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Boreal Owl Mating Habitat in the Northwestern United States

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Boreal Owl Mating Habitat in the Northwestern United States j RapteaRes. 30(3):123-129 ¸ 1996 The Raptor ResearchFoundation, Inc. BOREAL OWL MATING HABITAT IN THE NORTHWESTERN UNITED STATES VICKI HERREN, STANLEYH. ANDERSON,AND LEONARDE RUGGIERO1 WyomingCooperative Fish and WildlifeResearch Unit, Box3166, Laramie,Wyoming 82071 U.S.A. A•ST•,•CT.--Weexamined boreal owl (Aegoliusfunereus)mating habitat in the Sierra Madre range of the Medicine Bow National Forestin Wyoming in the northwesternUnited Statesduring 1992-93. In noc- turnal surveys,we found 22 boreal owl singinglocations which we comparedto 68 random locationsin the studyarea. Owls used standsdominated by Engelmannspruce (Piceaengelmannii) and subalpinefir (Abieslasiocarpa) more often (77%) than standsof lodgepolepine (Pinusconteata). Stand sizeranged from0.2-122.8 ha thoughadjacent stand type (forest or opening)was not investigated. All borealowls were found in areaswith old forest characteristicsincluding high basalareas of trees, tall snags,many large down logs,and a tall overstorycanopy. Some boreal owlssang from old foreststands adjacent to clearcuts,one as closeas 15 m. Older forestsmay provide nest holes for boreal owlswhich are obligate cavity nesters. KEYWORDS: borealowl; Aegolius funereus; mating habitat;, old growth; Sierra Madre Feaest;, clearcut. Habitat de apareamientode Aegoliusfunereusen el noroestede los EstadosUnidos RESUMEN.--Durante1992 y 1993, examinamosel h/tbitat de apareamiento de Aegoliusfunereus en la Cordillera de Sierra Madre del Medicine Bow National Forest en Wyoming, al noroeste de los Estados Unidos. En recorridos nocturnos, encontramos 22 sitios de canto de este bfiho, los que comparamos con 68 sitiosal azar en el/trea de estudio.Los bfihosusaron m/rs a menudo (77%) parchesdominados por Piceaengelmannii y Abieslasiocarpa que parchesde Pinus conteata.E1 rango del tamafio del parche fue de 0.2 a 122.8 ha. Todos los bfihos fueron encontrados en /treas con caracteristicasde bosque antiguo, incluyendograndes/treas basales de firboles,ramas espigadas, grandes troncos caidos y grandes sotobosques.Algunos individuos de esta especiese encuentranhasta alrededor de 15 m de clarosde bosque.Antiguos bosquespueden proveer de sitiosde nidificaci6n para estosbfihos que nidifican obligadamenteen cavidades. [Traducci6n de Ivan Lazo] Boreal owl (Aegoliusfunereus)mating and nesting During the mating season,male owls sing for a habitat is poorly understoodin the U.S. becauseof mate with a continuoushigh-pitched songthat can their nocturnal behavior in remote forests.During easily be heard from 1.5 km and, on clear, cold their late winter mating season in the western nights,up to 3.5 km (Bondrup-Nielsen1984). This states,boreal owls associatewith older high-eleva- allows an observer to locate owls without excessive tion forests generally composed of Engelmann intrusion. Owl singing locations indicate habitat spruce (Picea engelmannii,ES) and subalpine fir use and represent potential breeding sites (Mee- (Abies lasioca•pa,SA) (Webb 1982, Palmer 1986, han 1980, Bondrup-Nielsen 1984, Hayward et al. Hayward et al. 1993). High elevation patches of 1993). Breeding sites vary by vegetation type but ponderosa pine (Pinus ponderosa),Douglas fir boreal owls are obligate cavity nesters (Mikkola ( Pseudotsugamenziesii), and quaking aspen ( Populus 1983). tremuloides)are important when available(Hayward In this study,we sought to describehabitat used et al. 1993). These forest types also support the by boreal owlsduring the 1992-93 mating seasons. owl's primary prey, the southern red-backedvole We compared owl singing locationswith random (Clethrionomysgapperi) (Hayward et al. 1993). habitat locations within the study area. The re- search also illustrates the distribution of boreal owl • USDA Forest Service, Intermountain Research Station, singinglocations in a mosaicof forestpatches com- Box 8089, Missoula, MT 59807 U.S.A. prised mainly of lodgepole pine (Pinus contorta, 123 124 HERREN ET AL. VOL. 30, NO. 3 ests [scoring >38 on Marquardt's (1984) Old Growth Scorecard]. Timber on the Coon Creek watershed was harvestedwith numerous small clearcutsas part of a wa- Sierra Madre ter augmentafon experiment in the early 1990s. The East Fork was left as an undisturbed control. Mountains Elevationon the studyarea rangesfrom 2600-3300 m. Current land uses include logging and grazing; small- scale mining while harvesting for railroad ties occurred historically. The well-drained soils are 50-150 cm deep. Mean an- nual precipitationmeasures 86.4 cm (1983-93), 70% of which falls as snow from late September until late June. Snow survey data from 1993 show a maximum snow depth in late March 1993 of 235.2 cm (Gonyer 1994). The mean annual temperature is 0.6øC,with a low of -42.8øC and high of 30.6øC(Gonyer, 1994). METHODS Boreal Owl Surveys. We established 24 transects throughout the studyarea to locate male boreal owlsdur- ing the mating season.When possible,we surveyedby snowmobile on unplowed roads and on snowmobile routes traveled by other researchers.We surveyedmore remote areas by snowshoe on routes following stream coursesand ridges. A total of 122 listening stations(61 on each watershed) on the 24 transects allowed us to detect singing males in all areas of the two watersheds. Althoughaudibility of a boreal owl songis 100% within 700 m (Holmberg 1979), the stationsin this studywere a maximum distanceof 500 m apart to increasedetection of singingowls over broken topography. I I We surveyedfor singing owls for approximately6 hr 2km after dusk from late February through May of 1992-93. To account for temporal and seasonalvariations in sing- Figure 1. The two watersheds(2526 ha) in the study ing activity,we surveyedeach stationat leastthree times area located in the Sierra Madre mountains of Wyoming. over the two mating seasonsin different phasesof mating season (early season-late February, mid season-March and April, late season-May), at different times of the LP) and spruce/fir (SF) with interspersedclear- night (early,mid, late), during different moon phases(•fi, cuts.Together with prey and nest hole availability, •, 3•, full), and under different cloud cover (clear,partly cloudy,overcast), snowfall (none, light, moderate), and breeding habitat may be the most critical factor wind (none, light, medium) conditions.We did not sur- affecting the persistenceof owl populations.We vey under conditions of strong winds and heavysnowfall specificallydescribe (1) foreststructure at owl sing- which were the two factorsmost affectingcalling activity ing locationsrelative to the forested availablehab- in northcentral Colorado (Palmer 1986). Becauseof the potential bias of luring an owl from a itat on the study area, and (2) locationsof singing singing location toward the listening station, tape play owls relative to clearcuts. back was not used. All owl locations used in this study represent habitat used by spontaneouslysinging boreal STUDYAREA owls.When owl singing was heard, we either moved to- The Blackhall Mountain Study Area is located in the ward it until the actual singing tree was located or we Sierra Madre mountains of southcentral Wyoming in the identified the location by triangulating the site from two Medicine Bow National Forest (41øN, 107øW). Two con- stations(providing the singing continued while we trav- tiguouswatersheds, Coon Creek (1615 ha) and the Up- eled betweenstations). All singinglocations were marked per East Fork of the Encampment River (911 ha), are on a 1:24000 U.S. Geological Survey topographicmap. contained in the study area and lie near the Colorado We combined all locationsfrom both surveyyears to de- border, 38 km southof Riverside,Wyoming (Fig. 1). Both scribe habitat used by boreal owls during their mating drainages are heavily-forestedmosaics of SF and LP season.Locations that were used more than once during patches,with a few small meadowsalong the creeksand the studywere counted as a single location. ridgetops.Lodgepole pine covers58% of Coon Creek, Microhabitat.The U.S. Department of AgricultureFor- and 67% of the EastFork for 61% overall.The remaining est Service Rocky Mountain Forest and Range Experi- forests are SF. Half of all stands in both watersheds were ment Stationconducts long-term wildlife researchin the classifiedby the U.S. Forest Service (USFS) as older for- studyarea. A samplinggrid of 90 stationsin each water- SEPTEMBER 1996 BO•utL OWL MATING HABITAT 125 shed along north-south transects400 m apart has been ney U tests,we used Bonferroni-adjustedprobability level established for these wildlife studies. Stations are located for • --• 0.05 to P • 0.0022. 200 m apart, numbered and flagged. We used a subset To examine multivariate patterns in the habitat data, of those 180 stations to describe available habitat. To we used an exploratory discriminant function analysis. achieve a representative subsample,we listed stations Eleven variables (basal area, LP •38 cm dbh, ES 15-38 from both watershedsby cover type, discarded stations cm dbh, ES •38 cm dbh, SA 0-15 dbh, snags•39 cm that occurred in clearcuts,and choseevery third station dbh, snag height, logs 10-32 cm, logs •32 cm, canopy on the list until we had at least three times the number cover,and canopyheight) with low Pearsoncorrelations of owlsinging locations (N = 22) and approximately60% (r • 0.55) were usedin three direct discriminantanalyses LP stationsand 40% SF stations.Each location repre- (SPSS,Inc. 1990). Three analyseswere done becausethe sented suitablehabitat for boreal owlsduring their mat- two classificationgroups to separatein
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