NOTES

GRAY VIREOS WINTERING IN CAUFORNIA F-FPHANT TREES

PHILIP UNITT, San DiegoNatural History Museum, P. O. Box 121390, San Diego, California 92112-1390

The GrayVireo (Vireo vicinior)winters primarily in Sonoraand Baja California Sur (e.g., Barlowet al. 1999). There are scatteredwinter records from the Mexicanstates of Durango(Leukering and Bradley1997), Coahuila(Howell and Webb 1995), and San LuisPotosf (Fry et al. 1996). North of the Mexicanborder the onlywinter records are from the Big Bend area of Texas (Barlowand Wauer 1971) and southern, primarilysouthwestern, Arizona, north to the Kofa Mountains(Phillips et al. 1964). There are no publishedwinter records for Upper California. On 4 May 1999, aspart of a fieldsurvey for the San DiegoCounty bird atlas, Lori Hargrovevisited the canyonof Alma Wash, 6 milessouth of OcotilloWells and 3 to 5 mileswest of Split MountainRoad. She reportedto me largenumbers, perhaps a few thousand,of the Elephant Tree (Bursera microphylla), a concentrationfar greaterthan any reportedby Dice et al. (1992). The five standsof the ElephantTree enumeratedby Dice et al. in the Anza-BorregoDesert of easternSan DiegoCounty exhaustthe tree'sknown distribution in Upper California.A surveyby helicopteron 21 January2000 confirmedthe Alma Wash populationas by far the largestin the state(J. C. Dice pers. corem.) Bursera microphyllais locallycommon in Baja Californiaand Sonora(Turner et al. 1995). Bates(1992a) foundGray Vireos wintering in Sonorato be closelylinked to it, feeding primarily on its fruit and being, with the Ash-throatedFlycatcher (Myiarchuscinerascens), the primary disperserof its seeds,via regurgitation.He notedthat the winterrange of the vireoclosely matches the distributionof the tree, except in the Anza-BorregoDesert and a few canyonsin Arizona. There are no definite previouslypublished winter recordsfor Baja Californianorth of 28ø N (Grinnell 1928, Wilbur 1987), but the speciesoccurs in ElephantTrees at least in GuadalupeCanyon at the eastbase of the SierraJu•rez (8 February1997, R. Fischer; 12-13 March 2000, J. Morlan and R. Fischer;5-6 April 2000, R. E. Webster). Sincethe largeststand of the ElephantTree in Californiahad never been visited in winterby ornithologistsor birdwatchers,I reasoned that an undiscoveredpopulation of winteringGray Vireosmight occurthere. Therefore,to test the hypothesis,I organizedan expeditionon 4 and 5 December1999. Lori Hargrovemade supple- mentaryvisits to the bajadabelow the mouthof the canyonon 27 Novemberand 6 December. We founda minimumof fiveGray Vireos, all in areasof concentrationsof Elephant Trees,mixed with DesertLavender (H•ptis emor•i), CatclawAcacia (Acacia greggii), and Creosote Bush (Larrea tridentata)--the same habitat in which Gray Vireos winterin Sonora(Bates 1992a). At leasttwo were alongAlma Washon the bajada belowthe canyon(33 ø 03' 11-23" N, 116 ø 07' 53-08' 09" W; 33 ø 03' 22" N, 116 ø 08' 35" W), two were insidethe canyon(33 ø 03' 39-41" N, 116 ø 10' 6-16" W), one nearthe upperend known as Starfish Cove (33 ø 03' 39-43" N, ! 16ø 10' 46-57" W), and one was 0.7 mile southof Alma Wash in anothercanyon (33 ø 02' 47" N, 116 ø 09' 19"W) (Figure1). Alma Wash and StarfishCove are not namedon U.S. GeologicalSurvey topographic maps but are on mapsof the Anza-BorregoDesert, e.g., that of EarthwalkPress (1994). The bajadalabeled "Elephant Tree Area" on the USGS mapsactually has very few ElephantTrees except along the broadrocky wash. The birds were in small mixed-speciesflocks with Black-tailedGnatcatchers (Polioptila melanura), Verdins (Auriparus fiaviceps), Black-throatedSparrows (Amphispizabilineata), and Bewick'sWrens (Thryomanesbewickii). Barlow et al. (1999) alsoreported wintering Gray Vireos in mixed flocks,though this doesnot

258 WesternBirds 31:258-262, 2000 NOTES

toBorrego Spr/ngs • ". •,) Benson Dry Lake

Oeotillo Wells

miles

kilometers

HalthillDry Lake

• 200 m : ..- ,./ ,.., ß 400m ,_•.,

'., 600m ./_, /f.._%,

/ J .'ø StarfishCove '-'-?, • , • t - t• •_..,.... • ,..,/ •, ß • •.,."•,-,:1'"':.,,-,, ' t TM "1 ....•..--,•....'n.--, • bajaOa-.'. .'"

Valleinto•' • ....,• ...... •, .... ß...... •ountmsß g '• '•"'• • "'"•"' } t• ...... i',"-' ? Ct.,/'., ',.3 N, ' -- ? "•

,/ ., •_..,. ,½ L q c,-' I •y•suzzt •' ,'•'-•- • v, '-.•/ L " ' -" '""' • • (,-'"" /'-"":--'1 ,._/'ve' ..• Splitgountain

Figure1. Locationsof GrayVireos sighted in standsof the ElephantTree south of OcotilloWells, Anza-Borrego Desert State Park, December1999.

259 NOTES precludewinter territoriality as describedby Bates(1992b). Thoughwe were unsuc- cessfulin takingany photographs, we sawall the birdsextremely well, noting their size slightlysmaller than a Black-throatedSparrow, bill short and thick for a vireo, uniformlygray upperparts, whitish underparts, complete narrow white eye ring, lack of white lores. single narrow white wing bar, and narrow white edges on the secondaries.Though we did not observethe vireos feeding. they were all near numbersof femaleElephant Trees (the plant is largelydioecious). well festooned with fruit (Figure2). We broadcasta taped recordingof the Gray Vireo's songfrom time to time but locatedall butone of the birdsvisually. and the exceptionwas singing spontaneously. Anothersang in responseto the tapeafter being sighted. One birdgave a scoldingcall resemblingthe scoldof Hutton'sVireo (Vireo huttoni)•and two gavethe brief high- pitchedtrill describedand illustratedby Barlowet al. (1999)• a call unlikethose of othervireos occurring in California.Bates (pers. comm.) found this trill to be the most frequentvocalization of Gray Vireoswintering on the coastof Sonora. The numberof Gray Vireoswe observed,and their beingfound readily on a first attempt.imply that the speciesis a normalwinter visitor in the area.There had been no unusualrains that mightinduce an irregularspecies to extendits rangenorth into this extremelyarid region:indeed. annual rainfall in the Anza-BorregoDesert has beenwell belownormal since the wet E1Nifio winterof 1997-98. We notedonly the last remainsof ephemeralvegetation that proliferatedafter cloudburstson 11. 12. and 23 July 1999. Our discoverymight be construedas supportingother reports, publishedand unpublished,of the Gray Vireo in the Anza-BorregoDesert. Nevertheless, only one of theseis well supported.a sightingof an apparentmigrant at YaquiWell on 10 April 1988 by D. R. Willick(American Birds 42:482). The speciesis easilyconfused with eitherthe Plumbeous(V. plumbeus) or Bell'sVireo (V bellii),both known to occurin the

Figure2. FruitingElephant Tree (Burseramicrophylla) frequented by a Gray Vireo in Alma Wash.Anza-Borrego Desert State Park. 4 December1999. Photo by Jack W. Schlotte

260 NOTES

the Anza-BorregoDesert in winter(the Plumbeous in plantedpines, winter 1998-99, pers.obs.; Bell's in mesquites,24 January1984, SDNHM 42925). The Gray Vireo is virtuallyunknown away from its breedingand winteringranges as a migrant.The sole recordof a migrantfrom Californialisted by Grinnelland Miller (1944), of one collected at Mecca, RiversideCounty, on 26 March 1911 (vanRossem 1911) is in factin error. MichaelA. Pattenand I examinedthe specimen(UCLA 10697) andfound it is a Bell's Vireo, as specifiedon its label,apparently not the original.Since van Rossem(1911) reportedcollecting only one specimenof anyvireo on histrip to the SaltonSea region, thismust be the specimenhe originallyidentified as the Gray. I suggestthat the Gray Vireo normallymakes the commutebetween its breeding and winter ranges in a single nonstopflight. Undoubtedly much remains to be learnedof the Gray Vireo'sdistribu- tion;for example,the species extends farther north in Nevada,presumably as a breeder, than reportedby Johnson(1972)--J. V. Remsen(pers. comm.) noted one at LidaPass on 29 May 1976, and M. A. Patten (pers.comm.) noted anotherat Lida on 27 May 1995. The distributionof the ElephantTree in northernBaja California, local along the eastbase of the sierrasJuarez and SanPedro Martir, widespread between latitudes 30 ø and 28 ø N (Turneret al. 1995), suggeststhat winteringGray Vireos may be foundat manyplaces there in additionto GuadalupeCanyon. Our observationsin the Anza-BorregoDesert thus support Bates' hypothesis of a mutualdependence of the Gray Vireo and ElephantTree. The Gray Vireo has long been recognizedas a seriouslydeclining species in California, owing to brood parasitismby the Brown-headedCowbird (Molothrus ater)(e.g., Remsen1978). The ElephantTree itselfis consideredendangered in Californiaby the CaliforniaNative PlantSociety (Skinner and Pavlik 1994). Quitepossibly, the GrayVireos wintering in California,at the northwesternend of the winter range, breed in California,the westernend of the breedingrange. The viabilityof the ElephantTree in California couldbe affectedby an interactionbetween vireos and cowbirdsthat takesplace far from the trees themselves. I thank John Batesfor providinga taped recordingof the Gray Vireo he made in Sonora.Thanks to the other participantsin the expeditionto searchfor the vireos, MaryanneBache, Lori Hargrove, Mark David Hoefer, Ginger Rebstock,Robert Sanger,Jack W. Schlotte,David Seals, and James O. Zimmer.They enabled us to split into severalgroups and covermore areas,some repeatedly,than wouldhave been possibleotherwise. Thanks to CalTransand Pam Beare for the fundingtoward the San Diego Countybird atlas that providedfor Hargrove'sinitial visit. Thanks to Fritz Hertel, Universityof California,Los Angeles, for the loanof van Rossem'sspecimen. Thanksto RobbieFischer, Joseph Morlan, and RichardE. Websterfor their observa- tionsfrom BajaCalifornia. Thanks to John Batesand KathyMolina for their reviews.

LITERATURE CITED

Barlow,J. C., Leckie,S. N., andBaril, C. T. 1999. GrayVireo (Vireo vicinior), in The Birdsof North America (A. Poole and F. Gill, eds.),no. 447. BirdsN. Am., Inc., Philadelphia. Barlow,J. C., andWauer, R. H. 1971. The Gray Vireo (Vireo viciniorCoues; Aves: Vireonidae) wintering in the Big Bend region, west Texas. Can. J. Zool. 49:953-955. Bates,J. M. 1992a. Frugivoryon Bursera microphylla(Burseraceae) by wintering Gray Vireos(Vireo vicinior, Vireonidae)in the coastaldeserts of Sonora,Mexico. Southwest. Nat. 37:232-258. Bates, J. M. 1992b. Winter territorial behavior of Gray Vireos. Wilson Bull. 104:425-433.

261 NOTES

Dice, J. C., Jorgensen,M. C., Pilcher,K. L., Smith, C. H., and Th•riault, R. 1992. Known populationsof ElephantTrees (Bursera microphylla) in Anza-Borrego Desert State Park, San Diego County, California. Calif. Dept. Parks and Recreation,8885 Rio San Diego Dr., Suite 270, San Diego, CA 92108. EarthwalkPress. 1994. Anza-BorregoDesert region recreationmap. Earthwalk Press,Eureka, CA. Fry,A. J., Rojas-Soto,O. R., andNavarro S., A. G. 1996. A winterrecord of the Gray Vireo from San Luis Potosf,Mexico. W. Birds 27:54-55. Grinnell,J. 1928. A distributionalsummation of the ornithologyof LowerCalifornia. Univ. Calif. Publ. Zool. 32:1-300. Grinnell,J., and Miller,A. H. 1944. The distributionof the birdsof California.Pac. Coast Avifauna 27. Howell, S. N. G., and Webb, S. 1995. A Guide to the Birdsof Mexico and Northern CentralAmerica. Oxford Univ. Press,Oxford, England. Johnson,N. K. 1972. Breedingdistribution and habitat preference of the GrayVireo in Nevada. Calif. Birds 3:73-78. Leukering,T., and Bradley,J. 1997. Some observationsof birdson the central Mexican plateau.W. Birds28:177-180. Phillips,A., Marshall,J., and Monson, G. 1964. Birdsof Arizona. Univ. of Ariz. Press,Tucson. Remsen,J. V. 1978. Birdspecies of specialconcern in California.Calif. Dept. Fish& Game, WildlifeMgmt. BranchAdmin. Rep. 78-1. Skinner, M. W., and Pavlik, B. M., eds. 1994. Inventoryof Rare and Endangered VascularPlants of California.Calif. NativePlant Soc. Spec. Publ. 1, 5th ed. Turner, R. M., Bowers,J. E., and Burgess,T. L. 1995. SonoranDesert Plants: An EcologicalAtlas. Univ. of Ariz. Press,Tucson. Van Rossem,A. 1911. Winter birdsof the SaltonSea region.Condor 13:129-137. Wilbur,S. R. 1987. Birdsof Baja California.Univ. of Calif. Press,Berkeley.

Accepted 10 August 2000

262