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The Niche-Relationships of the California Thrasher?

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The Niche-Relationships of the California Thrasher? VøI'i•lXyXIV] GRINNELL,TheCalifornia Thrasher. 427 into the body of the partly eaten bantam and replacedit in the same spot where he found it. Next morning the seeminglyim- possiblewas madea practicalcertainty, for he foundthe body of a screechowl with the clawsof one foot firlnly imbeddedin the body of the bantam. He very kindly presentedme with the owl which, upon dissection,proved to be a female, its stomacheontalning a very considerableamount of bantam fleshand feathers,together with a great deal of wheat. (It seelnsprobable that the wheat was accidentallyswallowed with the crop of the bantam during the feast,but therewas so muchthat it seelnsstrange the owl did not discardit while eating). How a bird only 9.12 inchesin length• could have dealt out such havoc in so short a time is almost in- credible,but, althoughpurely circumstantial,the evldeneeagainst the owl appearedaltogether too strongfor evena reasonabledoubt. The doctorand I wishedto make as certain as possible,however, so the poisonedbantam was replacedand left for several days, but without any further results. For the abovementioned reasons I am rather doubtful as to the net value of this owl from an economicstandpoint, although birds in a wildstat• would not give theln such opportunitiesfor such wanton killing as birds enclosedin pens. THE NICHE-RELATIONSHIPS OF THE CALIFORNIA THRASHER? BY JOSEPH GRINNELL. TUE California Thrasher (Toxostomaredivivum) is one of the severaldistinct bird types which characterize theso-called "Cali- fornian Fauna." Its range is notably restricted, even more so than that of the Wren-Tit. Only at the south doesthe California Thrasher occurbeyond the limits of the state of California, and in that directiononly as far as the San PedroMartit Mountainsand Contribution from the Museum of Vertebrate Zo51ogyof the University of California. 428 GRINNELL,The California Thrasher. toct.[Auk San Quintin, not more than one hundredand sixty milesbelow the Mexican line in Lower California. An explanationof this restricteddistribution is probablyto be foundin the closeadjustment of the bird in variousphysiological and psychologicalrespects to a narrow range of environmental conditions. The nature of these critical conditions is to be learned throughan examinationof the bird's.•habita•.• It is desirableto make suchexamination at as many pointsin the generalrange of the speciesas possiblewith the objectof determiningthe elements commonto all thesepoints, and of thesethe onesnot in evidence beyond the limits of the bird's range. The followingstatements in this regardare summarizedfrom the writer's personalexperience combinedwith all the pertinent informationafforded in literature. The distributionof the CaliforniaThrasher as regardslife-zone is unmistakable. Both asobserved locally and overits entirerange the speciesshows close adherence to the Upper Sonorandivision of the Austral zone. Especiallyupwards, is it always sharply defined. For example,in approachingthe sea-coastnorth of San FranciscoBay, in SonomaCounty, where the vegetationis pre- vailingly Transition, thrashersare found only in the Sonoran "islands," namely southerly-facinghill slopes,where the maxi- mum insolationmanifests its effects in a distinctive chaparral containingsuch lower zoneplants as Adenostoma. Again, around Monterey, to find thrashersone must seek the warm hill-slopes back from the coastalbelt of conifers. EverywhereI have been, the thrashersseem to be very particularnot to ventureeven a few rods into Transition, whether the latter consistof conifersor of high-zonespecies of manzanita and deer brush, though the latter growth resemblesclosely in densityand general appearancethe Upper Sonoranchaparral adjacent. While sharplydelimited, as an invariablerule, at the upperedge of UpperSonoran, the CaliforniaThrasher is not soclosely restricted at the loweredge of this zone. Locally, individuals occur, and numbersmay do so where assoeiationalfactors favor, down well into Lower Sonoran. Instancesof this are particularlynumerous in the San Diegan district; for example,in the Lower Sonoran "washes" at the mouthsof the canyonsalong the southbase of the San Gabriel Mountains, as near San Fernando, Pasadena, and ¾ol. XXXI¾1 1917 J GRI•X•nL, The California Thrasher. 429 Azusa.A noticeablething in thisconnection, however, is that, on the desertslopes of the mountains,where Toxostoma lecontei occurson the desertfloor as an assoeiationalhomologue of T. redivivumin the Lower Sonoran zone, the latter "stays put" far ß • 3. To•ostornar. pasadenense '• -• '• • ßSpecimens examined • • Publishedrecords Figure 1. moredosely; that is, it straysbut little or not at all belowthe typicalconfines of its own zone,namely Upper Sonoran. The writer'sfield work in thevicinity of WalkerPass, Kern County, 430 GRINNELL,The California Thrasher. LOct.[-Auk providesgood illustrations of this. A tongueor belt of Lower Sonoran extends from the 3/•ohave Desert over the low axial moun- tain ridgeat the head of Kelso Creek and thencedown along the valley of the South Fork of the Kern River nearly to Isabella. Leeonte's Thrasher is a conspicuouselement in this Lower Sonoran invasion, but no California Thrashers were met with in this regionbelow the belt of goodUpper Sonoranon the flanking mountainsides, as markedby the presenceof diggerpine, blue oak, sumaeh, silk-tasselbush, and other good zone-plants. Similar zonal relationshipsare on record from San GorgonioPass, River- sideCounty, as well as elsewhere. Referencenow to'the generalrange of the bird underconsidera- tion (seep. 429), as comparedwith a llfe-zonemap of California (PacificCoast Avifauna No. 11, Pls. I, II), will showto a remas'k- abledegree how closely the formercoincides with the UpperSono- ran zone. The thrasheris, to be sure,one of the elementsupon the presenceof whichthis zonewas marked on the map; but it was only one of many, both plant and animal; and it is concordance wi];hthe aggregate that is significant. Diagnosis ofzonation simi- larly is possiblein scoresof placeswhere change in altitude(which as a rule meanschange in temperature)is the obviousfactor, as up the west flank of the Sierra Nevada, or the north wall of the South Fork valley, alreadyreferred to, in Kern County, or on the north wall of the San Jaeinto 3/•ountains. The California Thrasher is unquestionablydelimited in its rangein ultimate analysisby tem- peratureeonditlons. The isothermiearea it occupiesis in zonal pax'lance,Upper Sonoran. The secondorder of restrictionis faunal, using this term in its narrowedsense, indleating dependence upon atmospheric humidity. The CaliforniaThrasher does not rangeinteriorly into excessively arid country,although the UpperSonoran zone may, as aroundthe southern end of the Sierra Nevada, continue uninterruptedly towardsthe interior in a generallylatitudinal dh'eetion. This is true where extensiveareas are considered,but locally, as with zones,individuals or descent-linesmay have invaded short dis- tanees beyond the normally preferred conditions. An example of this situationis to be foundon the north and westslopes of the San Jacinto3/•ountains, where CaliforniaThrashers range around Vol. 1017XXXiV1 J GRINNELL,The California Thrasher. 431 onto arid chaparralslopes, intermingling with such arid Upper Sonoranbirds as Scott'sOriole and the Gray Vireo. It is question- able, however, as to what extent faunal restriction really operates in this case; for referenceto the zone map, again, showsthat a vast tract of LowerSonoran, lying to the eastof the desertdivides, extendscontinuously north to the head of OwensValley. Really the onlyunbroken bridge of UpperSonoran towards the eastfrom the west-Sierran habitat of Toxostoraa redivivura is around the southernend of the SierraNevada -- a very narrowand longroute of possibleemigration, with consequentfactors unfavorableto invasion,irrespective of either temperatureor humidity, such as interruptedassociational requirements and small aggregatearea. In this particularbird, therefore,faunal restriction may be of minor importance,as comparedwith zonal and associationatcontrols. That faunal conditionshave had their influenceon the species, however,is shownby the fact of geographicvariation within its range. The thrasher throughoutits habitat-as-a-whole,is sub- jected to different degreesof humidity. Amount of rainfall is, in a generalway, an index of atmospherichumidity, thoughnot withoutconspicuous exceptions. Comparingthe mapof the ranges of the subspeciesof T. redivivura(p. 429) with a climaticmap of the State, direct concordanceis observedbetween areas of stated rain- fall on the latter and the rangesof the respectivesubspecies. It will be seen that the race T. r. pasadenenseoccupies an area of relativelylow humidity,the race T. r. sonorateof higherhumidity and the race T. r. redivivuraof highesthumidity, in fact a portion of California'sfog-belt. The distinctivecolor-tones developed are, respectively,of gray, slate and brown casts. In the thrasher, therefore,we may look to faunatinfluences as havingmost to do with differentiationwithin the species. In this caseit is the faunal variationover the occupiedcountry which is apparentlyresponsible for the intra-specificbudding, or, in otherwords, the originationof new specificdivarications. •q•erever it occurs,and in whicheverof the three subspeciesit is represented,the CaliforniaThrasher evinces' strong associational predilections.It is a characteristicelement in California'sfamous chaparralbelt. Where this belt is broadestand best developed, as in the San Diegandistrict
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