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VI'ilXyXIV GRINNELL,heCaliforniahrasher. 427
into the body of the partly eaten bantam and replaced t in the
same spot where he found it. Next morningthe seemingly m-
possible as madea practicalcertainty, or he foundthe body of a
screech wl with the clawsof one foot firlnly imbedded n the body
of the bantam. He very kindly presentedme with the owl which,
upon dissection, roved to be a female, ts stomacheontalninga
very considerablemountof bantam fleshand feathers, ogether
with a great deal of wheat. (It seelnsprobable hat the wheat
was accidentallyswallowedwith the crop of the bantam during
the feast,but therewas so much hat it seelns trange he owl did
not discard t while eating). How a bird only 9.12 inchesn length
could have dealt out such havoc in so short a time is almost in-
credible,but, althoughpurely circumstantial,he evldenee gainst
the owl appeared ltogether oo strong or evena reasonable oubt.
The doctorand I wished o make as certain as possible, owever,
so the poisonedbantam was replacedand left for severaldays,
but without any further results. For the abovementioned easons
I am rather doubtful as to the net value of this owl from an
economictandpoint,lthoughirdsn a wildstatwould ot
give theln such opportunities or such wanton killing as birds
enclosedn pens.
THE NICHE-RELATIONSHIPS OF THE CALIFORNIA
THRASHER?
BY JOSEH GRINNELL.
TUE California Thrasher (Toxostomaedivivum) s one of the
severalistinctird ypes hichharacterizeheso-calledCali-
fornian Fauna. Its range is notably restricted, even more so
than that of the Wren-Tit. Only at the south does he California
Thrasher occurbeyond he 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.
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[Auk
428 GRINNELL,heCaliforniahrasher. toct.
San Quintin, not more than one hundredand sixty milesbelow he
Mexican line in Lower California.
An explanationof this restricteddistribution s probably o be
found n the closeadjustmentof the bird in variousphysiological
and psychologicalespects o a narrow range of environmental
conditions. The nature of these critical conditions is to be learned
throughan examination f the bird's habita.t is desirableo
make suchexamination t as many points n the general angeof
the species s possible ith the objectof determininghe elements
common o all thesepoints,and of these he onesnot in evidence
beyond the limits of the bird's range. The followingstatements
in this regardare summarizedrom the writer's personal xperience
combinedwith all the pertinent nformationafforded n literature.
The distributionof the CaliforniaThrasheras regards ife-zone
is unmistakable. Both asobservedocallyand over ts entire ange
the species hows loseadherenceo the Upper Sonorandivision
of the Austral zone. Especiallyupwards, s it always sharply
defined. For example, n approachinghe sea-coast orth of San
FranciscoBay, in SonomaCounty, where the vegetation s pre-
vailingly Transition, thrashersare found only in the Sonoran
islands, namely southerly-facing ill slopes,where the maxi-
mum insolationmanifests ts effects in a distinctive chaparral
containing uch ower zoneplants as Adenostoma. Again, around
Monterey, to find thrashers ne must seek the warm hill-slopes
back from the coastalbelt of conifers. Everywhere have been,
the thrashers eem o be very particularnot to ventureeven a few
rods into Transition, whether the latter consistof conifersor of
high-zonespeciesof manzanita and deer brush, though the latter
growth resembles losely n densityand general appearancehe
Upper Sonoranchaparraladjacent.
While sharplydelimited,as an invariable ule, at the upperedge
of UpperSonoran,he CaliforniaThrashers not socloselyestricted
at the loweredge of this zone. Locally, individuals occur, and
numbersmay do so where assoeiationalactors favor, down well
into Lower Sonoran. Instancesof this are particularlynumerous
in the San Diegan district; for example, n the Lower Sonoran
washes at the mouthsof the canyons longthe southbaseof the
San Gabriel Mountains, as near San Fernando, Pasadena, and
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ol. XXXI1
1917 J GRIXnL, The California Thrasher. 429
Azusa.A noticeablehing n thisconnection,owever,s that,
on the desert lopes f the mountains, hereToxostomaecontei
occursn the desertlooras an assoeiationalomologuef T.
redivivumn theLower onoranone,he atter staysut ar
3. Toostorna. pasadenense
' - ' Specimensxamined
Publishedecords
Figure 1.
moredosely; that is, it straysbut little or not at all below he
typicalconfinesf its ownzone,namelyUpperSonoran.The
writer'sieldwork n thevicinity f Walker ass, ernCounty,
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[-Auk
430 GRINNELL,heCaliforniahrasher. LOct.
provides ood llustrations f 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 thencedownalong the
valley of the South Fork of the Kern River nearly to Isabella.
Leeonte's Thrasher is a conspicuouslement in this Lower
Sonoran invasion, but no California Thrashers were met with in
this regionbelow he belt of goodUpper Sonoran n the flanking
mountain ides, s markedby the presencef diggerpine,blueoak,
sumaeh, silk-tasselbush, and other good zone-plants. Similar
zonal relationships re on record from San GorgonioPass, River-
sideCounty,as well as elsewhere.
Reference owto' he general angeof the bird underconsidera-
tion (seep. 429), as comparedwith a llfe-zonemap of California
(PacificCoastAvifaunaNo. 11, Pls. I, II), will show o a remas'k-
abledegree owclosely he formercoincides ith the UpperSono-
ran zone. The thrasher s, to be sure,one of the elementsupon the
presencef which his zonewasmarkedon the map; but it was
only one of many, both plant and animal; and it is concordance
wi];hheaggregatehat ssignificant. iagnosisfzonationimi-
larly is possiblen scores f placeswherechangen altitude(which
as a rule means hangen temperature)s the obviousactor,as up
the west flank of the Sierra Nevada, or the north wall of the South
Fork valley, alreadyreferred o, in Kern County, or on the north
wall of the San Jaeinto 3/ountains. The California Thrasher is
unquestionablyelimited n its range n ultimate analysis y tem-
peratureeonditlons.The isothermie rea it occupiess in zonal
pax'lance, pper Sonoran.
The secondorder of restriction s faunal, using this term in its
narrowed ense,ndleatingdependenceponatmospheric umidity.
The CaliforniaThrasherdoesnot range nteriorly nto excessively
arid country,although he UpperSonoran onemay, as around he
southern end of the Sierra Nevada, continue uninterruptedly
towardsthe interior in a generally atitudinal dh'eetion. This
is true where extensiveareas are considered, ut locally, as with
zones, ndividualsor descent-lines ay have invadedshort dis-
tanees beyond the normally preferred conditions. An example
of this situation s to be foundon the north and westslopes f the
San Jacinto3/ountains,where CaliforniaThrashers ange around
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Vol. XXXiV1
1017 J GRINNELL,heCaliforniahrasher. 431
onto arid chaparralslopes,nterminglingwith such arid Upper
Sonoranbirds as Scott'sOrioleand the Gray Vireo. It is question-
able, however, as to what extent faunal restriction really operates
in this case; for reference o the zone map, again, shows hat a
vast tract of LowerSonoran,ying to the eastof the desertdivides,
extends ontinuously orth to the head of OwensValley. Really
the onlyunbroken ridgeof UpperSonoranowards he east rom
the west-Sierran habitat of Toxostoraa redivivura is around the
southern ndof the SierraNevada - a very narrowand ongroute
of possible migration,with consequentactors unfavorable o
invasion, rrespective f either temperatureor humidity, such as
interruptedassociationalequirements nd small aggregate rea.
In this particularbird, therefore, aunalrestrictionmay be of minor
importance,as comparedwith zonal and associationat ontrols.
That faunal conditions ave had their influenceon the species,
however, s shownby the fact of geographic ariationwithin its
range. The thrasher throughout ts habitat-as-a-whole,s sub-
jected to different degrees f humidity. Amount of rainfall is,
in a generalway, an index of atmospheric umidity, thoughnot
withoutconspicuousxceptions. Comparinghe mapof the ranges
of the subspeciesf T. redivivurap. 429) with a climaticmapof the
State, direct concordances observedbetween areas of stated rain-
fall on the latter and the rangesof the respective ubspecies. t
will be seen that the race T. r. pasadenenseccupiesan area of
relatively ow humidity, he race T. r. sonoratef higherhumidity
and the race T. r. redivivura f highesthumidity, in fact a portion
of California's og-belt. The distinctivecolor-tones eveloped re,
respectively,of gray, slate and brown casts. In the thrasher,
therefore,we may look to faunat nfluences s havingmost to do
with differentiationwithin the species. In this case t is the faunal
variationover the occupied ountrywhich s apparently esponsible
for the intra-specificudding, r, in otherwords, he origination f
new specific ivarications.
qerever it occurs, nd in whichever f the three subspeciest is
represented,he CaliforniaThrasherevinces' trongassociational
predilections. t is a characteristiclement n California'samous
chaparralbelt. Where this belt is broadestand best developed,
as in the San Diegandistrictand in the foothill regions ordering
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[Auk
432 GRINNELL,heCaliforniahrasher. [Oct.
the great interior valleys, there the Thrasher abounds. The
writer's personal ield acquaintancewith this bird givesbasis or
the followinganalysisof habitat relations.
The California Thrasher is a habitual forager beneath dense
and continuous over. Furthermore, probably two-thirdsof its
foraging s done on the ground. In seeking ood aboveground,
as when patronizingcascarabushes, he thrasher arely mounts
to an exposedposition,but only goesas high as is essential o
securing he coveted ruits. The bird may be characterized s
semi-terrestrial,but always dependentupon vegetationalcover;
and this covermust be of the chaparral ype, open next to the
ground,with stronglynterlacing ranch-work nd evergreeneafy
canopy lose bove- not orestunder-growth,r close-set,pright
stems as in new-growthwillow, or matted leafage as in rank-
growing nnualherbage.
The Thrasher s relativelyomnivorousn its diet. Beal (Bio-
logicalSurveyBulletinno. 30, p. 55) examined 2 stomachs f
Toxostomaedivivum nd found hat 59 percentof the foodwasof a
vegetable ature and 41 animal. A large part of this food con-
sistedof ground-beetles,nts,andseeds, uchas are undoubtedly
obtainedby workingover he litter beneath haparral. The bird's
mostconspicuoustructural eature, he longcurvedbill, is used o
whiskaside he litter, and also o dig, pick-fashion,nto softearth
where nsectsie concealed. Groundmuch requented y Thrashers
shows umerousittle pits in the sollsurface,ess han an inchdeep,
steepon onesideand with a little heapof earthpiledup on the
oppositeide. Asalreadyntimated,heThrasher t timesascends
to thefoliage bove,or fruit anddoubtlessomensects. Much n
thewayof berries ndseedsmayalsoberecoveredrom he ground
in what s evidently he Thrasher's wnspecialized ethodof food-
getting. Evengrantinghisspecialization,do not seewhy the
chaparral,lone, hould fford he exclusiveorage-ground;or the
samemodeof food-gettingught o be just as usefulon the forest
floor,or evenon the meadow. The furtherfact, of widelyomni-
vorousdiet, leadsoneto concludehat it is not any peculiarityof
food-source,r way of getting t it, that alone imits he Thrasher
associationally.We must look farther.
The amateurobserver, r collector f specimens,s struckby the
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1917 J GRXNNLL, he CaliforniaThrasher.
433
apparent shyness of the Thrasher--by the easewith which it
eludescloseobservation,or, if thoroughlyalarmed, escapes etec-
tion altogether. For this protective effect the bird is dependent
upon appropriatecover, the chaparral, and upon its ability to
co-operaten makinguse of this cover. The Thrasherhas strong
feet and legs,and muscular highs,an equipmentwhichbetokens
powersfrunning;he &il sconspicuouslyong, s n manyun-
ning birds; and correlatlvely he wings are short, rounded,and
soft-leathered, ndicating little 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, rom two to six feet above he ground, n busheswhich
are usuallya part of its typicalchaparralhabitat. In only excep-
tional casess the chosen estingsite located n a bushor scrubby
tree, isolatedmoreor less rom the main body of the chaparral.
Thesevariouscircumstances,hichemphasize ependencepon
cover, and adaptation in physical structure and temperament
thereto,go to demonstratehe nature of the ultimate associational
niche occupiedby the California Thrasher. This is one of the
minor nicheswhichwith their occupantsll togethermake up the
chaparral association. t is, of course,axiomatic that no two
species egularly establishedn a single auna have precisely he
same iche elatlonships.
As a final statementwith regard to the CaliforniaThrasher,
we may conclude,hen, that its range s determined y a narrow
phaseof conditions btainingn the Chaparralassociation,ithin
the California auna, and within the Upper Sonoran ife-zone.