker(Sphyrapicus varius) and Red-headedWoodpecker (MeIanerpeserythrocephaIus), and mainlyinsectivorous. Insectivorous birdsplay an important role in a forestcommunity by influencing destructiveinsect populations (Koplin 1972;Dickson et aI. 1979; and Temple et aI.1979) in three ways: (1) directlythrough consumption, (2) indirectlyby spreading pathogens to insectpopulations and (3) by altering theinsect microhabitat.

Woodpecker populations inparticular have been known to

exhibit functional and numericalresponses to localized outbreaks ofinsect infestations. Kendeigh (1947)documented increased consumptionof spruce budworm (Choristoneurafumiferana) by woodpeckers during anoutbreak in Ontario forests. Besidesaccelerating the decline of anoutbreak, and perhaps moreimportantly, insectivorous birds playa major role in the retardation ofinsect populations before they reachoutbreak levels. Species mostinvolved in this respect are non­migratory residents like woodpeckers,chickadees (Parus spp.) andnuthatches (Sitta spp.). These birdshave the greatest impact on insectpopulations during the winter whentheir diet consists mainly of sedentaryinsect larvae. Resident bird specieslimit the number of insects emergingin the spring, thus reducing theseverity of summer outbreaks. Mostinsectivorous birds feed by pecking,which disrupts the microhabitat ofthe insect prey thus having adetrimental effect on the over-wintersurvival of the remaining insects(Otvos 1979).

Ontario's primary excavators notonly play an important role in insectsuppression, but also in the provisionof nesting cavities for other species.Secondary cavity-nesters are unable(or rarely attempt I to excavate theirown cavity, and are thus dependentupon natural cavities or those builtby other species. When a cavity builtby a primary excavator is abandoned,it may then provide a nesting site fora secondary cavity-nester. Some

VOLUME 10 NUMBER 3

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secondary cavity-nesters are veryselective in their choice of a cavity tothe point of becoming dependentupon a particular species of primaryexcavator. For example, bothBufflehead (Bucephala albeola)(Bellrose 1976; Scott et al. 1977; andHarrison 1984) and American Kestrel(Falco sparverius) (Scott et al. 1977)have demonstrated a distinctpreference for abandoned CommonFlicke!" nesting sites. In order forcavity-nesting birds to perform theirrole in the forest ecosystem, theymust be provided with suitablenesting habitat in the form of snags.

Snags in Ontario ForestsWe seem to know a great deal

about cavity-nesting species, but verylittle is known about the snags whichprovide the nesting substrate criticalto the reproductive success of thesespecies. Insufficient knowledge of therole snags play in meeting therequirements of cavity-using wildlifein Ontario has in the past forcedresource managers to develop habitatprescriptions based upon studiesconducted in the northeastern UnitedStates. Concern for the lack ofinformation on snags prompted theOntario Ministry of NaturalResources to conduct a study todetermine the abundance andcharacteristics of snags in standsrepresenting various forest types. Iwas involved in this study and wouldlike to present a brief summary of theresults from the report (MacDonald1990).

MethodsThe survey was conducted during

the summer of 1989 within the LeslieM. Frost Natural Resource Centremanagement unit encompassing parts

ONTARIO BIRDS DECEMBER 1992

of Sherborne, Stanhope, Ridout,Havelock and Hindon townships inMuskoka D.M. and HaliburtonCounty. Forty-four stands (978 halrepresenting a variety of hardwoodand conifer forest types typical of theGreat Lakes - St. Lawrence ForestRegion were surveyed. The standsranged in age from 80 to 160 yearsand varied in disturbance history(managed forests and those which arerelatively undisturbed from loggingand fire were included in the study).

Stands were sampled by cruisinga continuous strip 10 m wide in a zig­zag formation throughout the standresulting in a sampling intensity of5%. For the purposes of this survey,snags were defined as standing deadtrees greater then 10.2 cm indiameter at breast height (1.4 m) andgreater than 1.8 m in height. For eachsnag encountered, the followinginformation was recorded: species,diameter, height, state ofdecomposition (whether hard or soft),and presence of excavated cavities.

ResultsThe mean density of snags per

hectare of all stands surveyed was53.1 snags/ha (range 16.3-97.3).Stands dominated by intolerantspecies, white birch (Betulapapyrifera) and poplars (Populus spp.),had the highest average density ofsnags. Undisturbed stands had thelowest average number of snags perhectare. White pine (Pinus strobus)and sugar maple (Acer saccharum)represented the most abundant snagspecies. Seventy-five percent of thesnags recorded were within the10.2-25.4 cm diameter class.Undisturbed stands containedproportionally more large-diameter

97

Figure 1: Pileated Woodpeckers on snag. Drawing by Chris Kerrigan.

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snags (»50 cm) than any other foresttype. Large-diameter snags wereutilized most frequently in relation totheir abundance. Cavities excavatedin live trees were found mainly inwhite pine and sugar maple.

DiscussionThe availability of suitable

nesting habitat is critical to thereproductive success of all cavity­nesters. Numerous studies indicatethat cavity-nesting species densitiesare strongly correlated with snagdensity (Balda cited by Back 1979;Land et al. 1989; Howard et al. 1986;Zarnowitz and Manual 1985; Raphealand White 1984). Snag density in thestands surveyed is similar to thatreported in the United States byCarey (1983) who found that snagdensities ranged from 22.4-55.1/ha inmaple/beech/birch forests with oldgrowth stands having the lowestdensity. Cavity-nesting bird density isalso closely correlated with thedensity of large-diameter snags(Rapheal and White 1984). Mostspecies which nest in snags haveindividual requirements regardingsnag diameter. For example, PileatedWoodpeckers (Dryocopus pileatus)require snags »35 cm in diameter(Peck and James 1983). Largediameter snags are capable ofsupporting the greatest number ofsnag-dependent species. A large­diameter snag with limbs intact canprovide a nesting site in the trunk forspecies which require large cavities(e.g. Pileated Woodpecker). whileproviding sites for cavities in thebranches for species which requiresmaller-diameter substrate, e.g.Black-capped Chickadee (Parusatricapillus).

ONTARIO BIRDS DECEMBER 1992

Figure 2: Northern Hawk Owl in nestcavity. Drawing by Mark Reeder.

Reduction of available snags mayresult in increased competition fornesting sites, poor reproductivesuccess, and heavier dependenceupon artificial nesting structures.European Starlings (Sturnus vulgaris)are known to be aggressivecompetitors with the CommonFlicker, Bufflehead, Yellow-belliedSapsucker, House Wren (Troglodytesaedon) and numerous other speciesfor nesting cavities. Competition may

also result from mammals whichrequire cavities to raise their young,e.g. flying squirrels (Glaueomys spp.).If sufficient cavities are not available,some species may be forced toexcavate or build their own nests.The first choice of a nesting site forBarred Owls (Strix varia) is a brokentopped snag, or a tree with a largecavity. When these sites are notavailable, Barred Owls may attemptto build their own stick nest or use ahawk/crow/raven/squirrel nest.Although attempts may be made torepair nests, nesting is oftenunsuccessful due to poorlyconstructed nests offering little or noprotection to eggs and young (Bent1938; Stokes and Stokes 1989).

As the result of decreased snagavailability due to fuelwood cuttingand the clearing of forested land,many of Ontario's secondary cavity­nesters have become heavilydependent upon nest boxes; examplesare House Wren, Eastern Bluebird(Sialia sialis), Wood Duck (Aixsponsa), Tree Swallow (Iridoproenebieolor), and Purple Martin (Prognesubis) (Peck and James 1987). Nestboxes are only suitable for secondarynesters; primary excavators require anatural snag in which to excavatecavities. Nest boxes do not providesufficient insulation for winter roostsand even though they are used bymany species, they do not providefeeding and roosting sites whichnatural snags can, and by no meansprovide habitat for the multitude ofspecies of microorganisms, fungi,insects, birds and mammals whichnatural snags do.

ConclusionsCurrent Ontario Ministry of

99

Natural Resources guidelinesregarding snags in timbermanagement in the study area(Central Region) require that aminimum of 6 cavity trees greaterthan 25 cm in diameter per hectarebe maintained within stands allocatedfor harvest (Watton 1989). Incomparison to snags surveyed inmanaged stands, on average thisrepresents 46% of existing snags. Theguidelines represent 36.1% of theaverage density of snags found inundisturbed stands within the studyarea. During logging operations, snagswhich pose a safety hazard areremoved in accordance with theOccupational Health and Safety Act.Forest and wildlife managers arecontinuously collecting moreinformation to provide a basis fordetermining exactly how to managefor snags and snag-dependent speciesin Ontario forests.

Standing dead trees represent anessential component of any forestecosystem, and are critical for themaintenance of healthy populationsof all cavity-using wildlife. It is myhope, and the hope of those whocontributed to this survey and otherslike it, that the data collected willhelp satisfy the need for pertinentinformation regarding snags whichwill form the basis of snagmanagement in Ontario forests.

AcknowledgementsI would like to again thank all thoseindividuals who contributed to theSnag Survey in 1989. Thanks to theeditors of Ontario Birds for theirconstructive criticism of the firstdraft, especially to Ron Pittaway forall of his friendly advice, and to MikeTurner for help with the table.

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Literature citedBack, G.N. 1979. Avian communities and

management guidelines of the aspen-birchforest. Pp. 67-79 in Management of NorthCentral and Northeastern Forests forNongame Birds. USDA Forest ServiceGeneral Technical Report NC-51. NorthCentral Forest Experiment Station.St. Paul. Minnesota.

Bellrose, F.e. 1976. Ducks. Geese and Swans ofNorth America. Stackpole Books. Harrisburg.Pennsylvania.

Bent, A. e. 1938. Life Histories of NorthAmerican Birds of Prey. Part 2. United StatesNational Museum Bulletin 170.

Bent, A.C. 1939. Life Histories of NorthAmerican Woodpeckers. United StatesNational Museum Bulletin 174.

Bent, A.e. 1948. Life Histories of NorthAmerican Nuthatches. Wrens. Thrashers. andAllies. United States National MuseumBulletin 195.

Carey, A.B. 1983. Cavities in trees in hardwoodforests. In Snag Habitat Management.(Davies. J.W.. G.A. Goodwin. and R.A.Ockenfels. (eds.). USDA Forest ServiceGeneral Technical Report RM-99.

Dickson, J.G., R.N. Conner, R.R. Fleet,J.A. Jackson, andJ.C. Kroll (eds./. 1979.The Role of Insectivorous Birds in ForestEcosystems. Academic Press. New York.

Harrison, e. 1984. A Field Guide to the Nests,Eggs. and Nestlings of North AmericanBirds. Collins, Toronto.

Howard, R.A., S.K. Swallow, andR.J. Gutierrez. 1986. Primary cavity-siteselection by birds. Journal of WildlifeManagement 50: 571-575.

Kendeigh, S.C. 1947. Bird population studies inconiferous forest biome during a sprucebudworm outbreak. Ontario Department ofLands and Forests Biological Bulletin 1:31-34.

Koplin, J.R. 1972. Measuring predator impact ofwoodpeckers on spruce beetles. Journal ofWildlife Management 36: 308-320.

Land, D., W.R. Marion, and T.E. O'Meara.1989. Snag availability and cavity-nestingbirds in slash pine plantations. Journal ofWildlife Management 53: 1165-1171.

MacDonald, e. 1990. Forest Snag InventorySurvey. Leslie M. Frost Natural ResourceCentre Report. Ontario Ministry of NaturalResources. Dorset.

Otvos, I.S. 1979. The effects of insectivorousbird activities in forest ecosystems: anevaluation. In The Role of InsectivorousBirds in Forest Ecosystems. (Dickson, J.G.et al., eds.) Academic Press, New York.

Peck, G.K. and R.D. James. 1983. BreedingBirds of Ontario: Nidiology and Distribution.Volume 1: Nonpasserines. Life SciencesMiscellaneous Publications. Royal OntarioMuseum. Toronto.

Peck, G.K. and R.D. James. 1987. BreedingBirds of Ontario: Nidiology and Distribution.Volume 2: Passerines. Life SciencesMiscellaneous Publications. Royal OntarioMuseum. Toronto.

Rapheal, M. G. and M. White. 1984. Use ofsnags by cavity-nesting birds in the SierraNevada. Wildlife Monographs 86: 1-66.

Scott, V.E., K.E. Evans, D.H. Patton, ande.P. Stone. 1977. Cavity-nesting birds ofNorth American forests. USDA Forest ServiceAgriculture Handbook 511, Washington, D.C.

Stokes, DW. and L.Q. Stokes. 1989. A Guide toBird Behaviour. Volume 3. Little. Brown andCompany Ltd., Toronto.

Temple, S.A., M.j. Mossman, and B. Ambuel.1979. The ecology and management of aviancommunities in mixed hardwood-coniferousforests. Pp. 132-153 in Management ofNorth Central and Northeastern Forests forNongame Birds. USDA Forest ServiceGeneral Technical Report NC-51. NorthCentral Forest Experiment Station. St. Paul.Minnesota.

Thomas, J. W. (tech. ed./ 1979. Wildlife habitatsin managed forests: the Blue Mountains ofOregon and Washington. USDA ForestService Agriculture Handbook 533.Washington, D.C.

Watton, D.G. 1989. Algonquin RegionalGuidelines: Areas of Concern. OntarioMinistry of Natural Resources, Huntsville.

Zarnowitz, J.E. and D.A. Manual. 1985. Theeffects of forest management on cavity­nesting birds in northwest Washington.Journal of Wildlife Management 49: 225-263.

Christy MacDonald, Box 1324, Red Lake, Ontario POV 2MO.

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