The Ontario Great Gray Owl Irruption of2004-2005: … OB Vol...North American Birds (Bannon et. a!....

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The Ontario Great Gray OwlIrruption of 2004-2005:

Mortality, Sex, Molt and Age

Mark K. Peck and Glenn B. Murphy

IntroductionLarge scale and irregular irruptionsof Great Gray Owls (Strix nebulosa) have been reported manytimes in Ontario and throughouteastern North America (Nero 1980,James 1989a, Bull and Duncan1993). These irruptions often occurduring periods when northern smallmammal populations are low, causing Great Gray Owls to leave theirboreal forest breeding grounds inthe autumn and wander south insearch of food.

During the fall of 2004 and thefirst half of 2005, Great Gray Owlsmoved into southern environs inhigh numbers. It was undoubtedlyone of the largest irruptions everrecorded. According to reports inNorth American Birds (Bannon et.a!. 2005, Currie 2005, Granlund2005, Koes and Taylor 2005), thismassive influx moved intoMinnesota, Wisconsin, Ontario andQuebec, with large numbers of owlsbeing found at the edge of the boreal forest in Alberta, Saskatchewanand Manitoba also.

Within Ontario, Great GrayOwls were first documented inSeptember in northern Ontario, andcontinued their southward movement, concentrating in several sites

ONTARIO BIRDS DECEMBER 2005

throughout southern Ontario byearly 2005. The distribution andmovement of the Ontario birds aredetailed in the associated OntarioBirds article by Jones (2005). Theirruption was reported on televisionand radio, and was written-up in several local and national newspapers.Daily reports were found on theONTBIRDS listserv. Birders andnon-birders alike were provided withthe wonderful chance to observebirds in open rural sites, urban parksand even in backyards. Initial observations included attempts to sex andage the birds in the field, with manybirders using the information provided by Pittaway and Iron on theOntario Field Ornithologists' website (www.ofo.ca). An updated version is presented elsewhere in thisissue (Pittaway and Iron 2005).

The irruption provided aunique opportunity to investigatethe mortality, sex, molt and age ofowls moving south also. Along withobservations, there was considerable human contact. Owls werebanded, rehabilitated, accidentallykilled, mounted and prepared asspecimens, resulting in valuabledata being collected. This articlesummarizes the information collected from the following sources:

Ontario Ministry of NaturalResources (OMNR) Certificates ofReporting: Persons possessing amount or specimen of a speciallyprotected raptor species mustobtain a Certificate of Reportingfrom an OMNR district office.Ministry staff kindly provided summaries of 444 Great Gray Owls registered in the autumn of 2004 andthe first half of 2005 (Table 1).

OMNR Peterborough: LorraineNorris, Senior Fish and WildlifeTechnical Specialist, obtainedweight, wing chord and photographs of spread wings from 57Great Gray Owls. Primary covertswere also collected from most ofthe owls and were given to theRoyal Ontario Museum (ROM)where they will be added to the permanent collections.

Taxidermists: After obtainingCertificates of Reporting, manyowls were taken to taxidermists formounting. Ken Morrison, JimVogel, Ron Armstrong, Jim Jacksonand Rick Poulin saved carcasses orkept detailed notes on 30 GreatGray Owls during the mounting ofspecimens. Unfortunately, 12 additional carcasses were destroyedduring a freezer breakdown andwere unavailable for analysis.

Bird banding: During the fall of2004 and winter of 2005, Nigel Shaw(Innisfil area), Brian Ratcliff(Thunder Bay area), MylesFalconer (Perth area) and John

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Lemon (Sudbury area) banded 100Great Gray Owls. Age and sexinformation on the owls was generously made available to the authors.

Wild Bird Clinic, OntarioVeterinary College, University ofGuelph: Dr. Katharine Welch provided data from 14 Great Gray Owlautopsies performed at the clinic.

Wildlife Centres, Humane Societiesand the Society for the Preventionof Cruelty to Animals (SPCA):Injured birds taken to animal carefacilities were nursed back tohealth and released or, if euthanized, were returned to the nearestOMNR office. The ROM received28 Great Gray Owls from wildlifecare facilities.

Royal Ontario Museum (ROM):With the assistance of OMNR staffand the ONTBIRDS listserv, theauthors at the ROM requested anyunwanted dead owls, hoping to addspecimens to the ornithology permanent collections. The request wasanswered by numerous individuals,resulting in an additional 39 GreatGray Owls being turned in to theOMNR or sent directly to the ROM(Figure 1). Birds were prepared asstudy skins, skeletons and spreadwings. Tissue and feather sampleswere collected and have beenadded to the permanent collectionsalso. Upon request, all specimenswill be made available for morphometric, molecular and/or isotopicsignature research.

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Table 1: Area summaries of Great Gray Owls obtained from Certificates ofReporting, Ontario Ministry of Natural Resources (2005).

2004 2005AREA Oct Nov Dec Jan Feb Mar Apr May TOTALUnknown location 1 1 2

Algoma District 5 13 5 3 3 29Bruce County 1 1

Cochrane District 1 4 1 1 4 11

Durham Region 7 4 5 16Frontenac County 1 3 1 1 6

Haliburton County 1 1 2

Hastings County 11 10 15 3 1 40Kenora District 1 10 6 1 4 2 2 26Lanark County 10 4 4 18Leeds & Grenville County 2 2 3 7

Lennox & Addington County 2 1 3

Manitoulin District 1 3 1 1 6

Middlesex County 1 1

Muskoka District 1 1 2 4

Nipissing District 1 1

Northumberland County 2 2 3 1 8

Ottawa 9 6 1 1 1 18Parry Sound District 1 1

Peterborough County 13 5 6 1 1 26Prescott & Russell County 3 1 4

Rainy River District 9 13 5 1 1 2 31Renfrew County 1 1 3 1 6

Simcoe County 1 9 13 13 4 40Stormont, Dundas &

Glengarry County 1 1

Sudbury District 1 6 6 3 4 3 23Thunder Bay District 2 46 23 4 1 1 4 2 83Timiskaming 1 1 2

Kawartha Lakes 3 4 5 2 14York Region 5 5 3 1 14

TOTAL 5 81 66 99 76 76 25 16 444


MORTALITYCertificates of ReportingMost of the known Great Gray Owlcasualties were first reported to district offices of the OMNR. Eachoffice tracks all specially protectedraptor species, determining cause ofdeath, date of acquisition, locationwhere the specimen was found andother associated data. Table 1 summarizes the date and area whereeach owl was found.

Many of the northern districtshad their greatest mortality reportedduring November and December2004. Birds continued to movesouth, with increasing mortalityreported in southern counties andregions in January, February and

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March 2005. During the first fewmonths of 2005, mortality in thenorthern districts quickly decreased,suggesting birds were leaving thenorth and moving south in a concentrated fashion.

In the north, mortality wasgreatest in Algoma, Rainy Riverand Thunder Bay, and may be areflection of higher human interaction/populations in those areas.Southern Ontario mortality washighest in Hastings, Peterboroughand Simcoe counties, all areaswhere owls had concentrated inlarge numbers (Jones 2005), suggesting that food resources wereplentiful and starvation was not amajor cause of mortality.

Figure 1: Glenn Murphy on 19 April 2005 with some of the Great Gray Owls donated to the Royal Ontario Museum by private individuals, animal welfare organizations and the OMNR. Photo by Brian Boyle, ©ROM.

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Additional SpecimensKay McKeever of The OwlFoundation in Vineland admitted 43Great Gray Owls from November2004 through June 2005. Six of thebirds subsequently have beenreleased in western Manitoba withthe assistance of Jim Duncan,Biodiversity Conservation Section,Manitoba Conservation. Eight of thebirds have died and 29 remain in thecare of The Owl Foundation staff(Kay McKeever, pers. comm.). TheWildbird Care Centre in Ottawaadmitted 23 birds, released four andhad 19 succumb to their injuries. SueMeech of the Sandy Pines WildlifeCentre in Napanee took in 16 birds.Two of the birds were sent on to TheOwl Foundation, one was releasedand 13 died. An additional 30 owlswere reported from the University ofGuelph or were turned in directly tothe ROM without Certificates ofReporting.

In total, there is documentationfor 541 Great Gray Owls beinginjured (40) or killed (501) duringthe irruption. How many of theseowls died and went unreported isimpossible to determine. This isconsiderably more than the 51 deadbirds noted during the 1983-1984irruption (James 1989b).

In Minnesota, 750 owls werereported to have died in 2005 andan as yet undetermined numberperished in Wisconsin (Granlund2005). Jim Duncan (pers. comm.)reported lower than average numbers were killed in Manitoba in2004-2005.


Cause of DeathThere were 414 Great Gray Owlsfound dead along roadsides inOntario. Many were observed incollisions with vehicles, and birdsfound dead on roadsides with noadditional information were presumed to have met a similar fate.Six were found dead in traplines,nine died of starvation, five died incollisions with trains, two were window kills, one died fromAspergillosus, one expired frompulmonary congestion, and one waspresumed electrocuted after it wasfound dead under a hydro line. Inaddition, one bird flew into a tractor, another died after it flew into aparked truck and a third owl wasreported "just falling out of thesky"! Cause of death for 59 birdswas undetermined.

There were no reports of owlsbeing shot in Ontario, but theAssociated Press reported four menin Minnesota had been chargedwith poaching over a dozen GreatGray Owls in that state.

SEXTable 2 details the number of malesand females sexed internally, usinggonads, from ROM specimens anddonated carcasses. Females outnumbered males almost two to one.This is consistent with findings elsewhere that suggest females tend towander more widely while malesremain more sedentary (Duncan1987, James 1989b). Males may alsohave migrated later as is suggestedby their higher March mortality

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Table 2: Number of male and female Great Gray Owls sexed from ROM specimensand carcasses donated by taxidermists.

2004 2005Oct Nov Dec Jan Feb Mar Apr May Unknown TOTAL

Females 0 2 3 17 17 10 2 0 14 65

Males 0 0 1 5 2 11 1 0 15 35

number. Results are consistent withearlier findings from Manitoba andnorthern Minnesota (Bull andDuncan 1993). We also found thatmany of the early female mortalities were second winter birds, suggesting that young females maymove out of the north first.

During the preparation of specimens, individual owls wereweighed and measured (Table 3).Owls were weighed with an electronic balance to the nearest gram.Six females weighing between 696and 943 g were later determined tohave died of starvation. Captiveand emaciated birds were not usedin the sample. Weights from allother owls were within the rangesreported by Bull and Duncan(1993), with the exception of onemale that weighed 1435 g. This birdwas considered an outlier, and

removed from the sample.Minimum weights in both malesand females of these Ontario owlswere approximately 150 g higherthan had been reported previously(Bull and Duncan 1993).

Unflattened wing chord, tarsusand foot pad lengths were alsorecorded. Wing chord measurementswere similar to measurements takenin Manitoba (Duncan 1992). Footpad length was measured from thebase of the talon of the halux to thebase of the talon on the middle toe.Tail length was not measuredbecause of extensive feather wear ordamage to many of the specimens(Figure 2). Measurements originallywere taken to determine sex ofunknown birds from banding resultsusing a discriminant function analysis developed by Duncan (1996).However, lack of tail and foot pad

Table 3: Great Gray Owl minimum and maximum weight and measurement valuesobtained from birds turned in to the ROM.

Weight (g) Wing (mm) Tarsus (mm) Footpad (mm)

Female Male Female Male Female Male Female Male

Number 47 18 37 14 37 13 30 15

Minimum 1051* 845* 400 390 46 44 59.15 58.5

Maximum 1664 1135** 455 425 57 55 72.25 63.8

* captive and emaciated birds were removed from the sample** one male outlier weighing 1435 g was removed from the sample

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measurements in owls of known andunknown sex prevented us fromcompleting the study. Measurementswere included in this article to allowfor future comparisons.

Using weight and wing chordmeasurements from banding resultsand owls processed by OMNR(Lorraine Norris, pers. comrn.), weestimated sex of birds using rangevalues provided by Duncan (1992)and the minimum weight valueobtained from ROM preparedspecimens (Table 4). Overlappingrange values between sexes doesnot allow for complete segregationin Great Gray Owls. Flattened wingchord measurements taken duringOMNR processing were comparedwith values obtained by Johnsgard

(1988). ROM birds were included inthe table to show the amount ofoverlap in owls of known sex.

Measurements taken duringOMNR processing and duringbanding programs also suggest astrong bias toward females in allareas of the province (Table 4).Very few known males weighed lessthan 900 g (reported minimumvalue of females) and we do notbelieve this weight accurately represents the males in our sample.Specimens prepared at the ROMshowed no overlap between thesexes at weights less than 1051 g,with the exception of the emaciatedfemales mentioned above. Webelieve birds weighing less than1051 g may be designated males,

Figure 2: Great Gray Owl rectrices. The heavily worn, faded rectrices of a secondwinter (HY 2003) are presented on the left. The juvenile feathers have not beenmolted. The white tips normally seen on these feathers have been worn off. Recentlygrown adult rectrices are presented on the right. Photo by Mark K. Peck, ©ROM.

o TARIO BIRDS DECEMBER 2005

providing a more accurate representation of the male to femaleratio found in Ontario.

Unflattened wing chord alsoappeared to underestimate malenumbers. The flattened wing chordsprovided by Norris better represented the male to female ratios foundelsewhere. Additional study isrequired to confirm these findings.

PREY ITEMSStomach contents from ROM specimens, carcasses from taxidermistsand two pellets brought to the ROMwere analyzed by the authors, withadditional confirmation of skeletalmaterial provided by Bill Kilburn,University ofToronto. All prey itemswere removed from the crop andgizzard, and cleaned of fur and tissue, both manually and usingDermestid beetles in the bug roomof the ROM. Sixty-five stomachswere dissected, of which 31 wereempty. Meadow Vole (Microtus

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pennsylvanicus) was the most common prey item found in 30 of theremaining 34 gizzards. Both pelletsalso contained Meadow Vole skulls.In addition to Meadow Voles, otherprey items found in gizzards included: Star-nosed Mole (Condyluracristata) in four, Hairy-tailed Mole(Parascalops breweri) in one, Shorttailed Shrew (Blarina brevicauda) infive, Common Shrew (Sorexcinereus) in four and House Mouse(Mus musculus) in two. Evidence oflarger prey items was not found inany of the stomachs. A search ofGreat Gray Owl reports in the2004/2005 ONTBIRDS archivesfailed to reveal any other prey itemsbeing noted.

Granlund (2005) reported GreatGray Owls in Minnesota andWisconsin pursuing doves, rabbits,squirrels, small dogs and cats andeven a fur hat. In the Ontario irruption of 1983-1984, James (1989b)reported several larger prey items

Table 4: Weight and wing chord measurements from Great Gray Owls of knownsex (ROM). Estimates of sex using banding results (Shaw, Ratcliff, Falconer andLemon), and owls processed at OMNR (Norris). Sample size in brackets.

Weight Wing chord Wing chord

unflattened flattened

<900 g <1051 g >1200 g <391 mm >429 mm <430mm >447 mm

(ROM)

male male female male female male female

ROM 3 (24) 17 (24) 39 (47) 1 (14) 25 (38) NA NA

Shaw 0(47) 9 (47) 25 (47) 0(47) 33 (47) NA NA

Ratcliff 1 (32) 11 (32) 10 (32) 0(32) 22 (32) NA NA

Falconer 0(13) 1 (13) 10 (13) 0(13) 8 (13) NA NA

Lemon 0(4) 0(4) 3 (4) 0(9) 7 (9) NA NA

Norris 1 (56) 13 (56) 33 (56) NA NA 7 (57) 28 (57)

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also, including a possible Ermine(Mustela erminea), a Snowshoe Hare(Lepus americanus) , an EasternCottontail (Sylvilagus floridanus),and a Beaver (Castor canadensis).Avian prey items included a domestic chicken and a Northern Goshawk(Accipiter gentilis).

SUBCUTANEOUS FATMany banded birds, carcasses andspecimens were examined for subcutaneous fat. Most owls weredetermined to be healthy, with considerable fat deposits at the time ofbanding/death. Of the 55 femaleowls checked, three were describedas having light fat, four had moderate fat and 42 were described ashaving heavy or extremely heavyfat. The remaining six birds had nofat, and all were reported to havedied of starvation. Of the sixfemales that died of starvation, twowere from the Toronto area, twowere found near Lake Simcoe andindividual birds were picked up inTobermory and Ottawa.

A similar pattern was found inmale owls. Twenty-one birds weredescribed as having heavy fat, onehad moderate fat, three had light fatand one bird had no fat.Interestingly, the latter bird wasfound dead along the roadside, withthree voles in its stomach.

ESTIMATING AGEInternalMuseum specimens and taxidermycarcasses were aged internally byexamining skull ossification, long


bone ossification, and the presence orabsence of the bursa of Fabricius, asmall organ located near the cloaca.The bursa is grown during thenestling stage and regresses as thebird reaches maturity, usually disappearing by April following the firstyear in other owl species. It is commonly used for ageing museum specimens, and has been successfully usedto age Great Horned Owls (Bubovirginianus; Weller 1965), SnowyOwls (Bubo scandiaca; Josephson1980) and Ural Owls (Strix uralensis;Pietiainen and Kolunen 1986).Completeness of skull ossificationand the ossification of long boneswere checked by the authors, with asupplemental examination by KevinSeymour, Department of NaturalHistory, ROM. Using these criteria,no first winter birds were found in theROM specimens. However, preparations of skeletons did allow for easyidentification of starved birds.Starved owls had very little fat left inthe bones, causing skeletal materialto appear greaseless, pale and dry.

ExternalGreat Gray Owls do not molt theirprimaries (P), secondaries (S), primary coverts (PC) or tail feathers(rectrices) during the first prebasicmolt (Pyle 1997). Most juvenalfeathers are easily distinguishedfrom adult feathers by their lightbeige/white tips. Molt of flightfeathers begins in early to late summer, follows a regular pattern, but ispartial, and may take several yearsto complete. This incomplete molt

has been used in Europe as aneffective technique for ageingGreat Gray Owls (Pyle 1997). Neroand Copeland (1997) have suggested that Great Gray Owls may showan inhibited molt of flight feathersif inadequate nutrition during thesummer months does not allow forthe necessary energy requirementsof feather replacement.

Using banding results, photographs and specimens brought in tothe ROM, we examined flight feathers on Great Gray Owls to assessmolt pattern and age structure.Rectrices in younger birds and roadside casualties often showed heavywear or extensive damage (Figure 2)and were not used in our study. Infield situations, heavy wear of rectrices may assist in ageing youngerbirds (Pittaway and Iron 2005).

First Winter (HY [Hatch Year]2004): Juvenal flight feathers aregrown in May and June and areretained during the first prebasicmolt. All primaries (with the exception of PI0), secondaries and rectrices are white-tipped and more heavily barred than adult feathers. Thewhite tips on some of the feathersmay be slightly worn but the feathersshould still be dark and should notshow much evidence of fading.Compared to adult feathers, the rectrices are narrower, more pointedand may be slightly frayed by springof the following year. Juvenal feathers may be of slightly poorer qualityand will show some wear and fading,but should still be darker than juve-

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nal feathers grown in previous years(Jim Duncan, pers. comm.). Only oneowl was confirmed as a first winterbird (Myles Falconer, pers. comm.).This owl was banded in the Pertharea on 23 February 2005.

Second Winter (HY 2003): Secondwinter owls retained all or most oftheir white-tipped juvenal feathers,but the primaries, secondaries andrectrices were heavily worn. Thefeathers also showed heavy fading,appearing a lighter brown, whencompared to adult feathers. In thisage class, newly molted feathers,when found, were usually at theinner secondaries, S9 or S10 (Figure3). Ageing of the innermost secondaries, SII-SI3, often referred to astertials in passerines, is difficult andoften problematic. These feathersare different in appearance fromthe other secondaries, lacking thewhite tip, and often show less fadingthan other flight feathers. Darkercoloration in adult feather rachis(central shaft) often provided a useful character that can be easilychecked. After the innermost secondaries have been molted, themolt moves outward toward the tipof the wing. During the second winter' molted feathers do not appearto extend beyond S8.

Third Winter (HY 2002): In thethird winter, supplementary moltcentres are usually found at S5 andat P5 (Figure 4). Secondary moltcontinues to move distally from theinnermost secondaries toward the

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Figure 3: 5pread wing of a second winter (HY 2003) Great Gray Owl showing the numbering of primaries and secondaries. PI-PI0 and 51-59 have retained their juvenal"white" tipped feathers and show considerable wear and colour fading. 510 is a newlymolted feather. 511-513 are juvenal feathers but are different in appearance, lacking thewhite tips, and are difficult to score accurately. Photo by Mark K. Peck, ©ROM.

primaries. A second secondary moltcentre begins later at S5 and alsomoves distally toward the primaries. Primary molt is centrifugal, progressing in both directions, usuallybeginning at P5, but initiation at P4and P6 was also recorded. By theend of this molt cycle, one to threeprimaries had been replaced.Primary coverts appear to moltprior to the primaries and are readily distinguished between adult(pe5-PC7) and juvenile plumages.

Fourth/Fifth Winter (HY 2000/2001):Many of the juvenal feathers havenow been replaced with dark-tippedadult feathers (Figure 5). Pattern ofONTARIO BIRDS DECEMBER 2005

molt corresponds with the detailsnoted in third winter birds, with threeto nine primaries having beenreplaced by autumn.

> Fourth Winter (HY earlier than2001): All juvenal feathers have nowbeen replaced with adult feathers(Figure 6). P5 and P6 may sometimes show fading at the tips due towear and care must be taken toavoid confusion with juvenal feathers. The pattern of the wing molt isstill evident due to the colour fadingof older feathers. From the limitedinformation we had on this age class,there is a suggestion that adult moltmay not follow the same pattern

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Figure 4: Spread wing of a third/fourth winter (HY 200112002) Great Gray Owl. Newfeathers are found at P5, P6, S5 and S9 - S13. Flight feather molt in young Great GrayOwls is not completed annually but does foUow a regular pattern. Primary molt is centrifugal, progressing in both directions, usually beginning at P5. Secondary molt startsat the innermost secondaries and moves distally toward the primaries. A second secondary molt centre begins later at S5 and also moves distally toward the primaries.Primary coverts appear to molt prior to the primaries and are distinguishable betweenthe adult (pC5-PC7) and juvenile plumage. Photo by Mark K. Peck, ©ROM.

observed in younger birds. It is possible that adults may replace specificflight feathers as they become wornrather than in a regular pattern as isseen in younger birds.

It is important to note thatflight feather molt may vary considerably depending on the nutrientresources available during the molting period, confirming the findingsof Nero and Copeland (1997). Inyears when northern small mammalnumbers are low, molt may be minimal or even completely inhibited,thus increasing the difficulty of age-

ing birds accurately. Variation inindividual fitness, sex, age and raising of young could all impact moltstrategies for Great Gray Owls.

Although difficult, estimatingage classes of Great Gray Owls provides valuable information on thepopulation structure of this speciesand may also provide insight intonorthern breeding conditions. In2004, several Ontario Breeding BirdAtlas field crews reported on thelow numbers of small mammals inthe north (Don Sutherland, pers.comm.; Glenn Coady, pers. comm.).According to Jim Duncan (pers.

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Figure 5: Spread wing of a fourth/fifth winter (HY2000/200l) Great Gray Owl.Juvenal feathers have now been replaced at Pl-P7, S5 and S7-13. Pattern of moltcorresponds with the details noted in Figure 3. Photo by Mark K. Peck, ©ROM.

Figure 6: Spread wing of a > fourth winter Great Gray Owl. All juvenal feathers havenow been replaced. The pattern of the wing molt is stiD evident due to the colour fading of older feathers. P8, P4-P6, PI, Sl, S2, S4, and S8-S13 all appear to be recentlymolted, suggesting older birds may not follow the same molt pattern observed inyounger birds. Photo by Mark K. Peck, ©ROM.o TARlO BIRDS DECEMBER 2005

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Figure 7: Great Gray Owl age class totals, estimated from wing molt.

comm.), Great Gray Owls in hisManitoba study area had little or noproductivity in 2004 also. This wouldaccount for the lack of juvenilesfound in the 2004-2005 irruption(Figure 7) and may have been thetrigger for the southward movementof the owls. Juvenile birds were notreported in Manitoba or Minnesotaas far as we have been able to determine (Jim Duncan, pers. comm.;Nigel Shaw, pers. comm.).

Second and third winter birdsmade up the majority of the birdsfound in our sample. This suggeststhat productivity in the north washigh in those two years, with manyyounger birds successfully survivingthrough to the autumn of 2004. It is

unlikely that these numbers accurately reflect the age structure of allGreat Gray Owls in the north ofOntario and Manitoba. Juvenileand younger age class birds wandering or moving out of suboptimalhabitat is probably the usual situation during invasions and providesfurther evidence for food stress asthe likely cause for the irruption(Duncan 1987). Older, more experienced birds, occupying optimalhabitat, would be more likely tosurvive and stay in the north.

SUMMARYDuring the Great Gray Owl irruption of 2004-2005, information onmortality, sex, molt and age was col-

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lected from owls brought in to theOntario Ministry of NaturalResources, taxidermists, bird banders, wildlife rehabilitators and theRoyal Ontario Museum. There were541 owls found dead (501) orinjured (40) throughout Ontario.The majority of birds were founddead along roadsides. Other causesof death included starvation,traplines, trains, and windows. Mostowls appeared healthy, with considerable subcutaneous fat deposits atthe time of banding/death.

In a comparison of known sexowls, females outnumbered males,65 to 35. Sexing of unknown owlsusing weight and wing chord valuesalso pointed to a greater number offemales moving south during theirruption.

Estimates of age classes wereobtained using internal examinations and flight feather molt patterns. The only first winter GreatGray Owl found was banded in thePerth area, indicating poor productivity in the north in 2004. Molt patterns suggested most Great GrayOwls, 142 of 186, were second orthird winter in age.

AcknowledgementsA very special thanks to RonArmstrong, Jim Duncan, Myles


Falconer, Jean Iron, Colin Jones,Bill Kilburn, Jim Jackson, JohnLemon, Kay McKeever, SueMeech, Mike Martin, JuliaMonkman, Ken Morrison, CathyNihei (and staff), Lorraine Norris,Ron Pittaway, Rick Poulin, PeterPyle, Brian Ratcliff, KevinSeymour, Nigel Shaw, ShaunThompson, Suzanne Van Exan, JimVogel, Katharine Welch and JudithWilson for their sizable contributions to this article.

Thanks also to staff of theOntario Ministry of NaturalResources for their professionalismand support of the project. Wewould also like to thank all of theindividuals who assisted us with thisproject by bringing owls to theOntario Ministry of NaturalResources, Wildlife Centres or theRoyal Ontario Museum. As is oftenthe case, we thank the Ontario FieldOrnithologists and Ontario Birdseditors for their knowledge, patienceand assistance. Also, we would liketo thank the ONTBIRDS listserv,coordinated by Mark Cranford, forhelp with initial communicationsand archival material.

Partial funding for this projectwas provided by the Department ofNatural History, Royal OntarioMuseum.

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David. 2005. Quebec. North AmericanBirds 59: 234-235.

Bull, E.L. and J.R. Duncan. 1993. GreatGray Owl (Strix nebulosa). In The Birds ofNorth America, No. 41 (A. Poole and F.Gill, editors). Academy of NaturalSciences, Philadelphia, Pennsylvania, andAmerican Ornithologists' Union,Washington, D.C.

Currie, H.G. 2005. Ontario. North AmericanBirds. 59: 258-260.

Duncan, J.R.. 1987. Movement strategies, mortality, and behavior of radio-marked GreatGray Owls in southeastern Manitoba andnorthern Minnesota. Pp.101-107 in Biologyand Conservation of Northern Forest Owls.Symposium proceedings, February 3-7,1987, Winnipeg, Manitoba (R.W. Nero, R.J.Clark, R.J. Knapton, and R.H. Hamre, editors). General Technical Report RM-142.USDA Forest Service, Rocky MountainForest and Range Experiment Station, FortCollins, Colorado.

Duncan, J.R.. 1992. Influence of prey abundance and snow cover on Great Gray Owlbreeding dispersaL Doctoral dissertation,University of Manitoba, Winnipeg.

Duncan, J.R•• 1996. Techniques to sex andage Great Gray Owls: A bird in the handworth two in the bush? Birders Journal 5:240-246.

Granlund, J. 2005. Western Great Lakes.North American Birds. 59: 270-272.

James, R.D. 1989a. The Ontario Great GrayOwl invasion of 1983-1984: Numbers,dates, and 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.

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Johnsgard, P.A. 1988. North American Owls.Smithsonian Institution Press.Washington, D.C.

Jones, CoD. 2005. The Ontario Great GrayOwl Invasion of 2004-2005: Numbers,dates and distribution. Ontario Birds 23:106-121.

Josephson, B. 1980. Aging and sexing SnowyOwls. Journal of Field Ornithology 51:149-160.

Koes, R.F. and P. Taylor. 2005. PrairieProvinces. North American Birds. 59:283-284.

Nero, R.W. 1980. The Great Gray OwlPhantom of the northern forest.Smithsonian Institution Press,Washington, D.C.

Nero, R.W. and H.W.R. Copeland. 1997. Sexand age composition of Great Gray Owls(Strix nebulosa), Winter 1995/1996. Pp.587-590 in Biology and Conservation ofOwls of the Northern Hemisphere. Secondinternational symposium, February 5-9,1997, Winnipeg, Manitoba (J.R. Duncan,D.H. Johnson, T.H. Nicholls, editors).General Technical Report NC-190. USDAForest Service, North Central ResearchStation, St. Paul, Minnesota.

Pietiainen, H. and H. Kolunen. 1986. Agedetermination of breeding Ural OwlsStrix uralensis. Ornis Fennica 63: 26-27.

Pittaway R. and J. Iron. 2005. Ageing and variation of Great Gray Owls. Ontario Birds 23:138-146.

Pyle, P. 1997. Flight-feather molt patternsand age in North American owls.Monographs in Field Ornithology 2: 1-32.

Weller, M.W. 1965. Bursa regression, gonadcycle, and molt of the Great Horned OwLBird-Banding 36: 102-112.

Mark K. Peck, Department of Natural History, Royal Ontario Museum, 100Queen's Park, Toronto, Ontario M5S 2C6

Glenn B. Murphy, Department of Natural History, Royal Ontario Museum,100 Queen's Park, Toronto, Ontario M5S 2C6

VOLUME 23 NUMBER 3

The Ontario Great Gray Owl Irruption of2004-2005: … OB Vol...North American Birds (Bannon et. a!....

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