FURTHER DATA ON FOOD lIT. MS OF NORTHERN SAW-WHET OWLS (AEGOLIHS A CADICHS BROOKSI) ON THE QUEEN CHARLOTTE ISLANDS, BRITISH COLUMBIA

SPENCERG. SEALY,Department of Zoology,University of Manitoba,Winnipeg, Manitoba R3T 2N2, Canada

Mammals dominate the list of prey items taken by the widespread nominate subspeciesof the Northern Saw-whet Owl (Aegoliusacadicus acadicus)throughout its range, includingthe PacificNorthwest (Forsman and Maser 1970, Boula 1982, Grove 1985). Aegolius acadicusbrooksi, the only other subspeciesrecognized, is endemicto the Queen Charlotte Islandsor Haida Gwaii, BritishColumbia (Cory 1918, AOU 1957, Sealy 1998), whence limited information has shown that its diet includesinverte- brates-primarily from the intertidalzone--as well as vertebratetaxa in addition to small mammals. Previous data on brooksi came from examina- tion of digestivetracts of 16 adultsand onejuvenile salvaged between 1977 and 1986, primarily in nonbreedingseasons, and from stable-carbon- isotopeanalyses of tissuesof 12 of these specimens(Hobson and Sealy 1991). In additionto marineinvertebrates, two individualshad taken Dusky Shrews(Sorex monticola),and a publishedreference was notedto fledg- lingswith remainsof passerinebirds in their digestivetracts. Cowan (1989) listedthe NorthernSaw-whet Owl amongthe birdsof the QueenCharlotte Islandsthat forageprimarily away from the seaor shore,but littleinforma- tion was available on the food habits of brooksi at that time. Since the publicationof Hobsonand Sealy's(1991) results,further data on fooditems of brooksihave become available, and I givethem here.

METHODS AND RESULTS Sourcesof Informationon Prey Items Informationon foodhabits obtained since Hobson and Sealy(1991) has comefrom (1) a specimenin the CowanMuseum of Zoology,University of BritishColumbia (UBC), (2) contentsof digestivetracts (hereafter stomachs) of nine adultsand one juvenile,examined 1990-1997, (3) examinationof 11 pellets,and (4) direct observationsof individualsforaging. Whole and fragmentaryinvertebrates and vertebrateprey items from stomachsand pellets are stored in the Universityof Manitoba Museum of Zoology (UMZM). Invertebratesand lowerjaws of mammalianprey were compared to reference material collectedon the Queen Charlotte Islandsin 1986 (Hobson and Sealy 1991) and housed in the Royal British Columbia Museumand UMZM. Remainsof Ancient Murrelets(Synthliboramphus antiquus) were identifiedby their distinctivedown feathersand bone fragments;other bird remainswere identifiedfrom portionsof mandibles and entire primaries.Food items from all sourcesexamined in this study (stomachcontents, pellets, literature, museum specimen, and field observa- tions) are summarizedTable 1.

200 Western Birds 30:200-205, 1999 FOOD ITEMS OF NORTHERN SAW-WHET OWLS

Table 1 Prey itemsof Aegoliusacadicus brooksi

Speciesø Sampleswith species (%)b

Invertebrates Amphipoda(Orchestria traskiana, Orchestoidea californiana) 46.3 Isopoda(Ligia pallasii) 7.3 Diptera(Coelopa vanduzeei) 7.3 Unidentifiedinsect parts 2.4 Arachnida(Anyphaena sp.) 2.4 Amphibians WesternToad (Bufo boreas) 2.4 Birds AncientMurrelet (Synthliboramphus antiquus) 7.3 Chestnut-backedChickadee (Poecile rufescens) 2.4 Golden-crownedKinglet (Regulus satrapa) 2.4 Hermit Thrush(Catharus guttatus) 2.4 Unidentified bird remains 4.9 Mammals DuskyShrew (Sorex monticola) 9.8 DeerMouse (Peromyscus sp. ) 7.3

øFrom41 sources:stomachs, including those examined by Hobsonand Sealy (1991), containing at leasta trace of prey remains(n = 26), pellets(n = 11), recordsfrom the literature(n -- 2), a museumspecimen (n = 1), andan observationof foragingwhere the preyitem was identified (n -- 1). More than one prey speciessometimes occurred in the samestomach or pellet. bPresenceonly, not absolute numbers of individualprey items.

Contents of Stomachs Hobsonand Sealy (1991) recordedbrooksi eating predominantly inverte- brates: sand-hoppers(the amphipodsOrchestria traskiana and Orches- toidea californiana), seaweedflies (Coelopa vanduzeei),isopods (Ligia pallasii), and a juvenilespider (either Anyphaena aperta or A. pacifica). The spidersare not known to inhabit beaches;A. aperta occurson tree foliage,A. pacificain forestlitter (R. A. Canningspers. comm.). The other groupsinhabit beaches in the regionof the uppertide line and splashzone. Additionalrecords of invertebrateprey taken by brooksihave been ob- tained.A female(UBC 921) collected1 September1946 bearsthe notation "spider,"removed from the bird'sdigestive tract. On 7 March 1995 Phred Collins(pers. comm.) noted the remainsof sand-hoppersin a pelletregurgi- tatedby a femaleowl hit by a vehicle;later ! notedamphipod fragments in its stomach.The stomachof a male (4 November 1994) contained47 intact individualsplus fragments of Orchestoideacaliforniana and a partialupper mandible,piece of cranium,and feathersof a Chestnut-backedChickadee (Poecilerufescens). The stomachof a male (undated)contained amphipod fragments,and a female(15 January1995) and a male (19 March 1996) salvagednear Massethad eaten63+ and 54+ O. californiana,respectively. The stomachof a female(19 November1996) containedamphipod frag- ments,that of a juvenilekilled by a catin August1997 nearSkidegate Inlet 16 amphipodsplus fragments (O. californiana)and five isopods (Ligia pallasii).

201 FOOD ITEMS OF NORTHERN SAW-WHET OWLS

Patch(1922) removedpasserine feathers from stomachs of two fledglings collectedon 21 July 1919 that "will probablyprove to be [those]of some warbler." I did not locate these feathers. Stomachs of two adult females takenin 1946 containeda few feathers,which Ian McTaggart-Cowan(pers. comm.) believedwere from an owl. Stomachsof a female (1 October 1990) from Moresby Island and of a male (21 November 1996) from Masset containeda DuskyShrew and severalshrew hairs, respectively.

Contents of Pellets I examined11 pellets, 10 collectedon EastLimestone Island (see map in Sealy 1998) by Robert Kelly in June 1997, from a roost used by two individuals,and one from Graham Island(examined by Collins).The pellets from EastLimestone Island averaged 30.3 + 4.7 (standarddeviation) mm long and 15.4 + 0.7 mm wide and containedthe remainsof two downy Ancient Murrelets,one Hermit Thrush (Catharusguttatus), one Golden- crownedKinglet (Regulus satrapa), two DuskyShrews, and three deer mice (Peromyscussp.). Observationsof NorthernSaw-whet Owls Hunting or CarryingPrey Recentobservations of saw-whetowls at or travelingto or from the tide linepoint to foragingin the intertidalzone or upperbeach. On 5 April 1971 on LangaraIsland, I nettedone (Sealy1998)just abovethe splashzone on a sandbeach, about 30 m from the nearesttrees. The capturesite suggests travel to or from the beach. Over a 2-year period through 1997, Frank Reindl(pers. comm.) saw about 15 NorthernSaw-whet Owls flying toward and from the beachalong the coasthighway between Tlell and Skidegate. Finally,on 10 September1997, Kelly (pers.comm.) observed foraging on sand beacheson two islandsoff the west coast of MoresbyIsland. On HelgesonIsland, he saw a saw-whetowl foragingat 2300 alonga line of rottingkelp washed by waveson a risingtide; for about10 minutes,the owl hopped and flew back and forth along a 2-m strip of kelp. A second individualwas observedstanding in rotting kelp at the upper tide line on adjacentSaunders Island later the samenight. Sand-hoppers,isopods, and seaweedflies are particularlynumerous along the upper beachesat this stageof the tidal cycle(pers. obs.). On 17 June 1974, about15 km southof Sandspit,R. WayneCampbell (pers.comm.) saw a NorthernSaw-whet Owl perchedon a snag,holding a WesternToad (Bufo boreas)in its claws.Minutes later, the owl carriedthe toad into the woods. One afternoonin June 1997 on Anthony Island, Collins(pers. comm.) saw a saw-whetowl investigatetwo Hermit Thrush nests,but at each nest the thrushesrepelled the owl. Numerousobservers have suggestedor reported that brooksi takes newly hatchedAncient Murreletchicks at nightas they descendthe slopesfrom their burrowsto the sea. Blood et al. (1979) watchedone chasean Ancient Murrelet chickon the floor of the colonyon Lyell Islandbut did not ascertainif it was successful. Also on Lyell Island,Rodway et al. (1988) heard saw-whetowls calling at night duringthe murrelets'breeding season. On EastLimestone and Reef islands,Gaston (1992) foundAncient Murrelet down feathers in pelletsand

202 FOOD ITEMS OF NORTHERN SAW-WHET OWLS heard owlscalling in the colonymore frequentlywhen the chicksdeparted for the seathan at othertimes of the year.On 31 May 1996, Campbelland others watched a saw-whetowl fly off with a downy Ancient Murrelet captureden route to the sea and observeda secondone eatinga murrelet chick. In the late 1970s, Tom E. Reimchen (pers. comm.) observedone pursuinga bat overDrizzle Lake. The labelsof threespecimens of nominate acadicus from southeast Alaska note the remains of a Little Brown Bat (Myotislu½ifugus) removed from the stomachin one,a shrewin the second, and a vole (Microtus sp.) in the third.

DISCUSSION

This new informationextends Hobson and Sealy's(1991) findingsthat Aegoliusacadicus brooksi feeds on a wide array of taxa. Prey itemsand intendedprey identifiedso far representseveral groups of invertebrates,an amphibian,small birds, and smallmammals. The largenumbers of inverte- bratesin the samples,however, signify the importanceof marine-derived proteinin the diet of theseowls, at leastat the end of the breedingseason and duringthe nonbreedingseason. On the surface,available information suggests that brooksitakes a wider rangeof prey than doesnominate acadicus. Nonmammalian prey consti- tutes less than 5% of the prey species of both eastern and western populationsof acadicus (Cannings1993). Lists of prey items from the PacificNorthwest are dominatedby speciesof Peromyscusand •icrotus. Of 36 individualsremoved from pelletscollected in Oregon,all butone were smallmammals (Forsman and Maser 1970). Also in Oregon, of 77 prey from 57 pellets, 74 (96%) were mammals(Boula 1982). In Washington, Grove (1985) identified770 prey items from more than 900 pellets,of whichmore than 95% were smallmammals. The apparentdominance of smallmammals in the diet of acadicus,however, may reflectthe lack of informationon preytaken by coastalpopulations of thissubspecies. Further- more, the prey itemssampled above came onlyfrom pellets,whereas data on the diet of brooksi have come from observationsof owls foraging, stomachcontents, and isotopeanalysis of tissues,as well as pelletexamina- tion. Coastal populationsof acadicus possiblyconcentrate on marine invertebratesat certaintimes of the year,but thishas not beenconfirmed. Analysisof stablecarbon isotopespermits the determinationof the relativecontributions of proteinderived from marineand terrestrialecosys- tems to the dietsof birds.By examiningboth muscleand bone collagen, Hobsonand Sealy(1991) ascertainedboth short-term(i.e., on the order of severalweeks) and long-term (i.e., approachingthe lifetimeof the individual) dietaryinformation. The collagensignal in youngbirds, however, reflects the dietat a time whenthe bonesare growing.Hobson and Sealy(1991) found that most brooksisampled had eaten some marine protein;two females salvagedin autumnhad fed exclusivelyon marine protein. The remaining individualsshowed little, if any, long-termmarine contribution to their diets. The measurementsindicated that marine-basedfoods make up 10% to 15% of the lifetimediet of the owls.Muscle tissue of an Octoberjuvenile (Sealy 1998) showed a marine contributionto the diet of approximately50%

203 FOOD ITEMS OF NORTHERN SAW-WHET OWLS

(Hobsonand Sealy 1991). Isopodsand amphipodstaken by the juvenilein 1997 revealthat marine-derivedfoods may be taken soonafter fiedging, possiblyas soonas the youngowls can fly and forageon their own. Stable-carbon-isotopeand stomach-contentanalyses suggest that some brooksispecialize on proteinderived from the marineenvironment during the nonbreedingseason. Others take primarilyterrestrial prey itemsor both marineand terrestrialfood, as indicatedby a stomachthat containeda bird and intertidalinvertebrates. Prey used may reflectthe locationof home rangesof individualowls, whether adjacentto the coast or inland, and whetherthe sitesare stableyear round.How far owlstravel to obtainmarine prey is not known. Northern Saw-whetOwls also are opportunisticforagers, as some indi- vidualstake Ancient Murrelet chicks that are availableonly for 5 to 6 weeks each year (Sealy1976, Gaston1992). Gaston(1992) suspectedthat they specializeon Perornyscuson East LimestoneIsland, and the remains of these mice in some pelletscollected there supporthis contention.Shrews and deer mice are widelydistributed on Graham and Moresbyislands, and each small island where brooksi has been reported (see Sealy 1998) supportspopulations of Perornyscus(Foster 1965), breedingmurrelets and passerines(Campbell 1969, Rodwayet al. 1988) and, of course,terrestrial and marine invertebrates.

SUMMARY

Prey itemstaken by Aegoliusacadicus brooksi on the Queen Charlotte Islandswere obtainedfrom 41 sources:stomach contents (including those examinedby Hobsonand Sealy 1991), pellets,records from the literature, a museumspecimen, and observationsof foragingor carryingprey. Twelve specieswere identifiedamong the preyitems: five invertebrates (mostly from the intertidal),a toad, a seabird,three passerines,and two smallmammals.

ACKNOWLEDGMENTS

Nathalie Macfarlane(Queen Charlotte Islands/HaidaGwaii Museum)and Michael C. E. MacNall (Royal British ColumbiaMuseum) allowed me to examine digestive contentsremoved from salvagedspecimens of Aegoliusacadicus brooksi. Daniel D. Gibson(University of AlaskaMuseum, Fairbanks) provided catalog data on Alaska specimenswith diet informationand greafiy improvedthe manuscriptwith his editorialcomments. Theodor R. Swem raisedan importantpoint regardingthe dietarydiversity of the subspeciesof Northern Saw-whetOwl. Donald A. Blood supplieda copy of an unpublishedmanuscript, and R. Wayne Campbell, Phred Collins, Ian McTaggart-Cowan, Colin French, Anthony J. Gaston, Anne E. Hetherington,Peter Hamel, Margo Hearne, CharlotteHusband, Robert Kelly, Frank Reindl,Tom E. Reimchen,and Sean Sharpeprovided helpful information, shared observations,tracked down salvagedspecimens, collected pellets, provided a permit, or helpedin otherways. Robert A. Cannings,Charles Dondale, Phillip Lambert, and J. R. Vockerothidentified invertebrate prey items and voucher specimensthat facilitatedidentification of newmaterial. L. A. Din loanedspecimens from the Cowan Museum, UBC. Keith A. Hobson commentedconstructively on a draft of the manuscript.Financial support was providedby a researchgrant from the Natural Sciencesand EngineeringResearch Council of Canada.

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LITERATURE CITED

AmericanOrnithologists' Union. 1957. Check-listof North AmericanBirds, 5th ed. Am. Ornithol. Union, Baltimore. Blood,D. A., Anweiler,G. G., and Cutter,M. J. 1979. Surveyof AncientMurrelet colonyat Dodge Point, Lyell Island,May 1979. Unpubl. report to Rayonier Canada(B.C.) Ltd. (orderfrom WilsonOrnithological Library, Mus. Zool., Univ. Mich., Ann Arbor, MI 48109-1079). Boula, K. M. 1982. Food habits and roost-sitesof Northern Saw-whet Owls in northeasternOregon. Murrelet 63:92-93. Campbell,R. W. 1969. Springbird observations on LangaraIsland, British Columbia. BlueJay 27:155-159. Cannings,R. J. 1993. NorthernSaw-whet Owl (Aegoliusacadicus), in The Birdsof North America (A. Poole and E B. Gill, eds.), no. 42. Acad. Nat. Sci., Philadelphia. Cory, C. B. 1918. Catalogueof birdsof the Americas,part II, no.1. Zool. Ser. Field Mus. Nat. Hist. 13 (publ. 197). Cowan, I. M. 1989. Birdsand mammalson the Queen Charlotte Islands,in The Outer Shores(G. G. E. Scudderand N. Gessler,eds.), pp. 175-186. Queen Charlotte Is. Mus. Press,Skidegate, B.C. Forsman,E., and Maser,C. 1970. Saw-whetOwl preyson Red Tree Mice. Murrelet 51:10. Foster,J. B. 1965. The evolutionof the mammalsof the Queen CharlotteIslands, BritishColumbia. Occ. PapersBrit. Col. Prov. Mus. 14. Gaston,A. J. 1992. The AncientMurrelet: A naturalhistory in the QueenCharlotte Islands.T. and A. D. Poyser,London. Grove,R. A. 1985. NorthernSaw-whet Owl winterfood and roostinghabits in north- centralWashington. Murrelet 66:21-24. Hobson,K. A., and Sealy,S. G. 1991. Marine proteincontributions to the diet of Northern Saw-whet Owls on the Queen Charlotte Islands:A stable-isotope approach.Auk 108:437-440. Patch, C. A. 1922. A biologicalreconnaisance on Graham Islandof the Queen CharlotteGroup. Can. Field-Nat.36:133-136. Rodway,M. S., Lemon, M. J. F., and Kaiser,G. W. 1988. BritishColumbia seabird colonyinventory: Report 1--East coastMoresby Island. Can. WildlifeServ., Pacificand Yukonregion, Tech. Rep. Ser. 50. Sealy,S. G. 1976. Biologyof nestingAncient Murrelets. Condor 78:294-306. Sealy, S. G. 1998. The subspeciesof the Northern Saw-whetOwl on the Queen Charlotte Islands:An islandendemic and a nonbreedingvisitant. W. Birds 29:21-28.

Accepted 13 September 1999

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