Download movement patterns of wintering grassland sparrows in arizona

The Auk 117(3):748-759, 2000
MOVEMENT
PATTERNS
OF WINTERING
IN ARIZONA
GRASSLAND
SPARROWS
CALEB E. GORDON •
Department
of Ecology
andEvolutionary
Biology,Universityof Arizona,Tucson,
Arizona85721,USA
ABSTRACT.--I
used mark-recaptureanalysisand radio telemetryto characterizewinter
movementpatternsof sixgrasslandsparrowsin southeastern
Arizona.Mark-recapturedata
weregeneratedby bandingbirdscapturedduringrepeatedflush-nettingsessions
conducted
on a seriesof 7-ha plotsover three consecutive
winters.This resultedin 2,641capturesof
2,006individual sparrowsof the six species.Radiotelemetrywasconductedconcurrently
on 20 individualsof four of thesespecies.Recapturedataandradiotelemetryindicatedthat
Cassin'sSparrow(Aimophila
cassinii)and Grasshopper
Sparrow(Ammodramus
savannarum)
werethe mostsedentary,
followedby Baird'sSparrow(Ammodramus
bairdii),VesperSparrow
(Pooecetes
gramineus),
SavannahSparrow (Passerculus
sandwichensis),
and Brewer'sSparrow
(Spizellabreweri).
Grasshopper,
Baird's,Savannah,and Vespersparrowstendedto remain
within fixedhomerangesduring winter.With the exceptionof SavannahSparrows,whose
movementbehaviorvariedamongstudysites,movementpatternsremainedconstantwithin
speciesacrossyearsand studysitesdespiteradicalfluctuations
in the absoluteandrelative
abundances
of all species.
Interspecificdifferences
in movementpatternsuggestthatspecies
in thissystempartitionnichespaceaccordingto theregional-coexistence
mechanism.
Abundancesof the most sedentaryspecies,Cassin's,Grasshopper,and Baird'ssparrows,were
poorlyor negativelycorrelatedwith summerrainfall at the between-yearlandscapescale,
whereasabundancesof the more mobile Savannah,Vesper,and Brewer'ssparrowswere
stronglypositivelycorrelated.This is consistent
with the theoreticalpredictionthat movement constrainslarge-scalehabitatselection,favoringmobilespeciesin fragmentedenvironments.Received
28 April 1999, accepted
10 February2000.
where the unpredictabilTHEMOVEMENT
BEHAVIOR
of an organismde- desertenvironments,
terminesthe scaleat whichit perceivesandre- ity of rainfallcreatesseeddistributions
thatare
spondsto the spatialsubdivisionof its envi- highly patchyin time and space(Davies1984,
ronment (Wiens 1976, Levin 1992). Movement
Clark 1997, Dean 1997).
also is a fundamentalelementof populationIn contrast,an organismwith a fixedhome
levelprocesses
suchasmetapopulation
dynam- rangemovesaroundrepeatedlywithin an area
ics, geneflow, habitat selection,and foraging thatis smallrelativeto its abilityto travela givbehavior Understandingthe patterns, con- en distance.Suchhomerangesmay or maynot
straints,and adaptabilityof movementbehav- be defendedagainstconspecifics.
Thistype of
ior is an important step toward understanding movement is suited for exploiting relatively
the dynamicsof an organism'secologicalrela- predictableresources.
In suchcases,gainingfationships.
miliarity with a particularareais morecostefThe range of movementpatternsin nature fectivethan engagingin long-distance
explorcan be viewed as a continuum ranging from atory movements(Sinclair1984).
random wanderingto movementswithin a
fixed home range.Randomwanderingis well
suitedfor exploitingunpredictable,
patchyresources.In suchcases,the costof exploratory
movementis offsetby the benefitof increased
encounterswith resourcepatches(Noy-Meir
Although random wanderingsand fixed
home-rangemovements
generallyarelinkedto
distincttypes of resourcedistributions,both
strategiescanbe viablein many environments.
Indeed, the use of multiple movementstrate-
1973, Andersson1980, Sinclair 1984). This rationalehas beenusedto explainthe high degree of nomadismamonggranivorousbirds in
lar landscapes
hasreceiveda greatdealof attention(e.g.Hutchinson1951,LevinsandCul-
gies for exploiting single resourcesin particu-
ver 1971, Horn and MacArthur 1972, Tilman
1994). In modelsof regional coexistence,
ecologically similar speciescoexistvia a tradeoff
E-mail: [email protected]
748
July2000]
Sparrow
Movement
Patterns
betweenwidespreadexplorationfor and local
exploitationof resources(Brown 1989, Tilman
1994).This tradeoffimpliesthat increasedefficiencyat seekingnew patchescan only be
gainedby somesacrificeof an organism's
within-patchefficiency,
andviceversa.Thisconcept
hasbeen'invokedto explainnichedifferences
amonggranivorousmammalsand birds in desert ecosystems(Mares and Rosenzweig1978,
Brown et al. 1979, Thompsonet al. 1991). In
suchcases,granivorousbirds are describedas
"cream-skimmers"that explorelarge areasto
find the richestpatchesin the landscape.This
implies that birds distribute themselvesto
matchthe productionof resourcesat the landscape scale.Granivorous rodents, in contrast,
are more sedentarybut are more locally efficient, such that they can survive on resource
densitiestoo low to supportbirds. This notion
of birds as highly mobile cream-skimmersis
consistent
with patternsof nomadismthathave
beendocumentedin many seed-eatingbirds of
arid regions of Australia (Davies 1984, Clark
1997)and SouthAfrica (Dean 1997).Movement
patternsof granivorousbirds in the American
southwest have never been documented.
Ratherthan falling on a fixedpoint alongthe
nomadism/ home-rangecontinuum,an organism may adjust its movementstrategyin responseto shifting environmentalconditions.
Behavioralplasticityis particularlyvaluablein
highly fluctuatingand unpredictableenvironmentswherethebenefitsof flexibilityoutweigh
the costsof not beingableto specializeon, and
therebymaximizeefficiencyfor, oneparticular
strategy(Tripp and Colazo 1997).Plasticityof
behaviorand/or growthform hasbeenfound
in a variety of desert organisms (Fox 1990,
Pfennig 1992).
Given the variability and heterogeneityof
their environment,plasticitywould appear to
be advantageousfor sparrowswintering in Arizonagrasslands.
Productivityin theseecosystems is largely limited by rainfall (McClaran
1995).The rainfall that occursduringthe summer monsoonseasonlargely determinesthe
distributionof habitatconditionsfor wintering
sparrows.Summer rainfall controlsthe productionof grassseedsthat comprisethe sparrows' primary food source(PullJam1983, C.
Gordonunpubl.data)and the vegetativecover
that providesimportantprotectionfrom predators(PullJamand Mills 1977,Lima and Valone
749
1991).Summerrainfall in this regionis patchy
at the scale of thunderstorm
cells that are 2 to
8 km wide and is highly variableand unpredictable among years (Noy-Meir 1973, McClaran 1995). For this reason,thesesparrows
may be faced with radically different spatial
distributions
of suitable habitat from winter to
winter. PullJam and Parker (1979) measured
seedproductionat two grasslandstudy plotsin
southeastern
Arizona
over
four
successive
yearsand found that fluctuationsexceedingan
order of magnitudewere common.On oneplot,
seed productionincreasedmore than two orders of magnitude in responseto a 64% increasein summerrainfall betweenyears.
My study addressestwo major questions.
First, can variation in movement patterns explain the coexistenceof severalecological(ly
similar speciesin this system?Second,does
movement
behavior constrain habitat selection
in this system?To theseends, I developnew
field and analyticaltechniques
for studyingthe
local movement behavior of grasslandsparrows.I usethesein conjunction
with radio telemetry to characterizemovementpatternsof
six speciesof granivoroussparrowson their
winteringgroundsin the grasslandsof southeasternArizonaduringthreeconsecutive
winters. In particular, I focuson overall sedentariness,the tendencytoward fixed home-range
versus nomadic movements,and plasticity of
within-seasonmovementpatterns.I discussthe
observedmovementpatternsas strategiesthat
are moreor lessadaptedfor the spatiotemporal
distributionof resources,patches,and biological interactions
in the environment.
MATERIALS
AND METHODS
Studysites.--Fieldwork was conductedat three
study sitesin southeastern
Arizona. The Research
Ranch(TRR) is a 3,200-hapreservelocated8 km
southeast
of thetownof Elginin SantaCruz County,
Arizona. TRR is dominatedby upland grassland
vegetation
andliesat theedgeof a large(ca.900km2)
regionof grasslandin the SonoitaValley.TRR has
not been grazed by cattle since 1967. Study plots
werelocatedin the northwesternquarterof TRRbetween1,450and 1,500m elevation.The EmpireCienegaResourceConservationArea (EC) is alsolocated within the SonoitaValleygrasslands,
roughly10
km from TRR. It lies in Pima and Santa Cruz counties
and is leasedto a privaterancherfor cattlegrazing.
Within the EC, fieldwork was conducted in the
southeastern corner of the Davis Pasture, which is lo-
750
CALEB
E. GORDON
[Auk, Vol. 117
catedroughly3 km west-northwest
of Elginbetween weedyhabitatsaswell asgrassland(Rising1996,C.
1,450 and 1,480 m elevation. The Buenos Aires Na-
tional Wildlife Refuge(BANWR) is locatedin the
southernAltar Valleyin Pima Countyroughly150
km westof thepreviousstudysites.Thisvalleycontainsa muchsmallerpatchof grasslandhabitat(ca.
100km2) that occursalmostentirelywithin the refuge. BANWR has not been grazedby cattlesince
1985.Studyplotswere locatedin the BorregoGrasslandssectionof the refugebetween1,050and 1,210
m elevation.
Vegetation.--All
threestudyareaswere locatedin
open semidesert or plains grassland vegetation
(McClaran1995).Rainfallfluctuateswidely in this
habitat,rangingbetween200 and 400 mm per year;
50 to 60%of therainfallusuallyfallsduringthesummer monsoonseasonfrom July to September(McClaran 1995).Thishabitatis dominatedby a variety
of perennialbunchgrasses,
althoughmanyforbsand
several small woody perennials also are common.
The mostabundantgrassesincludeseveralspeciesof
Bouteloua
and Eragrostis,
includingthe widespread
exoticEragrostis
lehmanniana.
Theonlywoodyplants
taller than 1 m on the studyplotswere a few small
mesquitetrees(Prosopis
velutina).Thesewere more
abundant at the slightly lower and drier BANWR
site,wheretheytendedto beconcentrated
in arroyos
off of study plots. Severalspeciesof oaks (Quercus)
occurredin arroyosnear study plots at TRR.
Studyspecies.--The
studytaxaconsisted
of sixspeciesof New World sparrows(Emberizinae):Cassin's
Sparrow (Aimophilacassinii),Brewer'sSparrow (Spizella breweri),Vesper Sparrow (Pooecetes
gramineus),
Savannah
Sparrow(Passerculus
sandwichensis),
Grasshopper Sparrow (Ammodramus
savannarum),
and
Baird'sSparrow(Ammodramus
bairdii).Exceptfor the
smaller Brewer'sSparrow,these speciesexhibit a
greatdealof overlapin bodysizeandbill size,which
suggestsa high degreeof overlapin the seedsthat
comprisetheirwinterdiet (Pulliam1983,C. Gordon
unpubl. data). All of thesespeciesco-occurin midelevationgrasslands
of southeastern
Arizona.
Baird's,Savannah,Vesper,and Brewer'ssparrows
are long-distance
migrantsthat do notbreedlocally.
Theybeginto arrivein southeastern
Arizona in late
Augustand are absentfrom the areaby early May
(Phillipset al. 1964).Cassin'sSparrowsbreedlocally,
but it is unknownwhetherthewinteringindividuals
I studiedwerelocalbreeders.Grasshopper
Sparrows
winteringin southeastern
Arizonaare composedof
locally breeding (C. Gordon unpubl. data) and migrant birds, but the proportionalrepresentationof
thesetwo groupsis unknown.Grasshopper,
Baird's,
and Cassin'ssparrowsare non-flockingspeciesthat
are exclusively(Grasshopper
and Baird'ssparrows)
or largely (Cassin'sSparrow)restrictedto grassland
habitat(Rising1996).Savannah,Vesper,and Brewer's sparrowsare flockingspeciesthat use a wider
spectrumof vegetationtypesincludingshrubbyand
Gordon pers. obs.).
Flush-netting.--AtBANWR and TRR, I established
six permanent7-ha flush-nettingplots. Each plot
consistedof a 96-m net line with a 3.5-haflushing
zonefanningout on eachside.Foreachdayof flushnetting,crewsof 13 to 30 people(average22) assembledat 0830MST at the field siteto performthe followingfield protocol.Eight mistnets(2 x 12 m, 36
mm mesh)weresetup alongthe net line at the first
of the sixplots.Whennetswereready,thefieldcrew
fannedoutalongthe300-mbackedgeof oneflushing
zone.With people spreadout evenly alongthe peripheryof the flushingzone,a signalwasgivenand
the groupwalkedthroughthe flushingzonetoward
the net line, causingsparrowsto flush toward the
nets.Thecrewrepeatedtheprocedureforthesecond
flushingzoneon the othersideof the net line.
After workingbothsidesof a studyplot,eachnetted bird was banded with individuallynumbered
aluminumleg bandsor their bandnumberrecorded
if a recapture.We thendisassembled
thenetsat the
firstplot andrepeatedtheprocedureat thenextplot.
In this way,we flush-nettedall six plotsat a study
sitein randomsequence
on eachday of netting.
Betweenthe first week of Januaryand the first
week of March 1996,flush-nettingcrewsperformed
the aboveprocedureon sevenMondaysat TRR. At
the sametime of yearin 1997and 1998,flush-netting
wasdoneon sevenWednesdays
at TRRandon nine
Saturdaysat BANWR. Between Januaryand early
March1999,threeflushingdayswereconducted
on
the samestudyplotsat TRR and BANWR.
Radio telemetry.--I placed radio transmitterson
three Baird'sSparrowsand two GrasshopperSparrowsin lateFebruary1997at theEC andTRRstudy
sites.I attached1.0 g radio transmitters(modelBD2, HolohilSystemsLtd.) overtherumpwith leg-loop
harnesses
(RappoleandTipton1991)madeof cotton
candlewick.The radioshave a batterylife of six to
eightweeks.Beginningthe day afterradioswereattached,I attemptedto locateeachbird onceper day
on 12 days over a seven-weekperiod. Locationsof
initial netting and all subsequentrelocationswere
recordedusing a global positioningsystem(GPS)
unit accurate to within
2 m but not corrected for mil-
itary signalscrambling.I conductedthis sameprocedureon sevenVesperSparrowsand eightSavannahSparrowsbetweenlateJanuaryandearlyMarch
1998 at BANWR.
Recaptureanalysis.--Goodness-of-fit
tests from
program RELEASE(Burnhamet al. 1987) showed
that the mark-recapturedata setsdid not fit the assumptionsof theJolly-Seber
modelsusedin current
mark-recaptureanalyses(P < 0.01 for all data sets).
Therefore,I createda statisticcalledthe recapture
eventrate (RER) that I used as an index of sedentariness,with higherRER valuesreflectingsedentary
behaviorand/or lower mortalityduring the study
July2000]
Sparrow
Movement
Patterns
period. RER is definedsimply as the numberof recaptureeventsdivided by the numberof recapture
opportunities.The numberof recaptureopportunities for a given bird is the number of netting days
subsequentto the initial captureof that bird during
a given season.This statisticmeasuresthe overall
tendencyof birds to be recaptured.
I testedthe significanceof differencesin RER between data setswith a Monte Carlo simulationprocedure. For each simulation,I compareda pair of
datasetsby preservingthe mark-recapture
patterns
of individual birds but randomly reshufflingindi-
751
the averageof the RER valuesof eachindividual in
the data set. This statisticweights all individuals
equally and might be called the averageindividual
recapture event rate (AI). If individuals are drawn
from a homogeneous
population,thesetwo statistics
shouldbe identical. However,if birds capturedearly
in the seasonbehavedifferentlyfrom thosecaptured
late in the season, these statistics would differ. The
earlybirdshavemoreopportunitiesto be recaptured
thanthelatebirdsandare,therefore,heavilyweighted in the RER but not in the AI.
I used a simulationprocedureto test for statistically significantdifferencesbetweenthe RERand AI
was then recalculatedfor the two sampledata sets. statisticsfor all data setswith at least 20 recapture
This procedure was repeated 1,000 times for each events.For each simulation,I preservedthe exact
pairwisetestandthentheobserved
RERvalueswere structureof initial capturesby datefor all individual
comparedwith the distributionsof RERvaluesgen- birds within a given data set.I simulatedrecapture
eratedby the simulations.
data for eachdata setby settingthe probabilityof reNet learning potentially could confoundRER sta- captureat eachopportunityequalto theRERfor that
tisticsby reducingcaptureprobabilityfor previously data set. I repeatedthis procedure1,000 times and
capturedbirds. For datasetswith largenumbersof comparedobserveddifferencesbetweenAI and RER
recaptures(>20), I testedfor net learningeffectsby with the samplingdistributionsof thesestatistics
creating an expected binomial distribution of cap- generatedby the simulations.
turesunder the null hypothesisof no net learning
(capture probabilitiesare independentof previous
RESULTS
capture).To do this, I generatedmaximum-likelihood estimatesof binomial parametersas outlined
viduals between the two data sets. The RER statistic
Flush-netting.--Analysisof movement pat-
below.
A preliminaryvalueof N (populationsize)wasvisually estimatedfrom the asymptoteof a plot of cumulative
individual
birds banded
with
time. The
tern was based on the first three field seasons
of flush-netting,whichyielded2,641captures
of 2,006individualsparrowsamongthe sixfocal speciesand two studysitesaslistedin Table
1. Of the 574 within-seasonrecaptures,only
sevenoccurredat a plot otherthan the onein
whichthe bird had beeninitially banded.Only
two of these(one GrasshopperSparrowand
oneSavannahSparrow)requiredmovementof
probabilityof capturinga givenbird in a givenflush,
p, was calculatedas x (the averagenumberof birds
capturedon a samplingdate) divided by N, and 1 p = q was the probability of not capturing a given
bird .in a given flush.The probabilityof not capturing a given bird in i samplingdatesis q•,and q' N,
therefore,is the numberof birds in the population
that were nevercapturedin i samplingdates.This more than 100 m. Therefore, I based subsenumber was then added to the total number of birds
that were actuallycapturedat leastoncein the sampling period to producea new estimateof N. This
procedurewas then iterated with the new estimate
of N until the parameterestimatesconverged
onvalues that did not changein subsequentiterations.
These parameterswere then used to generateexpected binomial frequency distributions for the
numberof capturesfor eachbird in the dataset.Observeddistributionswere then comparedwith the
expecteddistributions using chi-squaregoodnessof-fit tests.
I calculated RER statisticsfor each data set by
lumpingall recaptureeventsand opportunitiesover
all individualswithin datasets.This weightseachrecapture opportunity equally, but individual birds
with more recapture opportunities (first captured
earlierin the season)areweightedmoreheavilythan
individualswith fewerrecaptureopportunities(first
capturedlaterin theseason).An alternativeis to take
quentanalysesof movementpatternsfrom the
banding data on same-plotrecapturesas described above.
The simulationtestsshowedpronouncedinterspecificdifferencesin RER (Table2). Cassin's
Sparrowsand Grasshopper
Sparrowswere the
most sedentary,followedby Baird'sSparrows,
VesperSparrows,and finally SavannahSparrows and Brewer'sSparrows.Within species,
RER varied little amongdata sets(Table3). !
found no significant differences in RER betweendatasets(year x studysite)for any spe-
ciesexceptSavannahSparrow,which was significantlymoresedentaryat the TRRstudysite
than at BANWR. This pattern was consistent
across years.
Five data sets (four for GrasshopperSparrow, one for Cassin's Sparrow) contained
752
CALEB
E. GORDON
[Auk,Vol. 117
TABLE1. Number of individualscapturedand within-seasonrecapturesfor six grasslandsparrows.Data
are numberof individualbirds, with numberof recaptureeventsin parentheses.
TRR = The Research
Ranch,BANWR = BuenosAiresNationalWildlife Refuge.
Study site
1996
1997
1998
Totala
2 (3)
43 (28)
53 (34)
TRR
BANWR
0 (0)
--
Cassin'sSparrow
0 (0)
9 (3)
TRR
BANWR
164 (71)
--
GrasshopperSparrow
16 (8)
278 (153)
388 (149)
189 (85)
989 (466)
TRR
BANWR
36 (5)
--
Baird's Sparrow
15 (3)
9 (2)
28 (7)
18 (4)
100 (21)
TRR
BANWR
1 (0)
--
Vesper Sparrow
7 (0)
194 (15)
141 (14)
83 (8)
420 (37)
TRR
BANWR
29 (2)
--
Savannah Sparrow
7 (0)
106 (1)
105 (9)
119 (2)
365 (14)
TRR
BANWR
0 (0)
--
Brewer's Sparrow
0 (0)
72 (2)
0 (0)
8 (0)
79 (2)
aSpecies
totalsforbothstudysites.Thetotalnumberof individualscapturedfor eachspecies
is slightlylessthanthesumofsubtotals
because
individualscapturedin multiple field seasons
are countedseparatelyin eachdatasetin whichtheyoccurred,but only oncein the totals.
enoughrecaptureeventsto usethe maximum- broken down by weekly intervals to examine
likelihoodnet-learningtestsdescribedabove. the probabilityof recapturingindividualsasa
In all of these data sets, observed distributions functionof the increasingtime betweenfirst
of captures were not significantly different and subsequentcaptures.For a randomlywanfrom thoseexpectedunderthenull hypothesis deringindividual,the probabilityof recapture
of independenceof capture events (Table 4). should decrease with time as it "diffuses"
This suggests
that a strongnet-learningeffect away from its startingpoint. Sparrowswith
was not present for Cassin'sSparrows and fixed home rangesmay experiencesomedeGrasshopper
Sparrows.
creasein recaptureprobability due to mortalEachrecaptureeventand opportunityfor recapturecanbe definedby the amountof time
TABLE3. Intraspecificvariation in recaptureevent
elapsedbetweenit andtheinitial captureof an
rate (RER) amongyears and betweenstudy sites
individual bird. Thus, the RER statistic can be
for four grasslandsparrows.Only datasetswith at
least15 individualsand specieswith at leasttwo
such data sets are listed.
TABLE2. Interspecificdifferencesin sedentariness
of sparrowsas measuredby recaptureeventrate
Species
1996
1997
(RER). CASP = Cassin'sSparrow,GRSP = GrassThe Research Ranch
hopperSparrow,BAIS= Baird'sSparrow,VESP=
VesperSparrow,SAVS= SavannahSparrow,BRSP GrasshopperSparrow 0.1100 0.1270
= Brewer'sSparrow.
Baird'sSparrow
0.0382 0.0455
VesperSparrow
--Species RER CASPGRSPBAIS VESP SAVSBRSP SavannahSparrowa
0.0220
--
1998
0.0917
0.0588
0.0234
0.0216
Buenos
AiresNational
WildlifeRefuge
CASP
0.1110
--
GRSP
BAIS
VESP
SAVS
0.0943
0.0446
0.0179
0.0085
ns
*
**
**
-**
**
**
BRSP
0.0044
**
**
**
*
ns
--
Grasshopper
Sparrow
--
0.1000 0.0777
Baird's Sparrow
VesperSparrow
SavannahSparrow
----
-0.0146
0.0018
0.0404
0.0192
0.0036
aRERwassignificantlyhigherat The Research
Ranchthanat Buenns, P :> 0.05; *, P < 0.05; **, P < 0.01 (Monte Carlo simulations).
os Aires NWR (P < 0.05; Monte Carlo simulations).
July2000]
Sparrow
Movement
Patterns
753
TABLE
4. Estimatedbinomialparametersandchi-squaregoodness-of-fit
testsfor net-learningeffectin sparrows.Thetestcompares
observeddistributions
of individualsin capturefrequencyclasses
with expected
distributionbasedon the assumption
of independence
of captureprobabilities
(nonet learning).
Species
Data set
Cassin'sSparrow
Grasshopper
Sparrow
Grasshopper
Sparrow
Grasshopper
Sparrow
Grasshopper
Sparrow
1998BuenosAires NationalWildlife Refuge
1998BuenosAires NationalWildlife Refuge
Na
pb
p
65
315
0.121
0.097
0.67
0.30c
1998 The Research Ranch
686
0.112
0.47 c
1997BuenosAires National Wildlife Refuge
415
0.116
0.25•
1996 The Research Ranch
265
0.129
0.20
Estimatedpopulationsize.
Estimatedprobabilityof captureof givenindividual on givennetting day.
A singletrap-happyindividualremovedfrom analysis.
ity, but otherwisethe probabilityof recapture for GrasshopperSparrowsand Cassin'sSparshould remain relatively constant through rowsto remainwithin fixedhomerangesdurtime.
ing this time period.It alsodemonstrates
that
The probabilityof recapturingGrasshopper mortality was not significantin these cases.
Sparrows(all data sets combined)decreased GrasshopperSparrowsand Cassin'sSparrows
with time (Fig. 1). However,theRERat thelon- were the only specieswith enoughrecapture
gesttimeinterval(eightweeks)wasstillhigher eventsfor this analysisto be performed.
than the total RER of the next-mostsedentary
Cassin'sSparrows (1998 BANWR data set)
species,Baird'sSparrow(Table2). In the 1996 did not show a significantdifferencebetween
TRR data set for GrasshopperSparrowsand AI and RER, but GrasshopperSparrowsdid
the 1998 BANWR data set for Cassin'sSpar- show this difference.For all four of the large
rows, RER remainedalmostconstantthrough datasetsfor Grasshopper
Sparrows,A! values
time (Fig. 1). In thesecases,individualswere weresignificantlylowerthan the RER(P < 0.05
just aslikely to be recapturedeightweeksfol- for 1998 at BANWR; P < 0.01 for 1998 at TRR,
lowing initial captureas in the first week po- 1997at BANWR, and 1996at TRR). Grasshopstcapture.This demonstrates
a strongtendency per Sparrowsthat were captured early had
higher probabilitiesof recapturethan birds
that were not initially captureduntil later in
-- GrasshopperSparrow1996 TRR
the season.
:
0.25
Cassin'sSparrow1998 BANWR
Radiotelemetry.--Of
the20 birdsthatreceived
radio transmitters,three disappearedbefore
the battery shouldhave expired,and 10 were
found deadbeforethe batteryexpired,having
beenkilled by predators.! relocated18 birds at
---A---Total GrasshopperSparrow
0.2
0.15
least twice for a total of 115 relocations.
To characterize
movementpatterns,I plotted
net distancemovedbetweeneachpair of loca-
0.1
tionsas a function of time and performedlinear
regression(Fig. 2). The regressionsfor all four
specieshad slopesbelow 2.6 m per day and R2
0.05
1
2
3
4
5
6
7
8
Intervalbetween captures(weeks)
FIG.1. Recaptureeventrate as a functionof the
time interval betweenfirst and subsequentrecapture
valuesbelow0.029.This suggeststhat individualsof all four speciestendedto remainwithin
fixedhomeranges.Thus,interspecific
variation
in sedentariness
may haveresultedfrom variationin home-rangesize amongspecies.
To describe the extent of movement, I also
eventsfor total Grasshopper
Sparrowsand Grass- comparedthe averagenet distancemovedbehopper Sparrowsin 1996 at The ResearchRanch tween all pairs of locationsfor individuals of
(TRR) and Cassin'sSparrows at BuenosAires National Wildlife Refuge(BANWR).
eachspecies.Becausenet distancemovedwas
not correlated with time, these distances can be
754
CALEB
E. GORDON
[Auk, Vol. 117
Baird's Sparrow
Savannah Sparrow
300
8001
600
200
iI1#1
0 i
i
0
10
200
100
i-
i
i
20
30
40
0
!
0
10
20
GrasshopperSparrow
Vesper Sparrow
200
800
]
, ,
150
, *
100
600
400
50
0, el ,I i1,_1teI,P I 4 • *
0
10
20
30
40
,
50
0
0
10
20
Time interval(days)
FIG.2. Net distances(distancesbetweeneachpair of radio telemetryrelocations)movedwith time for
four grasslandsparrows.
thoughtof as the net distancemovedbetween
any two locationsof a bird. Thesedata ranked
speciesfrom mostto leastsedentaryin exactly
the sameorderastheRERfromthenettingdata
in Table2. The Grasshopper
Sparrowwas the
mostsedentarywith an averagenet movement
of 84 m, followed by Baird's (113 m), Vesper
(142m) andSavannah(186m) sparrows.All interspecificdifferenceswere significant (twotailedt-tests,P < 0.05).Theseobservedaverage
ity amongindividualGrasshopper
Sparrowsis
that two types of individuals are presentin
winter, somewith fixedhomerangesand others that move nomadically.This is consistent
with manyterritorialsystems
which"floaters"
represent an excessof individuals relative to
the numberof availableterritories(Shutlerand
Weatherhead1994, Stutchbury1994,Westcott
and Smith 1994).A discrepancyin movement
behavioramongindividual Grasshopper
Spardistances were overestimates because of GPS
rows also may correspondto a differencebesignal scramblingand the error of the GPS tween migrant and residentpopulationsthat
unit. GPS locationsfor a fixed point taken on coexiston thesestudy sitesin winter It is also
the daysof radio telemetryshowedan average possiblethat differences
in recaptureprobabilnet movement
of 50.2 m.
ities amongindividualswere causedby other
It is importantto note that mortality cannot behavioral differences that could lead to differbe distinguished
from emigrationassources
of
ent capture probabilities,such as different
low RER. Corroborationof interspecificdifferor flyingheights.
encesin RER by the telemetrydata suggests flushingresponses
Extent of movement.--Cassin's
Sparrowsand
that RER does indeed reflect differences in moGrasshopper
Sparrows,
and
to
a
lesserextent
bility among species.Furthermore,the conBaird's
Sparrows,
were
the
most
sedentary
spestancyof RER with weekly intervalssuggests
cies.
The
extent
and
pattern
of
movements
in
thatthe amountof mortalitythatoccurreddurthese
sparrows
are
consistent
with
those
obing the study period was extremelysmall, at
least for Cassin'sSparrowsand Grasshopper served for desert rodents (Brown 1989,Brown
and Zeng 1989). Thus, the notion that these
Sparrows (Fig. 1).
birds are highly mobile"cream-skimmers"in
this systemmay not be accurate(Mares and
DISCUSSION
Rosenzweig1978,Brownet al. 1979,Thompson
Variation
among
individuals.--One
possibleex- et al. 1991). Somebirds, particularly Cassin's
planationfor differencesin recaptureprobabil- Sparrowsand grassland-obligate
Baird'sSpar-
July2000]
Sparrow
Movement
Patterns
755
rows and GrasshopperSparrows,may behave vannahSparrowsshowedsignificantlydiffermore like the rodents in the above models of
ent local movement patterns between study
desertgranivorecoexistence,
playingthe roles sites, movement patterns of Grasshopper,
of highly sedentarybut highly efficientforag- Baird's,and Vespersparrowswere fixed across
ers.
yearsand study sites(Table3). This behavioral
The sparrowsin this study exhibitedsignif- constancy
is remarkablegiventhe radicalflucicant interspecificvariation in movementpat- tuations(up to 20-fold) in abundanceof sparterns. Nonetheless,telemetry data indicated rows acrosssites and years (Table 1). Fluctuathat eventhe leastsedentaryspeciestendedto tions near an order of magnitude were obremainwithin fixedhomeranges(Fig.2). This servedfor all focal species.Given the imporpatternmightseemto conflictwith thenomad- tance of densities of interspecific and
ic behaviors that have been documented
in
intraspecificcompetitorson habitatselectionin
seed-eating
birdsin otherrainfall-limitedeco- various theoretical and empirically demonsystems(Davies 1984, Clark 1997,Dean 1997). strated situations(Fretwell and Lucas1970,RoIn suchsystems,
nomadism
hasbeensuggested senzweig1981,Pimm et al. 1985),it is surprisas a way for consumersto copewith extreme ing that suchvariationin the densityof potenspatial and temporal variability in resource tial competitorswould haveno effecton movedistribution (Noy-Meir 1973, Davies 1984, ment behavior.One potentialexplanationfor
Clark 1997, Dean 1997). Nonetheless,the ob- this pattern is that competitionamongsparservedsedentariness
of sparrowsin this study rows may not be an importantecologicalfactor
was not inconsistent with nomadic movements
in this system.Anotherpossibilityis that the
at different scalesof time and space.In fact, movementbehavior of Grasshopper,Baird's
sparrowsmay havecombinedbetween-year
re- and Vespersparrowsis fixedbecauseof various
gional scalemobility with within-winter sed- physiological,morphological or other conentarinessto effectivelyexploit the specific straints,and that thesespeciesare unableto
scalesof environmentalheterogeneityand var- adapttheirmovementbehaviorquicklyenough
iability in this system.The dramaticfluctua- to trackrapid changes
in theirenvironment.
A
tionsin sparrowabundance
acrosssiteswithin third possibilityis thatthedensityadjustments
years and acrossyears within sites (Table 1) that birds makein responseto environmental
suggestedthat the distributionsof sparrows variability obviate the need for additional bevariedsignificantlybetweenyearsat a regional havioraladjustments.
scale.Thispatterncouldresultif sparrowsunMovement
patternsandcommunitystructure.derwent a brief period of exploratorymove- FollowingHutchinson's
(1951)conceptof a fumentupon arriving on the winteringgrounds. gitive species,severaltheoreticalstudieshave
This would allow them to match their distrisuggestedthat variationin movementpatterns
bution to the variableand patchydistribution functionsas a nicheaxis,allowingthe coexisof resourcesin any given year, driven by the tence of otherwiseecologicallyidenticalspesummer rains.
cies in competitively structured communities
In contrastto the between-yearvariability of (e.g. Levins and Culver 1971, Horn and Macis
the summer rainfall pattern, the pattern of Arthur 1972,Tilman 1994).This coexistence
summer rainfall remains fixed over the course
permitted by a tradeoff between efficiencyat
of any givenwinter. For this reason,the distri- exploringfor new patchesand efficiencyat exbutionsof seedsand grasscoverremain rela- ploiting a particular patch (Brown 1989, Tiltively constantduring the courseof a winter.It man 1994). In other words, the best colonizer
is unlikelythat sparrowsalter thisdistribution of new patchescannotalsobe the bestcompetsignificantlyduringthe courseof mostwinters, itor within patches.
becausethey consumeonly 20 to 30% of the
In recent decades,the validity of equilibriseed resourcesin many winters (PullJamand um-based, competition-structuredcoexistence
Dunning 1987). Fixed home ranges during theorieshas been questioned(Connell 1975,
midwintermay be an adaptationto exploitthis Connorand Simberloff1979,Strong1983),parwithin-winter constancyby remainingseden- ticularly in highly fluctuating environments
tary and conservingenergy.
such as grasslandand shrubsteppe(Wiens
Plasticity
ofmovement
patterns.--Although
Sa- 1974, Rotenberryand Wiens 1980). The ratio-
756
CALEB
E. GOl•OON
[Auk,Vol. 117
nale for this argumentis that in communities that they shouldbe able to make a living on
wherebiologicalinteractions
neverreachequi- patcheswith lower resourcedensities.
librium
and
consumers
do
not
saturate
re-
sources,competitivepressureshould be low.
Withoutcompetition,no reasonexistsfor niches to be competitively structured. Resource
partitioning is not required for speciesto coexist. The wide fluctuationsin sparrow abundanceacrosstime and space(Table1) suggest
thatwintergrassland
sparrowassemblages
in-
Significanceof movementstrategiesin a fragmentedlandscape.--Recent
modelshave shown
that sedentaryspecies,includingcompetitive
dominants, are more adversely affected by
landscapefragmentationthan are moremobile
species(Dytham 1994;Tilman et al. 1994,1997;
Moilanenand Hanski 1995).If suitablepatches
becomewidely separatedin space,sedentary
speciesare unableto find and colonizethem.
deed are far from being in equilibrium. Furthermore,sparrowsare not believedto saturate Danielson (1991) and Pulliam and Danielson
seed resourcesduring most winters (Pulliam (1991)modeledthe effectof sedentarybehavior
on habitat selectionby varying the amountof
and Brand 1975, Pulliam and Parker 1979, Pulliam and Dunning 1987).Therefore,it may not patchesthat individualssampledbeforethey
be appropriateto invokethe notionof resource chosea patchto inhabit.In their simulations,
partitioningto explainthe significantdiffer- individuals that sampled a high number of
patchesoccupieda higher percentageof the
betterhabitatthan individualswith low patch
competitive coexistencemay not apply to co- sampling.In landscapeswhere high-quality
habitat patcheswere rare, the ability to find
existingspeciesof grasslandsparrows.
Nonetheless,
competitivestructuringshould goodpatcheslimitedpopulationgrowth,anda
not be ruled out completely.Competitiondoes high level of exploratorypatchsamplingwas
encesin movement patterns among speciesre-
cordedin this study. In short, the notion of
not need to be intense or constant to affect the
distributionof nichesin a community(Connell
1980). PullJam(1985) suggestedthat competition and resourcepartitioningamongsparrows
winteringin Arizona grasslands
are important
only in rare yearswhen seedproductionis extremelylow and competitionfor seedsmay be
especiallyintense.Furthermore,evenif predatorslimit sparrowpopulationsbelow their ability to saturatefood resources,nichepartitioning may be required for coexistence(Holt
1977).Therefore,differences
in movementpat-
necessaryfor speciesto survive.
This has an important implicationfor the
managementof highly sedentaryspeciessuch
as GrasshopperSparrows, Baird's Sparrows,
and Cassin'sSparrows.In additionto beingaffectedby pure increasesor decreases
in their
preferredhabitattypes,sedentaryspecies
may
suffer increased
adverse effects relative to more
mobile speciesif the distributionof suitable
habitat becomesfragmented.One important
caveatis that in this study,the mobility of sparrows was measured at the scale of several hect-
ares.This small scalemay not accuratelyreflect
terns may function as an important coexistence regionalmovements,
which perhapsare more
mechanismfor ecologicallysimilar grassland appropriatelymatchedto the spatiotemporal
sparrowswintering in Arizona grasslands.
scaleof environmentalvariabilityin this sysEvenin the absenceof nichepartitioningin tem.Figure3 suggests
thattheselocal,within-
responseto competition,the tradeoff between year mobility differencesindeedreflectmovewithin-patch efficiency and among-patch ment constraintsat the regionalscaleas well.
searchingmay posesignificantconstraintsfor At the between-yearand betweenstudy site
sparrows. Speciesthat sacrificedsome ability scales,abundances
of the more locallysedento explorethe landscapemay have done so to tary specieswere more poorly correlatedwith
gain efficiencyat the local scale.Indeed, the summerrainfall (a negativecorrelation
in the
three mostsedentaryspeciesin my study,Cas- caseof Baird'sSparrow)than were abundances
sin's Sparrow, Grasshopper Sparrow, and of the moremobilespecies.AlternativehypothBaird'sSparrow,are short-wingedrelativeto esescannotbe ruled out, but a likely explanaothersparrowsand areknownto be weak fliers tion for this patternis that sedentarybehavior
(Rising1996).Thisleadsto the predictionthat constrainsthe ability of GrasshopperSparthese sedentaryspeciesare more locally effi- rows, Baird'sSparrows,and Cassin'sSparrows
cient than are the more mobile sparrows,and to matchtheir winter spatial distributionsto
July2000]
Sparrow
Movement
Patterns
Baird's Sparrow
40R2
=0.18
30
Vesper Sparrow
2ooR2= 0.64
ø
•
150
20 ••
100
10
50
0
03
757
0
100
0
200
300
0
GrasshopperSparrow
100
200
Savannah Sparrow
500R2
420R2
4oo=0.21,
400 =0.38•
300 80••e
,o
20
200
100
o
0
300
40
,
100
*,
,
200
o
300
0
100
200
,
300
o
Cassin's Sparrow
o
Brewer'sSparrow
40
70
30
50
,o
40
20
30
20
10
10
0
i
,
,
100
200
300
•
0
.
.
100
200
300
Total July-Septemberrainfall (mm)
FIc. 3. Relationship
betweensummerrainfalland sparrowcapturesin subsequent
winter fromJanuary
to March1996to 1999.The threemostsedentaryspeciesat left are muchmorepoorly(or negatively)correlatedwith rainfallthanthe threemoremobilespeciesat right,suggesting
thatmovementconstraints
play
an importantrolein large-scalehabitatselection.Eachdatapoint represents
the totalnumberof individuals
capturedin sevenflush-nettingsessions
at a singlestudysiteper winter.The 1997and 1998BANWRdata
setswere standardizedto sevenflush-nettingdatesby discardingdatafrom the eighthand ninth netting
sessions.
I extrapolatedthe 1999totalsfrom threeto sevendatesby dividingthe three-weektotalsby the
averageproportionof birdsin a seven-week
datasetcapturedby thethird week.Tocalculatetheseaverages,
I divided the total numberof birdscaughtby the third weekby the total numberof birds that had been
caughtby the seventhweekin all datasetswith at leastsevennettingdates.Julyto Septemberrainfalldata
arefrom rain gaugesat the headquarters
of TRR andBANWR,1 to 5 km from all studyplots.R2valuesare
givenonly for specieswith sevendatapoints.Brewer'sSparrowsand Cassin's
Sparrowsdo not occurregularly at the higher-elevation
TRRstudysite;hence,onlythreedatapointsrepresenting
threeflush-netting
seasonsat BANWR are presented.
the distributionof preferredconditions(high
rainfall areas)in the landscape.
The grassland-obligate
GrasshopperSparrows and Baird'sSparrowsundoubtedlyhave
sufferedfrom widespreadconversionof grass-
lands to shrublands
in southeastern
Arizona
overthe past120years(Bahre1991).The sedentarybehaviorof thesespeciessuggeststhat
managersneedto considerthe distributionof
habitatpatchesin the landscape,aswell asthe
758
CALEB
E. GORDON
[Auk,Vol. 117
structuralcomposition
of the patches.In other CLARK, M. E 1997. A review of studies of the breeding biology of Australianbirds from 1986-95:Biwords, it's not just a questionof creatingthe
asesand consequences.
Emu 97:283-289.
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J.
H.
1975.
Some
mechanismsproducing
may matterwherethe patchis relativeto other
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A model and
patcheswithin the range of existingpopulaevidencefrom field experiments.Pages461-490
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suchasCassin's
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DANIELSON,
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