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Distribution and Density of Owls at Monte Bello Open Space Preserve, Santa Clara County, California

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Distribution and Density of Owls at Monte Bello Open Space Preserve, Santa Clara County, California DISTRIBUTION AND DENSITY OF OWLS AT MONTE BELLO OPEN SPACE PRESERVE, SANTA CLARA COUNTY, CALIFORNIA PAUL L. NOBLE, San FranciscoBay Bird Observatory,P. O. Box 247, Alviso, California 95002 From March 1986 throughJune 1987 I censusedowls 72 km south of San Franciscoin the northernend of Monte Bello Open Space Preserve,in SantaClara County. My objectiveswere to identifythe speciesand determine the densityof owlsin varioushabitats. Vocal responsesto tape-recordedcalls were noted and comparedto weather conditions,lunar phases,and time of year. STUDY AREA AND METHODS The study area, approximately300 ha, includesStevens Creek and its headwaters and ranges in elevation from 520 to 950 m. I used aerial photographsand vegetationmaps to determinethe areasof the five habitat types representedthere. Douglas-firforest, composedmostly of second growth Douglas-fir (Pseudotsugarnenziesii) with some Canyon Live Oak (Quercuschrysolepis) and Madrone (Arbutusrnenziesii), encompasses 75 ha. Canopy closureby these speciesapproaches 100%. This habitat is confinedmainly to the northeast-facingslopes of StevensCreek Canyon. Broadleafevergreen forest encompasses 75 ha mainlyof Canyon Live Oak. Other membersof the communityinclude Valley Oak (Quercus1obata), California Bay (Urnbellularia californica), and Madrone. A meadow, covering125 ha, is composedof non-nativeEuropean annual grasses with a few scatteredpatches of native perennialbunchgrasses and native annuals. On more rocky areas Coyote Brush (Baccharispilularis) grows in scattered clumps. Chaparral, composedof Chamise (Adenostornafasiculaturn) and manzanita(Arctostaphylos spp.), coversapproximately 22 ha on south-and west-facingslopes. A thin strip along StevensCreek and its main feeder streamsis a riparianwoodland of Big-leafMaple (Acer rnacrophy!!urn),Tan- oak (Lithocarpusdensifiora), California Bay, and some willow ($alix sp.). This habitatis bounded on both sidesby the Douglas-fircommunity. I usedtaped recordingsof owl callsto elicitresponses while I walkedthe censusroute. Tapes were played at stationsat 100-m intervals.Calls of each speciesof owl were played for 15 seconds,followed by a minute of silence. Callingowls were then mapped. Often owlswould not respondto the tapes immediatelyso I usuallystayed at each stationfor 5 to 10 minutes. I censusedthe area 2 or 3 times a month for a total of 38 censuses,spend- ing a total of 69 hoursin the field for an average4.6 hoursper month. The censusroute, limited to developed and some undevelopedtrails, allowed coverage of 95% of the study area. Most of the censuseswere conducted during calm, clear nights,but three were conductedduring strongwind or rain for information on the relationshipbetween owl calling and weather conditions.Censuses were generallyconducted between I hour after sunset Western Birds 21:11-16, 1990 11 OWLS AT MONTE BELLO PRESERVE and 1 hour beforesunrise. In May and June 1987 1put in two all-nightvigils to listento unsolicitedcalling and to recordon tape the owls'vocalizations. RESULTS Six speciesof owlswere found in the studyarea: the Barn Owl (Tyroalba), WesternScreech-Owl (Otus kennicottii),Northern Saw-whet Owl (Aegolius acadicus),Great Horned Owl (Bubo oirginianus),Northern Pygmy Owl (Glaucidiumgnoma), and Long-earedOwl (Asio otus). Owls were most vocalfrom late Decemberthrough March. Callingof all owl speciesexcept the Pygmydecreased through the latespring and summer.The periodfrom late Septemberthrough mid-November was generally quiet. The phasesof the moon seemedto have littleeffect on calling(Table 1). Weathercondi- tions,however, did have an effect.Although only three censuses(less than 10% of the total) were duringpoor weather, callingwas sharplyreduced during these periods. Western Screech-Owl The mostabundant owl encounteredin the studyarea with a densityof 0.4 pairs/ha. Calling maleswere often encounteredapproximately 100 m apart in preferred habitat. Western Screech-Owlsshowed a preference for the Canyon Live Oaks and were rarely encounteredoutside this habitat (Table 2). These owls readily respondedto taped callsand were most vocal in late Decemberand January.I usuallyheard screechowls while playingtapes; I rarely heard unsolicitedcalling. Often severalminutes passed before an owl wouldrespond to my calls.The firstowl's response, in turn, wouldstart other screechowls callinguntil severalwould be callingall around me. Western Screech-Owlsresponded to taped callswith the typical"bouncing ball" call (Tyler 1978). Mated pairs often respondedand began to duet near each other. The initialcalls switched to" trilling as pairsapproached one another. Other callsnoted were loud barkingby the adultsaccompanied by juveniles and puppy-likebarking uttered primarily by fledgedjuveniles in ,Juneand ,July.Calling by the adultsfell off markedlyby the end of March (Figure1). Table 1 Effect of Lunar Phases on Numbers of Owls Heard Number Number of Averagenumber Lunar phase of censuses owls heard heard per census New 14 103 7.3 First quarter 7 43 6.1 Full 9 64 7.1 Third quarter 8 64 8.0 12 OWLS AT MONTE BELLO PRESERVE Northern Saw-whet Owl Confined to the Douglas-firforest and the associatedriparian areas along StevensCreek (Table 2), the Saw-whet is the second most common owl in the study area with density of 0.25 pairs/ha. Saw-whet Owls are very aggressiveduring the courtshipand nestingseason. They would respondto my taped callsinstantly, and I often had to duck to avoid an attackingowl. Saw-whets,like WesternScreech-Owls, tended to respondonly to the taped calls, not to call unsolicited.Most vocal November through February (Figure 1), these owls respondedto the taped callswith two types of calls.One call consistedof a singlenote repeated 120 to 180 timesper minute. The pitch and volume increasedas the owls became agitated. The other call was a nasalwhine, increasingin pitch at the end, whichlasted 1.5 to 2.0 seconds. Othercalls included a loud angrychatter and variousloud barksand whistles. To solicitfood from adultsjuvenile Saw-whet Owls emit a callthat resembles a soft hiss (Jon Winter pers. comm.). I observeda juvenileSaw-whet Owl along StevensCreek in mid-August1986. Great Horned Owl This owl ranged over all habitatsin the study area (Table 2). Two pairs were presentthroughout the study.The one nestobserved, a bulky platform of sticks,was in a large Douglas-fir30-40 m from the ground and 3 m out from the trunk. A singleowlet fledgedin mid-June 1986 and continuedto beg for food from the adultsinto June of 1987. Great Horned Owls in my study area respondedto any owl call played but were located mostlyby unsolicitedcalling. Other owl species(Western Screech-Owls in particular) became quiet around a callingGreat Horned Owl. ß Screech Owl [• Saw-whet Owl NOV EEC JAN FEB MAR APR MAY JUN JUL AUG SEP OCT Figure 1. Average number of WesternScreech-Owl and Saw-whet Owls callingper census each month. 13 OWLS AT MONTE BELLO PRESERVE Distributionof Owl Occurrencesby Speciesand Habitat Percentageof Occurrences Percentage W. N. N. Great of study Screech Saw-whet Pygmy Horned Habitat type area Owl Owl Owl Owl Broadleaf evergreen forest 25 98 4 5 31 Douglas-firforest 25 2 80 65 39 Meadow 42 0 0 0 25 Chaparral 6 0 0 0 5 Riparian 1 0 16 30 0 Chi-squarevalue 276.4 363.6 428.1 16.32 Significancelevel p < 0.005 p < 0.005 p < 0.005 p < 0.25 Total observations 94 46 20 75 Northern Pygmy Owl NorthernPygmy-Owls were restrictedto the Douglas-firsand oaksborder- ing StevensCreek (Table2). Three pairswere encounteredalong the entire 4.8 km of the creekin the studyarea. The birdscalled in the two hoursafter twilightin the morningand the lasthour of twilightin the evening(Figure 2). I never heard a Pygmy Owl call at night. The Pygmy Owls were generally locatedby unsolicitedcalling and were equallyvocal throughout the year. I did not locateany nestingPygmy Owls, but W. Bousman(pers. comm.) saw two juvenilesin the studyarea in late July 1981. 3.5 3 0.5 o 12 1 2 3 4 5 6 7 8 9 lO 11 12 1 2 3 4 5 6 7 8 9 lO 11 Figure 2. Callingfrequency of Northern Pygmy Owls. 14 OWLS AT MONTE BELLO PRESERVE Barn Owl Encounteredonly eighttimes during the study,this species was seen flying low over the grasslandsor trees. I saw single birds in March, June, September, and November 1986 and in January, February, April, and June 1987. I found a possiblenest site in a largedead CanyonLive Oak whereI usuallysaw the owl, but I never saw any evidenceof breeding. Long-earedOwl I first encountered this owl on 20 November 1986, hunting over a meadow. In late DecemberI again saw this speciescalling from a willow copse.In February1987 1 witnesseda pair copulatingat a potentialnest site. On 14 May 1987 I saw three recentlyfledged young being fed by an adult closeto the nesttree. The nestwas placed in a Canyon Live Oak approxi- mately20 m from the groundand 5 m out from the trunk. It appearedto be an old flattenedsquirrel nest. Several pellets found below the nestcontained the remainsof CaliforniaVoles (fi4icrotuscalifornicus); no other prey were identified. The male's call was a low hooting to which the female often respondedwith a higher-pitchedcall or cat-likemews. The fledglings'food- beggingcalls resembled a violinbeing lightly stroked. These calls were low in volumeand did not carryfar. The fledglingswere attendedby the adultsuntil June 1987. DISCUSSION Populationdensities for Western Screech-Owlsin the southwesternU.S. have been previouslyreported to be 2.25 pairs/ha with territoriesspaced closer than 100 m (Miller and Miller 1951). These densitiesare over five times greaterthan those
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