Barred Owl Habitat Use as Determined by Radiotelemetry Author(s): Thomas H. Nicholls and Dwain W. Warner Reviewed work(s): Source: The Journal of Wildlife Management, Vol. 36, No. 2 (Apr., 1972), pp. 213-224 Published by: Allen Press Stable URL: http://www.jstor.org/stable/3799054 . Accessed: 03/02/2012 16:55

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http://www.jstor.org BARREDOWL HABITATUSE AS DETERMINEDBY RADIOTELEMETRY

THOMAS H. NICHOLLS, Department of Ecology and Behavioral Biology, University of , DWAIN W. WARNER,Bell Museum of Natural History, Universily of Minnesota, Minneapolis

Abstract: Radio transmitterswere successfullyplaced on 10 barredowls (Strixvaria) during1965-66, and the movementof the owls werefollowed with an automaticradio-tracking system for a totalof 1,182 days. Morethan 28,000 owl locationswere sampledfrom some two million locationsrecorded. These locations,along with habitatavailability information, were programmedfor computeranalysis to deter- mine intensityof habitatuse. Barredowls showedconsistent and highlysignificant preference or avoid- ancefor differenthabitats despite changing seasons, phenology, weather conditions, and years. The order of habitatpreference in decreasingintensity of use uZasoak ( Quercusspp. ) woods,mixed hardwoods and conifers,white cedar (Thufaoccidentalis) swamps, -, alder (Alnus spp.) swamps,marshes, and open fields.

Thispaper reports the use of radiotelem- Traditionalmarking methods seldom etry to determinethe intensityof use of producethe continuous,minute-to-minute varioushabitat types by barredowls. The datanecessary for evaluatingshort-duration recently acceleratedinterest in predatory movementsand activities of wild birdsthat animalsand the difficultiesinvolved in the will accuratelyprovide information on in- study of generally nocturnaland little tensityof habitatuse. In this study,radio known avian predatorspresented a dual transmitterswere developedfor andplaced challengeto the techniqueof radio-track-on 10 barredowls that were capturedwith ing. Japanesemist nets. Movementsof the owls Radio-trackingtechniques have been were monitoredby the Universityof Min- used most extensivelyin studies of mam- nesota's Cedar Creek AutomaticRadio- mals,which generally have a greatercapac- trackingStation (Cochran et al. 1965). Of ity for carryingradio transmittersthan some two million owl locations ( fixes) birds. Severalinvestigators have tried to recorded,more than 28,000were used in equip birdswith miniatureradio transmit- data analysis. These locationswere used ters for field studies,but few have done so to determinethe intensityof use of seven successfully( Southern1965, Graberand differenthabitat types during all seasons. Wunderle1966, Cochran et al. 1967). The authorsthank Dr. D. B. Siniff,who Habitatuse by birdshas been studiedby wrotethe computerprograms used in data visualobservation, by recaptureof marked analysis;L. B. Kuechleand C. Ransom,who birds,and by the appearancein studyareas built the radio transmittersused in this of species intermittentlycaptured by var- study;D. B. Lawrencefor the aerialphoto- ious sampling methods. In addition to graphs;and MarieA. Gravdahl,who pre- determininghabitat use, data from these pared the line drawings.Appreciation is types of studies have been the basis for extendedto Dr. W. H. Marshall,Director major interpretationsof responseto en- of the Universityof MinnesotaCedar Creek vironmentalchange, home range,territory, NaturalHistory Area; to A. Peterson,Resi- migration,dispersal, and populationstruc- dent Managerof CedarCreek; to P. Rice, ture. our technician;and to the manypersonnel of CedarCreek who gave freely of their 1 Present address: North Central Experi- help. The authorsacknowledge the support ment Station, USDA Forest Service, Folwell Ave- nue, St. Paul, Minnesota. of the NationalInstitutes of HealthTrain- 213 S^,;,t, ...... , I

214 Journal of Wildlife Management, Vol. 36, No. 2, April 1972

ing Grant5 T1 AI 188 to Dr. D. W. Warner OCCURRENCEOF SEASONS and the U. S. Atomic Energy Commission CEDARCREEK NATURAL HISTORYAREA Grant COO-133b67to Dr. J. R. Tester. M INNESOTA

STUDY AREA This investigationwas carriedout on the CedarCreek Natural History Area, 30 miles northof Minneapolis,Minnesota, in Anoka and Isanti counties. The some 5,000 acres are ownedand administered by the Univer- sity of Minnesota.Habitat within the study areahas been describedin detailby Pierce ( 1954), Brayet al. ( 1959), and others.The topographyis flat to gently rolling, with sandyupland areas interspersed with bogs, cattail ( Typha latifolia) marshes,swamps, four small lakes, a creek, and a drainage ditch. Upland areas typicallyhave sandy soil andwere classified by Brayet al. (1959) as tallgrassprairie, deciduous-angiosperm EARLY LATE EARLY LATE and forest, and mixed conifer- SPRING SPRING SUMMER FALL FALL WINTER angiospermforest. The predominantup- land are (, Fig. 1. Occurrenceof seasons-Cedar Creek NaturolHistory Q. ellipsoidalis,Q. alba, Q. rubra) andpines Area. (Pinus strobus, P. resinosa, P. banksiana), with scatteredsugar maple (Acer saccha- rum), whitebirch (Betula papyrifera), and (Salix spp.), white birch,and aspen ( Pop- basswood( Tilia americana) . Thereis often ulus tremuloides). a dense hazel (Corylus cornuta, C. ameri- The generalclimatological characteristics cana) understory.Small farms and mar- of the Cedar Creek NaturalHistory Area ginal croplandsurround the area. There have been recordedin Baker and Strub were some cultivatedfields of rye (Secale ( 1965) and Pierce ( 1954) . The general cereale), alfalfa (Medicago sativa), corn seasonal and phenologicalcharacteristics (Zea ma?ys),and soybeans(Glycine max). occurringduring this study are illustrated Manyfields were not cultivated,and most in Fig. 1. fields were separatedby rowsof treeswith understoriesof wild grape ( Vitis spp.), Phenology Rubus species, and chokecherry( Prunus Early Spring. Owls are nesting, occa- vtrgtnta. * . \) . sional snow storms occur, killing frost is The lowland areas have muck or peat common,ice and snow are melting, the soils usually poorly drained. These are ground is bare, grasses are beginningto normallydominated by white cedar,tama- grow, buds expand and leaves grow rack ( Larix laricina), and alder ( Alnus to about one-halfsize, frogs start calling, rugosa), with suchassociated species as red and migratorybirds return. osierdogwood (Cornus stolonifera), willow Late Spring. Youngowls are about to BARREDOWL HABITATUSE * Nicholls and Warner 215 leave or have left the nests,tree leavesare fully grown, herbs and shrubs are fully developed,and mosquitoesand wood ticks (Dermacentorvariabilis) are numerous. Summer. Owls are in family groups, adult owls start moltingnear the end of summer, mosquitoes are present, wood ticks disappearabout mid-July,deer flies ( Chrysopssp. ) are active,grasshoppers are numerous,tadpoles hatch, some birdsstart migrationin Augustand September, berries Fig. 2. Left: owl transmitteras it appeared before back and nuts maturein late summer,and July strap wos attached and acrylicapplied. Right: the 70-gram temperaturesrange from 43 to 88 F. transmitter-harnessused on barred owls. The transmitter Early Fall. Adult owls completemolt, operated 175 to 200 doys with a continuoussignal and had a range of up to 3 miles when a MalloryZM12 battery was killing frosts begin, mosquitoesand deer used for power. flies disappear,leaves begin to turn color ( most leaves are fallen by mid-October exceptfor some dead leaves persistingon with the back strapon the midlineof the the oaks), and most of the Passerinebirds backbetween the wings. Theharnesses and leave the area. transmitterson all birds were completely Late Fall. Waterfowlmigrate through coveredby feathers an advantagein cold the area,waters begin to freeze,snowstorms weather because transmitterswere kept start,and saw-whetowls migratethrough warm. the area. Automatic Radio-trackingSystem Winter. Owls increase their daytime hunting,owls begin courtshipactivities in The University of Minnesota'sCedar Februaryand March,most of the water CreekRadio-tracking System (Cochranet areasare frozen,snowstorms are common, al. 1965) utilizes two rotatingdirectional snowcover is normallycontinuous, and sub- receivingantennas. Each radio transmitted zero temperaturesare commonin January at a differentfrequency. When the radio and February. antennaspointed in the directionof an owl with a transmitter,the radio frequency MATERIALAND METHODS energy emitted from the transmitterwas detectedby receiversat a centrallaboratory. Owl Transmitter Permanentrecords of receivedsignals, as A successfultransmitter-harness was de- displayedby indicatorlights on receivers, veloped ( Nichollsand Warner1968 ) from were made by photography.The data on a solid strandof No. 12 or No. 14 cop- the filmwere read and tabulated to conform per electricalwire formedinto a harness to a samplinginterval that normallywas with a neck loop and body loop to which one fix every 15 minutesduring darkness transmitterparts were attached (Fig. 2). and one fix every 30 minutesduring day- "Transmitterparts and batterywere encap- time,or as nearlyas possibleto thisinterval. sulatedin a waterproof,cold-resistant, du- Degreebearings obtained from the antennas rableacrylic ... hardenough to withstand permittedowls to be locatedon maps by biting and clawing."The transmitterwas triangulation.The date,identification code locatednear the frontof the owl'ssternum, for eachowl, time, and degree bearing from 216 Journal of Wildlife lManagement,Vol. 36, No. 2, April 1972 each tower were punched on computer For example7if both an open field and an cardsand analyzedby a digital computer oak woods appearedin the same 1.6-acre that calculated movement and location square and the computerlocation map parametersand drew a map of movements showedmost locations occurring in the oak for each animal (Siniff and Tester 1965> woods, the squarewas coded as an oak Siniff1966). woodshabitat. The habitatinformation for In any animal-trackingsystem using tri- eachsquare, along with all locationsoccur- angulationthere may be discrepanciesbe- ring in each squarefor each barredowl) tween the true locationand the locations were programmedfor computeranalysis. found by triangulation.Important sources Theoutput from the program was presented of errorsand limitationsof the automatic as the numberand percentageof locations radio-trackingsystem have been discussed occurringin each habitat type for any by Heezenand Tester ( 1967) and Sargeant desiredunit of time et al. ( 1965). Cochranet al. ( 1965) deter- Habitats were categorizedinto seven mined that combinedsystem and reading types fourupland types (oak woodsnoak- errorswere no greaterthan +0.3° with savannaopen field, and mixedhardwoods strongsignals on a calm day, and +0.5° and conifers) and three lowland types with strongsignals on a windyday. ( white cedar swamp, alder swamp, and marsh). Brief descriptionsof these types HabitatAnalysis follow. The CedarCreek area was griddedinto a The oak woodstype usuallyconsisted of systemconsisting of 2,080,1.6-acre (0.002S a mixtureof two or threespecies of oak ln squaremile) squares.Squares were num- moderateto high densities.Trees varied in bered from 1 to 2,080 for computeriden- heightfrom 20 to 65 feet. Few shrubsand tification.Choice of squaresize was arbi- herbaceousplants grew underthese dellse trary,but 1.6-acresquares were convenient stands. The oak-savannahabitat consisted in termsof accuracyof the radio-trackingof scatteredoak treeswhere densities were systemand also permitteddetailed classifi- low enoughso that grassesand otherher- cation of habitattypes. Squaresize was baceousvegetation were the actual dom- largeenough so thateach square contained inantsof the community.The mixedhard- at least one possibledegree-bearing inter- woodand conifer type, depending upon the section. area, had variouscombinations of sugar Eachsquare was assigneda habitattype maplenbasswood red oak, white birch, that was determinedfrom aerial photo- northernpin oak, and white)red, and jack graphs,vegetation maps7 and field observa- pine. Few shrubsand herbaceousplants tions. Occasionally,more than one habitat grewunder the mixedhardwood and conifer type was presentin a 1.6-acresquare. To canopy. Open-fieldhabitats consisted of determinewhich habitat type was used by pastures,cultivated fields, and with the owl in such cases, a computer-drawnoccasional clumps of trees. Mostopen fields mapwas madeof all locationsfor eachowl werebordered by woodedareas. and placedover a vegetationmap of Cedar White cedar was the dominantlowland Creekthat also had the 1.6-acregrid. The foresttree. In someareas white cedar over- squarewas coded for the most important lapped and mixed with tamarack.White habitatin termsof the owl'spreference, as cedarreplaces tamarack in latesuccessional indicatedby the computermap of locations stages at Cedar Creek. The more dense BARRED0WL HABITATUSE * Nicholls and Warner 217 stands of white-cedarwere effective in type by the total numberof observedfixes reducingthe amountof light reachingthe obtainedfor each owl. forestfloor, and only a few shrubsand herbs Dataon the distributionof owl fixeswith toleratedthese low light conditions.The respect to different types of available alder swamp habitat consisted of dense habitatwere tested for significantdiffer- stands of alder in moist lowland areas. ences by chi-squaremethods of analysis. Somealder stands were so densethat it was The probabilitylevel of 0.05percent or less difficultto walk throughthem. An occa- was used to indicate significance. The sionaltamarack was found in this habitat hypothesisfor the chi-squaretest was stated type. Marshareas were characterizedby as follows: If an owl entered different open wetlandswith cattails,sedges, and habitatsby chance alone, the numberof grasses,generally without trees but some- radiofixes in each habitatwill be propor- timeshaving scattered clumps of red osier tional to availabilityof the habitattypes dogwood,alder, and willowspecies. within the home range of the individual A determinationof home range was owl. necessaryin evaluatinghabitat use. The To reduceerrors in location,fixes were termhome range, as usedin this study,was taken only when owls were not flying, defined as that area used, traversed,or becausethe two antennasseldom received regularlysurrounded by an owl duringits signalsat exactlythe sametime. Whenthe normalactivities of food-gathering,mating, samplinginterval could not be maintained nesting,caring for young,and seekingshel- because of random uncontrollabledata ter. The areaof eachowl's home range was gaps,we consideredthat the percentageof determinedby multiplyingby 1.6 (acres) time unknownwas unbiasedtoward any the numberof squaresin whichone or more specifichabitat type. locations were obtained. Home range boundarieswere determinedby drawing RESULTSAND DISCUSSION lines aroundthe outermost1.6-acre squares withfixes in them. Thosefew squareswith- Barredowls adapted quickly to the trans- out fixes,but insidethe homerange bound- mitter-harnessand it did not appear to ary,were considered part of the homerange influencetheir naturalactivities or cause becausethey were regularlysurrounded by injurieseven afterprolonged use. Onlyone squareswith fixes. barredowl died duringthe study and its After the acreage of each owl's home death was believed to be due to natural rangewas determined,the totalnumber of causes. Severalowls carriedtransmitters as acres of each of the seven habitattypes long as 180 days before the transmitters presentwithin the home range was also failed. determined.Habitat preference and inten- Ten barredowls were successfullyradio- sity of use were determinedby comparing trackedfor a total of 1,182 days. During the observednumber of fixes occurringin this time, 28,338 locations ( fixes) were eachhabitat type andthe expectednumber sampledfrom some two millionowl loca- of fixes that would have oucurredif owls tions automaticallyrecorded by the radio- had enteredthe differenthabitat types by trackingsystem ( Table 1) . Sufficientdata chancealone. Theexpected number of fixes for habitatanalysis were obtained from 9 of was obtaindby multiplyingthe percentage the 10 owls and comprisedsome 26,841 of the totalacreage present for eachhabitat radio locations.The averagehome range 218 Journal of Wildlife Management, Vol. 36, No. 2, April 1972

Table 1. Summaryof tronsmitterdays, dota doys, and numberof fixes obtained from burred owls during 1965-66 on the Cedor CreekNaturol HistoryArea.

DATE RELEASED END OF APPROX:[MATE BARRED WITH TRACKING TRANSMITTER DATA NUMBEROF OWL TRANSMrrTER PERIOD DAYSa DAYSb FIXESSAMPLED NOTES 701 S19-65 8- 9-65 84 77 2,251 Stoppedtransmitting 702 3-22-66 S-19-66 58 56 1,027 Stoppedtransmitting 7()3 7-156S 9-20 65 67 65 2,479 Stoppedtransmitting 704 11-12-65 2-13-66 94 92 4,653 Owl died 707 2-15-66 W15-66 181 65 2,122 Stoppedtransmitting 709 2-23-66 9-11-66 200 197 6,431 Stoppedtransmitting 710 3- 8-66 9-11-66 186 185 5,043 Stoppedtransmitting 711 3-14-66 5-17-66 64 14 400 Owl often out of re- ceiver range 712 3-14-66 5- S-66 52 51 2,587 Owl left area 714 3-22-66 10- W66 196 99 1,345 Stoppedtransnaitting Total 1,182 901 28,338

a Transmitter days = the number of days the transmitter was functional. b Data days _ the number of days in which usable locations were obtained. size for the nine owls was 565 acres. Home of fixes, dle particularwoods the owl was rangesize variedfrom 213 to 912 acres. using. For example,the heavilyused oak Computermaps of owl locations and woodsin the northwestcorner of Fig. 3 was movementswere madefor all owls. Fig. 3 filledin with fixes. A pictureof thatwood- is an exampleof such a map for a 45-day lot can be seen at the top of Fig. 4 for period from August24 to September20, comparisorl.The open fields north and 1965,for barredowl No. 703. Each plus southof this oak woodsreceived little use. markon the mapdenotes one or moreradio The large oak woods to the south of the fixesobtained for the owl in thatparticular small open field in the middle of Fig. 4 location.Lines between plus marks indicate was intensivelyused, as can be seen by movementsbetween successive locations. the numerousplus markson the computer Computermaps, when placed over vege- map in Fig. 3. tation maps of the study area ( Fig. 3 ), Comparisonof mapsin Figs. 3 and5 can illustratehow intensivelythe owl used dif- be madeby keepingin mindthe reference ferent areas within its home range. The pointsof the radio-trackingtowers) Cedar woodedareas were almostcompletely out- Creek,Cedar Bog Lake, and the various lined with radio fixes, whereasthe open characteristicpatterns of the deciduous fields and marshon either side of Cedar woodlots. Barredowl 703 had a 258-acre Creek had few fixes. In some cases the homerange for the 6S-dayperiod between map areas representingdeciduous woods July 1S and September20, 1965 (Fig. 5). were completelycovered with overlapping Each squareis 1.6 acresin size, and num- plus symbolsdenoting owl locations. The bers indicate the total radio fixes falling lines crossingthe open areasshowed that within each square. A large numberof owls frequentlyflew back and forth be- radiofixes occurred in the deciduouswoods tween differentwoodlots within the home comparedwith the few fixes in the white range. By knowingthe characteristicout- areas,which denote open fields and marshes line of each woods, it was often easy to (Fig. 5). Numerousradio fixes were ob- detexmine,from the computermap pattern tainedin someedge squares.For example, s-l El at +:f:f:i:t00 ...... ^" .E E . S;D7 t-.... E E - E.:it: £ sw ' D .;E : s .: :L: -Li....-v:b N? . ::i::: -::, : -:: j E i * ; 7 f f y . 0

BARREDOWL HABITATUSE * Nichollsand Warnet 219

1b

Fig. 3. A computer-drawnmap of locations and movements > 1 l/4 mule g of barredowl 703 fromAugust 24 to September20, 1965. A total of 1,055 fixes were used in compilingthe map. White Fig. 5. The home ronge of barredowl 703 from July 15 to areas on map are wooded, and shoded areas are open fields September20, 1965, was 258 acres. Eachsquare is 1.6 acres or marshes. Note the heavy use of wooded areas and the in size; the numbersin squaresindicate the total numberof little use of open areas and marshes. radio fixes folling within eoch square. The white areas on the mop indicateopen fields or marshes. Note how little use was made of these areas comparedwith the intensivelyused the largestdeciduous woods (woodlot E, deciduouswoods. Fig. 5) in the home rangehad both field andoak woods habitats in someof the same borderoutlined by fixes along the open squareson its northernedge. The question field and the northernedge of woodlotE to be answeredwas whetherthe fixes in (Fig.3). An aerialview of the samewoods- thosesquares were located in the openfield field edge is seen in Fig. 4. or in the oakwoods. Computermaps made Less use was made of the white cedar at 10-dayintervals during each owl'strack- swampin the northeasternpart of the home ing periodanswered this questionfor they range. A mixed hardwoodand conifer showedexactly where the owl was. In the 98 percentof the cited,more than .. example : ... in the edge squaresoccurred in the fixes : .. cy { Si.d i

: : : ;: 0: ::::: ':i: :::::LE.i:, oak woods. This explainedthe straight iSE:-E 0 :f.i.E:f:E, 7 ,;:S :4:: :

Sv ' &0 _

E -- . A Tower . , \

;: *6^i<'' 0(s0 iwer > A : -t ( § }/4 1 0"^ . ' M$*vs, ,,t ,V,,,,,,I j , m !le l, I -0-'- . u.,^<^jfi ;^>a , §8 >,c89 '.;%egi; S*iM3 Je5ai3 '::0; ' ; f j :0 Fig. 6. Computermop of the fixes and movementsof 709 from April 9 to April 19, 1966, based on 480 fixes. Fig. 4. Aerial view of open-field and oak woods hobitats. The intensivelyused area to the west of the loke is a mixed The objects in the center of the open field are experimental hardwoodand conifer habitat that is surroundedby a little- duck pens. used white cedor swamphabitat. 220 Journal of Wildlife AIanagement, Vol. 36, No. 2, April 1972 habitat(woodlot C, Fig. 5) in the middle of a lowlandcedar swamp received inten- sive use. By comparison,squares having only white cedar in them had few fixes. Computermaps showed that most fixes occurredin themixed hardwood and conifer habitatin those edge squaresalso having the whitecedar habitat. The samerelation- ship was true for the deciduouswoods (woodlot F, Fig. 5) to the south of the white cedarswamp. Few or no fixes were obtainedin the openmarsh area located on eitherside of CedarCreek. Fig. 7. Aerial photographof Cedar Bog Lakeshowing the circulormixed hardwoodand conifer island, adjacent to the A differentbarred owl intensivelyused a lake, which was completely outlined by fixes, as seen in mixedhardwood and coniferhabitat ( Fig. Fig. 6. 6). A 10-daycomputer map of the move- mentsof No. 709 fromApril 9 to April19, individualowl, by usingthe chi-squaretest 1966,is shown. The numerousfixes to the to determinesignificant differences between west of CedarBog Lakeoutlined an almost observedand expected numberof radio circularisland of mixed hardwoodsand fixesfor each habitat type, is shownin Table conifersthat was completelysurrounded by 2. The resultsof these tests, summarized a lowlandwhite cedar swamp. An aerial in Table3, showedwhich available habitats photographof the same area showed the were preferredby individualowls. The circularisland to the upperleft of the lake, resultswere consistentfor all owls, with surroundedby the cedar swamp, which the exceptionof No. 707. This owl made receivedlittle use (Fig. 7). intensiveuse of the oak-savannahabitat in Barredowls showeddefinite and signifi- contrastto five other owls that had this cant (P < 0.05) preferenceor avoidanceof habitatin their home rangesbut did not differenthabitat types. An exampleof how makeintensive use of it. habitatpreference was obtainedfor each A total of 52 habitatcomparisons were

Table 2. Order of hobitat preferenceas indicated by the chi-square test for significant differences between observed and expected numberof radio fixes in each hobitat type for barred owl 709 {sex unknown)from February23 to Sep- tember 11, 1966.

PERCENTAGEOF NUMBEROF RADIOFIXES HABITAT ACREAGE TOTALACRES X2 TYPE AVAILABLE AVAILABLE ObSerVeda EXPeCtedb VALUE Mixedhardwoods and conifers 126 14 2,081 903 1,786.90*** Oak woods 279 30 2,869 1,93S 644.03*** Wkte cedar swamp 238 26 1,296 1,677 116.96*** Adderswamp 42 5 4 323 331.64*** Marsh 97 11 52 709 683.99* ** Open field 130 14 148 903 734.01*** Total 912 100 6,450 6,450 ***P<0.005 (1 df). a Fixes were normally taken every 15 minutes during darkness and every 30 minutes during daylight. b The expected number of radio fixes was obtained by multiplying the percentage of the total acreage available for each habitat type by the total number of observed radio fixes. BARRED0\N7L HABITAT USE * Nzchollsand Warner 221

Table 3. Percentageof total fixes occurringin each habitat type and habitat selection as indicoted by the chi-square test for significontdifferences between observed and expected numberof radio fixes in each habitat type during the pe- riod in which eoch owl was radio-tracked.

BARREDOWL NUMBER,SEX, ANDDATES

NUMBEROF 701 F 702 F 703 704 M 707 709 710 712 714 M FIXESAND 5 - 19 - 65 3 - 22 - 66 7 - 15 - 65 1l - 12 - 65 2 - 15 - 66 2 - 23 - 66 3 - 10 - 66 3 - 14 - 66 3 - 2_ - 66 HABITAT to to to to to to to to to TYPE 8-9-65 5-19-66 9-20-65 2-13-66 8-15-66 9-11-66 9-1 1-66 5-5-66 11-4-66

Number of fixes 1,990a 942 2,478 4,653 1,705 6,450 5,031 2,264 1,328 Oak woods 94b (54)c 81b (38) 53b (27) 84b (43) 49b (23) 45b (30) 81b (37) 73b (35) 84b (36) Mixed hardwoods and conifers Npd NP 37b(17) NP 21e (15) 32b(14) NP NP NP White-cedar swamp <1t(1) 3e (2) 5f (20) NP l°f (21) 20f (26)

a Fixes were normally taken every 15 minutes during d Habitat not present in home range. darkness and every 30 minutes during daylight. e p > 0.05. bHabitat selected for; significant (P<0.05). tHabitat selected against; significant (P<0.05). c Percentage of total acres of each habitat available in home ral;vge. madeusing ffie chi-squaretest to determine marshes,and open fields. The order of which habitatswere intensivelyused by preferencewas similaramong all nine owls. individualowls and which were not. Forty- Wheredifferences among the owlsoccurred, nine comparisonsshowed significant (P < differencesin habitat preferenceusually 0.05) differencesbetween observed and consistedof a one- or two-placeshift in expectednumber of radiofixes and three the orderof preference. showedno significantdifferences. Of the 49 Seasonaldifferences or shifts in habitat showingsignificance, 12 showed that the use were not apparent.Barred owls radio- observednumber of radio fixes was more trackedduring spring (Nos. 702 and 712), than the expectednumber of radio fixes summer( Nos.701 and703), winter,spring, with respect to habitat availability. All andsummer (Nos. 707, 709, and 710), spring, exceptone of these 12 occurredin the oak summer,and fall (No. 714), and winter woods or mixed hardwoodand conifer (No. 704) all used the availablehabitats habitats. The one exceptionoccurred in withsimilar intensity. This was truedespite the oak-savannahabitat. Thirty-sevenof changingweather and phenologicalcondi- the 49 showedthat the observednumber of radio fixes was less than expected. All tions the generalcharacteristics of which occurredin either the oak-savanna,white are outlinedin Fig. 1. cedarswamp, alder swamp, marsh, or open- Therewere no major differences in habitat field habitats. Of the three showingnon- use by differentowls in differentyears. significance,one each occurred in the mixed Numbers701 and 703 were radio-tracked hardwoodand conifer,white cedarswamp, during196S, and Nos. 702, 707, 709, 710, andmarsh habitats. 712,and 714 were tracked in 1966. Number The generalorder of preferencein de- 704 was trackedin both 1965 and 1966. creasingintensity of use was oak woods, There were no major differencesin the mixedhardwoods and conifers,white cedar relativeintensity of habitatuse when male swamps, oak-savannas, alder swamps, owls werecompared with femaleowls. Sex 222 Journal of Wildlife Management, Vol. 36, No. 2, April 1972 determinationis difficult in barredowls, fromthe numerousperches. The two pre- and the sex of only four owls was deter- ferredupland habitats appeared to have all mined. Numbers704 and 714 were males the items necessaryfor survival of the and Nos. 701 and 702were females. barredowl. Preferencesfor thesetwo habi- Table3 also gives the percentageof ffie tats may have playedan importantrole in total fixes occurringin each habitattype causingthe owls to favorhunting grounds presentin each owl's home range during where the availableprey were of charac- the entire period in which each owl was teristictypes. radio-tracked.These results again em- Owls have extraordinaryvisual adapta- phasizethe importanceof the oak woods tions (Conraderand Conrader1965). The and the mixedhardwood and coniferhabi- forward-directedeyes are limitedto a total tats, whereowls spentthe highestpercent- visualfield of only about110 degrees,but age of their time. Less time was spent in the mobilityof the head enablesthe owl to the white cedar swamp and oak-savannasee aroundit. Thereis an overlapof the habitats;considerably less time was spent visualfields fromthe two eyesvgiving ie in the alderswamp, marsh, and open-field owl a centralangle of binoculaxvision of habitats. about70 degrees,compared with only about Our resultsraise some interestingques- 6 degreesfor songbirds.Because of this tionsfor furtherstudy as to ie reasonswhy wide stereoscopicvision, ie owl has ex- suchconsistent pattetos of habitatuse were cellentdepth perception a necessaryfactor found. We can only suggestsome possible in its survival.The physical arrangement of explanationsof why barredowls selected the habitatmust be of a properstructure to certainhabitat types in preferenceto others. enablethe owlsto use theireyes to the best Two of the sevenhabitats were preferred advantagewhile huntingprey. Such was overthe otherfive. The preferredhabitats the case in the intensivelyused oak woods were the oak woodsand, if presentin the andmixed hardwood and coniferhabitats. homerange of an individualowl, the mixed The ears, like the eyes, of ie owl are hardwoodand conifertypes. Theseupland well-developedand efficientorgans (Con- woodedareas were normally free of a dense raderand Conrader1965). The ear open- understory,and therewere few herbaceous ings are large. The openingsand the ear plants growing on the forest floor. The folds differin size, shape,and positionon physicalcharacteristics of iese twohabitats oppositesides of the head. These asym- appearedto be idealfor huntingprey. The metriescause soundsto be a little louder lack of brushmade it easy for owls to see, in one ear than in the other. Becausethe fly, and attack terrestrialprey without ear openingsare widely separatedby the hittingbranches or leavesen routeand giv- broadhead, sound waves reach the two ears ing the intendedprey warningof an im- at minutelydifferent intervals. These slight pendingattack. The oak woodshad many differencesof intensityand intervalenable deador dyingtrees that probablyprovided some owls to locate prey in almosttotal numeroushomes for prey species such as darkness.Drier conditions generally existed mice and squirrels.The barredowls pre- in the uplanddeciduous woods as compared ferredto nestand seek shelter in the hollow with the white cedar, marsh, and alder trees that were plentifulin the oak woods habitats,and prey species probablymade andmixed hardwoods and coniferhabitats. more noise while moving through dried Barredowls were observedhunting prey leavesand dead branchesscattered on the BARREDOWL HABITATUSE * Nicholls and Warner 223 ground. This characteristicof the habitat suitable nest trees, and hunting perches. may have been importantto barredowls, Prey speciesappeared to be abundantbut whosesurvival depends largely upon locat- were probablynot utilized to any extent ing prey by sounds. becauseof otherunfavorable factors. The white cedarswamp habitat had few Althoughbarred owls intensivelyused nestingand sheltercavities even in older certainhabitats more than others, little used trees,probably because the treeswere more habitats might also be importantto the resistantto decay fungi than were the owls. Habitatsgenerally avoided by owls deciduoustrees in the hardwoodtypes. For may be a source of prey that eventually the samereason, cedar trees probably pro- disperseinto habitats used nlore intensively videdrelatively few homesfor preyspecies. by owls. Forexample, terrestrial prey living The characteristicsof this habitatfor hunt- in white cedarswamps, alder swamps, and ing preydid not seemto be as idealas those marshes when they are relatively dry, in the deciduouswoods. The densefoliage migratein wet seasonsto adjoiningdrier would make flying and seeing more dif- uplandareas and may be utilizedas foodby ficult, and soundswere probablymuffled barredowls. by damp or wet vegetationon the forest Habitatselection must be complementary floor. to the biologicalrequirements of the indi- The oak-savannahabitat svas not utilized vidualspecies. It seemslogical for animals as intensivelyas expectedexcept by No. to use a habitator a combinationof habitats 707. The oak-savannawas a relativelyopen that most nearly contains all the items habitat and most trees were smaller in necessaryfor survival.The survivalof the height and diameter,so few hollow trees barredowl appearsto depend upon the were present. Prey appearedto be abun- availabilityof suitablefood, areas for court- dantin thishabitat, but otherfactors of the ship and mating,shelter for nesting and area apparentlydid not favor its use by concealment,and perchesfor huntingand mostof the barredowls studied. resting all of whichmust be in accordance Alder swampswere avoidedby barred with the owl'sbiological requirements and owls. The physical arrangementof this abilities. This study indicatedthat barred habitatwas such that large owls would have owls favoredhabitats that appearedto be had a difficulttime flying or attackingprey mostsuitable for the successfulcompletion becauseof the high densityof alderstems of theirlife cycle. and branches.Some alder swamps were so densethat a man wouldhave extremedif- CONCLUSIONS ficultypenetrating them, and they had no Radiotelemetrywas a useful technique suitablenest trees. for studyingthe movementsof barredowls. The marshhabitat was also avoidedby They carriedradio transmitterswell, and the owls. There were few or no suitable therewas no indicationthat the transluitters perches in this habitat, nor were there caused injuries or influenced the owls' adequateshady retreats where owls could naturalactivities. Accurate determinations perchand conceal themselves during usually of home range size were possible using inactivedaylight periods. Nesting or shelter radio-tracking.Habitat preference and in- cavitieswere virtuallynonexistent. tensityof use were determinedwith con- The open-fieldhabitat was the leastused. fidence. Those habitats (oak woods and It generallylacked cover for concealment, mixedhardwoods and conifers) havingall 224 Journal of Wildlife Management, Vol. 36, No. 2, April 1972 or most of the requirementsnecessary for CONRADER,J., AND C. CONRADER.1965. Owls. WisconsinTales and Trails6 ( 4 ) :2S29. survivalof the barredowl were also the GRABER,R. R. ANDS. L. WUNDERLE.1966. same habitatsmost preferred and mostin- Telemetricobservations of a robin ( Turdus tensivelyused, with no significantvariations migratorius). Auk 83( 4):67s677. HEEZEN,K. L., ANDJ. R. TESTER.1967. Evalua- with regardto sex, differentindividuals) tion of radio-trackingby triangulaffonwith phenologicalchanges, changing weather specialreference to deermovements. J. Wildl. conditions,or years. The methodof habitat Mgmt.31 ( 1 ) :12S141. NICHOLLS,T. H., ANDD. W WARNER.1968. A analysispresented in this reportmay be harness for attaching radio transmittersto useful in studies of habitatuse by other large owls. Bird-Banding39(3):209-214. speciesof animals. PIERCE,R. L. 1954. Vegetationcover types and land use historyof the CedarCreek Natural HistoryReservation, Anoka and Isanti Coun- LITERATURECITED ties, Minnesota.M.S. Thesis. Univ. of Min- nesota. 137pp. BAKER,D. G., ANDJ. H. STRUB,JR. 1965. Cli- SARGEANT,A. B., J. E. FORBES,AND D. W. WARNER. mate of Minnesota. Part III. Temperature 196S. Accuracyof dataobtained through the and its application. Univ. of Minnesota Agr. Cedar CreekAutomatic Radio TrackingSys- Expt. Sta. 62pp. tem. MinnesotaMuseum Nat. Hist. Tech. BRAYAJ. R., D. B. LAWRENCE,AND L. C. PEARSON. Rept. 10. [20pp.] Mimeo. 1959. Primaryproduction in some Minnesota SINIFF,D. B. 1966. Computerprograms for terrestrial communities for 1957. Oikos 10 analyzingradio-tracking data. MinnesotaMu- (pt.1):3849. seumNat. Hist.Tech. Rept. 12. 22pp. Mimeo. COCHRAN,W. W., G. G. MONTGOMERY,AND R. R. , ANDJ. R. TESTER.1965. Computer GRABER.1967. Migratory flights of Hylo- analysis of animal-movementdata obtained cichlathrushes in spring: a radiotelemetry by telemetry.BioScience 15[ 2 ) :104-108. study. Living Bird 6:2lS225. SOUTHERN,W. E. 1965. Biotelemety: a neur , D. W. WARNER)J. R. TESTER,AND V. B. techniquefor wildlife research. LisringB;rd KUECHLE. 196S. Automatic radio-tracking 4:4548. system for monitoring animal movements. BioScience 15 ( 2 ): 98-100. Receivedfor publicationApril 30, 1971.