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The 1995-1997 Great Gray Owl Invasionsin the Peterborough Area

Doug Sadler

IntroductionThe Great Gray Owl (Strix nebu­losa) breeds in boreal forest fromthe west coast eastward to Ontarioand Quebec (Prevett 1987,Morneau 1996). It makes periodicflights in winter to southern Canadaand the northern United States,including southern Ontario (Bulland Duncan 1993). I gatheredrecords of Great Gray Owls from alarge area around Peterboroughduring winter invasions in 1978-79,1983-84, 1995-96 and 1996-97, aswell as during more isolated occur­rences in January-April 1992 andJanuary 1993. The region fromwhich sightings were collected dur­ing the 1995-1997 invasions includ­ed all of Peterborough County, westto Lake Simcoe (and ThorahIsland), the Kawartha Lakes, andnorth and eastward into Haliburtonand Hastings Counties. This articledescribes how reports of sightingswere obtained, the number ofreported owls in each invasion, andsome characteristics of the owlsobserved.

MethodsMy information on the dates andlocations of Great Gray Owls dur­ing these invasions was derived inpart through a "communications

network" developed while writing aweekly column on all aspects of thenatural world and our relationswith it in the PeterboroughExaminer for nearly forty years.Reports also came in during the1995-1997 invasions after threenewspapers (in Peterborough,Lindsay and Bancroft), TV andradio ran items, often with myphone number and pictures of theowls. In 1997, flyers requestinginformation about the owls wereposted in corner stores and otherlocations within the area, as well.

Significant information was fedto me through local personnel ofthe Ontario Ministry of NaturalResources. Much information camefrom dead birds reported to theMNR office; some came from taxi­dermists. Tim Dyson, an experi­enced bird bander and taxidermistwho specializes in raptors, under­took efforts to band a number ofthe owls, which also yielded valu­able data.

As a result of previous experi­ence' during the last two invasions Ikept detailed reports, pinning peo­ple down to specific sites, which Irecorded in some detail with dates,and on a map. Quite rightly, peopleask how I know they were all GreatGrays, and how I was able to tell

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that these represented separatebirds. The answers are not unequiv­ocal. However, no identificationerrors were detected. The owls werealmost all seen along roadsides indaylight and allowed closeapproach. Many mentioned thegreat facial disks and yellow eyes.Most callers were not declared"naturalists"; certainly few had everseen any kind of owl before.

But how did I tell whether thesewere new birds, repeats, or the samewhich had moved on? It was notalways possible. But a number offactors helped. Some birds keptrecurring at the same locations,often daily, over weeks. In manycases, when investigated this provedto involve two, often a pair as deter­mined by size differential, or evenas many as five birds. Sometimesthese were all found on one visit. In1997, along one stretch of roadwhen out attempting to band theowls, Tim Dyson saw no fewer thanseven, with five in view at one time.Road kills and dates helped in anestimate of the number of owls atanyone location.

Birds were sometimes capturedfor banding at the same place,. andtheir recorded characteristics com­pared. An intuitive estimate of thesize of winter territories quicklydeveloped and events seemed toback this up. As a rule of thumb,birds found two or more kilometresapart were counted as different,once the invaders appeared to havesettled down to a temporary stabili-

ONTARIO BIRDS AUGUST 1998

ty. Brunton and Pittaway (1971)noted that invading Great GrayOwls near Ottawa "set up definitehome ranges" (one of which wasapproximately 112 acres), "andtended to stay within those bound­aries".

No doubt there were unavoid­able errors, but these were coun­tered by presumed cases wheremultiples were not detected, and bythe probability that many owls notvisible from the car or roadsidewere missed. It became obviousthat most sightings came from trav­elled routes. The estimate of num­bers was almost certainly conserva­tive.

Numbers of OwlsDuring the winter of 1978-79, I hadrecords of 34 Great Gray Owls.Estimates of the total number ofGreat Grays in all of southernOntario that winter ranged from 61(Goodwin 1979) to at least 112(Vickery and Yunick 1979). In 1983­84, my recorded sightings increasedto 97 owls. James (1989a) noted that407 Great Gray Owls were report­ed to the American Birds OntarioRegion editor that winter, in a flightthat "exceeded in numbers any pre­viously-recorded movement" ineastern North America.

In the winter of 1995-96, wewatched in astonishment as num­bers climbed steadily to more than330 different Great Gray Owls inmy study area. Ridout (1996) laterreported: "Great Gray Owl sight-

ings totalled >600 birds across s.Ontario during March. This conser­vative estimate places this past win­ter's s. invasion as likely the largestever experienced in the province."In 1996-97, a completely unexpect­ed 265 owls appeared here, as an"echo flight" occurred that wasalmost as large as the previous win­ter (Ridout 1997). We wereastounded at the unprecedentedresponse of those reporting duringthese invasions. My wife and I wereoverwhelmed. At the beginning, thephone never stopped ringing; wehad to take it off the hook oversuppertime in order to eat!

The map that resulted from allthis action showed a concentrationof sites just south of the edge of theShield (with more isolated sitesalong the north and south fringes).This distribution was probably part­ly because it included marginallands with open areas suitable forrodent hunting, but not muchunbroken forest, similar to the

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habitat used by these owls nearOttawa reported by Brunton andPittaway (1971). It also had a betterroad network and more peopleoccupying all-year homes, thusmore road traffic to provide sight­ings. A further, very significant fac­tor was that it was where localmedia had the most effective cover­age. The reporting pattern was infact to a large degree an artifact ofall this rather than an actual andimpartial picture. We might assumethat the total area and intensity ofthe invasion were much broaderthan I was able to record.

The occurrence of messagesreporting different sightings, bymonth, for the invasions of 1995-96and 1996-97, is presented in Table 1.The last report in the 1995-96 inva­sion was on 15 June, while a roadkillreported on 16 May was the last inthe 1996-97 flight. As the latter birdhad a worn tail, it may well havebeen a release from hospitalization.

The figures in Table 1 should be

Table 1: Occurrence of messages reporting Great Gray Owl sightings by month

Invasion Oct. Nov. Dec. Jan. Feb. Mar. Apr. May June Total Messages

1995-961996-97

11

3o

22

2 44 27124 211 67

86 1124 2

5o

425331

interpreted with caution, sincecallers would obviously report theirinitial sightings, but only rarely sub­sequent ones (let alone their lastone), in spite of my requests thatthey do so. However, I learned toask whether the report was the

caller's first, and quite often learnedabout previous sightings of thesame or another bird.

It is hard to tell how many casu­alties there were since not all wouldbe noticed or reported. At least15 % of recorded owls became

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known traffic victims in 1995-96,with fewer reported the next win­ter. Twenty-seven dead Great GrayOwls were reported to the MNRfrom the district over the winter of1996-97.

Age and ConditionEvidence of plumage in captured orinjured birds, as well as observa­tions in the field from 1995-96, tendto show that most were fledged in1994. Was this a particularly goodyear for nest success in the northwoods? It seems that immaturebirds may be the first to be forcedout when food is in short supply,followed by females (Duncan1987). Established males are mostlikely to remain behind. Those thatstay back often die of starvation(Duncan and Hayward 1994).Plumages noted in 1997 showedthat far more were rather olderbirds. Perhaps some were the sameyoung birds that arrived in 1996 buta year older and wiser, occupyingterritories they found productivethe previous season. Seven respon­dents noted that birds appeared atexactly the same place each year(even the same lookout posts).Probably others did not bother toreport such a happening.

Most fatalities examined were ingood physical condition. Only asmall handful had died of starva­tion; this might have been allied todisease or ageing, since foodseemed plentiful. Other informa­tion that might be secured from

ONTARIO BIRDS AUGUST 1998

such sources in future includes sex,age, breeding readiness, and varia­tions in plumage.

Duration of InvasionsIn view of the unexpected way thatsome owls lingered, even into Junein 1996, there was widespread spec­ulation as to whether some mightstay to breed. Breeding has beenknown on some rare past occasionsin southern Ontario (Forbes et al.1992). A report surfaced in April1997, when an owl was reported,that the caller had seen one in July1996 in the same locality. But nohard evidence of summer occupa­tion has surfaced. Few signs ofawakening sexual activity werereported, but toward the end someowls appeared more closely paired,and there were two reports of inter­active flight display, and also unusu­al hooting calls which may havebeen from this species. One owI wasseen by Peter Burke turning "feetup" at an intruding individual on 29March 1997.

FoodGreat Gray Owls prey primarily onsmall mammals, especially rodents(James 1989b), and "voles(Microtus spp.) dominate theirdiets over most of their range"(Duncan and Hayward 1994). Suchexamination as we were able to doof food pellets and of the stomachcontents of fatalities generally con­formed to these findings, but alsoincluded some less usual prey.

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Figure 1: Great Gray Owl with Eastern Chipmunk, north of Kirkfield, 25 May 1996.Photo by Sam Barone.

Two pellets and ten stomachswere examined. Those from birds

under treatment were ignored. Fourstomachs were empty. The remain-

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der yielded: 14 Meadow Voles(Microtus pennsylvanicus) , oneWhite-footed Mouse (Peromyscusleucopus) , two Star-nosed Moles(Condylura cristata) , four Short­tailed Shrews (Blarina brevicauda),two Meadow Jumping Mice (Zapushudsonius), and even a Black­capped Chickadee (Parus atricapil­Ius). Visual reports of captures.added Red Squirrel (Tamiasciurushudsonicus), Eastern Chipmunk(Tamias striatus) , and Ermine(Mustela erminea). While unusual,squirrels, chipmunks and weaselshave been previously reported asprey (Brunton and Reynolds 1984,Bull and Duncan 1993). It did notprove possible to be more specificabout seasonality of prey use. Inany case, numbers were too smallfor generalization.

Causes of InvasionsGreat Gray Owl invasions arebelieved to be caused primarily bycrashes of prey populations in thebreeding range (Duncan 1987, Bulland Duncan 1993, Duncan andHayward 1994, Pittaway 1997). Inaddition, "particularly good repro­ductive success of owls prior tomovements may accentuate themagnitude of their invasions"(Shuford and Desante 1979).

There has been frequent specu­lation that snow depth and crustingin the north would affect availabili­ty of food for these owls and resultin southward flights (e.g., Shufordand Desante 1979, Kaufman 1997).

ONTARIO BIRDS AUGUST 1998

However, Robert Nero (1980) andJim Duncan (Duncan and Hayward1994) in Manitoba have disputedthis assumption, pointing out thatthere are at times movementsn<;>rthward to places of deepersnow; and that these owls are capa­ble of diving successfully into deepsnow (45 cm) and even snow withsubstantially iced layers. Reportsfrom northern Ontario indicate thatin 1995-96, the icy crust in someareas was such that people andeven moose (Alces alces) wereforced to walk on top of it. Deepsnow itself is not as much of a haz­ard to the owls as we might think,since mice often tunnel to the sur­face for ventilation. Perhaps it is theadded difficulty in locating orreaching prey that affects overallsuccess.

There is much we do not yet fullyunderstand about this bird of mys­tery. We must look forward to thenext invasion, whenever that maybe, and be better prepared to learnfrom it. It should be an excitingexperience.

AcknowledgementsI want to thank the many peoplewho reported their sightings ofGreat Gray Owls to me, and madethis study possible. Tim Dyson inau­gurated the flyer project in 1997,and provided important informa­tion through banding owls. MikeBarker and Barry Snyder of theOntario Ministry of NaturalResources were especially helpful

in supplying information aboutsightings. Lastly, I appreciated thesuggestions of Dan Brunton, RossJames and Ron Tozer, who under­took critical reviews of an earlierdraft of this article.

Literature CitedBrunton, D.F. and R. Pittaway. 1971.

Observations of the Great Gray Owl onwinter range. Canadian Field-Naturalist85: 315-322.

Brunton, D.F. and W.D. Reynolds. 1984.Winter predation on an ermine by a GreatGray Owl. Blue Jay 42: 171-173.

Bull, E.L. and J.R. Duncan. 1993. GreatGray Owl (Strix nebulosa). In The Birdsof North America, No. 41 (A. Poole and EGill, editors). Academy of NaturalSciences, Philadelphia, and AmericanOrnithologists' Union, Washington, D.C.

Duncan, J.R. 1987. Movement strategies,mortality, and behavior of radio-markedGreat Gray Owls in southwesternManitoba and northern Minnesota. Pp.101-107 in Biology and Conservation ofNorthern Forest Owls (R.W. Nero, R.J.Clark, R.J. Knapton and R.H. Hamre, edi­tors). General Technical Report RM-142.United States Department of Agriculture,Forest Service, Fort Collins, Colorado.

Duncan, J.R. and P.H. Hayward. 1994. Reviewof technical knowledge: Great Gray Owls.Pp. 159-175 in Flammulated, Boreal, andGreat Gray Owls in the United States: aTechnical Conservation Assessment (G.D.Hayward and J. Verner, editors). GeneralTechnical Report RM-253. United StatesDepartment of Agriculture, Forest Service,Fort Collins, Colorado.

Forbes, G., M. Runtz and R. Tozer. 1992.New breeding record for Great Gray Owl:most southerly in Canada. Ontario Birds10: 117-118.

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Goodwin, C.E. 1979. The winter season:December 1, 1978 - February 28, 1979.Ontario Region. American Birds 33: 276­279.

James, R.D. 1989a. The Ontario Great GrayOwl invasion of 1983-1984: numbers datesand distribution. Ontario Birds 7: 5-15. '

James, R.D. 1989b. The Ontario Great GrayOwl invasion of 1983-84: habitat, behav­iour, food, health, age, and sex. OntarioBirds 7: 55-61.

Kaufman, K. 1997. Changing seasons. FieldNotes 51: 718-721.

Morneau, F. 1996. Great Gray Owl (Strixnebulosa). Pp. 602-605 in The BreedingBirds of Quebec: Atlas of the BreedingBirds of Southern Quebec (J. Gauthierand Y. Aubry, editors). Association quebe­coise des groupes d'ornithologues,Province of Quebec Society for the Pro­tection of Birds, Canadian WildlifeService, Environment Canada, QuebecRegion, Montreal.

Nero, R.W. 1980. The Great Gray Owl,Phantom of the Northern Forest.Smithsonian Institution PressWashington, D.C. '

Pittaway, R. 1997. Owls and snow: whynorthern forest owls invade the south.OFO News 15(1): 8.

Prevett, J.P. 1987. Great Gray Owl (Strixnebulosa). P 210 in Atlas of the BreedingBirds of Ontario (M.D. Cadman, PEJ.Eagles and EM. Helleiner, compilers).University of Waterloo Press, Waterloo,Ontario.

Ridout, R. 1996. The spring season: March 1- May 31, 1996. Ontario Region. FieldNotes 50: 273-276.

Ridout, R. 1997. The winter season:December 1, 1996 - February 28, 1997.Ontario Region. Field Notes 51: 744-747.

Shuford, D. and D. DeSante.1979. The chang­ing seasons. American Birds 33: 255-262.

Vickery, P.D. and R.P. YURick. 1979. The1978-1979 Great Gray Owl incursionacross northeastern North America.American Birds 33: 242-244.

Doug Sadler, R.R. 4, Peterborough, Ontario K9J 6X5

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