WESTERN BIRDS Volume 20, Number 1, 1989 DIETS OF FIVE SPECIES OF DESERT OWLS 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 eleva- tion, 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. ß•r COARARONBARN-OWL. ß LONG-EARED OWL V GREATHORNED OWL ß WESTERNSCREECH-O#L o BURROWINGOWL N H G F Mojave Desert ColoradoDesert 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• • 00•0•0000000 • 00•0•000000 • 00••000•0 • 000•0000000 • 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 80 70 60 • ,A,ß COMMONGREAT HORNEDBARN-OWL OWL 50- _, 30- z lO Perognathus 80- 70- 60- 50- 40- 30 20- 10- A S 0 N D J F MA M J J A S 0 N D J FM AM J J A 1•6 MONTH 198• Figure2. Monthlyfrequencies of kangaroorats and pocketmice in the dietsof three owl species in the Coachella Valley. 7 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