Spotlight Surveys for Grassland Owls on San Clemente Island, California

2005·5.4 MB·English

by  Anne M Condon

Wilson Bulletin 117(2):177—184, 2005
SPOTLIGHT SURVEYS FOR GRASSLAND OWLS ON
SAN CLEMENTE ISLAND, CALIFORNIA
ANNE M. CONDON,' ERIC L. KERSHNER,'^ BRIAN L. SULLIVAN,' ^
DOUGLASS M. COOPER,' AND DAVID K. GARCELON^
—
ABSTRACT. Accordingto Breeding Bird Survey data, grasslandbirds areamongthemostimperiledspecies
in North America. Within this group, grassland owls show steep population declines across the United States.
Despite these declines, questions still remain regarding the seasonal and geographic distribution of grassland
owls. On San Clemente Island (SCI), California, grassland owls are known to occur, but nothing is known about
their distribution or abundance. To increase our understanding of owl populations on SCI, we used night-time
spotlighting to survey for grassland owls from October 2001 to October 2002. We recorded 733 detections of
three species of owls: Barn Owl {Tyto alba). Burrowing Owl {Athene cunicularia), and Short-eared Owl {Asio
flammeus). Barn (8.3 ± 0.8 owls/hr) and Burrowing owls (2.2 ± 0.7 owls/hr) were the most frequently detected
species, whereas Short-eared Owls were rarely detected (0.2 ± 0.1 owls/hr). We detected owls during all night-
time hours surveyed and detected Barn Owls in every month of the study. We detected Burrowing Owls only
from October to March and Short-eared Owls from December to April, suggesting that they are winter visitors.
Despite the bias of increased detectability using roadside surveys, spotlighting from a vehicle enabled us to
efficiently cover a large proportion of the island (compared to walking surveys) and survey multiple grassland
species using one survey technique. Received 8August 2003, accepted 22 February 2005.
Grassland birds are among the most imper- landscape by changing the shrub component
iled wildlife in North America (Peterjohn and ofthe native coastal chaparral habitats to open
Sauer 1999, Sauer et al. 2004), and, within grasslands (Coblentz 1980; BLS and ELK un-
this group, owls are considered species of publ. data). In 1993, however, feral grazers
; conservation concern in most North American were removed from SCI; as a result, succes-
regions (Wellicome and Haug 1995, Herkert sional change has been allowed to take place
: et al. 1996, Sheffield 1997, U.S. Fish and and the grasslands are reverting to more nat-
Wildlife Service 2002). Biologists visiting and ural, shrubby communities.
working on San Clemente Island (SCI), Cali- Due to successional change, the conserva-
I' fornia, have observed Barn (Tyto alba). Bur- tion status of the island’s owls, and our lack
rowing (Athene cunicularia), and Short-eared of knowledge about grassland owls on SCI,
(Asio flammeus) owls at various times of the we examined the presence/absence, relative
year and have documented breeding by Barn abundance, and distribution of grassland owls
' Owls (BLS and ELK unpubl. data). However, on SCI. We hope to provide a better under-
little else is known about the owl populations standing of how grassland owls use SCI and
I' on SCI. determine how the successional transition of
Grasslands compose 30% (—4,300 ha) of grassland habitats may effect these owl pop-
'
I SCI’s vegetation community; thus, there is ulations in the future.
I ample habitat for grassland owls. The pres- METHODS
ence of large, open grasslands on SCI has re-
! suited from the island’s history of ranching Study area.—SCl (32° 50' N, 118° 30' W)
I and the introduction of feral grazers and ex- is located approximately 109 km northwest of
I otic grasses in the mid-lSOOs (Andrew 1998). San Diego, California, and is the southern-
I' fhese introductions dramatically altered the mhaositslCaanldifisor3n4iakmChalnonngelanIdsl2a.n4d-,6.f4hekm14w,i6d0e3.-
I A relatively level, open plateau runs the
I
' Inst, for Wildlife .Studies, 2515 C'amino del Rio length of the island, with elevations ranging
South, Ste. 334, San Diego, ('A n2l()8, USA. from sea level to 599 m. Deep canyons of
’ Inst, for Wildlife Studies. B.O. Box 1 104. Areata, varying lengths incise the jilatcau from the
C'A 95518. USA.
'Chirrent atklress: ('ornell Lab of Ornithology, 159 east and west sides, 'fcmpcraturcs range from
.Sapsucker Wt)ods Rd., Ithaca, NY 14850. U.SA. 6 to 37° C' and mean annual precipitation is
*Corresponding tuithor; e-mail: kershnerC^'iws.org 17.8 cm (C’alifornia State Northridge, Depart-
177
178 THE WILSON BULLETIN • Vol. 117, No. 2, June 2005
ment ofGeography unpubl. data, 1998-2002). tober 2002) for a total of 104 surveys (13 all-
Fog is common, especially in the summer. island surveys). We tried to survey as many
Prevailing winds are from the west, and windy of the eight transects in one night as possible,
days are frequent throughout the year (typi- and each month we randomized the order in
cally Beaufort scores of 2-3). SCI is admin- which we surveyed the eight transects. We
istered by the U.S. Navy and is used foractive conducted surveys by driving transects in a
military training as part of the Southern Cal- truck at night and using spotlights to locate
ifornia Offshore Range; however, the U.S. owls (hereafter spotlighting). The driver and
Navy has an environmental program to protect passenger, equipped with 750,000-candle
natural and cultural resources (U.S. Depart- power spotlights, scanned both sides of the
ment of the Navy 2001). road, making full sweeps of the plateau and
Suitable owl habitat (i.e., grasslands and road ahead while driving 16-32 km/hr (de-
maritime desert scrub) is found predominantly pending on road conditions). We used binoc-
on, and surrounding, the island’s large central ulars (7 X 42 and 8 X 42) to identify species
plateau. Grasslands comprise native and non- and plotted the locations of flying or perched
native species (Avena spp., Bromus spp., Nas- birds on a topographic map. We recorded the
sella pulchra) and scattered shrubs such as time ofdetection, species, and behavior at de-
coyote brush (Baccharis pilularis)\ however, tection (e.g., perched, hunting, flushed) for
after the removal of feral grazers in 1993, each individual located.
shrub cover has increased (J. Dunn pers. Some transects (e.g., R3, R4, and R5) in-
comm.). Maritime desert scrub is dominated cluded multiple dead-end spur roads; in these
by boxthorn (Lycium californicum), snake situations we backtracked over the same road
cactus {Bergerocactus emoryi), cholla {Opun- in order to resume the survey, only recording
tiaprolifera), prickly pear cactus (Opuntia lit- observations while traveling in the initial di-
toralis), California sagebrush {Artemisia cal- rection along the spur. From March to October
ifornica), and morning glory (Calystegia rna- 2002, one transect (R2) was shortened to 6 km
crostegia). See Raven (1963) and Kellogg and due to a change in the accessibility to that part
Kellogg (1994) fora more detailed description of SCI. We surveyed the shortened route for
of SCI’s vegetation.— those 8 months, and adjusted the total distance
Survey technique. Because of the inacces- surveyed to 76 km, rather than 80 km.
sible nature (i.e., steep, rocky canyons) ofpo- Under optimal weather conditions (e.g.,
tential nesting habitat for some owl species clear skies, no fog), the maximum distance at
and the limited availability of personnel to which we could reliably detect owls was ap-
search the vast grassland expanses, we sur- proximately 250 m (determined using a Barn
veyed for owls along established island roads. Owl replica, spotlight, and digital rangefin-
We established eight 10-km transects (Fig. 1). der). We did not conduct surveys in fog or
We
selected transect starting points randomly rain, or when wind exceeded a score of 6 on
while ensuring that no two transects over- the Beaufort scale (—40-50 km/hr). Temper-
lapped. Although SCI roads were not estab- atures between 12 and 14° C, cloud cover be-
lished randomly and roadside surveys are as- tween 0 and 25%, and wind speed of 2 or 3
sociated with certain biases (Bart et al. 1995, on the Beaufort scale (—8-19 km/hr) were
Keller and Scallan 1999), the layout of the typical survey conditions.
island road system offered access to most of Because SCI is an active training facility
the open grassland and maritime desert scrub for the U.S. Navy, designing a straightforward
habitat; the view from the roads was typically survey design was challenging. We had to ad-
unobstructed on either side. Our survey tran- just ourmethodology to account forgeograph-
sects sampled approximately 55% (77.5 km) ic and temporal (both seasonal and hourly) ac-
of the available roads on SCI, and provided a cess restrictions, usually on short-notice. On
representative sample of owl habitat across some nights we were denied access to certain
the island (i.e., they traversed —34% of the areas of the island. When we were unable to
grassland on the island). survey all transects in one night, we finished
We surveyed each transect once per month the surveys on the next available date when
over a 13-month period (October 2001 to Oc- access was granted. These restrictions created
Condon et al. • SPOTLIGHTING OWLS ON SAN CLEMENTE ISLAND 179
uneven survey coverage; no surveys were control over when each transect could be sur-
conducted during some time-blocks (i.e., 10, veyed, these constraints simply aided the ran-
wI eI, awnedre12gihvrenpasrtegsuulnasreta)ccaensds,. duI'rliiensge oatchceersss, domDia/taatioannalyo.fsio.su.r—deI'soigdne.termine |')atterns of
constraints prevented us from standardi/ing temporal and seasonal activity, ue
( I )
how transects were surveyed over various summed the amount of time surveyed in each
time-blocks, which coidd iiinuence survey re- hour alter sunset for each month ot the study,
sults if there are time-dependent associatiofis (2) totaled the number ofowl detections uith-
in owl acti\ity. However, because we hatl little in each 1-hr time block, and (3) calculated the
180 THE WILSON BULLETIN
• Vol. 117, No. 2, June 2005
12
BarnOwl
4.4
BurrowingOwl
10 -
Short-earedOwl
14.6
14.6 1.0
10.5
8 - 0.9
20.8 0.3
1/5 c®
s
4 - 0.5
0.3
2 -
6 7 8 10 11 12 13
Houraftersunset
surFvIeGy.s2o.n SBaanrnCleBmuernrtoewiInsgl,anad.ndCaSlhiofrortn-ieaa,re2d00o1wlanddet2ec0t0i2o.nsS/uhrrvdeuyrienffgorIt-h(trotballochkrssuafrtveeryesdun)ssethofwornsapbootvlieghbtairnsg
number of owl detections/hr of effort sur- 3.3-10.7) over the course of the study. We
veyed for each time block. The number ofde- consistently detected Barn Owls 1-8 hr after
tections/hr represents relative abundance rath- sunset, despite varying levels of effort (Fig.
er than the number of individuals, as any one 2), and detected the fewest Bam Owls/hr 9
owl might have been observed on more than and 13 hr after sunset. Almost half (46%, 259
one occasion on a given night, despite our ef- of561), ofall Barn Owl observations were on
forts to reduce double counting. Means are transects R4 and R5. For all months, we de-
presented ± SE. tected 2.2 ± 0.7 (range = 0—4.0) Burrowing
RESULTS Owls/hr; excluding months when Burrowing
We completed 68 survey hr on 104 tran- wOweldsetweecrteedpr4e.0su±ma0b.l6ypaebrsehnr.tBfurrormotwhienigslOawndl,
sects (i.e., 8 transects surveyed 13 times). The activity was limited to 1—6 hr after sunset or
mean number of transects surveyed per night early morning hours (i.e., 13 hr after sunset;
was 2.9 ± 0.25 (range - 1-8). Most of our Fig. 2). We observed 68% of the Burrowing
survey efforts were between 2 and 6 hr after Owls on transect Rl, R2, and R3. We detected
sunset in = 64.9 hr, 96%). Each transect re- 0.2 ±0.1 (range = 0-2.2) Short-eared Owls/
quired 30-70 min (mean completion time = hr for all months, and 0.4 ± 0.3 per hr ex-
39.2 min) to survey, depending on road con- cluding months when this species was pre-
ditions and the numb—er of owls observed. sumably absent from the island. We detected
recPorredseednc7e3/3aboswelncdee.tecDtuiroinsngof13thsruerevesypesc,iewse: sSuhnosrett-eaanrdedonOwalllstrbaentswecetesnex1ceapntdR67.hr after
OSBhaworlrnst-o{ennar8=e9d o5{6fn1)1=,041B1(u)8r5ro.ow6lw%si).ngtWrae(n/?sdeec=ttse.c1t6Be1ud)r,rBoaawrn-nd cyeoarWrd,se.woWhbisecehrovbessduepBrpvaoermdtsOtwhpelrsegvreievoaeutrseystbmronenuetdmhibneogrftrohefe-
OinwglsOownls9 (o8n.6%4)7. W(4e5.d2i%d),notadnedtecSthoarnty-eoawrlesd tBoabrenr 2O0w0ls2/(hFrigi.n3)J.unBeurarnodwitnhge OfwelwesstweirneOocb--
on 8 of 104 (7.7%) tra—nsects. served only from October 2001 to March
Relative abundance. We recorded a mean 2002 and in October 2002; during those times,
of 8.3 ± 0.8 Barn Owl detections/hr (range = they were detected on 84% (47/56) of the
Condon et al. • SPOTLIGHTING OWLS ON SAN CLEMENTE ISLAND 181
Surveymonth
FIG. 3. Barn, Burrowing, and Short-eared owl detections/hr by month for spotlighting surveys on San
Clemente Island, California, October 2001-October 2002. Survey effort (total hr surveyed) for each month
shown above bars.
transects. Their absence between April and idents only. Burrowing Owls were the second
September indicates that they are primarily most common species detected October
winter visitors. We observed Short-eared through March; we recorded no detections
Owls on five occasions between 12 December during the breeding season. Burrowing Owls
2001 and 7 April 2002. from northern breeding grounds migrate south
—
Owl behavior. Forty-seven percent of the in September and October and north in March
—
Barn Owls were first detected in flight {n and April (Haug et al. 1993), consistent with
262), and 53% were perched {n = 299). Barn when we observed them on SCI. Occurrence
Owls perched primarily on utility wires (n = of Short-eared Owls is irregular; they arrive
157, 53% of perched observations), but also in large numbers and winter on SCI only dur-
were seen on power poles, fences, junk piles, ing certain years (BLS and ELK unpubl. data).
buildings, rocks, signs, shrubs, the ground, The importance of SCI for wintering owls
and the road (n = 89). Burrowing Owls were brings up an important question: how will the
d=ete5c1t)edanbdothwhiennflpigehrtch(e3d2%(6o8f%,detnect=ion1s1,0)n. natural succession of grassland habitat (in the
absence of feral grazers) impact Barn, Bur-
Burrowing Owls were most commonly found
= rowing, and Short-eared owls on SCI? As the
on dirt/gravel roads (n 66), but they also
grasslands become shrubbier, reverting to a
I tp8h)eercoghfreodSuhnoodnrtu(-tneial=irteyd1w8i)O.rwelsW,sejuoinnbksfeplriivlgehetsd,, r7co3ocmk%sp,a(/ra/en=dd tmiocripeatneatsuoramle“ipmrpea-cgtrsazoinng”wincotnedriitnigono,wlwepoapn--
with 27% (n = 3) that were perched. ulations, as all three species prefer large, open
grassland habitats with little shrub or tree cov-
I DISCUSSION er (Marti 1992, Haug et al. 1993, Holt and
We detected all three species of grassland Leasure 1993). Burrowing Owls use short-
owl on SCI, and our data suggest that SCI is structured vegetation, including uncultivated
an important wintering ground for each spe- fields, for foraging (Haug and Oliphant
—
cies. We found Barn Owls year-round sug- 1990) presumably due to increased visibility
gesting resident status, whereas Burrowing of prey in those types (Konrad and Gilmer
and Short-eared owls appear to be winter res- 19S4). Reforestation (i.e., succession in this
182 THE WILSON BULLETIN • Vol. 117, No. 2, June 2005
case) generally constitutes habitat loss for roosting birds in forest habitats (Lindenmayer 1
Short-eared Owls (Holt and Leasure 1993). et al. 1996), as well as roosting seabirds and I
Owl behavior.—AW three species are waterfowl (Snow et al. 1990, King et al. 1994, ^
thought to be primarily crepuscular foragers, Whitworth et al. 1997). Debus (1995), how-
especially in winter (Marti 1992, Haug et al. ever, found that spotlighting without audio
1993, Holt and Leasure 1993). However, we cues while driving between survey points was
found that all three species were active during ineffective for detecting forest owls.
ttohebefirasctti8vehrduarfitnerg salulnsheotu.rsBaorfnthOewnlisghta.ppIetairs ateWmeetbehloidevfeorthastursvpeoytilnigghtgirnagssilsaannd aopwplrsoprbie--
more difficult to assess Burrowing Owl activ- cause these species use mostly open habitats
ity because, during the months when this spe- and are visible at night while foraging (i.e.,
cies was detected, we surveyed only 8 of the low quartering flight over vegetation or scan-
13 1-hr blocks after sunset. It appears that ning for prey while perched). Furthermore,
Burrowing Owls are equally active in early most grassland owls are light colored on their
morning (13 hr after sunset) and early evening ventral side, improving detection when illu-
hours (Fig. 2). We detected Short-eared Owls minated (Marti 1992, Holt and Leasure 1993,
primarily in the first hour after sunset. Marks et al. 1994). Grassland owls are typi-
On SCI, Barn Owls regularly use power cally less vocal than forest owls, especially
lines for perching, and they may be drawn to outside of the breeding season, making tradi-
roadsides where other man-made structures tional call-playback techniques less effective
serve as perches. Bam Owls are well adapted in winter (Heintzelman 1965, Haug et al.
thounutsiinnggnuerabranroaldasn,dsecsappeecsiaallnydihnawvientaehrab(iKtono-f 11999934,, THoolmtsaentdalL.e2a0s0u1r,e C1o9n93w,ayMaarnkdsSeitmoaln.
idgeteetctaeld. w19e9r9e).peOrvcehredh,alafnodf oafll tBheasme, O5w3l%s 200U3s)i.ng high-powered spotlights, we were
wmeorset oBnaumtilOitwylwiroebss.erTvhateiotnrsansweecrtse wsiitthuattheed aSbClFestogreaffsiscliaenndtlhyabsiutravteyfora floarrgaegipnrgopoow—rltsi.onThoef
along the main road, and utility poles are sit- alternative to roadside spotlig—hting hiking
uatWeed aldoetnegctitesdentthieremlaejnogrthi.ty of Burrowing tacarkoesnssurbusgtganetdiatlelryralionngeart.nAighsttudywocuolmdparhianveg
Owls on dirt/gravel roads, where they might three survey techniques for Burrowing Owls
be attracted to the bare ground in an otherwise indicated that line-transect surveys are the
dense, grassy habitat. Burrowing owls are least effective means of surveying in the
known to forage in uncultivated fields (Haug breeding season (Conway and Simon 2003).
evtaiasl.e1t99a3l.) a20n0d3)a.lonSgixtthye-eeidgghetspoefrrcoeantdso(fGearl-l Tshuursv,eysspmoitglhigthtgiinvge ufsorthoewblesstdcuhrainncgertooacdosvideer
Burrowing Owls detected were perched on the a large area with the least numberofobservers
ground, 60% of which were on dirt/gravel while collecting valuable data.
roads. Burrowing Owls appearto forage along We believe spotlighting may be useful for
or perch on dirt/gravel roads more than paved determining the status of owls where they are
roads. We detected them most often on three potentially at risk, or where data are lacking,
transects, approximately 66% of which were especially during the non-breeding season
dirt/gravel roads (n = 17 km). Unlike larger when these species might not be as responsive
grassland owls. Burrowing Owls forage pri- to tape playback methods (Haug et al. 1993).
marily on the ground and are not as visible in Spotlighting might be a useful tool in esti-
flight (Haug et al. 1993). mating presence/absence in any open area,
Use of spotlighting as a sun'ey technique during any time ofyear. It might be especially
fuosredgrtaescshlnainqdueowfolrs.s—urSvpeoytilnigghmtuilntgipisleawwiliddleilfye ulysefiunl ttohecoMnidudcwtesstteartnewiadnedsuwrevseytse,rnespUenciitaeld-
taxa at night (e.g., mammals [Focardi et al. States, where long stretches of road cut
2001], spiders [Martin and Major 2001], and through open habitat suitable for these spe-
reptiles and amphibians [Corben and Fellers cies. This technique may also be used to
2001]). Spotlighting has been used to locate quickly locate regularly used areas or ascer-
Condon et al. • SPOTLIGHTING OWLS ON SAN CLEMENTE ISLAND 183
tain periods of activity for more intensive Focardi, S., a. M. De Marinis, M. Rizzotto, and A.
. monitoring or research efforts. Pucci. 2001. Comparative evaluation of thermal
I infrared imaging and spotlighting to survey wild-
Potential drawbacks to spotlighting surveys
life. Wildlife Society Bulletin 29:133-139.
I taraeki(n1g) srpooatdlingohitsien,g (f2o)rtihlelepgoaslsipboialcithyinogf,mainsd- Gerva2i0s0,3.j.Sap.a,cDe.uKs.eRoasnednbpeesrtgi,ciadendeRx.poGs.urAentrhiosknyo.f
(3) being limited to habitat adjacent to roads, male Burrowing Owls in an agricultural land-
which may not accurately represent overall scape. Journal of Wildlife Management 67:155-
habitat and may inflate or decrease detection 164.
daenpdeSncdailnlganon19s9p9e)c.ieIsn (oBuarrtsteutdyal,.w1e99f5o,uKnedllienr- Haug1,99E3.. aB.u,rrBo.wian.gMOiwlllsa{pA,thaennedcMu.nicSu.larMiaar)t.eTlhl.e
Birds ofNorth America, no. 61.
creased detectability (possibly due to in- Haug, E. A. and L. W. Oliphant. 1990. Movements,
creased abundance) near roads for Barn and activity patterns, and habitat use of Burrowing
Burrowing owls, which may be influenced by Owls in Saskatchewan. Journal of Wildlife Man-
the increased number of perches and the fact agement 54:27-35.
that most roads were dirt. Thus, the potential Heintzelman, D. S. 1965. Distribution and population
density ofBarn Owls in Lehigh and Northampton
biases, as well as safety issues, associated
counties, Pennsylvania. Cassinia 49:2-19.
with roadside surveys should be thoroughly Herkert, j. R., D. W. Sample, and R. E. Warner.
evaluated prior to using this technique in other 1996. Management ofMidwestern grasslandland-
situations. scapes for the conservation of migratory birds.
Pages 89-116 in Management of Midwestern
ACKNOWLEDGMENTS
landscapes for the conservation of Neotropical
birds (E R. Thompson, III, Ed.). General Techni-
This study was funded by the Commander in Chief, cal Report NC-187, USDA Forest Service, North
Pacific Fleet, through Commander, Navy Region Central Forest Experiment Station, Detroit, Mich-
Southwest Environmental Department, under contract
' igan.
number N6871 1-99-C-6665 to the Institute for Wild- Holt, D. W. and S. M. Leasure. 1993. Short-eared
life Studies, with Naval Facilities Engineering Com- Owl (Asiofiammeus). The Birds of North Amer-
mand Southwest Division. We thank the U.S. Navy
ica, no. 62.
! Region Southwest, Natural Resources Office, and Keller, C. M. E. and J. T. Scallan. 1999. Potential
CSteoocuhmtnhmicwaaelnstdsu—pDSpioavrnits.iDoiWne,ego,NthaaCvanalklifJo.FranWci.ialiWtfaioelrsk,loEgnBi.gstiiMncieallelrsaiannpdg, KellrBoorgaegde,sdiidEn.egMBb.iiarsdaensSdurdJv.ueeLy.troKouetlhelasb.oigtCgao.tnd1co9hr9a4n.1g0eS1s:a5n0a-lC5ol7ne.g-
: and D. Rosenberg for reviewing early drafts of this mente Island vegetation condition and trend and
manuscript.
the elements of ecological restoration. Prepared
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184 THE WILSON BULLETIN Vol. 117, No. 2, June 2005
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