Grinnell 1917 the Niche-relationship of the California Thrasher

8/10/2019 Grinnell 1917 the Niche-relationship of the California Thrasher

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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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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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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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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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'Vol. XXXI]

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.

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Grinnell 1917 the Niche-relationship of the California Thrasher