THE STATUS OF THE BURROWING OWL IN SAN DIEGO ......Burrowing Owls were widespread in this county and...

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Our effort to determine the status of theBurrowing Owl in San Diego County (Fig. 1) ispart of the urgent implementation tasksassociated with the San Diego Multiple SpeciesConservation Program (MSCP). The MSCP is alocal Habitat Conservation Plan (HCP)implemented under the State’s NaturalCommunity Conservation Planning (NCCP)Program of which the city and County of SanDiego, as well as several other cities, areparticipating members. It encompasses thesouthwest quadrant of the county in which the

vast majority of the county human populationresides (Figs. 2-4).

Each HCP/NCCP requires a monitoringprogram to determine the effectiveness of thatplan. The Biological Monitoring Plan for theMSCP (Ogden 1996) recommended that “Allknown Burrowing Owl breeding localitieswithin the preserve should be monitored forlevel of occupation; thus, grassland plotdelineation needs to account for the knowndistribution of Burrowing Owls.” Clearly, themonitoring of Burrowing Owls is a critical

Proceedings of the California Burrowing Owl Symposium 90-102© The Institute for Bird Populations 2007

THE STATUS OF THE BURROWING OWL IN SAN DIEGO COUNTY, CALIFORNIA

JEFFREY L. LINCER1

Wildlife Research Institute, Inc. P.O. Box 2209

Ramona, CA 92065 USA

PETER H. BLOOM2

Western Foundation of Vertebrate Zoology439 Calle San Pablo

Camarillo, CA 93012 USA

Abstract. Burrowing Owl (Athene cunicularia) populations in San Diego County appear tohave decreased through the early 1900s in conjunction with human population growth andconcomitant habitat loss. By the late 1970s/early 1980s, there were perhaps 250-300 pairsleft, but as of 2003 the population had been reduced to no more than 25-30 resident pairs(note: recent surveys revealed additional owls suggesting that there were, at most, 46 pairsin the county as of 2007). Primary threats to the remaining small Burrowing Owl populationin San Diego County are: reduced habitat suitability and fragmentation of remainingsuitable habitat; conflicts with management of listed species, especially the California LeastTern (Sterna antillarum browni) and Western Snowy Plover (Charadrius alexandrinus nivosus);and human disturbance (including their pets and vehicles). In addition, predators, preyabundance and availability, and colony size are likely factors influencing populationdynamics, the relative importance of which are difficult to characterize. Given county owlpopulation declines and current threats, a comprehensive management program is essentialto protect remaining habitat and owls. If such a plan, or similar action, is not implementedsoon, extirpation of the Burrowing Owl in San Diego County seems imminent.

Key Words: Athene cunicularia, breeding, Burrowing Owl, California, habitat loss,management, population trends, San Diego County, status.

_______________________________________1 E-mail: [email protected] E-mail: [email protected]


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component of the MSCP. With the exception ofMarine Corps Base Camp Pendleton (Bloom1980) and Fallbrook Detachment (Bloom 1996),comprehensive studies of this species within thecounty or the MSCP were lacking prior tostudies by the Wildlife Research Institute, Inc.(WRI 2002, 2003, 2004, 2005).

However, Burrowing Owl population declineshad begun early in some places and they wereobvious to many biologists. Grinnell and Miller(1944:202), reported, “Numbers in favorablelocations large; originally common, even‘abundant.’ Latterly becoming scarce in settledparts of the state.” Sams and Stott (1959:21)indicated, “Resident, formerly fairly common insuitable areas.” During the 1970s and early1980s, this species was still widespread in SanDiego County and colonies of 40-60 pairs couldbe found in Escondido and the Warner Ranch (J.Oakley, WRI, personal communication) withsmaller colonies of 30-40 pairs along the coast, onOtay Mesa, and along the Mexican border (basedon interviews with local biologists and birders).

The objectives of the subject project were to:• Determine the location of historical and

remaining breeding Burrowing Owlswithin San Diego County;

• Identify those conditions that seemed toinfluence county population trends; and

• Identify needed management, includingenhancement opportunities, and researchneeds.

STUDY AREAThe study area included all of San Diego County(approximately 6.67 million ha) located in thesouthwest corner of the state (Fig. 1). It has acoastal plain, with foothills, mountains, anddesert, respectively, to the east. The coastal area,where most Burrowing Owls have been located,ranged from sea level up to approximately 700m. Precipitation ranges from approximately fiveto 102 cm for the county and 25 to 46 cm per yearfor the areas west of the foothills (T. Oberbauer,San Diego County, personal communication).

METHODSInformation for this study came from fivegeneral sources: museum and other records; thescientific literature (Clark et al. 1997, Lincer andClark, this volume); outdoor-oriented public;local experts; and our own fieldwork. Wereviewed historical egg and skin collections atseveral museums (i.e., San Diego NaturalHistory Museum, Field Museum of NaturalHistory, National Museum of Natural History,American Museum of Natural History, WesternFoundation of Vertebrate Zoology, Santa BarbaraMuseum of Natural History, U.C. Museum ofVertebrate Zoology, California Academy ofSciences, Carnegie Museum of Natural History,University of Minnesota Bell Museum, andUCLA Dickey Bird and Mammal Collection). Weinterviewed key professionals in the San Diegoornithological community (see Acknowledge-ments). One of the authors (PHB), within abroader study of southwestern California raptorpopulations, surveyed the largest patches ofpotential Burrowing Owl habitat in the countylocated on Marine Corps Base Camp Pendleton(309,000 ha) annually between 1971-2004 andadjacent Fallbrook Detachment (21,876 ha)between 1994-2004. Finally, major sources of datafor this investigation came from studies within

FIGURE 1. Study area location in San Diego County,CA.

JEFFREY L. LINCER AND PETER H. BLOOM

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the MSCP, funded by the City of San Diego andthe California Department of Fish and Game(CDFG), which addressed the status of theBurrowing Owl (WRI 2003) as well as otherraptors within the MSCP (WRI 2002, 2004, 2005).

Because of patchy historical data sets formuch of San Diego County, we were limited topresenting estimates of the owl’s status for threeperiods: late 1800s/early 1900s, the late1970s/early 1980s, and recently (primarily 2000-2003).

RESULTSMuseum records from 10 institutions (spanningroughly 1880 to 1939) revealed 54 BurrowingOwl egg sets and 18 birds collected during thebreeding season in San Diego County (Fig. 2).Interviews with long-term resident biologists,historical checklists, and perspectives providedby authors (Garrett and Dunn 1981, Unitt 1984,and Zeiner et al. 1990) indicate that the

Burrowing Owl population was stillsubstantially healthy during the 1970s and early1980s (Fig. 3).

The last time period for which we have fairlycomprehensive San Diego County BurrowingOwl data is 2000-2003 (Fig. 4). During thisperiod, WRI conducted a series of raptorinvestigations (WRI 2002, 2003, 2004, 2005) andthe San Diego Museum of Natural Historyconducted part of its five-year study for aCounty Bird Atlas (Unitt 2004). During thiswindow, we estimate that 25-30 pairs ofBurrowing Owls remained in the county (seeupdate in Discussion).

DISCUSSION

HISTORICAL AND CURRENT DISTRIBUTION

Burrowing Owls have disappeared and/orpopulations declined in several southernCalifornia and San Francisco Bay counties and

FIGURE 2. Historical locations of breeding Burrowing Owls in San Diego County, CA.


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in coastal areas (DeSante et al. 1997, Klute et al.2003, Kidd et al. this volume). This trend is notlimited to California; as of 1992, 16 (67%) of 24states and provinces polled reported BurrowingOwl population declines and none reported anincrease (James and Espie 1997). As a result ofwidespread concern, this species was the subjectof two recent international symposia (Lincer andSteenhof 1997, Wellicome and Holroyd 2001) aswell as the 2003 California Burrowing OwlSymposium (this volume). Concern for thisspecies is not just local or even regional; it hasalso declined in several large regions, mostnotably the northeastern Great Plains andCanada (Klute et al. 2003).

The international trend is reflected by SanDiego County as a microcosm. References to“former breeding” areas by Unitt (1984:108) andAbbott (in Bent 1961:391) indicate thatBurrowing Owls were widespread in this countyand common, primarily from the coast and thegrassy interior, during the late 1800s and early

1900s (Fig. 2). Abbott described numerousBurrowing Owls still living on scattered vacantland amongst a growing City of San Diego in1921. This is consistent with observations ofStephens (1919:17), who stated that the owl wasa “…common resident in open ground from theseashore to the higher foothills.”

However, Grinnell and Miller (1944:202)noted that it was “...becoming scarce in settledparts of the state.” Although Burrowing Owlsare not common in natural deserts (D.Rosenberg, Oregon State University, personalcommunication), the lack of historical recordsfrom the desert may also reflect egg collectoraccess ability. Evidence for some desert-dwellingBurrowing Owls is supported by an overviewduring the 1950s by Sams and Stott (1959:21),who indicated that the species “Occurs in opencountry both in the coastal lowlands and thedesert (italics added).”

Although Unitt (1984) reported that theBurrowing Owl was still present in the 1970s in

FIGURE 3. Breeding Burrowing Owl locations in San Diego County, CA in the late 1970s and early 1980s.


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many areas, by the time he reviewed its status(1984:109), he considered it an “Uncommon anddeclining resident in grassland, agriculturalland, and coastal dunes” and went on to say,“...urbanization has greatly restricted the extentof suitable habitat [in San Diego County].” Atthat time, owls were known to have bred alongSan Diego’s coast, in the Ramona Grasslands(Loy 1986, Bloom 1994 in BFMA 1997), andalong a north-south line from El Cajon to justeast of San Ysidro (see Map 30 of Unitt 1984).

In the late 1970s and early 1980s, we estimatethere may have been 250-300 pairs welldistributed in suitable habitat throughout thecounty, based on our review (Fig. 3). Given thedocumented county trends in Burrowing Owlnumbers and associated owl habitat loss, therecould have been substantially more owls at theturn of the century.

Burrowing owls currently occupy somehistorical sites (e.g., North Island, south SanDiego coastal area, and Otay Mesa) (Unitt 1984)

although in much reduced numbers (Fig. 4).They are also present at some locations thatwere not included in Unitt’s 1984 review. Recentobservations of some individuals (e.g., onRancho Jamul [D. Lawhead, CaliforniaDepartment of Parks and Recreation, personalcommunication], and Rolling Hills, whereartificial burrows are showing some success [D.Mayer, California Department of Fish andGame, personal communication] may be areflection of increased management attentionand organized searching (i.e., through theCounty Bird Atlas and focused WRI raptorsurveys). Owls were clearly absent from manydeveloped places where they were formerlyfound (e.g., north-central county, coastal areas,including south of La Jolla, Point Loma, and thearea around the City of San Diego).

Surprisingly, even the relatively vast andpristine habitats of Camp Pendleton currentlysupport no breeding Burrowing Owls althougha mix of wintering and migrating individuals

FIGURE 4. Breeding Burrowing Owl locations in San Diego County, CA during 2000-2003.


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persist (PHB). The juxtaposed FallbrookDetachment population presently consists onlyof migrants (PHB, J. W. Kidd, Kidd BiologicalConsulting, personal communication). Theseobservations are consistent with those in Unitt(2004).

Burrowing Owl populations in San DiegoCounty are experiencing the same kind ofconsistent declines observed throughout thenortheastern Great Plains and Canada (Klute etal. 2003). In that regional decline factors varybased on specific local and regional populationstressors, it is unlikely that exactly the samefactors are responsible for declines in all affectedpopulations. It is equally likely, however, thatsome key factors were/are common to manydeclining populations.

The extirpation of the Camp Pendletonbreeding population is an interesting case studybecause it is a well documented, if poorlyunderstood, example of part of the San DiegoCounty population decline. Camp Pendletonand adjacent Fallbrook Detachment, at thenorthwestern corner of San Diego County, arelarge adjacent military reservations (52,000 and3,582 ha, respectively) with huge areas ofpotential Burrowing Owl habitat.

E. Harrison collected two egg sets in 1931from unknown locations on what was thenRancho Santa Margarita (E. Harrison, WesternFoundation for Vertebrate Zoology, personalcommunication). While this property was stillunder private ownership and management,considerably more of it was devoted toagriculture, particularly beans, (R.J. O’Neill,Rancho Mission Viejo, personal communi-cation). Given the propensity of someagricultural areas to support Burrowing Owls, itis likely that this breeding population was largerand more widely distributed historically thanwhen PHB first observed it in 1971 (see below).Historical evidence of breeding Burrowing Owlsis lacking for Fallbrook Detachment on CampPendleton’s eastern boundary despite a largeCalifornia ground squirrel (Spermophilusbeecheyi) population.

PHB surveyed Camp Pendleton during the1970s and documented approximately 15 pairs(Bloom 1980). Six of these were located inNovember Area at the south end of thereservation and nine widely distributed pairswere located in the X-Ray, Oscar, Kilo 1 and

mid-southern coastal zone of the Base (Bloom1980 and PHB, unpublished data). By the early1980s, Camp Pendleton’s breeding BurrowingOwl population dropped to only two pairs inKilo 1 and by the late 1980s only one breedingpair persisted (Fig. 2, PHB, unpublished data). Apair was last seen in 1994 (J. W. Kidd, KiddBiological Consulting, personal communi-cation). Appropriate Burrowing Owl nestinghabitat exists on the Fallbrook Detachment butno nests have been found. If breeding Burrow-ing Owls did exist on Fallbrook Detachment,they disappeared prior to 1993 (Bloom 1996).Thus, it appears that in the northwest portion ofthe county, including Camp Pendleton and theFallbrook Detachment, the Burrowing Owl hadbeen extirpated as a breeding species by 1995.

In 2003, based on the authors’ personalknowledge, there were 25-30 resident pairs ofBurrowing Owls in San Diego County locatedprimarily in the southern quarter of the countyand on North Island, which is due west of theCity of San Diego (Fig. 4). (Note: In 2006, anadditional 14 owl pairs were reported for theeastern Otay Mesa area (B. Jones, HELIXEnvironmental Planning, personal communica-tion) and two pairs successfully bred in theRamona Grasslands, which were from owls bredand released from WRI (JLL). It is possible thatthese owls were present in an area notpreviously surveyed or the result of recentreproductive success because of a break in arecent drought. Regardless of the reason, therecould currently (2007) be roughly 16 more owlpairs (for a total of 41-46 pairs) in the countythan we previously estimated.)

Although lack of data prevented exploring thehistorical wintering Burrowing Owl populationand trends, we can assume that all or most ofthe above resident pairs (and some of theirsurviving offspring) remain in the countyduring the winter, which would mean thatapproximately 148-168 local individuals winterin the county. These are, apparently, joined bymigratory wintering birds (JLL and PHB,personal observation) and Unitt (2004) also“...noted the burrowing owl in 20 [county bird]atlas squares where it evidently no longerbreeds.” We estimate that 50-100 of thesemigrant Burrowing Owls winter in, or migratethrough, Camp Pendleton and adjacentFallbrook Detachment alone. Given the


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distribution of suitable habitat and ourknowledge of wintering owls, another 100 owlslikely winter in San Diego County, for a totalestimate of approximately 300 to 370 owls thatcould winter here. However, data collected forthe County Bird Atlas (Unitt 2004) suggests thatthe wintering owl numbers may be lower.

REASONS FOR POPULATION DECLINES

Decreases in the county’s Burrowing Owlpopulation clearly coincide with increases in thehuman population according to the records ofthe San Diego Area Governments (SANDAG,Fig. 5). San Diego County experienced a majorhuman population increase around the City ofSan Diego in the late 1940s and early 1950s inresponse to the build-up in Navy personnelassociated with World War II. The county’shuman population continued to grow throughthe 1970s and early 1980s, coinciding with theconstruction of the interstate highway system.This facilitated movement into, and develop-ment of, previously inaccessible interior areas,converting easily developed grasslands intocitrus groves, other agricultural uses andhousing developments.

The history of several current and former owlcolonies provides insight into the reason for owlpopulation losses. To the extent that these typifythe environment faced by Burrowing Owlpopulations, colony history may shed light onwhat to expect in the future and what actionmay need to be taken.

North County. — One of the authors (JLL)visited many of the former Burrowing Owlcolony sites with J. Oakley, a biologist whoconducted Burrowing Owl studies in thenorthern half of the county in the 1970s and1980s. Several moderate size colonies (two tothree dozen pairs) persisted in his study areas(Oceanside, Carlsbad, Vista, San Marcos, WarnerRanch) through the late 1970s and into the early1980s (J. Oakley, WRI, personal communication).Many of these lingered for years in the urbanvacant lots between new development. Overtime, these colonies decreased in size andeventually disappeared. In some cases, thehabitat had been completely replaced by urbanand commercial uses, and in one case, by theexpansion of the Palomar Airport runway. Inother cases, the remaining habitat, althoughapparently viable was close to or surrounded by

FIGURE 5. San Diego County human population growth versus Burrowing Owl population estimates (humanpopulation data from San Diego Area Governments (SANDAG), San Diego, CA).

developed areas. In these cases, we concludedthat disturbance from human activity (e.g.,walking, jogging, off-road activity, dog walking) and loose and feral pets (chasing and,presumably, preying upon owls) would havemade the environment inhospitable for owls(Wesemann and Rowe 1985, Millsap and Bear2000). In still other locations, like the RamonaGrasslands, where Burrowing Owls previouslybred (Bloom 1994 in BFMA 1997, Loy 1986),habitat existing previously appears to be presentbut we observed no nesting owls and fewfossorial mammals. In one eastern county areawhere Oakley had worked, the Warner Ranch,only one owl pair remained in 2003 (Fig. 4).Habitat conditions appeared unchanged fromwhen approximately three dozen BurrowingOwl pairs were documented (Fig. 3, J. Oakley,WRI, personal communication) except there wasa complete absence of ground squirrels andother fossorial mammals (JLL).

Camp Pendleton. — The reasons for both earlyand even contemporary Burrowing Owldeclines on Camp Pendleton were likely in partdue to direct conversion of land uses from cattlegrazing and crop production to intense andlocalized military maneuvers, particularly in thelowlands and coastal areas. Camp Pendletonwas established in 1942 and has annuallyconducted considerable heavy track vehicleactivity, including amphibious landing vehiclesand tanks that may have crushed groundsquirrel and Burrowing Owl burrows and thuseliminated colonies of both species. A moresubtle loss of breeding habitat may haveresulted from the removal of intense cattlegrazing after the military took control thatprobably allowed tall non-native grasses (Avenaspp. and Bromus spp.) and sweet fennel(Foeniculum vulgare) to flourish.

Because of the introduction of exotic annualMediterranean grasses, dry climate, and militarytraining, Camp Pendleton probably burnedmore frequently than any other similar-sizedparcel of land in California. Occasional fires inBurrowing Owl habitat are probably a positiveevent, but at too high a frequency typeconversion and the effect on the prey base(especially arthropods, herpetofauna, and smallmammals) is probably detrimental and mayhave compounded the effects of militarymaneuvers on the species (PHB).

North Island and Other Military Bases. — NorthIsland supports one of two remaining BurrowingOwl colonies. It is located just west of the City ofSan Diego at the north end of a barrier peninsulathat forms the western edge of San Diego Bay.This Navy base has historically supported a largeBurrowing Owl population (Winchell 1994,Winchell and Pavelka 2004). During 1994–1995,at least 27 pairs were present (C. Winchell, U.S.Fish and Wildlife Service [USFWS], personalcommunication). According to B. Stewart(Wildlife Services, personal communication),there have been at least 14 pairs and/or activeburrows in recent years. In 1999, the number wasreduced to 11 breeding pairs (Winchell andPavelka 2004), but the population has decreasedsince then (WRI 2003, 2005). Even more recently(March 2002), during WRI’s field surveys, themaximum number of pairs, based on activeburrows, was between six and eight. The causalfactors responsible for this decline are not clear.However, predation by nesting PeregrineFalcons (Falco peregrinus) has been documented(C. Winchell, U.S. Fish and Wildlife Service,personal communication). In addition, theseowls are seen as a threat by those responsible forintensively managing nearby California LeastTern and Western Snowy Plover colonies. Forinstance, at least two Burrowing Owl breedingpairs that nested on the Ream Field AuxiliaryNaval Base (southern coastal San Diego County),but foraged on the adjacent Tijuana SloughNational Wildlife Refuge, were trapped andremoved because they were known predators ofCalifornia Least Terns on the refuge (PHB).

Under contract by responsible federalagencies, Burrowing Owls have been capturedand removed from federal lands from CampPendleton south to the Tijuana Slough NationalWildlife Refuge (PHB). Owls were either livetrapped and transported to the desert or, if un-trappable, they were shot or if injured duringtrapping, they were euthanized. Some wereheld in rehabilitation facilities until the end ofthe breeding season and hence produced noyoung that year and had dubious survivalpotential given inadequate release techniques(PHB). No reports of numbers taken are knownor have been provided by the agencies. A recentFreedom of Information Act request by theNational Audubon Society for informationrelated to the take of predators by Animal


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Services at California Least Tern and WesternSnowy Plover breeding sites was promised butnever provided (P. DeSimone, NationalAudubon Society, personal communication).

Management for the California Least Tern andWestern Snowy Plover (and, if continued, againstthe Burrowing Owl) by government agencies islikely to be a significant causative factor in thecontinuing decline of the Burrowing Owl on theSan Diego County coastline. Even if owls aretrapped from locations other than North Island(or any other breeding area), any reduction in thearea’s population has potentially importantimpacts to overall recruitment. Finally, as Navyfacilities expand and human activities increase,the normal tolls of habitat loss, associated preyreduction, and human disturbance will beexacted on this dwindling population. Althoughthe Navy may have, for many years, protectedthis colony from the developmental impacts thatcharacterized the mainland (because importanthabitat remained undisturbed at low levels ofmilitary activity), current and future expansionmay simply cause/continue a delayed negativeeffect.

Otay Border Area. — The Otay border area(several hundred ha), including Otay Mesa,supports the other remaining colony (HELIX2001, Recon 2001, EDAW 2001, Unitt 2004, andJLL). This area abuts, and is just north of, theMexican border fence at and to the east of theOtay Border Crossing and U.S. Customs Facility.This area on eastern Otay Mesa contained,perhaps, the largest remaining Burrowing Owlcolony in San Diego County in 2003. This scrubhabitat is kept sparsely vegetated by BorderPatrol activities and fires (Unitt 2004). It hasrecently supported as many as 22 owls (JLL),representing six to seven pairs and their young.It not only supports a relatively large, andperhaps the last, Burrowing Owl “colony” in theMSCP, but also numerous other sensitive species(WRI 2003).

In the winter of 2001/2002, a relatively largenumber of Burrowing Owls (>12) wintered at theabove site in open scrub and short grass habitat(JLL). By summer of 2002, the area had beenturned into a construction site lay down area for aborder culvert project (JLL). The habitat had beencleared, large equipment parked, and numerouslarge culverts stored on the owls’ wintering andbreeding site. This area is part of a County

Specific Plan, which portends additional majorland use changes including the construction of atriple border fence. Without proper environ-mental review, lead agency action, and creativemanagement, this large remaining BurrowingOwl population will be lost in the near future.

There are also currently several pairs ofBurrowing Owls along a narrow strip of land,near the Customs Facility, which is patrolled bythe Border Patrol. The owls along the border,and within a fenced and patrolled area, aresurviving in this internationally protected zonebut their breeding status is unknown.

Additional scattered owl pairs have breed onOtay Mesa, including on the Navy’s SatelliteSurveillance Station immediately north ofBrown Field approximately 17 km northwest ofthe Custom Facility (WRI 2003). In addition,habitat loss and human disturbance, includingvehicle traffic, decreases habitat values for thesebirds. Some owl habitat there, including a parcelowned by the City of San Diego, has beendevastated by off-road vehicle activity. Plans toexpand the Navy’s Satellite Surveillance Stationcould also affect the remaining two BurrowingOwl pairs within and adjacent to the station.

CONSERVATION IMPLICATIONSDevelopment and associated land use changeshave been responsible for the decline of theBurrowing Owl in San Diego County. Many ofthese factors are also operating over a largergeographic scale (Lincer and Steenhof 1997,Klute et al. 2003).

• Major factors causing local Burrowing Owldeclines include:- Direct loss of suitable habitat from

private and public development.- Habitat type conversion.- Loss of ground squirrels and other

fossorial mammals as a result of control(i.e., poisoning) efforts, habitat loss, andpredation by pet and feral dogs and cats.

- In the coastal region, conflicts with themanagement of listed species, like theCalifornia Least Tern and WesternSnowy Plover.

- Owl mortality and disturbance from petand feral dogs and cats.

• Modifying factors:- Habitat fragmentation and loss of habitat


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connectivity (James et al. 1997).- Disturbance caused by people hiking,

jogging, and walking dogs. • Possibly important factors:

- Vulnerability of small colonies tostochastic events (Johnson 1997).

- Border Patrol activities may have bothnegative impacts (on habitat andburrows) and a positive impact throughthe protection provided by securityfencing.

- Altered top predator dynamics that mayhave led to increased populations ofmeso-predators: e.g., coyotes (Canislatrans), badgers (Taxidea taxus), foxes(Urocyon cinereoargenteus), bobcats (Lynxrufous) and some other species, likeraccoons (Procyon lotor), opossums(Didelphis marsupialis), and skunks(Mephitis mephitis and Spilogale putorius).

The primary threats to the remaining smallBurrowing Owl population in San Diego Countyare:

• Inadequate habitat acreage and fragmen-tation of remaining suitable habitat.

• Conflicts with management of listed species(i.e., the California Least Tern and WesternSnowy Plover) in the coastal region.

• Disturbance by recreational vehicles andconstruction equipment.

• Inadequate regulatory protection and thelack of a comprehensive management planto reverse the ongoing declines.

Unnatural levels, and kinds, of predation willcontinue to play a role in the demise ofBurrowing Owls. Prey availability may belimiting in certain locations. Small colonies andscattered pairs will continue to be vulnerable tostochastic events.

Resource management needs include (1)understanding the status and trends of theBurrowing Owl population and (2) identifyingdemographic and other environmental factorsthat influence those trends.

We recommend the following to enhanceBurrowing Owl conservation in San DiegoCounty:

• Yearly monitoring of known BurrowingOwl locations in the county should beconducted until the current populationdecline is accepted as irreversible or thetrend is reversed and numbers stabilize. We

recommend monitoring occupancy, nestingsuccess and productivity of all knownbreeding pairs using consistent method-ology (Steenhof 1987, CBOC 1997).

• Quantify the relative importance ofpredators, disturbance, prey base, andminimal colony size, to the success ofBurrowing Owl colonies.

• Using appropriate pilot studies, develop aproactive approach to Burrowing Owlrelocation. Identify important site criteriaand potential receiver sites, examine likelycarrying capacity, and (if necessary)augment the sites’ ability to support owlsbefore relocating owls to those locations.Potential sites include both public andprivate parcels. Site improvements mayinclude habitat management andmanipulation, security, the installation ofartificial nest burrows, prey management,and introduction of fossorial mammals.There should be a cooperative effortinvolving all governmental levels andagencies (including resource agencies, theDepartment of Defense, HomelandSecurity, water authorities, and airportauthorities), as well as the private sector.

• Establish a burrowing owl captive breedingand reintroduction facility. Experimentalefforts at WRI (JLL) and Naval WeaponsStation, Seal Beach (PHB) have demon-strated success where captive bred andreleased owls have produced eggs andyoung in artificial burrows. Although pastBurrowing Owl reintroduction results havebeen mixed (Martell et al. 2001), successrates have been improved through carefulplanning and timely execution (Leupin andLow 2001). Every opportunity to increasesuccess rates should be explored if owlpopulation trends are to be reversed. Areintroduction program would substan-tially improve the chances of this speciespersisting in San Diego County. A breedingfacility could serve numerous purposesincluding (1) receiving and safely holding“problem” Burrowing Owls (e.g., thosetrapped at California Least Tern colonies),(2) receiving/holding injured, butunreleasable, potential breeding birds, (3)public education, and (4) providing asource of young birds and/or family units


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for reintroduction or augmentation ofextant colonies (Barclay 1987).

• Develop alternatives to trapping and re-locating or euthanizing Burrowing Owls inorder to protect least terns or othersensitive species, such as incorporatingthem into a captive breeding and reintro-duction program.

• Develop a comprehensive and coordinatedSan Diego County Burrowing OwlManagement and Monitoring Plan,involving all relevant parties. Publiceducation should be incorporated into thisproactive effort. Programs involvingprivate landowners elsewhere have beensuccessful and provide a good model(Hjertaas 1997). This could includeestablishing a Burrowing Owl Hotline thatcould quickly connect individuals withchallenging management needs with thosewho can provide timely solutions.

• Because (a) this species has been extirpatedin much of its former range, (b) it continuesto decline in most areas, (c) most (>70%) ofthe remaining state owl population isconcentrated in Imperial Valley livingunder man-made conditions subject tochange, and (d) it appears to get limitedprotection from changing land uses becauseit is not listed under state or federalendangered species acts, some increasedlevel of regulatory protection may be inorder. One alternative may be listing inspecific areas, such as San Diego County.

The San Diego County owl breeding popu-lation has decreased approximately 90 percentfrom what it was 25 to 30 years ago. It thennumbered in the hundreds of pairs and at thetime of this symposium it was reduced to nomore than perhaps 25-30 resident pairs. Evenwith the recently observed additional East OtayMesa and Ramona Grasslands owls, there arenot likely more than 46 pairs in the county. Ifcurrent threats continue and a comprehensivemanagement program is not implemented soon,the extirpation of the Burrowing Owl in SanDiego County is imminent.

ACKNOWLEDGEMENTSWe would like to thank the City of San Diego, theCDFG NCCP Local Assistance Program, and WRI

for funding. H. Cheong (City of San Diego) wasespecially helpful in the set-up, management, andsupport of our efforts. Thanks to J. Barclay, T.Roberts, and K. Hunting for reviewing the manu-script, and R. Zembal, K. Keane, B. Johnson, P.Unitt, D. Bittner, J. Hannan, L. Trulio, G. Holroyd,and D. Rosenberg for reviewing earlier versions ofthis manuscript, and R. Clark for assistance withthe literature. J. Kidd assisted with the collection ofmuseum data and provided field observations. P. Unitt (San Diego Natural History Museum) wasvery helpful by providing the database onBurrowing Owl observations and associated mapsfrom the San Diego County Bird Atlas Project.Many individuals, in numerous agencies andorganizations, provided assistance: W. Berry(M.C.B. Camp Pendleton); R. Knight (FallbrookDetachment); T. Oberbauer (San Diego County); B.Collins, M. Alfaro, L. Warner, K. Shampaine, C.Winchell (USFWS); D. Mayer, D. Lawhead, T.Stewart (CDFG); R. Fisher, M. Mendelsohn, M.Fuller, M. Kochert (USGS); J. Wells (U.S. ForestService); J. Caruso, J. Weber (Otay Lakes CityWater Supply); J. Miller (Center for BiologicalDiversity); J. Jehl (formerly with Hubbs-Sea WorldResearch Institute); C. Gailband (Chula VistaNature Center); P. Famolaro (SweetwaterAuthority); R. T. Donalson (Chugach Telecommu-nications & Computers, Inc.); T. Johniken, D.Larson, S. Barnhill, T. Conkle (U.S. Navy); B. R.Bonesteel, B. Stewart (USDA, APHIS-WildlifeServices); G. Holroyd, T. Wellicome (CanadianWildlife Service); R. Barber, R. Jones (PalomarAudubon Society); D. Bothe (Pardee Homes); S.Taylor (HELIX Environmental Planning); C.Schaefer (EDAW/ TAIC); J. Oakley, Co-Director,WRI Eagle Project, and volunteers from the WRIEagle Project and WRI’s Hawk Watch. Finally, wewould like to thank all the participants in theBurrowing Owl listserv ([email protected])for responding to our requests for information.

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THE STATUS OF THE BURROWING OWL IN SAN DIEGO ......Burrowing Owls were widespread in this county and...

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