Grinnell 1917
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Grinnell 1917 Grinnell 1917 Document Transcript

  • VøIi•lXyXIV] GRINNELL,TheCaliforniaThrasher. 427into the body of the partly eatenbantamand replacedit in thesamespotwherehe foundit. Next morningthe seeminglyim-possiblewasmadea practicalcertainty,for hefoundthebodyof ascreechowlwith the clawsof onefootfirlnly imbeddedin the bodyof the bantam. He very kindly presentedmewith the owl which,upondissection,provedto be a female,its stomacheontalningavery considerableamountof bantamfleshand feathers,togetherwith a great deal of wheat. (It seelnsprobablethat the wheatwas accidentallyswallowedwith the crop of the bantam duringthefeast,but therewassomuchthat it seelnsstrangethe owldidnot discardit whileeating). How a birdonly9.12inchesin length•could have dealt out such havoc in so short a time is almost in-credible,but, althoughpurelycircumstantial,the evldeneeagainstthe owlappearedaltogethertoostrongfor evena reasonabledoubt.The doctorand I wishedto makeascertainaspossible,however,so the poisonedbantam was replacedand left for severaldays,but withoutanyfurtherresults. For theabovementionedreasonsI am rather doubtful as to the net value of this owl from aneconomicstandpoint,althoughbirdsin a wildstat•wouldnotgive theln such opportunitiesfor suchwanton killing as birdsenclosedin pens.THE NICHE-RELATIONSHIPS OF THE CALIFORNIATHRASHER?BYJOSEPHGRINNELL.TUE CaliforniaThrasher (Toxostomaredivivum)is one of theseveraldistinctbirdtypeswhichcharacterizetheso-called"Cali-fornian Fauna." Its range is notably restricted,even more sothan thatof theWren-Tit. Only at the southdoestheCaliforniaThrasheroccurbeyondthe limits of the stateof California,and inthat directiononly asfar asthe SanPedroMartit MountainsandContribution from the Museum of Vertebrate Zo51ogyof the University of California.
  • [Auk428 GRINNELL,TheCaliforniaThrasher. toct.SanQuintin, not morethan onehundredandsixty milesbelowtheMexican line in Lower California.An explanationof this restricteddistributionis probablyto befoundin the closeadjustmentof the bird in variousphysiologicaland psychologicalrespectsto a narrow range of environmentalconditions. The nature of these critical conditions is to be learnedthroughan examinationof the birds.•habita•.•It is desirabletomakesuchexaminationat asmanypointsin the generalrangeofthespeciesaspossiblewith the objectof determiningtheelementscommonto all thesepoints,andof thesethe onesnot in evidencebeyondthe limits of the birdsrange. The followingstatementsin thisregardaresummarizedfromthewriterspersonalexperiencecombinedwith all the pertinentinformationaffordedin literature.The distributionof the CaliforniaThrasherasregardslife-zoneisunmistakable.Bothasobservedlocallyandoveritsentirerangethe speciesshowscloseadherenceto the Upper Sonorandivisionof the Austral zone. Especiallyupwards,is it alwayssharplydefined. For example,in approachingthesea-coastnorthof SanFranciscoBay, in SonomaCounty, wherethe vegetationis pre-vailingly Transition, thrashersare found only in the Sonoran"islands," namely southerly-facinghill slopes,where the maxi-mum insolationmanifestsits effectsin a distinctive chaparralcontainingsuchlowerzoneplantsasAdenostoma. Again,aroundMonterey,to find thrashersonemust seekthe warm hill-slopesbackfrom the coastalbelt of conifers. EverywhereI havebeen,the thrashersseemto bevery particularnot to ventureevena fewrodsinto Transition, whether the latter consistof conifersor ofhigh-zonespeciesof manzanitaand deerbrush,thoughthe lattergrowthresemblescloselyin densityand generalappearancetheUpper Sonoranchaparraladjacent.Whilesharplydelimited,asaninvariablerule,at theupperedgeofUpperSonoran,theCaliforniaThrasherisnotsocloselyrestrictedat the loweredgeof this zone. Locally, individualsoccur,andnumbersmay do sowhereassoeiationalfactorsfavor, downwellinto LowerSonoran. Instancesof this are particularlynumerousin the San Diegandistrict; for example,in the Lower Sonoran"washes"at the mouthsof the canyonsalongthe southbaseof theSan Gabriel Mountains, as near San Fernando, Pasadena, and
  • ¾ol.XXXI¾11917 J GRI•X•nL, The CaliforniaThrasher. 429Azusa.A noticeablethingin thisconnection,however,isthat,onthedesertslopesof themountains,whereToxostomaleconteioccursonthedesertfloorasanassoeiationalhomologueof T.redivivumintheLowerSonoranzone,thelatter"staysput"farß • 3. To•ostornar. pasadenense• -• • • ßSpecimensexamined• • PublishedrecordsFigure 1.moredosely; that is, it straysbut little or not at all belowthetypicalconfinesof its ownzone,namelyUpperSonoran.Thewritersfieldworkin thevicinityofWalkerPass,KernCounty,
  • [-Auk430 GRINNELL,TheCaliforniaThrasher. LOct.providesgoodillustrationsof this. A tongueor belt of LowerSonoranextendsfrom the 3/•ohaveDesert over the low axial moun-tain ridgeat the headof KelsoCreekandthencedownalongthevalley of the SouthFork of the Kern River nearly to Isabella.LeeontesThrasher is a conspicuouselement in this LowerSonoran invasion, but no California Thrashers were met with inthisregionbelowthe belt of goodUpperSonoranon the flankingmountainsides,asmarkedby thepresenceof diggerpine,blueoak,sumaeh,silk-tasselbush, and other good zone-plants. Similarzonalrelationshipsare on recordfrom San GorgonioPass,River-sideCounty,aswellaselsewhere.Referencenowtothe generalrangeof the birdunderconsidera-tion (seep. 429),ascomparedwith a llfe-zonemapof California(PacificCoastAvifaunaNo. 11,Pls.I, II), will showto a remask-abledegreehowcloselytheformercoincideswith theUpperSono-ran zone. The thrasheris, to besure,oneof the elementsuponthepresenceof whichthiszonewasmarkedon the map; but it wasonly oneof many, both plant and animal; and it is concordancewi];htheaggregatethatissignificant.Diagnosisofzonationsimi-larlyispossiblein scoresof placeswherechangein altitude(whichasa rulemeanschangein temperature)istheobviousfactor,asupthe westflankof the SierraNevada, or the north wall of the SouthFork valley,alreadyreferredto, in Kern County,or on the northwall of the San Jaeinto 3/•ountains. The California Thrasher isunquestionablydelimitedin its rangein ultimateanalysisby tem-peratureeonditlons.The isothermieareait occupiesis in zonalpaxlance,UpperSonoran.The secondorder of restrictionis faunal, usingthis term in itsnarrowedsense,indleatingdependenceuponatmospherichumidity.The CaliforniaThrasherdoesnot rangeinteriorlyinto excessivelyaridcountry,althoughtheUpperSonoranzonemay,asaroundthesouthernend of the Sierra Nevada, continue uninterruptedlytowardsthe interior in a generallylatitudinal dheetion. Thisis true whereextensiveareasare considered,but locally, as withzones,individualsor descent-linesmay have invadedshort dis-taneesbeyondthe normally preferredconditions. An exampleof thissituationisto befoundonthenorthandwestslopesof theSanJacinto3/•ountains,whereCaliforniaThrashersrangearound
  • Vol. XXXiV11017J GRINNELL,TheCaliforniaThrasher. 431onto arid chaparralslopes,interminglingwith sucharid UpperSonoranbirdsasScottsOrioleandtheGray Vireo. It isquestion-able, however,as to what extent faunal restrictionreally operatesin this case; for referenceto the zonemap, again,showsthat avasttractofLowerSonoran,lyingto theeastof thedesertdivides,extendscontinuouslynorth to the headof OwensValley. Reallytheonlyunbrokenbridgeof UpperSonorantowardstheeastfromthe west-Sierran habitat of Toxostoraa redivivura is around thesouthernendof theSierraNevada-- a verynarrowandlongrouteof possibleemigration,with consequentfactorsunfavorabletoinvasion,irrespectiveof either temperatureor humidity, suchasinterruptedassociationalrequirementsand smallaggregatearea.In thisparticularbird,therefore,faunalrestrictionmaybeof minorimportance,as comparedwith zonal and associationatcontrols.That faunalconditionshavehad their influenceon the species,however,is shownby the fact of geographicvariationwithin itsrange. The thrasherthroughoutits habitat-as-a-whole,is sub-jected to differentdegreesof humidity. Amountof rainfall is,in a generalway, an indexof atmospherichumidity,thoughnotwithoutconspicuousexceptions.Comparingthemapoftherangesofthesubspeciesof T. redivivura(p.429)witha climaticmapof theState, direct concordanceis observedbetweenareasof statedrain-fall on the latter and the rangesof the respectivesubspecies.Itwill be seenthat the race T. r. pasadenenseoccupiesan area ofrelativelylowhumidity,the raceT. r. sonorateof higherhumidityandthe race T. r. redivivuraof highesthumidity,in fact a portionof Californiasfog-belt. The distinctivecolor-tonesdevelopedare,respectively,of gray, slate and brown casts. In the thrasher,therefore,we may lookto faunatinfluencesashavingmostto dowith differentiationwithin the species. In thiscaseit isthe faunalvariationovertheoccupiedcountrywhichisapparentlyresponsiblefortheintra-specificbudding,or,in otherwords,theoriginationofnewspecificdivarications.•q•ereverit occurs,andin whicheverof the threesubspeciesit isrepresented,the CaliforniaThrasherevincesstrongassociationalpredilections.It isa characteristicelementin Californiasfamouschaparralbelt. Wherethis belt is broadestand bestdeveloped,asin the SanDiegandistrictandin the foothillregionsbordering
  • [Auk432 GRINNELL,TheCaliforniaThrasher. [Oct.the great interior valleys, there the Thrasher abounds.Thewriters personalfield acquaintancewith this bird givesbasisforthe followinganalysisof habitat relations.The California Thrasher is a habitual forager beneathdenseand continuouscover. Furthermore,probablytwo-thirdsof itsforagingis doneon the ground. In seekingfoodaboveground,as whenpatronizingcascarabushes,the thrasherrarely mountsto an exposedposition,but only goesas high as is essentialtosecuringthe covetedfruits. The bird may be characterizedassemi-terrestrial,but alwaysdependentupon vegetationalcover;and this covermustbe of the chaparraltype, opennext to theground,withstronglyinterlacingbranch-workandevergreenleafycanopycloseabove-- notforestunder-growth,orclose-set,uprightstemsas in new-growthwillow, or matted leafageas in rank-growingannualherbage.The Thrasheris relativelyomnivorousin its diet. Beal (Bio-logicalSurveyBulletinno. 30, p. 55) examined82 stomachsofToxostomaredivivumandfoundthat 59percentof thefoodwasof avegetablenatureand41 animal. A largepart of thisfoodcon-sistedof ground-beetles,ants,andseeds,suchasareundoubtedlyobtainedby workingoverthelitter beneathchaparral. The birdsmostconspicuousstructuralfeature,thelongcurvedbill, isusedtowhiskasidethelitter, andalsoto dig,pick-fashion,intosoftearthwhereinsectslieconcealed.GroundmuchfrequentedbyThrashersshowsnumerouslittle pitsin thesollsurface,lessthananinchdeep,steepononesideandwith a little heapof earthpiledup ontheoppositeside. Asalreadyintimated,theThrasherattimesascendstothefoliageabove,forfruitanddoubtlesssomeinsects.Muchinthewayofberriesandseedsmayalsoberecoveredfromthegroundin whatisevidentlytheThrashersownspecializedmethodoffood-getting.Evengrantingthisspecialization,I donotseewhythechaparral,alone,shouldaffordtheexclusiveforage-ground;forthesamemodeof food-gettingoughtto bejustasusefulontheforestfloor,or evenonthe meadow.The furtherfact,of widelyomni-vorousdiet,leadsoneto concludethat it is notanypeculiarityoffood-source,orwayofgettingat it, thatalonelimitstheThrasherassociationally.We must look farther.The amateurobserver,orcollectorofspecimens,isstruckby the
  • Vol.XXXI¾]1917 J GRXNN•LL,The CaliforniaThrasher. 433apparent"shyness"of the Thrasher--by the easewith whichiteludescloseobservation,or, if thoroughlyalarmed,escapesdetec-tion altogether. For this protectiveeffect the bird is dependentupon appropriatecover, the chaparral,and upon its ability toco-operatein makinguseof thiscover. The Thrasherhasstrongfeet andlegs,andmuscularthighs,an equipmentwhichbetokenspowersofrunning;thet&ilisconspicuouslylong,asinmanyrun-ning birds; and correlatlvelythe wingsare short,rounded,andsoft-leathered,indicatinglittle use of the flight function. Thecolorsof the bird are non-conspicuous-blended,dark and lightbrowns. The nests of the Thrasher are located in dense massesof foliage,from two to sixfeet abovethe ground,in busheswhichareusuallya part of its typicalchaparralhabitat. In onlyexcep-tionalcasesis the chosennestingsitelocatedin a bushor scrubbytree,isolatedmoreor lessfromthe mainbodyof the chaparral.Thesevariouscircumstances,whichemphasizedependenceuponcover, and adaptation in physical structure and temperamentthereto,goto demonstratethenatureof the ultimateassociationalnicheoccupiedby the California Thrasher. This is oneof theminornicheswhichwith their occupantsall togethermakeup thechaparralassociation.It is, of course,axiomaticthat no twospeciesregularlyestablishedin a singlefauna have preciselythesamenicherel•atlonships.As a final statementwith regardto the CaliforniaThrasher,wemayconclude,then,that its rangeisdeterminedby a narrowphaseof conditionsobtainingin theChaparralassociation,withinthe Californiafauna,andwithin the UpperSonoranlife-zone.