j. RaptorRes'. 37(2):117-124 ¸ 2003 The Raptor ResearchFoundation, Inc.
BREEDING
SEASON HABITAT NORTHERN
USE AND ECOLOGY PYGMY-OWLS
OF MALE
ALAN R. GIESE 1
Department ofFisheries and Wildlife,OregonStateUniversity, Corvallis,OR 97551 U.S.A. Emc
D. FORSMAN
U.S.ForestService, PacificNorthwest Research Station,3200 SWJefferson Way,Corvallis,OR 97331 U.S.A. ABSTRACT.--We examinedhabitat use,nestlocation,diet, and activitypatternsof radio-markedNorthern Pygmy-Owls (Glaucidium gnoma)during four breedingseasons(1994-97) in fragmentedforestson the OlympicPeninsula,Washington.We observedforagingin all availablevegetationcategories,but patterns of use were non-random.Structurallydiverseand older forestswere most heavilyused,openingsand patchesof saplingsreceivedthe leastuse, and use of edge wasintermediate.We locatedeight nests,all in woodpecker cavitiesin patchesof structurally-diversetbrest. We found no clear evidence that nests were locatednear edges.Northern Pygmy-Owls were diurnally active,and male owlsdeliveredfood to femalesduring nest establishmentand incubation,and to both femalesand nestlingsduring brooding. Date of fiedging varied tkom mid-Juneto mid-July.Fledgingwas synchronous,and minimum estimates of brood size ranged from 1-5. Diet included a mix of small birds, mammals and insects.Our results suggestthat the creationof openingsby clear-cutloggingis unlikelyto benefit Northern Pygmy-Owls, and that the replacementof structurallydiverseforestswith uniform forestsmay be detrimental. KEYWOP, DS: NorthernPygmy-Owl; Glaucidium gnoma;fragmentation;habitatuse.
ESTACION
REPRODUCTIVA,
USO DE HABITAT
Y ECOLOGIA
DEL BUHO
PIGMEO
MACHO
DEL
NORTE
RESUMEN.•Examinamos el usode hibitat, la localizaci6nde nidos,la dietay lospatronesde la actividadde bfihospigmeos(Glaucidium gnoma),durantecuatro estaciones reproductoras(1994-97) en bosquesfragmentadosen la peninsulaOlympic,en Washington.Observamosel forrajeo en todaslas categoriasde vegetaci6ndisponibles, cuyospatronesde usono estabandeterminadospot el azar.Losbosquesmilsviejosy estructuralmentediversosfileron los m•s usados,los clarosy parchesde •rboles fueron los mucho menos usados,la utilizaci6nde bordesfue intermedio.Localizamosocho nidos,todosen cavidades de carpinteros en parchesde bosquesestructuralmentediversos.No encontramosninguna evidenciaclara que los nidos estuvieransituadoscercade los bordes.Los bfihospigmeosdel norte fueron mis activosduranteel dia, los bfihosmachosentregaronel alimentoalas hembrasduranteel establecimiento de nidosy la incubaci6n, tanto a hembrasy pichonesdurante el empollamiento.La fecha de crecimientode plumasen que los polluelosdeNan abandonarel nido vari6 a parfir de mediadosde junio hastamediadosde julio. E1 crecimientode plumasfue sincr6nicoy lasestimaciones minimasdel tamafiode la nidadafiuctuaronentre 1 y 5. La dietaincluy6una mezclade pijaros,mamifkrose insectospequefios.Nuestrosresultados sugierenque espocoprobableque la creaci6nde clarospot la talabeneficieal bfiho pigmeodel norte y que el reemplazo de bosquesestructuralmente diversospot bosquesuniformespuede set perjudicial. [Traducci6n de C6sarMirquez]
Holt et al. (1990) labeled the Northern Pygmy- westernforestsremains scant,and publishedinforOwl (Glaucidium gnoma) North America's least- mation is often equivocal.Yet westernforestshave studied owl. Thirteen years later, detailed infor- experienced decades of intense resource extracmation for this locally-common inhabitant of tion pressure(Parry et al. 1983) and it is presently
impossibleto estimatehow suchchangesmight affect Northern Pygmy-Owls.Here, we report on • Presentaddress:Departmentof Biology,Arizona State University,Tempe,AZ 85287-4601U.S.A.;e-mailaddress: Northern Pygmy-Owl habitat use and ecology in
[email protected] forestsheavilyfragmentedby clear-cutlogging. 117
118
GmsEANDFORSMAN
VOL. 37, NO. 2
The range of the Northern Pygmy-Owlextends many owlseach day as possible(typicallytwo to eight) from Alaskato Central America,includingforests and beginningwhere we had finishedthe day before Thus,eachowlwaslocatedat all timesof the day,because from the west coast to the Rocky Mountains a new owl typicallytopped the list each day. Location (Johnsgard1988). They are diurnally active,use times ranged from about an hour before sunriseto about cavitynests,and are generalistpredatorsof small an hour after sunset. birds,mammals,and insects(Bent1938,Johnsgard We classifiedhabitat into five vegetationcategories on structuralcharacteristics. The Open-Sapling cat1988). Northern Pygmy-Owls havebeencommonly based egoryincludedareasdominatedby smallconifersaplings sightedin or near openings,leadingto statements (1-4 m tall) intermixed with extensiveareas (>25% covthat theypreferentiallyforagedin openings(Bent er) of bare ground and shrub cover,mostlyon recent The EarlyStem Exclusion categorywasdominat1938, AOU 1983, Johnsgard 1988), nested near clear-cuts. edges (Webb 1982, Reynolds et al. 1989), and ed by youngconifers(5-15 m tall) with fewopeningsin the overstoryand with dense,overlappinglimbs in the might benefit from partial forestclearing(Johns- understory.The LateStemExclusion categorywasdomigard 1988). In contrast,othershavesuggested that natedby medium-sized conifers(typically20-40 cm DBH partial forestclearingmaybe detrimentalto North- and >15 m tall) with a closedcanopyand open understory.The Structurally Diversecategorywascharacterized ern Pygmy-Owls(Marshall 1992). Haywardand Garton (1988) used call-response by a widevarietyof tree sizesand a multilayeredcanopy. It consisted primarilyof matureand older forests(typisurveysto examine resourcepartitioningby small cally>80 yr), but alsoincludedmid-agedstandscharac-
forest owls in Idaho
and concluded
that the North-
terizedby high structuraldiversity.Finally,the Edgecategory included all areaswithin 30 m of an intersection
ern Pygmy-Owl wasa habitatgeneralist.Although their workis the mostextensivestudyto date,many questionsremain abouthabitatuseby thisspecies. We usedradiotelemetryto studyhabitatusefor two behaviors,foragingand nesting.We were specificallyinterestedin whetherpartialforestclearing m•ght benefit this species.We alsodescribehome range size, diet, activitypatterns,nest characteristics,and nestingbehavior.For the purposesof this paper,we hereafteruse'Northern Pygmy-Owl' and
betweena patch of Open-Sapling, and any of the other categories. Thirty meterswasusedbecausepilot-study observationssuggested that foragingflights by Northern Pygmy-Owls rarely exceededthis distance.A foraging flight wasdefinedasanyflight which includedor ended with an attempt to captureprey. Becausepatchinessin the studyarea waspredominantlythe resultof clear-cut logging,patcheswere generallyeasyto delineateand classify.We delineatedpatch boundariesusing ARC/ INFO (ESRI, Redlands,CA) and digital orthophotographs.We visitedeachpatch and basedvegetationclas-
'owl' synonymously.
sifications for both used and available cover on visual
METHODS
inspection. We usedcompositionalanalysis(Aebischeret al. 1993)
The studyareawasa mixture of federal,state,and pri- and programRESELECT(Leban1994;availableat http. vate landson the northwestcornerof the OlympicPen- //ces.iisc.ernet.in/hpg/envis/resdocl 120.html) to comComposi•nsula,Washington.The area was hilly to mountainous pare relativeuseamongvegetationcategories. treatsthe individualasthe samplingunit, with elevationsranging from 50-1350 m. Mean annual tional analysis precipitationrangedfrom 150-250 cm. Naturalvegeta- accountsfor the unit-sumconstraintof proportions,and tion wasdominatedby coniferousforestsof westernhem-
allowsunique availabilityfor each individual.We usedan
lock (Tsugaheterophylla), Douglas-fir(Pseudotsuga menzie- F-testto determine study-widedeviationfrom random usfor differentialusebetweenvegszz),westernred-cedar (Thuja plicata),and silverfir (Abies age,and pairwiset-tests amabilis). Valley bottoms typically included variable etation categories. Use ratios were calculatedby dividing proportionate amountsof bigleaf maple (Acermacrophyllum) and red alder (Alnusrubra) (Hendersonet al. 1989). Even-aged use by proportionateavailability.Proportionateusewas patchesof primarily Douglas-firresultedfrom logging definedasthe proportionof locationsfor a givenowlin and
silviculture.
a given vegetation category.All locations were deter-
We locatedowlsby walkingloggingroadswhilevocally mined by homing to an owl with a hand-held receiver •m•tatingtheir calls.Surveys wereconductedduringApril until the owl waslocatedvisually,or until triangulation and May 1994-97,andwerenot designedto samplethe indicated that the owl was directly overhead.Locations study area equitably.Rather, we concentratedon areas were mappedin the field on aerialphotographsand subwherewehad observedowlspreviously, andsurveyed oth- sequentlydigitized using digital orthophotographs.Loer areaslessintensively. When found, owlswerecaptured cations of owls
