Diets of Five Species of Desert Owls

Home , Barn owl, Burrowing owl, Horned owl, Perognathus

WESTERN BIRDS

Volume 20, Number 1, 1989

CAMERON W. BARROWS, 53277 Avenida Diaz, La Quinta, California 92253

Common Barn-Owls(Tyro alba), Great Horned Owls (Bubo virginianus), Long-earedOwls (Asio otus), Western Screech-Owls(Otus kennicottii),and BurrowingOwls (Athene cunnicularia)all occur as year-round or seasonal residentsof southernCalifornia deserts. This speciesrichness provided me an opportunityto compare the diets of these owls both within and between desertregions. Within-region comparisons allowed analyses in situationswhere all owl specieshad accessto the sameprey base,eliminating differential prey availabilityas a factor.Interspecific differences under these conditions might then be due to differencesin predatorybehavior or prey preferences.Addi- tionally,I was ableto followmonth-to-month variation in dietsof threesym- patricowl speciesto determineif theirdietary fluctuations were synchronous or independent.

STUDY AREA AND METHODS

Dietswere determinedfrom the contentsof regurgitatedpellets found below the owls'diurnal roosts or burrowentrances. With the exceptionof one Long- eared Owl samplefrom Anza-BorregoState Park in 1980, the pelletswere collectedfrom December1985 throughthe summerof 1988. Unlessother- wise noted, the collectionsrepresent one visit to each site. A few siteswere occupied nearly continuously, allowing temporal analysis of theirinhabitants' diets. These roosts were located primarily in the Coachella ValleyPreserve, a 5260-ha naturalarea in RiversideCo., California(0 to 100 m elevation). This area is in the Colorado Desert subdivisionof the Sonoran Desert(Jaeger 1957) and is dominatedby CreosoteBush (Larreadioaricata) scruband scatteredDesert Fan Palm (Washingtoniafilifera) oases. Thousand, Horseshoe,Indian, and BiskraPalms are found within this preserve.Other samplesfrom within the CoachellaValley were taken at the Universityof Cali- fornia'sBoyd Deep CanyonResearch Center and SevenPalms oasis. Except for thoseof BurrowingOwls, 85% of thesesamples were collected in Desert Fan Palm oases.All thesecollection sites were dominatedby naturalvegeta- Western Birds 20:1-10, 1989 1 DIETS OF DESERT OWLS tion.For comparison, pellets were also gathered in areasthat included irrigated farmlands and downtown Indio. Additionalsites outside the CoachellaValley but withinthe ColoradoDesert includedtwo locationsnear the SE shore of the Salton Sea, Imperial Co. (-65 m elevation, Common Barn-Owl and Great Horned Owl), in the OrocopiaMountains, Riverside Co. (300 m elevation,Common Barn-Owl), at YaquiWell in Anza-BorregoState Park, San Diego Co. (300 m elevation, Long-earedOwl) and in ChemehueviWash, San BernardinoCo. (200 m elevation, Long-earedOwl). Three sitesoccupied by Common Barn-Owlswere at intermediateeleva- tions (650-900 m) and supportedvegetation common to both the Colorado and Mojave deserts.These sitesincluded the Oasisof Mara in Twentynine Palmsand Morongo Valley, both in San BernardinoCo., along with a loca- tion near Corn Spring, RiversideCo. The Mojave Desertsites (1000 to 1600 m elevation)were dominatedby JoshuaTrees (Yucca brevi.folia) and MojaveYucca (Y. schidigera).Common Barn-Owl diet sampleswere collectednear Cima Dome and in the Lanfair Valley,San BernardinoCo., and at Devil'sPunch Bowl, LosAngeles Co. A Long-earedOwl diet samplewas collectednear China Lake, Kern Co. Loca- tions of all sitesare shownin Figure 1.

RESULTS

SpeciesAccounts COMMONBARN-OWL. This year-round resident was commonly found roostingin palm oases,mine addits,and cliffs.At all sitessampled within the ColoradoDesert, pocket mice (Perognathussp.) were the mostcommon prey in Common Barn-Owl diets (Table1). This patternheld both betweensites and between years (at ThousandPalms Oasis), indicatinga regionaldiet similarity.In the MojaveDesert, and at two of the sitesat intermediateeleva- tions,kangaroo rats (Dipodomys sp.) dominated this owl's diet, again showing a regionalsimilarity. Common Barn-Owl dietsfrom Morongo Valley varied betweenyears from a preponderanceof pocketmice to a nearly equal pro- portionof pocketmice and kangaroorats, illustrating Morongo's intermediate position.Irrigating or urbanizingthe desertappears to resultin an increased frequencyof rodentsadapted to mesic conditionsin the owls' diet. House Mouse (Mus rnusculus)and Pocket Gopher (Thornornysbottae) remains becamemore abundant,while pocket mice and especiallykangaroo rats were reduced. Despitediet similaritieswithin regions and withinhabitats, there was a high degree of month-to-monthvariation at Thousand Palms Oasis (Figure 2). Throughoutmy study,the frequencyby monthof pocketmice or kangaroo rats varied from above 70% to near 0%. LONG-EAREDOWL. Theseowls are primarilywinter residents; most arrive in Octoberthrough December and leavethe area by March. However,a few stayto breed,with recordsfrom YaquiWell and MorongoValley. These owls occasionallyroost in palms,but were morecommon in PaloVerde (Cercidiurn floridurn),Ironwood (01neya tesota), and tamarisk(Tamarix sp.) trees in oases or desert washes. DIETS OF DESERT OWLS

Pelletanalyses indicated temporal shifts in Long-earedOwl prey selection (Table2). Diets from the CoachellaValley, in the winter of 1985-86, were dominatedby pocketmice (,52%);in 1986-87 kangaroorats predominated (72%); in 1987-88 pocketmice againdominated (67%). All sitesand years combined,this owl and Common Barn-Owls from the Colorado Desert had the highestpercentage of similarity,89%, of any owl speciespair in thisstudy. Percentageof similarityis the sum of the smallerfrequencies in all prey cate- goriesfor the two dietsbeing compared (Brower and Zar 1977). The result of this comparisonindicates that these owls have similarprey selection capabilities.However, month-to-monthcomparisons of adjacentCommon Barn-Owlsand Long-earedOwls at ThousandPalms (Figure 2) revealedstrik- ing differencesin their temporalpatterns of predationon pocket mice.

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Figure1. Siteswhere owl pelletswere collected.A = CoachellaValley; B -- Anza- BorregoState Park; C -- OrocopiaMountains; D -- Corn Spring/ChuckwallaMoun- tains;E -- ChemehueviWash; F = Lanfair Valley; G -- Cima Dome; H •- China Lake; I = Morongo Valley; J = TwentyninePalms; K -- Devil'sPunchbowl. 00•0•00000• •

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DIETS OF DESERT OWLS

GREAT HORNED OWL. Thisresident owl wasregularly found in two palm oasesI surveyedin the CoachellaValley. An additionalpair was in an aban- doned building on the SE shore of the Salton Sea. Great Horned Owls selectedthe broadestrange of prey sizesof any of the speciesconsidered here, taking large numbers of Audubon Cottontails (Syivilagusaudubonii) and woodrats (Neotoma sp.) along with arthropods, especiallyscorpions (Table 3). Overall diets of Great Horned Owls in the CoachellaValley had a moderatepercentage of simfiaritywith Common Barn- Owls (66%) and Long-earedOwls (65%) from the same region. WESTERN SCREECH-OWL. Western Screech-Owlsare year-round residentsof the palm oases.Large pellet accumulationswere locatedin three oases,although a few screech-owlpellets were found in almostevery palm oasisexamined. This owl'sdiet seemsto reflectthe availabilityof prey in and aroundthe palms (Table3). Deer mice (Peromyscussp.) and Spiny Pocket

Dipodomys 90

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Perognathus

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Figure2. Monthlyfrequencies of kangaroorats and pocketmice in the dietsof threeowl species in the Coachella Valley.

DIETS OF DESERT OWLS

Mice(Perognathus spinatus) are especially numerous in palmoases (pers. obs.); their abundanceappears to be reflectedin the WesternScreech-Owrs diet. BURROWING OWL. This year-roundresident was the only speciescon- sideredin thisstudy that wasnot at leastpartly associated with palm oases. BurrowingOwls are the mostinsectivorous of the fiveowl species(Table 3); one of their mostcommon prey was earwigs(Dermaptera). The similarlysized Burrowing Owl andWestern Screech-Owl were the most allopatricof anyspecies pair considered here; the percentageof similarityin theirdiets was 67%. BurrowingOwls were found lower on the alluvialfans, far from the palm oases. Two Common Barn Owl roosts,a Great Horned Owl roost, and a Long- eared Owl communal roost (the number of owls varied between 2 and 15) werein useregularly enough to allowa monthlydiet analysis (Figure 2). These roostswere about 1 km apart, separatedby a large washthat was a likely huntingarea for all threespecies. Over the 21-monthstudy, there was little if any similarityin the frequenciesof pocketmice taken by the owl species. The frequencyoscillations of kangaroorats in the dietsof CommonBarn- Owlsand GreatHorned Owls, the speciespair with the mostsimilar diet fluc- tuationsfor this prey, were synchronousonly 65% of the time.

DISCUSSION

Regionaland habitat-relatedsimilarities of CommonBarn-Owls in the Col- orado and Mojave Desertslend supportto a hypothesisthat owls'diets are a directreflection of prey availability(Errington 1932). Dietary differences betweenthe high and low desertsmay be explainedin part by the limited seasonalavailability of pocketmice in the MojaveDesert. In the Mojave,the mostcommon pocket mouse, Perognath us Iongimembris,hibernates during the wintermonths (Ingles 1965) and istherefore unavailable to the owlsmuch of the year.The ColoradoDesert has more pocket mouse species (Ingles 1965), manyof whichare activeyear round. Thesepocket mice are availablein all seasonsand are an inte•al part of the Common Barn-Owl'sdiet. Similarly, woodratlodges were observedto be much more abundantwithin the ranges of thoseCommon Barn-Owlsin the Mojave Desertthat took a highpercent- age of this rodent in their diets. Other studieshave alsodocumented a correlationbetween prey availability and owl diets.Schwartz and Bleich(1985) found that the proportionsof prey speciesin the dietsof CommonBarn-Owls at two Californialocations roughly matchedthe frequenciesof thoseprey in the surroundingenvironment. While their data is suggestiveof Common Barn-Owl predatorypatterns, they did not providea temporalanalysis to determinewhether the owlsactually tracked prey availability.Broad temporalrelationships were reportedby Evansand Emlen (1947), Fitch (1947), and Marti (1974). These studiesdocumented the seasonalavailability of certainprey matchingtheir occurrencein the owl diets;again, they didn'texamine the tendencyof owlsto trackthe abundance of those prey. In my analysis,I comparedthe dietsof threeowl specieshunting over the same time and spaceto interpretpredation patterns. The overallsimilarity DIETS OF DESERT OWLS in the dietsof Long-earedOwls, Common Barn-Owls, and, to a lesserextent, Great Horned Owlsindicates similar predatory capabilities and predilections. Month-to-monthcomparisons of the threeowl species'predation on pocket mice,the mostcommon prey (overall)for eachspecies, indicated no consis- tent trackingof availability.For Common Barn-Owlsand Long-earedOwls, there were strikingdifferences between the monthly ratesof predationon pocketmice and to a lesserextent on kangaroorats. The large proportion of kangaroorats in many Long-earedOwl pellets (duringthe winter of 1956-$7) from the CoachellaValley confirm that theserodents were avail- ableto CommonBarn-Owls. Yet, duringthat period,barn-owls took approx- imatelytwice as many pocketmice as kangaroorats, even when kangaroo rats constitutedas much as 72% of neighboringLong-eared Owls' diets. Geographicand habitat-relatedpatterns in desertowl dietscan be explained by prey availability.However, prey availabilityalone appearsto fall shortof explainingthe monthlyvariation of the owl dietsreported here. Selective preda- tionappears likely, and competitive interference and differences in habitatselec- tion may have influencedthe observeddiets.

ACKNOWLEDGMENTS

PeteBloom, Bob McKernan,and Allan Muth aidedin locatingLong-eared Owl pellets. Katherine Barrows,Pete Bloom, Mark Fisher, Tim Manviis, Allan Muth, Bruce Webb, and Jon Winter offeredsuggestions to improvedrafts of my manuscript.The Nature Conservancy,Bureau of Land Management,Joshua Tree National Monument,U.S. Fishand WildlifeService, California Department of Fishand Game, and Boyd Deep Canyon ResearchCenter generouslyallowed access to their facilitiesand land.

LITERATURE CITED

Brower,J. E., and Zar, J. H. 1977. Fieldand LaboratoryMethods for GeneralEcology. Win. C. Brown, Dubuque, IA. Errington,P. L. 1939-.Studies on the behaviorof the Great Horned Owl. WilsonBull. 44:212-220. Evans,F. C., and Emlen, J. T. 1947. Ecologicalnotes on the prey selectedby a Barn Owl. Condor 49:3-9. Fitch, H. S. 1947. Predationby owls in the Sierran foothillsof California. Condor 49:137-151. Ingles,L. G. 1965. Mammalsof the PacificStates. Stanford Univ. Press,Stanford, CA. Jaeger,E. C. 1957. The North AmericanDeserts. Stanford Univ. Press,Stanford, CA. Marti, C. D. 1974. The feedingecology of four syrnpatricowls. Condor 76:45-71. Schwartz,O. A., and Bleich,V. C. 1985. Optimalforaging in Barn Owls?Rodent fre- quenciesin diet and fauna. Bull. S. Calif. Acad. Sci. 84(1):41-45. Accepted 1 November 1988

10 Thefollowing article is the third in a serieson Californiararities edited by Morlan and Roberson.It isbased on materialssubmitted to the CaliforniaBird Records Committee(CBRC). The descriptionand circumstanceswere edited from the accountsof the observersand havebeen reviewed by them.Roberson prepared the distributionalsummary and Morlan preparedthe identificationsummary. In thisway we hopethat muchimportant information accumulated in CBRC files will become widely available.

Common Black-Hawk Sketch by Tim Manviis

FIRST RECORD OF THE COMMON BLACK-HAWK FOR CALIFORNIA

BRIAN E. DANIELS, 3471 Lama, Long Beach, California 90808 LOREN and DEBRA HAYS, 9896 Argyle, HuntingtonBeach, California92646 JOSEPH MORLAN, 417 TalbotAve., Albany, California94706 DON ROBERSON, 282 Grove Acre Ave., Pacific Grove, California 93950

On 13 April 1985, Danielsand the Haysesfound a Common Black-Hawk Buteoga!iusanthracinus at ThousandPalms Nature ConservancyPreserve, RiversideCounty, California, a lushCalifornia Fan PalmWashingtonia filifera oasiswith a creeklined by Fremont CottonwoodsPopulus fremontii. They firstsaw the hawkat 0700 flyingdownstream about 50 yardsaway. A Common RavenCorvus corax whose nest was nearbyharassed it. The hawk flew back overthe observers(within about 50 feet) and disappearedinto the palmtrees. At about0800 they sawit againabout a half mile upstream.It flew pastthem and then circled higher and higher, disappearingtoward the north. They observedthe bird for a total of about 15 minutes.Efforts to follow it by car were unsuccessful.They, and others,searched for the hawk later that day, but did not find it. The followingdescription was compiled from thoseof the threeobservers: A buteoninehawk aboutthe sizeof a Red-tailedHawk Buteojamaicensis (not directly comparedbut soaringin the samegeneral area) but differingby itsvery broad rounded wingsand short wide tail. It was perhapsthe same length as the adjacentraven but wasnoticeably bulkier and soaredwith a flat flightprofile. Western Birds 20:11-18, 1989 11