Mechanisms of Competitive Exclusion Between Two

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Mechanisms of Competitive Exclusion Between Two Species of Chipmunks
Author(s): James H. Brown
Source: Ecology, Vol. 52, No. 2 (Mar., 1971), pp. 305-311
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1934589 .
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MECHANISMS
EXCLUSION
OF COMPETITIVE
TWO SPECIES OF CHIPMUNKS'
JAMES
BETWEEN
H. BROWN
Department of Zoology, Universityof California, Los Angeles 90024
Abstract. Two species of chipmunks,Eutamias dorsalis and E. umbrinus exclude each other
fromcertainelevationson isolatedmountainrangesin the centralGreat Basin. Competitive
success is determinedby habitat;dorsalisexcludes umbrinusfromthe sparse pinion-juniper
forestsat lower elevationsand umbrinusexcludesdorsalisfromthe denserforestsat higher
habitat.
altitudes.The two speciesoccur togetheronly in a verynarrowstripof intermediate
of the two specieswithinthis overlap zone reObservationof the behaviorand interactions
sultedin the followingexplanationfor the mutualexclusion.E. dorsalis,the more aggressive
species,chases umbrinusfromthoseareas wherethe treesare so widely
and more terrestrial
shifts
advantageimmediately
spaced thatumbrinusmustfleeon the ground.The competitive
large and dense that
to the more social and arborealumbrinuswhenthe treesare sufficiently
In thesehabitatsumbrinusreadilyescapes dorsalisby fleeingthrough
theirbranchesinterlock.
thetreesover routesthatthe moreaggressivespeciescannotfollow.In such situationsthe agdisadvantageousbecause the more
gressivenatureof dorsalisactuallybecomescompetitively
social umbrinusis so numerousthatdorsaliswastesa greatdeal of timeand energyon fruitbetweenthe two speciesin aggressivebehaviorapparentlyrepreless chases. The differences
sent adaptationsto the densityof cover and food resourcesin their habitats.The main
aggresinteraction
betweenthesetwo chipmunks(interspecific
mechanismsof the competitive
sion, the abilityof the subordinatespecies to utilize some featureof the habitatto escape
fromthe dominantspecies,and aggressiveneglecton the part of the dominantspecies) may
exclusionbetweenhighlymobile animals.
in cases of competitive
be important
frequently
Competitiveexclusion has been documented both
in the laboratory (Gause 1934, Park 1962, and
others) and in the field (Connell 1961, Beauchamp
and Ullyott 1932, Hairston 1951, Tanner 1952,
Istock 1967, and others). However most of the cases
of competitiveexclusion in nature are based upon
circumstantial,distributionalevidence, and only Connell (1961) has done extensive experimentalwork
in the field and described mechanismsby which exclusion is effected.In his study of the interactions
between two species of sessile, intertidalbarnacles,
Connell showed that the distributionof one species
was limitedto the uppermostregion of the intertidal
zone because below that region it was physically
crowded out of the limited available space by a second species. This is of particular interestbecause
many of the observed cases of competitiveexclusion
in nature occur between species of highly mobile
vertebrateswhere the mechanismsof exclusion must
be very different.
The present paper describes the interactionsbetween two species of chipmunks (Eutamias dorsalis
and E. umbrinus) which exclude each other from
certainaltitudinalranges on numerousisolated mountain peaks in the central Great Basin. Hall (1946)
provided excellent distributionalevidence indicating
that competitiveexclusion occurred between the two
species. The present study was designed to answer
the question: How do the species utilize theirhabitats
and interactso as to exclude each otherfromall areas
but a narrow zone of overlap?
1-Received June1, 1970; acceptedAugust5, 1970.
METHODS
This study occupied most of two summers, 1968
and 1969. The firstsummerwas spent visitingmany
of the mountainrangesin centraland easternNevada
where one or both species of chipmunkwere known
to occur, in order to confirmHall's (1946) distributional data and become familiarwith the general distributionand ecology of the species. Three weeks
(August 5 to 26) of 1968 and most of the summer
(July 2 to August 7 and August 21 to September9)
of 1969 were spent in the Snake Range where a
study area was established on Baker Creek, elevation 2,300 m, 7 miles west of Baker, White Pine
County, Nevada. At this study area the microdistribution,habitat utilization,and interspecificinteractions of the chipmunkswere investigatedin detail
as describedbelow.
Microdistribution.-An area of approximately 1
mile2 (2.6 kM2) which included portions inhabited
by each species was censused regularly both summers.All parts of the area were covered at least twice
each summer and those places where the ranges of
the two species abutted or overlapped were observed
much more intensively. Censuses were made by
slowly walking through selected areas in the early
morning and plotting on a map the location and
species of each chipmunkobserved.
Interspecific aggression.-Although both species
were abundant in the studyarea both summers,indimobile and dispersed that
viduals were sufficiently
it was impossible to observe a significantnumber of
interspecificinteractions among animals in com-
JAMES H. BROWN
306
pletely natural circumstances.It was found that an
artificialfeedingstation,baited with crushedpeanuts,
attracted individuals of both species when it was
placed in an area where their altitudinaland habitat
ranges overlapped. The feeding station made it possible to observe numerous interspecific
-and intraspecificinteractionsbetween chipmunksand to compare the behavior of the two species in identical surroundings.The feeding stationwas maintained from
August 9 to 20, 1968, and from July4 to August 7
and August 23 to September 1, 1969. During these
periods observations were made 6 days per week
from sunrise,when the chipmunksfirstbecame active, until about 0930 hr, when activity began to
wane. Occasionally observationswere also made in
the late afternoon,but activitywas much less than
in the morning and they were not very profitable.
The chipmunks rapidly habituated to the presence
of the observer,who recorded each visit and aggressive encounter. In 1969 the individuals visitingthe
station were trapped, marked with colored plastic
discs rivetedthrougheach ear, and released. With the
chipmunks individually marked it was possible to
determine the total number of individuals visiting
the station,the frequencyof visitsby each individual,
and the fate of individualsinvolved in aggressiveinteractions.
Habitat utilization.-It was obvious that the two
species differedin the extent to which they were
arboreal or terrestrial.Two methods were used to
quantifythe nature of this difference.First, on four
differentdays the frequencywith which individuals
of each species used the various routes by which the
chipmunks approached the feeding station were recorded. Secondly, the amount of time spent in trees
or on the ground by individualsof each species was
recorded on 8 differentmornings.These latter observationswere made on undisturbedchipmunks at
least 300 m fromthe feedingstation.
RESULTS
AND DISCUSSION
General ecology and distribution.-Eutamias dorsalis and E. umbrinus are similar in size (body
weights are 55-68 and 51-80 g respectively) and
general body proportions.As chipmunksgo, they are
quite different
in coloration (dorsalis is pale, umbrinus brightlystriped) so thattheycan be distinguished
at a distance by an experienced observer. Both species feed mainly on the seeds and fruitsof a variety
of plants includingpifion(Pinus monophyla), juniper
(Juniperusosteosperma), mountain mahogany (Cercocarpus montanus), wild rose (Rosa sp.), prickly
pear (Opuntia sp.), and several grasses and forbs.
In addition,umbrinuswere observedfeedingon fungi
and the seeds and fruitof chokecherry(Prunus virginiana) while dorsaliswere seen feedingon the seeds
of cliffrose (Cowania mexicana). Both species pre-
Ecology, Vol. 52, No. 2
fer somewhat open, rocky habitats.Both live in burrows which they dig among rocks or at the base of
trees.
Both umbrinusand dorsalis are widely distributed
United States
in the forestedareas of the southwestern
in general and on the isolated mountainranges of the
central Great Basin in particular. On most of the
mountains of eastern and central Nevada (see Hall
1946 for details) both species are present and the
forestedhabitatsare partitionedaltitudinallybetween
them. E. dorsalis is restrictedto the sparse pifionjuniper associations on the lower slopes. At higher
elevations it is replaced by umbrinuswhich ranges
up to treeline.
There are two mountainranges where only one of
the two species occurs. E. dorsalis inhabitsthe Pilot
Range and umbrinusis foundin the Ruby Mountains.
Both of thesemountainrangesare found at the northern edge of the geographical distributionsof both
species and the absence of one may be attributedto
historical accident. Each of these mountain ranges
has large areas of habitat that is apparentlysuitable
for the missing species, but in its absence the other
species has expanded its altitudinalrange to include
all forested habitats from the lowest pinons and
junipersto treeline(Hall 1946; confirmedin the present study). This observationthat each species occupies a wider range of habitats in the absence of the
other than it does where both species occur together
is excellent circumstantialevidence that their distributions are limited by competitive exclusion when
both species inhabitthe same mountain.
No direct attemptwas made to determinethe resource for which the chipmunksare ultimatelycompeting,but two observationssuggest that it is food.
First,the animals dig theirown burrowsand suitable
sites appear to be so numerous that it is hard to
imagine that shelter could be a limiting resource.
Secondly,individualsof both species engage in rigorous conteststo obtain access to local concentrations
of food. This is particularlyobvious in summer and
early fall when seeds and fruitsare ripeningand the
chipmunks are storing them away for the winter.
The bait put out at the feeding station not only attractednumbers of both species to feed, but it also
apparentlyinduced several umbrinusto establishpermanentresidencein the immediatevicinity.It should
be noted that similarconcentrationsand interactions
of chipmunksoccur at natural concentrationsof food
as well as at the artificialfeedingstation.I have observed up to fiveumbrinusand two dorsalis (on differentoccasions) feedingin and under a singlepifion
with an exceptionallyheavy crop of cones. The preceding observation counters the possible objection
that the behavioral interactionsof chipmunksat the
feedingstationreportedhere were the aberrantresults
of totallyunnaturalconcentrationsof individuals.
EarlySpring1971
COMPETITION IN CHIPMUNKS
.~~~~~~~~~5
-0
'4+-+
'4
.'',',','',+
Ap.;~~~~
*.':.:
C'''
307
Ai
4~~~~--
KEY;
0
E. umbrinus + E dorsalis
L
and junipers
widely spaced
Pilons
Dirt roads
FIG.
1969.
-
j
Streams
1W Dense thickets
of aspen,
chokecherry,
rose
No trees or shrubs
(meadows or bare ground)
--- Contourlines
*
El! Pinionsand junipers,
g
branchesfrequently
interlocking
Rocky cliffs
Feeding station
-.20 kmI
of the two speciesof chipmunkson the studyarea duringJulyand August
1. The microdistributions
Microdistribution.-The location of sightings of
individualchipmunkson the studyarea in the Snake
Range during 1969 are shown in Figure 1. In 1968
the patternwas essentiallyidentical,and it is possible
to tell fromfieldnotes and specimensin the Museum
of Vertebrate Zoology (University of California,
Berkeley) that the distributionsof the two species
have not changed significantly
since fieldparties visited the Baker Creek area 40 years ago. The distributions of the species are largely nonoverlapping.
JAMES H. BROWN
308
\+
\
Ala
~~~OBSERVAIOPOSX
Ecolo-rvVol. 52,No. 2
+
FIG. 2. Detail of the habitatin the vicinity
of the feedingstationwhereboth speciesof chipmunksoccur.
Hatched areas indicatebare, rockyground;unhatchedareas, soil and littersubstrate.Large irregularoutlines
indicatethe canopyspreadof the followingtrees:pifon,circlewithcross;juniper,unshadedcircle;mountain
mahogany,shaded circle.
measuring more than 100 m in greatestdimension,
the zone of overlap is narrow relative to the move*
E. dorsalis
mentsof the chipmunksthemselves.
20 The small degree of distributionaloverlap and its
temporal stabilitysuggeststhat the shiftin competitive advantage fromone species to the othermust be
.-J
dependent upon some relativelypermanent feature
o
of their environment.It is apparent from Figure 1
that the distributionsare conspicuously correlated
Q~10
withthe densityand size of trees.The transitionfrom
one species to the other does not correspond to an
5
ecotone betweenplant communities,but occurs within
the pifion-juniper
association.
E. dorsalis is restrictedto the stands of pifion0-10
40-20
20-30
30-40
40-50
50-60
60-70
80-90
90-100
juniper where the trees are small and spaced so that
PER CENT
OF TIME ON GROUND
there is considerable open ground between them.
FIG. 3. Frequencydistribution
of the amountof time When the pifions and junipers become sufficiently
spenton the ground,as opposedto in trees,by each spe- dense that some of theirbranches
interlock,dorsalis
cies. The difference
betweenspeciesis highlysignificant
is replaced by umbrinus. An area of intermediate
(P < 0.001).
habitat,where both species occur, is mapped in deOnly in a narrow zone, varyingin width from a few tail in Figure 2. It consistsof interdigitating
areas of
meters to about 200 m, can both species be found. open, rocky ground and stands of pi-nonand juniper
Since these chipmunksfrequentlyhave home ranges with interlockingbranches. E. umbrinusalso inhabits
25
-
EDE.
'1
umbrinus
EarlySpring1971
\+\X
COMPETITION IN CHIPMUNKS
OBSERVATIO POST
309
+
FIG. 4. Paths utilizedby each speciesas it approachedthe bait station.Pathsof umbrinus
are indicatedby
completeshading,thoseof dorsalisby cross banding.Othersymbolsare explainedin Figure2.
other dense stands of trees including aspen groves,
chokecherrythickets,and, at higher elevations, forests of mixed conifers.
Several otherpairs of vertebratespecies have strikingly similar distributionson the mountains of the
Great Basin, and this may reflecta similar effectof
vegetation structureand competitiveinteractionon
these species. They include desert and bushy-tailed
woodrats (Neotoma lepida and N. cinerea), Scrub
and Steller's Jays (Alphelecoma coerulescens and
Cyanositta stelleri), and Plain Titmouse and Mountain Chickadee (Parus inornatus and P. gambelli).
The microdistributionsof the last-mentionedpair
correspond almost exactly to the local distributions
of E. dorsalis and E. umbrinus respectively.This
correspondence is maintained despite the much
greater mobility of the birds and I have watched
flocks of titmice fly over or around vegetation of
inappropriatedensitywithoutstoppingto forage. Effects of vegetationstructureon interspecificinteractions are apparentlyof quite general occurrence, at
least in vertebrates.
Rosenzweig and Winakur (1969) have recently
found habitat structureto have an importanteffect
of desert rodents and Cody
on the microdistribution
(1968) has analyzed its influenceon the competitive
relationshipsbetween species of birds in grasslands.
Habitat utilization.-Despite the fact thatboth species harvestmuch of theirfood in trees,umbrinusis
much more arborealthan dorsalis. It spends a greater
proportionof its time in trees than dorsalis (Fig. 3)
and when approaching the feeding station it tended
to dash out from a nearby clump of trees whereas
dorsalis usually approached on open, rocky ground
(Fig. 4). This verysignificantdifferencein the ability
of the two species to utilize trees is also apparent
from the following qualitative observations. When
approached by man, or a predator,dorsalis usually
runs away on the grounduntilout of sight,and when
disturbedit will often descend from a tree and flee
in this manner. On the other hand, umbrinususually
climbsthe nearestgood-sizedtree and "freezes"when
it is disturbed. In interspecificaggressiveencounters
an umbrinusfrequentlyescaped froma pursuingdorsalis by climbinga tree and eitherrunningout to the
tip of a long, thin branch or crossingto anothertree
throughthe branchesand the dorsalisfailed to follow.
Interspecificaggression.-The feedingstationswere
highlysuccessfulat attractingmembers of both species and a large number of interspecificand intra-
Ecology,Vol. 52,No. 2
JAMES H. BROWN
310
TABLE
at the feedingstationin 1969
1. Visitsand aggressiveinteractions
Aggressiveencounters
Visits
ummb
Total number...............
....
.....
Observedfrequencyb
Expected frequencyb
Number of individuals.......
dor
umb > umb
1,162
946
143
551
.449
18
11
.203
.304
13
dor> umba
umb> dora
310
83
dor > dor
166
.237
.201
9
.560
.495
21
theloseris thespeciesontheright.
is thespeciesontheleftofthe> symbol,
aThewinner
andare calculated
tospeciesidentity,
withrespect
wererandom
amongindividuals
ifencounters
arethoseexpected
encounters
ofaggressive
frequencies
bTheexpected
is highly
ofencounters
significant
ofvisitsbybinomial
frequencies
andexpected
between
observed
theobserved
expansion.Thedifference
(X2 = \33.2;
from
frequency
P< .005).
specific behavioral interactionswere observed. The
resultsof the observationsat the feedingstationduring 1969 are summarizedin Table 1. As soon as the
9
O---O=E.umbrinus
__
o
chipmunkswere coming to the station regularly,an
Z 8
dorsaIis
-=E.
interspecificdominance heirarchywas rapidly estab57
lished; higher ranking individuals chased away and
?5
defended the feeding area against subordinate an6~~~~~~~~imals. E. dorsalis was significantlymore aggressive
than umbrinus.Individuals of dorsalis occupied the
/ four highest positions in the dominance hierarchy,
3
of all interspecificencounters, and
won four-fifths
engaged in more interspecificand intraspecificinterI
l
l
j~~~~~~~~~~~~
Io
actions than predictedon the basis of the number of
10
30
5
20
25
15
10
-AUGUST
JULYEvisits of each species. Qualitativelyit was apparent
of each speFIG. 5. Numbersof individualchipmunks
that individualsof dorsalis invariablyapproached the
feeding station alone whereas those of umbrinus cies visitingthe feedingstationduringa 5-weekperiod
in 1969.
frequentlytraveledin pairs made up of varyingcombinationsof ages, sexes, and individuals.
The aggressivenature of dorsalis is apparentlyan in injuryto the umbrinusbecause they immediately
adaptation to its sparsely vegetated habitat, where fled to nearby trees and evaded the pursuingdorsalis
selectionfavors those chipmunksthat are sufficiently as describedin the previous section.
The data presented in Figure 5 suggest that the
aggressive to defend the scarce, widely distributed
food sources against nonspecificand heterospecific continuedutilizationof the bait by large numbersof
competitors.Where the local distributionsof the two umbrinusled to the abandonmentof the station by
species overlap, umbrinus is more numerous and several dorsalis. Certainly the number of dorsalis
more tolerantof other chipmunksthan dorsalis. This visitingthe stationdeclined and this decline was not
probably reflectsnot only reduced selection for the owing to mortalitybecause two marked dorsalis freaggressivedefense of a large area, but also selection quentlywere seen in the general area, but many yards
for some sort of loose social association because of fromthe station,aftertheir last visit. The following
the advantage of having additional animals nearbyto sequence of interspecificinteractions,frequentlyobdetect and give warningof concealed predators.Cer- served once more than five or six umbrinus were
tainly the opportunitiesfor a predator to approach visitingthe bait, probably accounts for the decline in
unobservedare much greaterin the denselyvegetated the number of dorsalis in the immediate vicinityof
the station.A dorsalis would chase an umbrinusfrom
habitats.
Between July 4 and August 7, 1969, the longest the bait into a nearby tree and returnto the station
period that the feeding station was in continuous only to find it occupied by another umbrinusbusily
operation,therewere interestingchanges in the num- feeding or filling its cheek pouches. The dorsalis
ber of individualsof each species that were using the would banish the second umbrinus to a tree, but
bait (Fig. 5). Although dorsalis began to visit the meanwhile the firstumbrinuswould have slipped in
bait first,new individualsof umbrinuswere attracted again to feed, and so the process would continue.
untiltheywere twice as numerousas dorsalis. Addi- Often the dorsalis would eventuallyleave the station
tional umbrinuswere recruitedto the station despite withouthaving fed. In such cases the aggressivenarepeated aggressive encounters (which they usually ture of dorsalis obviouslyworked to its disadvantage.
and competitivelydisadvantageous
lost) withdorsalis. These encountersseldom resulted Similar inefficient
v
I
t
Early Spring 1971
COMPETITION
IN CHIPMUNKS
311
effectsof
self-defeating
effectsof interspecificaggressionhave been described Miller 1967). The inefficient,
in several species of birds (Ripley 1961) and termed interspecificaggression in some circumstanceshave
been observed in several bird species (Ripley 1961).
aggressiveneglect.
Differencesin abilityto locomote and take refugein
CONCLUSIONS
certain portions of the habitat such as trees or tall
I conclude that mutual competitiveexclusion be- buildings,in addition to differencesin aggressiveness,
tween the two species of chipmunksis effectedin the are an importantpart of the interaction between
followingmanner.E. dorsalis is more aggressivethan Norway and roof rats (Ecke 1954). It certainlyseems
E. umbrinus and is better at moving over open that interspecificaggressionwhich seldom results in
widelyspaced that mortalityor even injury is a common element in
ground.When treesare sufficiently
its competitormust flee significantdistances on the many cases of competitiveexclusion between highly
defendslocalized resources mobile animals. The effectof the aggressionis counground,dorsalis efficiently
and excludes umbrinusby means of interspecificag- terbalanced by the abilityof the subordinatespecies
gressive encounters.The aggressionrapidly becomes to take refuge in some portion of the habitat from
ineffectivewhen the trees become so closely spaced which the dominant species cannot displace it.
that the more arboreal umbrinuscan readily escape.
ACKNOWLEDGMENTS
In fact, once the densityof trees reaches a critical
My wife,Astrid,providedinvaluableassistancewith
level, umbrinus (which is more numerous and more
tolerantof other chipmunksthan dorsalis) is able to all phasesof the project.G. A. Bartholomewand M. L.
read and criticizedan earlierdraftof
exclude dorsalis because the latter wastes excessive Cody have kindly
The cooperationof the personnelof the
the manuscript.
time and energyon fruitlesschases.
U. S. ForestServicein Bakerand Ely, Nevada, did much
The interactingbehavioral mechanisms of exclu- to makethefieldworkpleasant.
from
sion betweenthese chipmunksare verydifferent
LITERATURE CITED
the simple physical destructionby crowdingthat ocand P. Ullyott.1932. Competitive
A.,
R.
S.
Beauchamp,
curs in barnacles (Connell 1961). In fact the partricertainspeciesof freshwater
between
relationships
ticular combination of behavioral patterns in each
clads. J.Ecol. 20: 200-208.
species of chipmunkand the way that these interact Cody, M. L. 1968. On methodsof resourcedivisionin
in interspecificencounters seems highly specialized
grasslandbird communities.Amer. Naturalist102:
107-147.
for this particular situation and hardly likely to be
coma general featureof competitiveinteractions,even in Connell,J. H. 1961. The influenceof interspecific
of the
petitionand other factorson the distribution
verysimilar,highlymobile vertebrates.In this regard
barnacle Chthamalus stellatus. Ecology 42: 710-723.
it is interestingto compare the resultsof the present Ecke, D. H. 1954. An invasionof Norwayratsin southstudywiththe unpublishedresultsof a studyby D. H.
westGeorgia.J. Mammal. 35: 521-525.
Sheppard (reported in Miller 1967) of competition Gause, G. F. 1934. The strugglefor existence.Williams
and Wilkins,Baltimore.
between two other species of chipmunks (Eutamias
N. G. 1951. Interspeciescompetitionand its
Hairston,
minimum
and E. amoenus) in westernAlberta, Canof Approbableinfluenceon the verticaldistribution
ada. Some aspects of that interactionappear similar
palacian salamandersof the genusPlethedon.Ecology
to those between E. dorsalis and E. umbrinus,as the
32: 266-274.
competitorsdivide the habitat on the basis of density Hall, E. R. 1946. Mammals of Nevada. Universityof
CaliforniaPress,Berkeleyand Los Angeles.
of vegetation and interspecificaggression plays an
Istock, C. A. 1967. Transientcompetitivedisplacement
importantrole in exclusion. Others, however, are
bettles.Ecology48:
in naturalpopulationsof whirligig
quite different;the distributionof only one of the
929-937.
species is limitedby competitiveexclusion, and it is Miller,R. S. 1967. Patternand processin competition.
Adv. Ecol. Res. 4: 1-74.
the forest-dwellingspecies that is more aggressive.
and populations.SciT. 1962. Beetles,competition
Park,
From the fragmentarydata available, it also appears
ence 138: 1369-1375.
that most of the importantcomponentsof the inter- Ripley,S. D. 1961. Aggressiveneglectas a factorin inactions betweendorsalis and umbrinushave been imin birds.Auk 78: 366-371.
competition
terspecific
plicated in cases of exclusion between other species Rosenzweig,M. L., and J. Winakur.1969. Population
habitatsand enof both vertebratesand invertebrates.Interspecific ecologyof desertrodentcommunities:
Ecology50: 558-572.
complexity.
vironmental
aggression plays an important role in competitive Tanner,J. T. 1952. Black-cappedand Carolina Chickinteractionsbetween rats, mice, blackbirds, titmice,
adees in the southernAppalachian Mountains.Auk
hummingbirds,and ants (many sources reviewed by
69: 407-424.
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