TERRITORIAL
SONG SPARROW
INTERACTIONS
AND BEWICK'S
IN SYMPATRIC
WREN POPULATIONS
ROBERT E. GORTON, JR.
AI•STRACT.--The
similarityin song,habits,andhabitatin theSongSparrowandBewick'sWren
stimulated
a studyof theirinteractions
in western
Washington.
Bothspecies
respond
tomodels
and
songof the otherspecies
moreintensely
thanto the Black-capped
Chickadee,
but lessintensely
than to their own species.
Both birdsshoweda gradualdecrease
in aggressive
behaviorto conspecifics
over the breeding
season,
the Bewick'sWrenresponse
risingsomewhat
earlyin theseason
beforegraduallydeclining
over the restof the studyperiod.While the Bewick'sWren showeda strongdecrease
in response
levelsto theSongSparrowovertime,theSongSparrowshoweda significant
dropin aggressive
response
towardthe Bewick'sWren at the timethe first broodwasfledged,the response
rising
again at the time of the appearanceof the seconddutch.
The SongSparrowandtheBewick'sWrenrepresent
a sustained
bidirectional
system
of interspecificaggression.
Thissystemmaybe moresensitive
to changes
in breedingbehaviorovertheseason
thanis theintraspecific
system.Wheretheycoexisttheyforageat slightlydifferentheightsandon
slightlydifferentfood, but overlapsufficientlyso that aggression
and similarityof songhave
evolvedas still other mechanisms
to reducecompetition.--Department
of Biology,Universityof
PugetSound,Tacoma,Washington
98416.Presentaddress:
DepartmentofEntomology,
University
of Kansas, Lawrence, Kansas66045. Accepted27 February 1976.
A TERRITORY
is defined as "any defendedarea" (Noble 1939). It is a fixed area
from which intruders are discouragedby advertisement, threat, attack, or some
combination of these behaviors (Brown 1975).
Severalsuggestions
concerning
the ultimatefactorsresponsible
for the evolutionof
territorial behavior have been made. Some authors stressthe benefits accrued by
familiarity with an area and the necessityof preventingdisruptionof the events
between pair-bond formation and the fledgingof young (Howard 1920, Nice 1941).
Othersview territorialityas a spacingmechanismfor individualsand nests,checking
predationand disease(Hinde 1956).Tinbergenet al. (1967)and Horn (1968)verified
experimentallythat predationincreases
with populationdensity.Cody(1971)stated
that predationis the singlegreatestcauseof reproductivefailure in mostbirds. The
adaptive value of protectingpotentiallylimiting resourceshas been the subjectof
extensivediscussions(Brown 1964, arians and Willson 1964, Immelmann 1971).
Brown (1964)arguedthat aggressive
behavioris oftenemployedin the acquisitionof
defensiblegoalsthat tend to maximizeindividual survival and reproduction.
An increasingbodyof evidenceexistson consistentterritorial interactionsbetween
species(arians and Willson 1964;Cody 1968, 1969, 1974;Cody and Brown 1970)as
well as within them. In the caseswhere ecologicallysimilar speciesoccur together,
one would expectnatural selectionto favor ecologicaldivergenceand a consequent
reductionof competition.Lack of divergencecould mean (1) that insufficienttime
has elapsedfor the changesto be completed,(2) that someenvironmentalconditions
preventdivergence,e.g. structurallysimpleenvironments
or stronglystratifiedfeeding sites in structurally complex vegetation, or (3) that other speciesare already
exploitingsimilar resourcesin a diversifiedhabitat (arians and Willson 1964). When
adequateecologicaldivergenceis not possible,convergencemay occurin thosebehaviors and characteristicsassociatedwith spacingand territoriality. Such characteristics include song, aggressive display, and external morphology (color and
701
The Auk 94: 701-708. October 1977
702
ROBERTE. GOaTON,JR.
[Auk, Vol. 94
patterns). Interspecificterritoriality need not be an all or nothing phenomenon.Intermediate casesmay occur in which rather lessecologicallysimilar speciesexhibit only
partial territorial overlap (Cody 1974).
Bewick's Wren (Thryomanesbewickii) and the Song Sparrow (Melospizamelodia)
commonlyco-existin broadleaf forests,riparian woodlands, shrubbythickets, parks,
and gardens in northwestern Washington. The wren is mainly insectivorous and
feeds its young primarily Lepidopteran larvae and adults up to about 2 cm long
(Miller 1941, Bent 1948). The Song Sparrow eats both seedsand insects, although it
feedsits young largely or entirely on animal matter (Schoener1968), suchas caterpillars and lacewings(Tompa 1962). Accordingto Stiles(1973) the two speciesoverlap
somewhat in foraging heights, behavior, and technique (gleaning) in Washington
state. They establish territories at about the same time and resemble each other in
song through parts of their ranges (Bent 1948, 1968; Jewett et al. 1953; Peterson
1961). Spectrogramsof the songsof the two speciesillustrate graphically a remarkable resemblanceof song (Robbins et al. 1966). Miller (1941) noted that the Song
Sparrow was known to pursue Bewick's Wren when it saw the latter intruding in its
territory. L. S. Best, W. J. Erckmann, and L. A. Fairbanks (MS) found that Song
Sparrowsin the University of WashingtonArboretum in Seattlecould be inducedto
attack a Bewick's Wren mount accompaniedby its recorded song. These observations suggestthe possibilityof sustainedterritorial interactions.
MATERIALS AND METHODS
This study was undertakenapproximately5 mileswest of Burlington,Skagit County, Washington.
Observationswere made within an area of about 4.5 square miles containinga variety of habitat types:
meadow,swamp,pond, densebrush, clearings,unclearedloggedareas,and second-growth
deciduous,
evergreen,and mixedforests.The field studieswere madefrom 12 March to 13 June 1974.
I prepared two mounts each of the Song Sparrow and Bewick's Wren and one mount of the
Black-cappedChickadee(Parusatricapillus),all in a perchingpose.The birdsfrom which the mounts
were prepared were collectedfrom the study tract. Brown glasseyes were used in all mounts.
Songsof theBewick'sWren wererecordedin SkagitCountyon a Sony110-Acassette
recorder.Songsof
the SongSparrow (Cornell Cut 40) and the Black-cappedChickadee(Cornell Cut 24) were recordedin
Fullerton,California,and Michigan, respectively.
All recordedsongswereplayedin the field on a Sony
TC-72
cassette recorder.
The subjectbirds(thosebeingtested)werelocatedprimarily by song.The modelwas placedwithin the
territory of the subjectas closeto the singingbird as possible.The songwasthen playedat full volume and
the responseof the subjectto the modeland songnoted, along with temperature,habitat, and weather
conditions.The cassetterecorderwas not camouflagedbut was hidden wheneverpossible,always as close
to the model as possible.I was often distant, but probably visible, assuminga low profile during the
experiments,but on severaloccasionsattackson the model took place within inchesof me, indicatingno
fear responseinterfering with the responsesto the model. Individual birds were sometimestestedmore
than once,but nevermorethan four timesduringthe season.No bird wastestedtwicein the sameday.
Experimentswerestoppedduringrain or darkness.Early in the studyI pacedoff distances
fromthe model
to the subjects'nearestpoint of approach. Later I approximatedthe distancesby eye.
The SongSparrowwastestedagainstmodelspresentedin the sequence
SongSparrow,Bewick'sWren,
Black-cappedChickadee,SongSparrow;the Bewick'sWren againstBewick'sWren, SongSparrow,
Black-cappedChickadee,Bewick'sWren. Modelswerechangedafter a response
or a periodof from 5 to
15 min, alwaysallowingampletime for a response.
A minimumof 3 min betweentestswasallowedthe
subjects. Terminal conspecifictests were used to demonstrateany habituation occurringover the
experimentalperiod, and the Black-cappedChickadeewas usedas a controlto which neitherbird was
expected to react aggressively.
The responses
of the subjectswere scoredon a scaleof 0 to 4 (seeTable 1). Means and standarderrors
were calculatedfor all classesof data. The sign test was used to determine differencesin responsedata
obtained from experimentsinvolving a given subject species.The 2 x 2 contingencytest was used to
compareresponsedata between subject species(Conover 1974).
October1977]
SongSparrowand Bewick'sWrenInteractions
TABLE
703
1
SCORING SYSTEM FOR MEASURING RESPONSESTO MODELS AND SONGS
0
= no response
1.0 = somereaction:bird comeswithin 25 feet of the model or beginsto sing
1.5 = bird comeswithin 15 feet of the modeland beginsto sing;activeflying
2.0 = strongerresponse:alarm calls, tail flicking, scolding,puffing up, active flying and orientation
towards model, and associatedbehavior
2.5 = same responseas 2, and bird comeswithin 10 feet of the model
3.0 = same responseas 2, and bird comes within 5 feet of the model
3.5 = reaction just short of attack, bird comescloserthan 5 feet to the model
4.0 = bird attacks model
RESULTS
Mean seasonalresponsesfor the study period were calculated for the Song Sparrow and Bewick's Wren (Table 2). Both speciesexhibited a similar pattern: highest
responseto the conspecificsongand model, minimal responseto the chickadeecontrol,
and an intermediate responseto the interspecificsong and model. The absolute
numbers and percentagesof birds respondingin each categoryare given in Table 3.
It was of interest to determine responsepatterns as a function of the breeding
cycle. The 1974 season(12 March 1974-13 June 1974) was divided into four time
periodsof 18, 29, 32, and 14 days respectively,to allow a minimum of 6 individual
responsesin each period. No statisticallysignificantchangesin the mean responseof
the SongSparrowto its conspecificsongand model were notedamongany of the four
time periods (Fig. 1A). Likewise, no statistically significant changesin the mean
responseof Bewick's Wren to its conspecificsongand model were noted (Fig. 1C).
The SongSparrow'sresponseto the Bewick'sWren songand model(Fig. lB) shows
a statisticallysignificantdrop in the mean responsebetween the secondand third
periods(31 March-28 April and 29 April-30 May), while the mean responseof the
Bewick's Wren to the Song Sparrow songand model (Fig. 1D) showsa significant
drop betweenthe first and fourth periods(12 March-30 March and 31 May-13 June).
Song Sparrows were intenselyintraspecificallyterritorial over the study period
with a gradual decline in mean aggressiveresponseover time (Table 2, Fig. 1).
Tompa (1962), working with the Song Sparrow in British Columbia, also noted a
gradual declineof territorial behaviorduring the breedingseasonuntil late July and
August when molt began in adults. Early in the breedingseasonduring the first
period (12-30 March) aggressionwas high in many of the subjectbirds, but in other
birds avoidanceresponseswere noted, in which a bird singingfrom its perch immediatelyceasedsingingand flew away from the model, ofteninto low, densebrush.
Suchresponses
were notedon four occasions
during this period, one caseinvolving
what appearedto be the male of a mated pair. Numerousaltercationswere noted
betweenSongSparrowsuntil June, when the number of suchencountersdecreased.
On one occasion the model was placed between two adjoining territories. Two
males respondedaggressivelyto the model, but not to each other. They were very
TABLE
2
MEAN SEASONALRESPONSES
OF SUBJECTS
TO CONSPECIFICMODELSAND SONGS
x
Model and song
Subject
SS (SS)
SS2
2.24 • 0.215a
1.32 q- 0.194•
BW (BW)
0.25 q- 0.12•o
BCC (BCC)
BW
0.57 q- 0.11a4
1.28 q- 0.11a9
0.03 q- 0.03a•
x with oneSE. Subscripts
indicatesamplesize.s SS = SongSparrow,BW = Bewick'sWren, BCC = Black-capped
Chickadee.
704
ROBERTE. GORTON,JR.
[Auk, Vol. 94
3.5'
2.5
16
3.0
11
2.0
2.5
1.5-
2.O
1.01.5
0.5'
1.0
1
2
3
4
1.5'-
1
2
3
4
1.0
I0
1.0
O. 5
1
0.5
2
4
0
1
•
2
3
4
Fig.1.A.Meanresponses
ofSong
Sparrow
toconspecific
song
andmodel
asafunction
oftime,with2SE
on either sideof the mean. NumbersaboveSE indicatesamplesizein giventime period.Period 1, 12-30
March;period2, 31 March-28April; period3, 29 April-30 May; period4, 31 May-6 June.B. Mean
responses
of SongSparrowto Bewick'sWrensongandmodel.C. Meanresponses
of Bewick'sWrento
conspecific
songandmodel.D. Meanresponses
of Bewick'sWrento SongSparrowsongandmodel.
tolerantof eachotheron the territorialperiphery,but notto the presence
of a third
unknownintruder.On 7 MayI noticed
twopairsof SongSparrows
withbeaksfull of
insects,
indicating
thepresence
ofyoung.(Eggsareusually
foundfromearlyAprilto
lateMay, although
recorded
aslateasmid-July
(Bent1968).)
Tompa(1962)found
thebreeding
season
to extendfrommid-March
to lateJuly.Thefirstfledglings
with
parents
werenoticed
on21May, whenfourSongSparrows
wereseentogether,
one
of whichorientedtowardstheconspecific
modelat the onsetof therecordedsong.On
8 Junetwo moregroups,
oneof fourandoneof five birds,werenotedin close
association,
someof whichlackedthecentralbrownspotcharacteristic
of adults.
The nextdayoneof a pairof SongSparrows
walkedwithin3 cmof themodelwith
noaggressive
behavior,
lookedat it closely,
andflewoff.Theconspecific
testdone
hoursbeforewithanotherSongSparrow
resulted
in a quickattackonthemodel.A
1-daysurveyon22Junerevealed
fewsinging
birdsandmanysubadults.
The breeding
cyclefor the Bewick's
Wrentemporally
approximates
that of the
SongSparrow.
Jeweftet al. (1953)statedthatnesting
begins
in mid-Aprilto midMay with broodsappearing
fromlateMay to earlyJune.DawsonandBowles(1909)
reported
fresheggsfrommid-Aprilto mid-June.
Nestingmaybeginsomewhat
earlier, because
I noticedan individualwitha stickin itsbill on 18March.Territorial
songwasheardthroughout
March.Themeanresponse
forAprilwashigher,coincid-
ingwiththeonset
of nesting.
Overall,therewaslittlechange
overthestudyperiod.
The primaryweaponin territorialdisputes
for thispopulation
is thesong:61.5%of
the responses
(24 of 39) had an absolutevalue of 1. Singingduelswere common
betweenadjacentindividuals,andin oneinstance
two adjacentbirdspursuedand
attacked eachother in a large tree for nearly an hour, with short recesses.
October1977]
705
SongSparrowand Bewick'sWrenInteractions
TABLE
3
NUMBER AND PERCENT OF BIRDS RESPONDING TO EACH SCORING CATEGORY
Score
Subject model
and song
BW-BW (BW) •
0
6
15.4%
BW-SS (SS)
BW-BCC (BCC)
SS-SS (SS)
SS-BCC (BCC)
1«
14
25.6%
35.9%
17
12
50.0%
35.3%
2
4
Total
0
2«
1
0
0
0%
2.5%
0%
0%
100%
0
0
0
0
0
0%
0%
0%
0%
0%
8
20.6%
5
14.7%
3
3«
30
1
0
0
0
0
0
0
96.8%
3.2%
0%
0%
0%
0%
0%
0%
7
4
2
3
7.5%
3.8%
5.7%
5
0
0
3
0%
0%
7.3%
10
18.9%
SS-BW (BV•r)
1
10
6
11.3%
13.2%
12
11
3
29.3%
26.8%
7.3%
7
17.1%
12.2%
8
15.1%
13
24.5%
35
2
0
2
0
0
0
1
87.5%
5.0%
0%
5.0%
0%
0%
0%
2.5%
39
34
100%
31
100%
53
100%
41
100%
40
100%
• SS = Song Sparrow, BW = Bewick's Wren, BCC = Black-cappedChickadee.
As mentionedabove, the SongSparrow experimentwas performedin this sequence:SongSparrow, Bewick'sWren, Black-cappedChickadee,SongSparrow.A
Bewick's Wren in the neighborhoodcould thus respondto its conspecificin a sequencedesignedto testa SongSparrow.In 12 of 41 tests(29.3%) for territorialityin
the SongSparrow, the Bewick'sWren respondedto its conspecific.Likewise in 8 of
34 tests(23.5%) of the Bewick'sWren experiments,the SongSparrowrespondedto
its conspecific.Thus 26.7% of the tests(20 of 75) producedcircumstantialevidenceof
territorial overlap. Possiblyin at least a few instancesthe birds were near their
territorial boundaries, or were attracted out of their territories by a strange conspecificsong. Becauseno territorial mapping was done, the degreeof territorial
overlap in the two speciescannot be determinedquantitatively, but overlap often
seemedto occur in transition zonesbetween relatively open areas and structurally
more complexareas.When a conspecificrespondedto its own songand modelduring
a test involving the other species,it was possibleto watch individuals of the two
speciestogether.In one instancethe SongSparrow drove off the Bewick'sWren. In
sevenother casesthe Bewick'sWren either eliciteda singingresponsefrom the Song
Sparrow, actively avoidedthe SongSparrow, or, in one case,begansinging.In one
of the formercases,the sparrowmovedinto the treein whichthe Bewick'sWren was
singing.Each moveby the wren higherin the tree wasmatchedby a similarmoveby
the sparrow. This continueduntil the Bewick'sWren reachedthe top of the tree.
TABLE
4
ANALYSIS OF SEASONAL DATA TO COMPARE CONSPECIFIC AND INTERSPECIFIC RESPONSES FOR
GIVEN SPECIES,USING SIGN TEST
Pop. 1
Pop. 2
Test statistic
a-level
SS-SS(SSp
SS-SS(SS)
SS-BW (BW)
BW-BW (BW)
BW-BW (BW)
SS-BW (BW)
SS-BCC (BCC)
SS-BCC (BCC)
BW-SS (SS)
BW-BCC (BCC)
6.5
6.5
4.161
3.202
4.161
0.0001
0.0001
0.0001
0.0001
0.0001
BW-SS (SS)
BW-BCC (BCC)
1. 716
0.0005
• ss = SongSparrow,BW = Bewick'sWren, BCC = Black-cappedChickadee.
706
ROBERTE. GORTON,JR.
TABLE
[Auk, Vol. 94
5
ANALYSIS OF SEASONAL DATA TO TEST IF 8ONG 8PARROW RESPONDS TO CONSPECIFIC AND
INTERSPECIFIC MODELS AND 8ONGS AT SAME LEVELS AS BEWlCK'S WREN 1
Pop. 1
Pop. 2
Test statistic
Accept H0?
SS-SS(SS?
BW-SS (SS)
SS-BCC (BCC)
BW-BW (BW)
SS-BW (BW)
BW-BCC (BCC)
15.55
10.313
2.433
No
No
Yes
a-level
0.001
0.005
> 0.10
Using 2 x 2 contingencytest (class1: responses0-1V2; class2: responses2-4).
SS = SongSparrow, BW = Bewick'sWren, BCC = Black-cappedChickadee.
When the sparrowmade anothermoveup the tree, the wren flew off. The Song
Sparrowthensangseveraltimesandreturnedto its lowerperch.The entireepisode
lasted lessthan 2 min. Another Bewick's Wren approacheda perching SongSparrow
to singatop a high nearbytree. The SongSparrowtook up the song,movinghigher
up its ownperchto reply.The duelwentonseveralminutesuntilthe wrenflew off.
Analysisof Mean Responses.--TheSongSparrow respondeddifferentlyto the
conspecific
songand modelthanto the Bewick'sWren songand modeland differentlyto the Bewick'sWren songand modelthan to the chickadee
songand model
(P < 0.0001,signtestfor nonparametric
data).Threedistinctmeanresponses
to the
threemodelandsongpresentations
emerge.Likewise,the Bewick'sWren responded
differentlyto its conspecific
modeland songthan to the SongSparrowmodeland
songand differentlyto the SongSparrowmodeland songthan to the chickadee
modeland song(P < 0.0005),illustratingthe samethreedistinctresponse
classes
as
the Song Sparrow (seeTable 4).
The SongSparrowreactedmorevigorously
to its conspecific
songand modelthan
did the Bewick'sWren to its conspecific
songand model(P < 0.001, 2 x 2 contingencytestfor nonparametric
data).Similarly,eachresponded
differentlyto thesong
and model of the other (P < 0.005). Both respondedin the same manner to the
chickadeesongand model,suggesting
it was an effectivecontrol(Table 5).
DISCUSSION
The SongSparrowreactedmuchmore to its conspecificsongand modelthan did
the Bewick's Wren to its conspecificsong and model. The question arises:Why
should thesetwo speciesshow this differencein reaction intensity?The SongSparrowspickeda few favoritepercheswithin their territoriesfrom whichto sing,usually
no more than 2-3 m high. Bewick's Wrens, on the other hand, seemedto have no
specialsingingperchesbut sangin many placesof different heights,often 3 m or
more. Furthermore the Bewick's Wren seemedmore active within its territow than
the SongSparrow.Evenif the increased
heightcommonto the singingperchesof the
Bewick's Wren did not provide greater visibility, making it more difficult for an
intruderto go unnoticed,it seemslikely that the smalleramountof time spentin
more placeswould increasethe probabilityof encounteringan intruder, leadingto
his exclusion
from the area.
As mentionedabove, the SongSparrow respondedat different levels to its con-
specificsongand model,to the Bewick'sWren songand model,and to the BlackcappedChickadeesongand model.This indicatesthat the SongSparrowrecognizes
both of theseothertwo speciesand treatseach one differently.(One attack on the
Bewick'sWren model took placeprior to the onsetof the song.)This fact suggests
that thoseindividualsthat react aggressively
to the Bewick'sWren have a selective
October1977]
SongSparrowandBewick'
s WrenInteractions
707
advantage over those that do not. This is true if the resourcesupply is sufficient to
support only one breeding pair of one speciesoptimally. When responseis plotted
against time, a drop in responseis noted between the secondand third periods. The
third periodis the time of feedingyoungand fledging, a periodwhen parent birds are
busy with hungry fledglings in which they have a great deal of time and energy
invested. While it should be adaptive to defend a territory vigorouslyagainstconspecifics,it shouldbe lessadvantageousto defendthe territory againstother species,
with which competitionis lessintense,during the periodbetweenbroods.That the
mean responserisesagain during the fourth period is suggestive.During this period,
eggsare again in the nest, and an adequatefood supply is again a pressingconcern.
Thus the interspecificaggression,primarily involving singingand active flying, is
more sensitiveto different activity periods over the breeding season,the Bewick's
Wren being somewhatlessof a threat to limiting resourcesthan the conspecific.
The Bewick's Wren is considerablylessinterspecificallyaggressivethan the Song
Sparrow. A partial proximate explanation for this is that the Bewick's Wren is
generally lesspugnacious. Furthermore in actual combat the smaller wren standsless
of a chanceof emergingvictorious. Therefore selectionmay favor the evolution of
only that level of aggressionagainst a competitor that is most effective and least
energeticallyexpensiveper unit gain. As with the Song Sparrow, Bewick's Wren
respondsdifferently to each of the models and songs;responseis presumably a
function of niche overlap. The mean level of aggressiondecreasesover the breeding
season.As the seasonprogresses,greater demandsare placed on the parentsto bring
increasinglymore food to the nestas the nestlingsget larger. They are then spending
much of their time foragingfor the nestlingsand excludingconspecifics,which is
more critical at this point in the breedingcyclethan excludingother species.Finally
whenthe broodis fledged,aggression
shouldbe least,as is reflectedin the changesin
modes of aggressivelevels.
The birdsdivide the habitat vertically,the SongSparrowpreferringsimpler,more
openhabitatswith a lower foliageheightdiversitythan the Bewick'sWren, sothat
territorial overlap refersto overlap of territorial volumes. Thus, a speciesthat may
be toleratedat oneheightmay not be at another,while a conspecific
may be excluded
at any level within the territory. For example, a Song Sparrow intruding into the
territory of another at 6 m high is very likely to come down to 1 m if allowed to
remain; a Bewick's Wren is much lesslikely to do so, but would remain out of the
Song Sparrow's territorial volume. Several coexistencemechanisms are at work in
this system.Vertical feedingsite segregationhas been mentioned.Feedingbehavior,
and consequentlyforagingsubstrates,differ somewhat.Differencesin bill morphology suggestthat while they feed the young the same kinds and sizesof food, this food
may be in different placesso that someprey items available to the Bewick'sWren,
for examplein deep, narrow cracksin the trunks of trees, are unavailableto a Song
Sparrow. Cody (1974) found that the SongSparrowspends11% more time in the 2-5
m zone in the absenceof the Bewick's Wren, Orange-crownedWarbler, Yellow
Warbler, CommonBushtit, and Chestnut-backedChickadee,commoncompetitors
in this zone. It is apparentthat for this populationcompetitionis pronouncedenough
for the establishmentand maintenanceof a systemof interspecificaggression.
ACKNOWLEDGMENTS
I thank Howard Channingand the Cornell Laboratory of Ornithologyfor the songrecordings.My
grandmother,Mrs. Frieda Gorton, suppliedme with bed and board during the fieldwork. My father-in-
708
ROBERT
E. GORTON,
JR.
[Auk,Vol. 94
law, Barrie Smith, suppliedme with a tape recorder.I also thank E. L. Karlstrom, and G. Blanks of the
University of Puget Sound and D. R. Paulsonof the University of Washington for assistanceand encouragement. M. Perrone and D. Paulson also critically read the manuscript and made many valuable
suggestions.Katy Swansonand Karen Mesmer of the Quantitative Sciencesectionof the University of
Washingtonhelped with the statisticalanalyses.Walter Coole of Skagit Valley Collegeprovided faculty
privilegesand an officeduring my stay in Skagit County. Specialthanksgo to my wife, Terry Anne, who
participated in one way or another in every aspectof this study and manuscript. The study was part of a
Master of Sciencethesisat the University of Puget Sound.
LITERATURE CITED
BENT, A.C.
U.S.
1948. Life historiesof North American nuthatches,wrens, thrashers,and their allies.
Natl.
Mus.
Bull.
195.
1968. Life historiesof North American Cardinals, grosbeaks,buntings, towhees, finches,
sparrows,and their allies, part 3. U.S. Natl. Mus. Bull. 237.
BROWN,J. L. 1964. The evolutionof diversityin avianterritorialsystems.
WilsonBull. 76: 160-169.
1975. The evolution of behavior. New York, W. W. Norton Co.
CODY,M. L. 1968. Interspecificterritoriality among hummingbird species.Condor 70: 270-271.
1969. Convergencecharacteristicsin sympatric populations:A possiblerelation to interspecific territoriality. Condor 71: 222-239.
1971. Ecologicalaspectsof reproduction.Pp. 462-512 in Avian biology,vol. 1 (D. S. Farnet
and J. R. King, Eds.). New York, AcademicPress.
1974. Competition and structure of bird communities. Princeton, New Jersey, Princeton
Univ.
Press.
CODY, m. L., AND J. L. BROWN. 1970. Character convergencein Mexican finches. Evolution
24: 304-310.
CONOVER,W.J.
1974. Practicalnon-parametricstatistics.New York, Wiley Press.
DAWSON,
W. L., ANDJ. H. BOWLES. 1909. The birdsof Washington,2 vols.Seattle,OccidentalPub.
Co.
HINDE, R. A. 1956. Territories of birds. Ibis 98: 340-369.
HORN, H.S.
1968. The adaptive significanceof colonialnestingin the Brewer's Blackbird (Euphagus
cyanocephalus).Ecology49: 682-694.
HOWARD,E.H.
1920. Territory in bird life. London, Murray.
IMMELMANN, K. 1971. Ecologicalaspectsof periodic reproduction. Pp. 342-389 in Avian biology,
vol. 1 (D. S. Farner and J. R. King, Eds.). New York, Academic Press.
JEWETT,S. G., W. P. TAYLOR,W. T. SHAW,ANDJ. W. ALDRICH. 1953. Birdsof WashingtonState.
Seattle, Univ. WashingtonPress.
MILLER, E.V. 1941. Behavior of the Bewick's Wren. Condor 43: 81-99.
NICE, M. M. 1941. The role of territory in bird life. Amer. Midi. Naturalist 26: 441•[87.
NOBLE, G. K. 1939. The role of dominance in the social life of birds. Auk 56: 263-273.
ORIANS,G. H., AND m. F. WILLSON. 1964. Interspecificterritoriesof birds. Ecology45: 736-745.
PETERSON,R.T.
1961. A field guide to Western birds. Boston, Houghton-Mifflin.
ROBBINS,C. S., B. BRUUN,AND H. S. ZIM. 1966. A guide to field identification:Birds of North
America. New York, Golden Press.
SCHOENER,r. W. 1968. Sizesof feedingterritoriesamong birds. Ecology49: 123-141.
STILES, g. W. 1973. Bird communitystructurein alder forests.UnpublishedPh.D. dissertation,
Seattie, Univ. Washington.
TINBERGEN,N. IMPEKOVEN,AND D. FRANCK. 1967. An experimenton spacing-outas a defense
against predation. Behavior 28:307-321.
TOMPA,F.S.
1962. Territorial behavior:the main controllingfactorof a localSongSparrowpopula-
tion. Auk 79: 687-697.