128 DOUGLASA. BOYCE,JR. VOL. 19, No. 4 Adult male Prairie Falconand youngat a typicalMojave Desertnest-site. Artwork by N. John Schmitt. PRAIRIE FALCON PREY IN THE MOJAVE DESERT, CALIFORNIA DOUGLASA. BOYCE,JR. ABSTRACT-- Twenty-fivespecies of birds,9 speciesof mammals,5 speciesof reptilesand 1 speciesof insectwere representedin prey remains and castings from 19 Prairie Falcon (Falco mexicanus) nests in theMojave Desert, California, during1977 and 1978. Reptiles represented a greater proportion in thediet than is reported in mostother Prairie Falcon food studiesin the westernUnited States.The Horned Lark (Eremophilaalpestris), Mourning Dove (Zenaidura macroura), ValleyPocket Gopher (T}wmomys bottae) and DesertWoodrat (Neotoma lepida) were found in over50% of the nests. Eighty-fourpercent of theprey weighed less than 150 g. The mean prey weight was 107 g and equals 20% of theweight of male Prairie Falcons. The Prairie Falcon (Falcomexicanus)is considered sionallyCollared Lizard (Crotaphytuscollaris), while a generalistin prey selection(Bent 1938: Part 2). Bond (1936) reported exclusivelymammalian prey Mammals and birds are the most common prey in 41 castingsat another Mojave Desert nest. Be- taken, with specificprey frequenciesvarying re- cause little is known about food habits of Prairie gionally(Tyler 1923; Fowler1931; Enderson 1964; Falconsin the Mojave Desert, I studied this aspect Brown and Amadon 1968; Leedy 1972; Ogden of their biology. 1973; Denton 1975; Haak 1982). Reptilesand in- METHODS sectsare rarely recordedas prey (Table 1), although Prey remainsand castingswere collectedfrom 19 falcon nests Snyder and Wiley (1976) reported an unusualre- throughoutthe MojaveDesert (34 ø N 116øW)between March and lianceon insectsfor food. Typesof prey selectionby June 1977 and 1978 in order to providea qualitativesummary of Prairie Falconsnesting in the Mojave Desert con- Prairie Falcon food habits. Prey remains and castingswere also trastsharply with prey previouslyrecorded for this collectedfrom immediately below the nest site when known to area. Pierce (1935) and Fowler (1935) reported a havecome from no other raptor. Prey remainswere identifiedin the field or comparedwith specimensat Humboldt StateUniver- nestin the MojaveDesert where young Prairie Fal- sity, Arcata, Califbrnia. Fresh weights tbr prey items were ob- cons were raised entirely on a diet of reptiles -- tained from the Museum of Vertebrate Zoology, University of mainly Chuckwalla(Sauromalus obesus) and occa- California,Berkeley, California. I usedan adjustedweight Table 1. Frequency(c•)of birds, mammals,reptiles and insectprey remainsin Prairie Falconnests in the western United States. SOURCE LOCATION MAMMALS BIRDS REPTILES INSECTS Fowler ( 1931) Califbrnia 30c 70 0 0 McKinleyun publ. a Colorado 55 45 0 0 Marti and Braun (1975) Colorado 39 61 0 0 Ogden (1973) Idaho 53 33 14 trace S.R.B.P.b (1979) Idaho 22 72 6 0 Platt (1974) New Mexico 37 54 9 0 Voelkerunpubl. a Oklahoma 8 92 0 0 Porter and White (1973) Utah 8 92 0 0 Smith and Murphy (1973) Utah 31 50 0 19 This Study Mojave Desert 52 38 10 trace adatafrom Sherrod(1978:96, 97) bSnakeRiver Birds of Prey annual report 1979 Croundedto nearest 1% 129 RAPTORRESEARCH 19 (4):128-134 130 DOUGLASA. BOYCE,JR. VOL. 19, No. 4 of 500 g for rabbit speciesin Table 2 because,from the available observationsof encounterswith someprey speciesmay be difficult information,it is unlikelyPrairie Falcons are capableof carrying to make. Prey remains may also underestimatethe numbers of anything heavier (seediscussion below). smallrodents and birdsactually captured (Cade 1960).Because of Quantifying prey remainsand castingscollected from hawk numerouspotential biases, food habitsreported here are qualita- nestsduring the nestingseason is biased and unreliable(Errington tive not quantitative. 1932; Craighead and Craighead 1956). Somecastings and prey Delineationof the Mojave Desertboundaries closely parallels remains may be depositedby falconsbefore nestingbegins and the outer distributionallimits of theJoshua Tree (Yuccabrevifolia) persistuntil collectionduring the nestingseason. Furthermore, (Jaeger1957). Cresote(Larreadivaricata) and Burro Bush(Fanseria Fowler(1931) reported that adult Prairie Falconsremove uneaten dumosa)are also characteristic desert flora. Alkali sinks, creosote prey and castingsfrom nests.Haak (1982) reported that a larger bushscrub, shadscale scrub, Joshua Tree woodland,and Pinyon- variety of prey was found at neststhan was hunted, suggesting Juniper woodland form the major Mojave Desert floral com- over-representationof uncommonlyused prey at nests;however, munities (Munz and Keck 1959). Table 2. Prey items identified at Prairie Falconnests in the Mojave Desert. No.a %b ESTIMATED SPECIES NESTS NESTS WEIGHT(g) MAMMALS California GroundSquirrel (Spermophilusbeecheyi) 2 10.5 565 Mojave Ground Squirrel (Spermophilusmohavensis ) 5 26.0 177 Whitetail Antelope Squirrel (Ammospermophilusleucurus ) 4 21.0 113 Valley PocketGopher (Thomomysbottae ) 12 63.0 88 Pocket Mouse (Perognathu•sp.) 5 26.0 19 Kangaroo Rat (Dipodomysp.) 5 26.0 41 Deseret Woodrat (Neotomalepida) 11 58.0 105 Black-tailedJack Rabbit (Lepuscalifornicus) 2 10.5 500 Desert Cottontail (Sylvilagusaudubonii) 2 10.5 500 BIRDS Chukar (Alectorischukar) 3 15.8 500 Western Sandpiper* (Calidrismauri) 1 5.3 27 Rock Dove (Columbalivia) 2 10.5 393 Mourning Dove (Zenaidamacroura) 8 42.1 109 White-throateed Swift (Aeronautessaxatalis) 1 5.3 36 Western Kingbird (Tyrannusverticalis) 2 10.5 41 (Table 2 continued) WINTER 1985 PRAIRIEFALCON PREY IN CALIFORNIA 131 (Continuation of Table 2) BIRDS (cont'd) Say'sPheobe* (Sayornissaya) 1 5.3 25 Horned Lark (Eremophila alpestris ) 12 63.2 28 Cactus Wren* (Campylorhynchusbrunneicapillus) 1 5.3 37 Rock Wren (Salpinctesobsoletus) 3 15.8 11 SageThraasher* (Oreoscoptesmontanus) 1 5.3 44 LeCont's Thrasher* (Toxostomalacontei) 1 5.3 62 Mountain Bluebird (Sialia currucoides ) 1 5.3 2.7 LoggerheadShrike (Laniusludovicianus) 1 5.3 45 European Starling (Sturnusvulgaris) 2 10.5 77 Black-headed Grosbeak* (Pheucticusmelanocephalus) 3 15.8 46 White-crowned Sparrow (Zonotrichialeucophrys) 1 5.3 2 Western Meadowlark (Sturne lla neglecta) 5 26.3 103 Red-wingedBlackbird (Agelaiusphoeniceus) 2 10.5 56 Brewer's Blackbird (Euphaguscyanocephalm ) 3 15.8 58 Scott's Oriole* (lcterusparisorum ) 2 10.5 38 Northern Oriole* (Icterus galbula ) 1 5.3 27 Western Tanager (Pirangaludoviciana ) 4 21.1 31 House Sparrow (Pas, er domesticus) 4 21.1 27 House Finch (Carpodacusmexicanus) 1 5.3 20 REPTILES Deserl Iguana* (Dipsosaurusdorsali•) 1 5.3 56 Chuckwalla (Sauromalusobesus) 4 21.0 235 Zebra-tailed Lizard* 1 5.3 (Callisaurusdraconides ) 2 Desert Horned Lizard (Phrynosomaplatyrhinos ) 4 21.0 22 Western Whiptail (Cnemidophorustigris) 2 10.5 1 aThenumber of nestsin whicha specieswas recorded. bThenumber ofnests inwhich aspecies wasfound divided bythe number ofnests examined (N = 19);times 100 and reported asa percentage. *Speciesnot previouslyrecorded in the literature as Prairie Falcon prey. 132 DOUGLASA. BOYCE,JR. VOL. 19, No. 4 RESULTS AND DISCUSSION not found at Utah nests, they concluded that White-faced Ibis were too heavy for Prairie Falcons Thirty-nine speciesrepresenting 3 vertebrate to carry. Adult Chukar (Alectorischukar) and classeswere present in prey collectionsfrom 19 California Ground Squirrel (Spermophilusbeecheyi) nests (Table 2). Twenty-five ,speciesof birds, 9 weighbetween 500-565 g and were the next largest speciesof mammalsand 5 speciesof reptileswere prey items found in eyrie samplesand may have identified. Insect partswere rarely noted and only been brought to the nest by the female (863 g, 1 species,Armored StinkBeetle (Eleodes armata) was Enderson 1964). identified. The Horned Lark, Valley Pocket Birds. The Horned Lark and Mourning Dove Gopher and DesertWoodrat were presentin over were found in 63 and 42% of the nests,respect- 50% of the nests.The Mourning Dove was present tively. The presence of a Western Sandpiper in 48% of the nests. (Calidrismauri) (Table 2) demonstratesthe oppor- Although the number of avianspecies captured tunistic hunting nature of Prairie Falcons. The outnumbered mammalsby almost3 to 1, the mean Mojave River flows through the desert but is sub- weightfor birds(76 g) washalf of the meanweight terranean for much of its length. Sandpiper re- for mammals(179 g) suggestinggreatest energetic mains were found at a falcon nest 3.2 km from one return resultsfrom capturingmammals. Analysis of the few points where the river surfaces. The of 214 pellets provides further evidence that sandpiperwas the only prey item not observedin mammalsmight be capturedmore often than birds the field. or reptiles.Mammals were present in 72%, birdsin Reptiles. Most Prairie Falconfood studiesshow 24% and reptilesin 4% of the pelletsexamined. that reptiles are infrequently reported as prey In contrast to Pierce (1935) and Bond (1936) I (Table 1); however, reptiles did constitutea rela- found no instancewhere all or nearly all prey items tively high proportion of the prey in this study were from just one vertebrate class. No single (9.5%). Desert Horned Lizard (Phrynosoma speciesappears as primary prey on a desert-wide platyrhinos)and Chuckwallawere found at 20% of basis(Table 2). Some specieswere infrequent in the nests.Reptile remains,however, were recorded preyremains desert-wide but were locally frequent. in only 4% of the pellets.Reptile scales were found For example, the Black-headedGrosbeak (Pheuc- mixedwith mammalhair in castingsbut no castings