1. The California Thrasher has a highly restricted range, occurring only in California and parts of Baja California. 2. It is found exclusively in the Upper Sonoran life zone, preferring warm, south-facing hillsides with dense chaparral vegetation. 3. Within its range, the California Thrasher shows adaptations to different levels of humidity through three subspecies that vary in coloration and geographic distribution.

1. VøI'i•lXyXIV] GRINNELL,TheCaliforniaThrasher. 427
into the body of the partly eatenbantamand replacedit in the
samespotwherehe foundit. Next morningthe seeminglyim-
possiblewasmadea practicalcertainty,for hefoundthebodyof a
screechowlwith the clawsof onefootfirlnly imbeddedin the body
of the bantam. He very kindly presentedmewith the owl which,
upondissection,provedto be a female,its stomacheontalninga
very considerableamountof bantamfleshand feathers,together
with a great deal of wheat. (It seelnsprobablethat the wheat
was accidentallyswallowedwith the crop of the bantam during
thefeast,but therewassomuchthat it seelnsstrangethe owldid
not 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 evldeneeagainst
the owlappearedaltogethertoostrongfor evena reasonabledoubt.
The doctorand I wishedto makeascertainaspossible,however,
so the poisonedbantam was replacedand left for severaldays,
but withoutanyfurtherresults. For theabovementionedreasons
I am rather doubtful as to the net value of this owl from an
economicstandpoint,althoughbirdsin a wildstat•wouldnot
give theln such opportunitiesfor suchwanton killing as birds
enclosedin pens.
THE NICHE-RELATIONSHIPS OF THE CALIFORNIA
THRASHER?
BYJOSEPHGRINNELL.
TUE CaliforniaThrasher (Toxostomaredivivum)is one of the
severaldistinctbirdtypeswhichcharacterizetheso-called"Cali-
fornian Fauna." Its range is notably restricted,even more so
than thatof theWren-Tit. Only at the southdoestheCalifornia
Thrasheroccurbeyondthe limits of the stateof California,and in
that directiononly asfar asthe SanPedroMartit Mountainsand
Contribution from the Museum of Vertebrate Zo51ogyof the University of California.

2. [Auk
428 GRINNELL,TheCaliforniaThrasher. toct.
SanQuintin, not morethan onehundredandsixty milesbelowthe
Mexican line in Lower California.
An explanationof this restricteddistributionis probablyto be
foundin the closeadjustmentof the bird in variousphysiological
and psychologicalrespectsto 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
makesuchexaminationat asmanypointsin the generalrangeof
thespeciesaspossiblewith the objectof determiningtheelements
commonto all thesepoints,andof thesethe onesnot in evidence
beyondthe limits of the bird'srange. The followingstatements
in thisregardaresummarizedfromthewriter'spersonalexperience
combinedwith all the pertinentinformationaffordedin literature.
The distributionof the CaliforniaThrasherasregardslife-zone
isunmistakable.Bothasobservedlocallyandoveritsentirerange
the speciesshowscloseadherenceto the Upper Sonorandivision
of the Austral zone. Especiallyupwards,is it alwayssharply
defined. For example,in approachingthesea-coastnorthof San
FranciscoBay, 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 chaparral
containingsuchlowerzoneplantsasAdenostoma. Again,around
Monterey,to find thrashersonemust seekthe warm hill-slopes
backfrom the coastalbelt of conifers. EverywhereI havebeen,
the thrashersseemto bevery particularnot to ventureevena few
rodsinto Transition, whether the latter consistof conifersor of
high-zonespeciesof manzanitaand deerbrush,thoughthe latter
growthresemblescloselyin densityand generalappearancethe
Upper Sonoranchaparraladjacent.
Whilesharplydelimited,asaninvariablerule,at theupperedge
ofUpperSonoran,theCaliforniaThrasherisnotsocloselyrestricted
at the loweredgeof this zone. Locally, individualsoccur,and
numbersmay do sowhereassoeiationalfactorsfavor, downwell
into LowerSonoran. Instancesof this are particularlynumerous
in the San Diegandistrict; for example,in the Lower Sonoran
"washes"at the mouthsof the canyonsalongthe southbaseof the
San Gabriel Mountains, as near San Fernando, Pasadena, and

3. ¾ol.XXXI¾1
1917 J GRI•X•nL, The CaliforniaThrasher. 429
Azusa.A noticeablethingin thisconnection,however,isthat,
onthedesertslopesof themountains,whereToxostomalecontei
occursonthedesertfloorasanassoeiationalhomologueof T.
redivivumintheLowerSonoranzone,thelatter"staysput"far
ß • 3. To•ostornar. pasadenense
'• -• '• • ßSpecimensexamined• • Publishedrecords
Figure 1.
moredosely; that is, it straysbut little or not at all belowthe
typicalconfinesof its ownzone,namelyUpperSonoran.The
writer'sfieldworkin thevicinityofWalkerPass,KernCounty,

4. [-Auk
430 GRINNELL,TheCaliforniaThrasher. LOct.
providesgoodillustrationsof this. A tongueor belt of Lower
Sonoranextendsfrom the 3/•ohaveDesert over the low axial moun-
tain ridgeat the headof KelsoCreekandthencedownalongthe
valley of the SouthFork of the Kern River nearly to Isabella.
Leeonte'sThrasher is a conspicuouselement in this Lower
Sonoran invasion, but no California Thrashers were met with in
thisregionbelowthe belt of goodUpperSonoranon the flanking
mountainsides,asmarkedby thepresenceof diggerpine,blueoak,
sumaeh,silk-tasselbush, and other good zone-plants. Similar
zonalrelationshipsare on recordfrom San GorgonioPass,River-
sideCounty,aswellaselsewhere.
Referencenowto'the generalrangeof the birdunderconsidera-
tion (seep. 429),ascomparedwith a llfe-zonemapof California
(PacificCoastAvifaunaNo. 11,Pls.I, II), will showto a remas'k-
abledegreehowcloselytheformercoincideswith theUpperSono-
ran zone. The thrasheris, to besure,oneof the elementsuponthe
presenceof whichthiszonewasmarkedon the map; but it was
only oneof many, both plant and animal; and it is concordance
wi];htheaggregatethatissignificant.Diagnosisofzonationsimi-
larlyispossiblein scoresof placeswherechangein altitude(which
asa rulemeanschangein temperature)istheobviousfactor,asup
the westflankof the SierraNevada, or the north wall of the South
Fork valley,alreadyreferredto, in Kern County,or on the north
wall of the San Jaeinto 3/•ountains. The California Thrasher is
unquestionablydelimitedin its rangein ultimateanalysisby tem-
peratureeonditlons.The isothermieareait occupiesis in zonal
pax'lance,UpperSonoran.
The secondorder of restrictionis faunal, usingthis term in its
narrowedsense,indleatingdependenceuponatmospherichumidity.
The CaliforniaThrasherdoesnot rangeinteriorlyinto excessively
aridcountry,althoughtheUpperSonoranzonemay,asaroundthe
southernend of the Sierra Nevada, continue uninterruptedly
towardsthe interior in a generallylatitudinal dh'eetion. This
is true whereextensiveareasare considered,but locally, as with
zones,individualsor descent-linesmay have invadedshort dis-
taneesbeyondthe normally preferredconditions. An example
of thissituationisto befoundonthenorthandwestslopesof the
SanJacinto3/•ountains,whereCaliforniaThrashersrangearound

5. Vol. XXXiV1
1017J GRINNELL,TheCaliforniaThrasher. 431
onto arid chaparralslopes,interminglingwith sucharid Upper
SonoranbirdsasScott'sOrioleandtheGray Vireo. It isquestion-
able, however,as to what extent faunal restrictionreally operates
in this case; for referenceto the zonemap, again,showsthat a
vasttractofLowerSonoran,lyingto theeastof thedesertdivides,
extendscontinuouslynorth to the headof OwensValley. Really
theonlyunbrokenbridgeof UpperSonorantowardstheeastfrom
the west-Sierran habitat of Toxostoraa redivivura is around the
southernendof theSierraNevada-- a verynarrowandlongroute
of possibleemigration,with consequentfactorsunfavorableto
invasion,irrespectiveof either temperatureor humidity, suchas
interruptedassociationalrequirementsand smallaggregatearea.
In thisparticularbird,therefore,faunalrestrictionmaybeof minor
importance,as comparedwith zonal and associationatcontrols.
That faunalconditionshavehad their influenceon the species,
however,is shownby the fact of geographicvariationwithin its
range. The thrasherthroughoutits habitat-as-a-whole,is sub-
jected to differentdegreesof humidity. Amountof rainfall is,
in a generalway, an indexof atmospherichumidity,thoughnot
withoutconspicuousexceptions.Comparingthemapoftheranges
ofthesubspeciesof T. redivivura(p.429)witha climaticmapof the
State, direct concordanceis observedbetweenareasof statedrain-
fall on the latter and the rangesof the respectivesubspecies.It
will be seenthat the race T. r. pasadenenseoccupiesan area of
relativelylowhumidity,the raceT. r. sonorateof higherhumidity
andthe race T. r. redivivuraof highesthumidity,in fact a portion
of California'sfog-belt. The distinctivecolor-tonesdevelopedare,
respectively,of gray, slate and brown casts. In the thrasher,
therefore,we may lookto faunatinfluencesashavingmostto do
with differentiationwithin the species. In thiscaseit isthe faunal
variationovertheoccupiedcountrywhichisapparentlyresponsible
fortheintra-specificbudding,or,in otherwords,theoriginationof
newspecificdivarications.
•q•ereverit occurs,andin whicheverof the threesubspeciesit is
represented,the CaliforniaThrasherevinces'strongassociational
predilections.It isa characteristicelementin California'sfamous
chaparralbelt. Wherethis belt is broadestand bestdeveloped,
asin the SanDiegandistrictandin the foothillregionsbordering

6. [Auk
432 GRINNELL,TheCaliforniaThrasher. [Oct.
the great interior valleys, there the Thrasher abounds.The
writer's personalfield acquaintancewith this bird givesbasisfor
the followinganalysisof habitat relations.
The California Thrasher is a habitual forager beneathdense
and continuouscover. Furthermore,probablytwo-thirdsof its
foragingis doneon the ground. In seekingfoodaboveground,
as whenpatronizingcascarabushes,the thrasherrarely mounts
to an exposedposition,but only goesas high as is essentialto
securingthe covetedfruits. The bird may be characterizedas
semi-terrestrial,but alwaysdependentupon vegetationalcover;
and this covermustbe of the chaparraltype, opennext to the
ground,withstronglyinterlacingbranch-workandevergreenleafy
canopycloseabove-- notforestunder-growth,orclose-set,upright
stemsas in new-growthwillow, or matted leafageas in rank-
growingannualherbage.
The Thrasheris relativelyomnivorousin its diet. Beal (Bio-
logicalSurveyBulletinno. 30, p. 55) examined82 stomachsof
Toxostomaredivivumandfoundthat 59percentof thefoodwasof a
vegetablenatureand41 animal. A largepart of thisfoodcon-
sistedof ground-beetles,ants,andseeds,suchasareundoubtedly
obtainedby workingoverthelitter beneathchaparral. The bird's
mostconspicuousstructuralfeature,thelongcurvedbill, isusedto
whiskasidethelitter, andalsoto dig,pick-fashion,intosoftearth
whereinsectslieconcealed.GroundmuchfrequentedbyThrashers
showsnumerouslittle pitsin thesollsurface,lessthananinchdeep,
steepononesideandwith a little heapof earthpiledup onthe
oppositeside. Asalreadyintimated,theThrasherattimesascends
tothefoliageabove,forfruitanddoubtlesssomeinsects.Muchin
thewayofberriesandseedsmayalsoberecoveredfromtheground
in whatisevidentlytheThrasher'sownspecializedmethodoffood-
getting.Evengrantingthisspecialization,I donotseewhythe
chaparral,alone,shouldaffordtheexclusiveforage-ground;forthe
samemodeof food-gettingoughtto bejustasusefulontheforest
floor,or evenonthe meadow.The furtherfact,of widelyomni-
vorousdiet,leadsoneto concludethat it is notanypeculiarityof
food-source,orwayofgettingat it, thatalonelimitstheThrasher
associationally.We must look farther.
The amateurobserver,orcollectorofspecimens,isstruckby the

7. 'Vol.XXXI¾]
1917 J GRXNN•LL,The CaliforniaThrasher. 433
apparent"shyness"of the Thrasher--by the easewith whichit
eludescloseobservation,or, if thoroughlyalarmed,escapesdetec-
tion altogether. For this protectiveeffect the bird is dependent
upon appropriatecover, the chaparral,and upon its ability to
co-operatein makinguseof thiscover. The Thrasherhasstrong
feet andlegs,andmuscularthighs,an equipmentwhichbetokens
powersofrunning;thet&ilisconspicuouslylong,asinmanyrun-
ning birds; and correlatlvelythe wingsare short,rounded,and
soft-leathered,indicatinglittle use of the flight function. The
colorsof the bird are non-conspicuous-blended,dark and light
browns. The nests of the Thrasher are located in dense masses
of foliage,from two to sixfeet abovethe ground,in busheswhich
areusuallya part of its typicalchaparralhabitat. In onlyexcep-
tionalcasesis the chosennestingsitelocatedin a bushor scrubby
tree,isolatedmoreor lessfromthe mainbodyof the chaparral.
Thesevariouscircumstances,whichemphasizedependenceupon
cover, and adaptation in physical structure and temperament
thereto,goto demonstratethenatureof the ultimateassociational
nicheoccupiedby the California Thrasher. This is oneof the
minornicheswhichwith their occupantsall togethermakeup the
chaparralassociation.It is, of course,axiomaticthat no two
speciesregularlyestablishedin a singlefauna have preciselythe
samenicherel•atlonships.
As a final statementwith regardto the CaliforniaThrasher,
wemayconclude,then,that its rangeisdeterminedby a narrow
phaseof conditionsobtainingin theChaparralassociation,within
the Californiafauna,andwithin the UpperSonoranlife-zone.