The Auk 117(3):748-759, 2000

MOVEMENT PATTERNS OF WINTERING SPARROWS IN ARIZONA

CALEB E. GORDON • Departmentof Ecologyand Evolutionary Biology, University of Arizona,Tucson, Arizona 85721, USA

ABSTRACT.--Iused mark-recaptureanalysis and radio telemetryto characterizewinter movementpatterns of sixgrassland sparrows in southeasternArizona. Mark-recapture data weregenerated by bandingbirds captured during repeated flush-netting sessions conducted on a seriesof 7-ha plots over three consecutivewinters. This resultedin 2,641captures of 2,006individual sparrowsof the six species.Radio telemetry was conductedconcurrently on 20 individualsof four of thesespecies. Recapture data and radiotelemetry indicated that Cassin'sSparrow (Aimophila cassinii) and GrasshopperSparrow ( savannarum) werethe mostsedentary, followed by Baird'sSparrow (Ammodramus bairdii), Vesper Sparrow (Pooecetesgramineus), Savannah Sparrow (Passerculussandwichensis), and Brewer'sSparrow (Spizellabreweri). Grasshopper, Baird's, Savannah, and Vespersparrows tended to remain within fixedhome ranges during winter.With the exceptionof SavannahSparrows, whose movementbehavior varied amongstudy sites, movement patterns remained constant within speciesacross years and studysites despite radical fluctuations in the absoluteand relative abundancesof all species.Interspecific differences in movementpattern suggest that species in thissystem partition niche space according to theregional-coexistence mechanism. Abun- dancesof the most sedentaryspecies, Cassin's, Grasshopper, and Baird's sparrows,were poorly or negativelycorrelated with summerrainfall at the between-yearlandscape scale, whereasabundances of the more mobile Savannah,Vesper, and Brewer'ssparrows were stronglypositively correlated. This is consistentwith the theoreticalprediction that move- ment constrainslarge-scale selection,favoring mobile species in fragmentedenvi- ronments.Received 28 April 1999, accepted10 February2000.

THEMOVEMENT BEHAVIOR of an organismde- desertenvironments, where the unpredictabil- terminesthe scaleat whichit perceivesand re- ity of rainfall createsseed distributions that are spondsto the spatialsubdivision of its envi- highly patchyin time and space(Davies 1984, ronment (Wiens 1976, Levin 1992). Movement Clark 1997, Dean 1997). also is a fundamentalelement of population- In contrast,an organismwith a fixedhome levelprocesses such as metapopulation dynam- rangemoves around repeatedly within an area ics, gene flow, habitat selection,and foraging that is smallrelative to its ability to travela giv- behavior Understandingthe patterns, con- en distance.Such home ranges may or maynot straints,and adaptabilityof movementbehav- be defendedagainst conspecifics. This type of ior is an important step toward understanding movement is suited for exploiting relatively the dynamicsof an organism'secological rela- predictableresources. In suchcases, gaining fa- tionships. miliarity with a particulararea is morecost ef- The range of movementpatterns in nature fectivethan engagingin long-distanceexplor- can be viewed as a continuum ranging from atory movements(Sinclair 1984). random wanderingto movementswithin a Although random wanderingsand fixed fixed home range.Random wandering is well home-rangemovements generally are linked to suitedfor exploitingunpredictable, patchy re- distinct types of resourcedistributions, both sources.In suchcases, the costof exploratory strategiescan be viablein many environments. movementis offsetby the benefitof increased Indeed, the use of multiple movementstrate- encounterswith resourcepatches (Noy-Meir gies for exploiting single resourcesin particu- 1973, Andersson1980, Sinclair 1984). This ra- lar landscapeshas received a greatdeal of at- tionalehas beenused to explainthe high de- tention(e.g. Hutchinson 1951, Levins and Cul- gree of nomadismamong granivorous in ver 1971, Horn and MacArthur 1972, Tilman 1994). In models of regional coexistence,eco- E-mail: [email protected] logically similar speciescoexist via a tradeoff

748 July2000] SparrowMovement Patterns 749 betweenwidespread exploration for and local 1991).Summer rainfall in this regionis patchy exploitationof resources(Brown 1989, Tilman at the scale of thunderstorm cells that are 2 to 1994).This tradeoffimplies that increasedef- 8 km wide and is highly variableand unpre- ficiencyat seekingnew patchescan only be dictable among years (Noy-Meir 1973, Mc- gainedby somesacrifice of an organism'swith- Claran 1995). For this reason,these sparrows in-patchefficiency, and vice versa. This concept may be faced with radically different spatial has been'invoked to explainniche differences distributions of suitable habitat from winter to amonggranivorous mammals and birds in de- winter. PullJam and Parker (1979) measured sert ecosystems(Mares and Rosenzweig1978, seedproduction at two grasslandstudy plotsin Brown et al. 1979, Thompsonet al. 1991). In southeastern Arizona over four successive suchcases, granivorous birds are describedas years and found that fluctuationsexceeding an "cream-skimmers"that explorelarge areasto order of magnitudewere common.On oneplot, find the richestpatches in the landscape.This seed productionincreased more than two or- implies that birds distribute themselvesto ders of magnitude in responseto a 64% in- matchthe productionof resourcesat the land- creasein summer rainfall betweenyears. scape scale. Granivorous rodents, in contrast, My study addressestwo major questions. are more sedentarybut are more locally effi- First, can variation in movement patterns ex- cient, such that they can survive on resource plain the coexistenceof severalecological(ly densitiestoo low to supportbirds. This notion similar speciesin this system?Second, does of birds as highly mobile cream-skimmersis movement behavior constrain habitat selection consistentwith patternsof nomadismthat have in this system?To these ends, I developnew been documentedin many seed-eatingbirds of field and analyticaltechniques for studyingthe arid regions of Australia (Davies 1984, Clark local movement behavior of grassland spar- 1997) and SouthAfrica (Dean 1997).Movement rows.I use thesein conjunctionwith radio te- patternsof granivorousbirds in the American lemetry to characterizemovement patterns of southwest have never been documented. six speciesof granivoroussparrows on their Ratherthan falling on a fixedpoint alongthe wintering groundsin the grasslandsof south- nomadism/ home-rangecontinuum, an organ- easternArizona during threeconsecutive win- ism may adjust its movementstrategy in re- ters. In particular, I focus on overall sedentar- sponseto shifting environmentalconditions. iness,the tendencytoward fixed home-range Behavioralplasticity is particularlyvaluable in versus nomadic movements,and plasticity of highly fluctuatingand unpredictableenviron- within-seasonmovement patterns. I discussthe mentswhere the benefits of flexibilityoutweigh observedmovement patterns as strategiesthat the costsof not beingable to specializeon, and are more or lessadapted for the spatiotemporal therebymaximize efficiency for, oneparticular distributionof resources,patches, and biolog- ical interactions in the environment. strategy(Tripp and Colazo 1997).Plasticity of behaviorand/or growthform has beenfound in a variety of desert organisms (Fox 1990, MATERIALS AND METHODS Pfennig 1992). Studysites.--Field work was conductedat three Given the variability and heterogeneityof study sitesin southeasternArizona. The Research their environment,plasticity would appear to Ranch (TRR) is a 3,200-hapreserve located 8 km be advantageousfor sparrowswintering in Ar- southeastof the town of Elginin SantaCruz County, izonagrasslands. Productivity in theseecosys- Arizona. TRR is dominatedby upland grassland tems is largely limited by rainfall (McClaran vegetationand lies at theedge of a large(ca. 900 km 2) 1995).The rainfall that occursduring the sum- regionof grasslandin the SonoitaValley. TRR has mer monsoonseason largely determinesthe not been grazed by cattle since 1967. Study plots distributionof habitatconditions for wintering were locatedin the northwesternquarter of TRRbe- sparrows.Summer rainfall controlsthe pro- tween1,450 and 1,500m elevation.The EmpireCie- nega ResourceConservation Area (EC) is alsolocat- ductionof grassseeds that comprisethe spar- ed within the SonoitaValley , roughly 10 rows' primary food source(PullJam 1983, C. km from TRR. It lies in Pima and Santa Cruz counties Gordonunpubl. data) and the vegetativecover and is leasedto a privaterancher for cattlegrazing. that providesimportant protection from pred- Within the EC, fieldwork was conducted in the ators(PullJam and Mills 1977,Lima and Valone southeastern corner of the Davis Pasture, which is lo- 750 CALEBE. GORDON [Auk, Vol. 117 catedroughly 3 km west-northwestof Elginbetween weedyhabitats as well as grassland(Rising 1996, C. 1,450 and 1,480 m elevation. The Buenos Aires Na- Gordon pers. obs.). tional Wildlife Refuge(BANWR) is locatedin the Flush-netting.--AtBANWR and TRR, I established southernAltar Valleyin Pima Countyroughly 150 six permanent7-ha flush-nettingplots. Each plot km westof the previousstudy sites. This valley con- consistedof a 96-m net line with a 3.5-haflushing tains a muchsmaller patch of grasslandhabitat (ca. zonefanning out on eachside. For each day of flush- 100 km2) that occursalmost entirely within the ref- netting,crews of 13 to 30 people(average 22) assem- uge. BANWR has not been grazedby cattle since bled at 0830MST at the field siteto performthe fol- 1985.Study plots were locatedin the BorregoGrass- lowingfield protocol.Eight mist nets(2 x 12 m, 36 landssection of the refugebetween 1,050 and 1,210 mm mesh)were setup alongthe net line at the first m elevation. of the sixplots. When nets were ready, the field crew Vegetation.--Allthree study areaswere locatedin fannedout along the 300-m back edge of oneflushing open semidesert or plains grassland vegetation zone. With people spreadout evenly along the pe- (McClaran1995). Rainfall fluctuateswidely in this ripheryof the flushingzone, a signalwas given and habitat, rangingbetween 200 and 400 mm per year; the groupwalked through the flushingzone toward 50 to 60%of therainfall usually falls during the sum- the net line, causingsparrows to flush toward the mer monsoonseason from July to September(Mc- nets.The crew repeated the procedure for thesecond Claran 1995).This habitatis dominatedby a variety flushingzone on the otherside of the net line. of perennialbunchgrasses, although many forbs and After workingboth sides of a studyplot, each net- several small woody perennials also are common. ted was banded with individually numbered The mostabundant grasses include several species of aluminumleg bandsor their bandnumber recorded Boutelouaand Eragrostis,including the widespread if a recapture.We thendisassembled the netsat the exoticEragrostis lehmanniana. The only woodyplants firstplot andrepeated the procedure at thenext plot. taller than 1 m on the studyplots were a few small In this way, we flush-nettedall six plotsat a study mesquitetrees (Prosopis velutina). These were more sitein randomsequence on eachday of netting. abundant at the slightly lower and drier BANWR Betweenthe first week of Januaryand the first site,where they tended to be concentratedin arroyos week of March 1996,flush-netting crews performed off of study plots. Severalspecies of oaks (Quercus) the aboveprocedure on sevenMondays at TRR. At occurredin arroyosnear study plots at TRR. the sametime of yearin 1997and 1998,flush-netting Studyspecies.--The study taxa consisted of sixspe- wasdone on sevenWednesdays at TRRand on nine ciesof New World sparrows(Emberizinae): Cassin's Saturdaysat BANWR. Between Januaryand early Sparrow (Aimophilacassinii), Brewer's Sparrow (Spi- March1999, three flushing days were conducted on zella breweri),Vesper Sparrow (Pooecetesgramineus), the samestudy plots at TRR and BANWR. SavannahSparrow ( sandwichensis), Grass- Radio telemetry.--I placed radio transmitterson hopper Sparrow (Ammodramussavannarum), and three Baird'sSparrows and two GrasshopperSpar- Baird'sSparrow (Ammodramus bairdii). Except for the rowsin late February1997 at the EC and TRRstudy smaller Brewer'sSparrow, these speciesexhibit a sites.I attached1.0 g radio transmitters(model BD- greatdeal of overlapin bodysize and bill size,which 2, HolohilSystems Ltd.) overthe rump with leg-loop suggestsa high degreeof overlapin the seedsthat harnesses(Rappole and Tipton 1991) made of cotton comprisetheir winter diet (Pulliam1983, C. Gordon candlewick.The radioshave a batterylife of six to unpubl. data). All of thesespecies co-occur in mid- eightweeks. Beginning the day afterradios were at- elevationgrasslands of southeasternArizona. tached,I attemptedto locateeach bird onceper day Baird's,Savannah, Vesper, and Brewer'ssparrows on 12 days over a seven-weekperiod. Locationsof are long-distancemigrants that do not breedlocally. initial netting and all subsequentrelocations were Theybegin to arrive in southeasternArizona in late recordedusing a global positioningsystem (GPS) Augustand are absentfrom the area by early May unit accurate to within 2 m but not corrected for mil- (Phillipset al. 1964).Cassin's Sparrows breed locally, itary signalscrambling. I conductedthis samepro- but it is unknownwhether the winteringindividuals cedureon sevenVesper Sparrows and eightSavan- I studiedwere local breeders. Grasshopper Sparrows nah Sparrowsbetween late January and early March winteringin southeasternArizona are composedof 1998 at BANWR. locally breeding (C. Gordon unpubl. data) and mi- Recaptureanalysis.--Goodness-of-fit tests from grant birds, but the proportionalrepresentation of program RELEASE(Burnham et al. 1987) showed thesetwo groupsis unknown.Grasshopper, Baird's, that the mark-recapturedata setsdid not fit the as- and Cassin'ssparrows are non-flockingspecies that sumptionsof the Jolly-Sebermodels used in current are exclusively(Grasshopper and Baird'ssparrows) mark-recaptureanalyses (P < 0.01 for all data sets). or largely (Cassin'sSparrow) restrictedto grassland Therefore,I createda statisticcalled the recapture habitat(Rising 1996). Savannah, Vesper, and Brew- event rate (RER) that I used as an index of sedentar- er's sparrowsare flockingspecies that use a wider iness,with higher RER valuesreflecting sedentary spectrumof vegetationtypes including shrubby and behaviorand/or lower mortalityduring the study July2000] SparrowMovement Patterns 751 period. RER is definedsimply as the numberof re- the averageof the RER valuesof each individual in captureevents divided by the numberof recapture the data set. This statisticweights all individuals opportunities.The numberof recaptureopportuni- equally and might be called the averageindividual ties for a given bird is the number of netting days recapture event rate (AI). If individuals are drawn subsequentto the initial captureof that bird during from a homogeneouspopulation, these two statistics a given season.This statisticmeasures the overall shouldbe identical. However,if birds capturedearly tendencyof birds to be recaptured. in the seasonbehave differently from thosecaptured I testedthe significanceof differencesin RER be- late in the season, these statistics would differ. The tween data setswith a Monte Carlo simulationpro- earlybirds have more opportunities to be recaptured cedure. For each simulation, I compared a pair of than the latebirds and are,therefore, heavily weight- data setsby preservingthe mark-recapturepatterns ed in the RER but not in the AI. of individual birds but randomly reshufflingindi- I used a simulationprocedure to test for statisti- viduals between the two data sets. The RER statistic cally significantdifferences between the 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 pairwisetest and thenthe observed RER values were structureof initial capturesby datefor all individual comparedwith the distributionsof RERvalues gen- birds within a given data set. I simulatedrecapture eratedby the simulations. data for eachdata setby settingthe probabilityof re- Net learning potentially could confoundRER sta- captureat eachopportunity equal to the RERfor that tisticsby reducingcapture probability for previously data set. I repeatedthis procedure1,000 times and capturedbirds. For data setswith largenumbers of comparedobserved differences between AI and RER recaptures(>20), I testedfor net learningeffects by with the samplingdistributions of these statistics creating an expected binomial distribution of cap- generatedby the simulations. turesunder the null hypothesisof no net learning (capture probabilitiesare independentof previous capture).To do this, I generatedmaximum-likeli- RESULTS hood estimatesof binomial parametersas outlined below. Flush-netting.--Analysisof movement pat- A preliminaryvalue of N (populationsize) was vi- tern was based on the first three field seasons sually estimatedfrom the asymptoteof a plot of cu- of flush-netting,which yielded2,641 captures mulative individual birds banded with time. The of 2,006individual sparrowsamong the sixfo- probabilityof capturinga givenbird in a givenflush, cal speciesand two studysites as listed in Table p, was calculatedas x (the averagenumber of birds 1. Of the 574 within-seasonrecaptures, only capturedon a samplingdate) divided by N, and 1 - p = q was the probability of not capturing a given sevenoccurred at a plot other than the one in bird .in a given flush.The probabilityof not captur- whichthe bird had beeninitially banded.Only ing a given bird in i sampling dates is q•,and q' N, two of these (one GrasshopperSparrow and therefore,is the numberof birds in the population one SavannahSparrow) required movement of that were never capturedin i samplingdates. This more than 100 m. Therefore, I based subse- number was then added to the total number of birds quentanalyses of movementpatterns from the that were actuallycaptured at leastonce in the sam- banding data on same-plotrecaptures as de- pling period to producea new estimateof N. This scribed above. procedurewas then iterated with the new estimate The simulationtests showed pronounced in- of N until the parameterestimates converged on val- ues that did not changein subsequentiterations. terspecificdifferences in RER (Table2). Cassin's These parameterswere then used to generateex- Sparrowsand GrasshopperSparrows were the pected binomial frequency distributions for the most sedentary,followed by Baird's Sparrows, numberof capturesfor eachbird in the dataset. Ob- VesperSparrows, and finally SavannahSpar- serveddistributions were then comparedwith the rows and Brewer'sSparrows. Within species, expected distributions using chi-squaregoodness- RER varied little amongdata sets (Table3). ! of-fit tests. found no significant differences in RER be- I calculated RER statisticsfor each data set by tweendata sets(year x studysite) for any spe- lumpingall recaptureevents and opportunitiesover ciesexcept Savannah Sparrow, which was sig- all individuals within data sets.This weightseach re- capture opportunity equally, but individual birds nificantlymore sedentary at the TRRstudy site with more recapture opportunities (first captured than at BANWR. This pattern was consistent earlierin the season)are weightedmore heavily than across years. individualswith fewerrecapture opportunities (first Five data sets (four for GrasshopperSpar- capturedlater in the season).An alternativeis to take row, one for Cassin's Sparrow) contained 752 CALEBE. GORDON [Auk, Vol. 117

TABLE1. Number of individuals capturedand within-seasonrecaptures for six grasslandsparrows. Data are numberof individual birds, with numberof recaptureevents in parentheses.TRR = The Research Ranch,BANWR = BuenosAires NationalWildlife Refuge.

Study site 1996 1997 1998 Totala Cassin's Sparrow TRR 0 (0) 0 (0) 2 (3) 53 (34) BANWR -- 9 (3) 43 (28) GrasshopperSparrow TRR 164 (71) 16 (8) 388 (149) 989 (466) BANWR -- 278 (153) 189 (85) Baird's Sparrow TRR 36 (5) 15 (3) 28 (7) 100 (21) BANWR -- 9 (2) 18 (4) Vesper Sparrow TRR 1 (0) 7 (0) 141 (14) 420 (37) BANWR -- 194 (15) 83 (8) Savannah Sparrow TRR 29 (2) 7 (0) 105 (9) 365 (14) BANWR -- 106 (1) 119 (2) Brewer's Sparrow TRR 0 (0) 0 (0) 0 (0) 79 (2) BANWR -- 72 (2) 8 (0) aSpecies totals for bothstudy sites. The total number of individualscaptured for eachspecies is slightlyless than the sum of subtotalsbecause individualscaptured in multiple field seasonsare countedseparately in eachdata set in which theyoccurred, but only oncein the totals. enoughrecapture events to usethe maximum- broken down by weekly intervals to examine likelihoodnet-learning tests described above. the probabilityof recapturingindividuals as a In all of these data sets, observed distributions function of the increasingtime betweenfirst of captures were not significantly different and subsequentcaptures. For a randomlywan- from thoseexpected under the null hypothesis deringindividual, the probabilityof recapture of independenceof capture events (Table 4). should decrease with time as it "diffuses" This suggeststhat a strongnet-learning effect away from its startingpoint. Sparrowswith was not present for Cassin's Sparrows and fixed home rangesmay experiencesome de- GrasshopperSparrows. creasein recaptureprobability due to mortal- Eachrecapture event and opportunityfor re- capturecan be definedby the amountof time TABLE3. Intraspecificvariation in recaptureevent elapsedbetween it and the initial captureof an rate (RER) among years and betweenstudy sites individual bird. Thus, the RER statistic can be for four grasslandsparrows. Only datasets with at least 15 individualsand specieswith at leasttwo such data sets are listed. TABLE2. Interspecificdifferences in sedentariness of sparrowsas measuredby recaptureevent rate Species 1996 1997 1998 (RER). CASP = Cassin'sSparrow, GRSP = Grass- hopperSparrow, BAIS = Baird'sSparrow, VESP = The Research Ranch VesperSparrow, SAVS = SavannahSparrow, BRSP GrasshopperSparrow 0.1100 0.1270 0.0917 = Brewer'sSparrow. Baird's Sparrow 0.0382 0.0455 0.0588 VesperSparrow -- -- 0.0234 Species RER CASPGRSP BAIS VESP SAVSBRSP SavannahSparrow a 0.0220 -- 0.0216 CASP 0.1110 -- BuenosAires National Wildlife Refuge GRSP 0.0943 ns -- GrasshopperSparrow -- 0.1000 0.0777 BAIS 0.0446 * ** Baird's Sparrow -- -- 0.0404 VESP 0.0179 ** ** VesperSparrow -- 0.0146 0.0192 SAVS 0.0085 ** ** SavannahSparrow -- 0.0018 0.0036 BRSP 0.0044 ** ** ** * ns -- aRER was significantlyhigher at The ResearchRanch than at Buen- ns, P :> 0.05; *, P < 0.05; **, P < 0.01 (Monte Carlo simulations). os Aires NWR (P < 0.05; Monte Carlo simulations). July2000] SparrowMovement Patterns 753

TABLE4. Estimatedbinomial parameters and chi-squaregoodness-of-fit tests for net-learningeffect in spar- rows.The testcompares observed distributions of individualsin capturefrequency classes with expected distributionbased on the assumptionof independenceof captureprobabilities (no net learning).

Species Data set N a pb p Cassin'sSparrow 1998Buenos Aires NationalWildlife Refuge 65 0.121 0.67 GrasshopperSparrow 1998Buenos Aires NationalWildlife Refuge 315 0.097 0.30c GrasshopperSparrow 1998 The Research Ranch 686 0.112 0.47 c GrasshopperSparrow 1997 BuenosAires National Wildlife Refuge 415 0.116 0.25• GrasshopperSparrow 1996 The Research Ranch 265 0.129 0.20 Estimatedpopulation size. Estimatedprobability of captureof given individual on given netting day. A singletrap-happy individual removed from analysis. ity, but otherwisethe probabilityof recapture for GrasshopperSparrows and Cassin'sSpar- should remain relatively constant through rows to remainwithin fixedhome ranges dur- time. ing this time period.It also demonstratesthat The probabilityof recapturingGrasshopper mortality was not significantin these cases. Sparrows (all data sets combined)decreased GrasshopperSparrows and Cassin'sSparrows with time (Fig. 1). However,the RERat thelon- were the only specieswith enoughrecapture gesttime interval (eight weeks) was still higher eventsfor this analysisto be performed. than the total RER of the next-mostsedentary Cassin'sSparrows (1998 BANWR data set) species,Baird's Sparrow (Table 2). In the 1996 did not show a significantdifference between TRR data set for GrasshopperSparrows and AI and RER, but GrasshopperSparrows did the 1998 BANWR data set for Cassin'sSpar- show this difference.For all four of the large rows, RER remainedalmost constant through datasets for GrasshopperSparrows, A! values time (Fig. 1). In thesecases, individuals were weresignificantly lower than the RER(P < 0.05 just as likely to be recapturedeight weeks fol- for 1998 at BANWR; P < 0.01 for 1998 at TRR, lowing initial captureas in the first week po- 1997at BANWR, and 1996at TRR). Grasshop- stcapture.This demonstratesa strongtendency per Sparrowsthat were captured early had higher probabilitiesof recapture than birds that were not initially captureduntil later in -- GrasshopperSparrow 1996 TRR the season. 0.25 : Cassin'sSparrow 1998 BANWR Radiotelemetry.--Of the 20 birdsthat received ---A---Total GrasshopperSparrow radio transmitters,three disappearedbefore 0.2 the battery shouldhave expired, and 10 were found dead beforethe batteryexpired, having been killed by predators.! relocated18 birds at 0.15 least twice for a total of 115 relocations. To characterizemovement patterns, I plotted 0.1 net distancemoved between each pair of loca- tions as a function of time and performed linear 0.05 regression(Fig. 2). The regressionsfor all four specieshad slopesbelow 2.6 m per day and R2 valuesbelow 0.029. This suggeststhat individ- ualsof all four speciestended to remainwithin 1 2 3 4 5 6 7 8 fixedhome ranges. Thus, interspecific variation Interval between captures (weeks) in sedentarinessmay haveresulted from vari- ationin home-rangesize amongspecies. FIG. 1. Recaptureevent rate as a functionof the To describe the extent of movement, I also time interval between first and subsequentrecapture eventsfor total GrasshopperSparrows and Grass- comparedthe averagenet distancemoved be- hopper Sparrowsin 1996 at The ResearchRanch tween all pairs of locationsfor individuals of (TRR) and Cassin'sSparrows at BuenosAires Na- eachspecies. Because net distancemoved was tional Wildlife Refuge (BANWR). not correlated with time, these distances can be 754 CALEBE. GORDON [Auk, Vol. 117

Savannah Sparrow Baird's Sparrow

300

600 200

100 200iI1#1 80010 i i i- i i 0 ! 0 10 20 30 40 0 10 20

Vesper Sparrow GrasshopperSparrow

200 600 , , 150 400 , * 100 50 8000,] el ,I i1,_1te I,P I 4 • * , 0 0 10 20 30 40 50 0 10 20

Time interval(days) FIG.2. Net distances(distances between each pair of radio telemetryrelocations) moved with time for four grasslandsparrows. thoughtof as the net distancemoved between ity amongindividual Grasshopper Sparrows is any two locationsof a bird. Thesedata ranked that two types of individuals are presentin speciesfrom mostto least sedentaryin exactly winter, somewith fixedhome ranges and oth- the sameorder as the RERfrom thenetting data ers that move nomadically.This is consistent in Table2. The GrasshopperSparrow was the with manyterritorial systems which "floaters" mostsedentary with an averagenet movement represent an excessof individuals relative to of 84 m, followed by Baird's (113 m), Vesper the numberof availableterritories (Shutler and (142m) andSavannah (186 m) sparrows.All in- Weatherhead1994, Stutchbury1994, Westcott terspecificdifferences were significant (two- and Smith 1994).A discrepancyin movement tailed t-tests,P < 0.05).These observed average behavioramong individual GrasshopperSpar- distances were overestimates because of GPS rows also may correspondto a differencebe- signal scramblingand the error of the GPS tween migrant and residentpopulations that unit. GPS locationsfor a fixed point taken on coexiston thesestudy sitesin winter It is also the daysof radio telemetryshowed an average net movement of 50.2 m. possiblethat differencesin recaptureprobabil- ities amongindividuals were causedby other It is importantto note that mortality cannot behavioral differences that could lead to differ- be distinguishedfrom emigrationas sources of ent capture probabilities, such as different low RER. Corroborationof interspecificdiffer- encesin RER by the telemetrydata suggests flushingresponses or flying heights. that RER does indeed reflect differences in mo- Extent of movement.--Cassin'sSparrows and bility among species.Furthermore, the con- GrasshopperSparrows, and to a lesserextent stancyof RER with weekly intervals suggests Baird'sSparrows, were the mostsedentary spe- that the amountof mortalitythat occurreddur- cies.The extent and pattern of movementsin ing the study period was extremelysmall, at thesesparrows are consistentwith thoseob- least for Cassin'sSparrows and Grasshopper served for desert rodents (Brown 1989, Brown Sparrows (Fig. 1). and Zeng 1989). Thus, the notion that these birds are highly mobile"cream-skimmers" in DISCUSSION this systemmay not be accurate(Mares and Rosenzweig1978, Brown et al. 1979,Thompson Variationamong individuals.--One possible ex- et al. 1991). Some birds, particularly Cassin's planationfor differencesin recaptureprobabil- Sparrowsand grassland-obligateBaird's Spar- July2000] SparrowMovement Patterns 755 rows and GrasshopperSparrows, may behave vannah Sparrowsshowed significantly differ- more like the rodents in the above models of ent local movement patterns between study desertgranivore coexistence, playing the roles sites, movement patterns of Grasshopper, of highly sedentarybut highly efficientforag- Baird's,and Vespersparrows were fixed across ers. yearsand study sites(Table 3). This behavioral The sparrowsin this study exhibitedsignif- constancyis remarkablegiven the radicalfluc- icant interspecificvariation in movementpat- tuations(up to 20-fold) in abundanceof spar- terns. Nonetheless,telemetry data indicated rows acrosssites and years (Table 1). Fluctua- that eventhe least sedentaryspecies tended to tions near an order of magnitude were ob- remainwithin fixedhome ranges (Fig. 2). This servedfor all focal species.Given the impor- patternmight seemto conflictwith thenomad- tance of densities of interspecific and ic behaviors that have been documented in intraspecificcompetitors on habitatselection in seed-eatingbirds in otherrainfall-limited eco- various theoretical and empirically demon- systems(Davies 1984, Clark 1997, Dean 1997). strated situations (Fretwell and Lucas 1970,Ro- In suchsystems, nomadism has been suggested senzweig1981, Pimm et al. 1985),it is surpris- as a way for consumersto copewith extreme ing that suchvariation in the densityof poten- spatial and temporal variability in resource tial competitorswould haveno effecton move- distribution (Noy-Meir 1973, Davies 1984, ment behavior.One potential explanationfor Clark 1997, Dean 1997). Nonetheless,the ob- this pattern is that competitionamong spar- servedsedentariness of sparrowsin this study rows may not be an importantecological factor was not inconsistent with nomadic movements in this system.Another possibility is that the at different scalesof time and space.In fact, movementbehavior of Grasshopper,Baird's sparrowsmay havecombined between-year re- and Vespersparrows is fixedbecause of various gional scalemobility with within-winter sed- physiological,morphological or other con- entarinessto effectivelyexploit the specific straints,and that thesespecies are unableto scalesof environmentalheterogeneity and var- adapttheir movement behavior quickly enough iability in this system.The dramaticfluctua- to trackrapid changesin their environment.A tionsin sparrowabundance across sites within third possibilityis that the densityadjustments years and acrossyears within sites (Table 1) that birds make in responseto environmental suggestedthat the distributionsof sparrows variability obviate the need for additional be- variedsignificantly between years at a regional havioraladjustments. scale.This pattern couldresult if sparrowsun- Movementpatterns and community structure.- derwent a brief period of exploratorymove- FollowingHutchinson's (1951) concept of a fu- mentupon arriving on the winteringgrounds. gitive species,several theoretical studies have This would allow them to match their distri- suggestedthat variationin movementpatterns bution to the variableand patchydistribution functionsas a nicheaxis, allowing the coexis- of resourcesin any given year, driven by the tence of otherwiseecologically identical spe- summer rains. cies in competitively structured communities In contrastto the between-yearvariability of (e.g. Levins and Culver 1971, Horn and Mac- the summer rainfall pattern, the pattern of Arthur 1972,Tilman 1994).This coexistenceis 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 ex- butionsof seedsand grasscover remain rela- ploiting a particular patch (Brown 1989, Til- tively constantduring the courseof a winter.It man 1994). In other words, the best colonizer is unlikelythat sparrowsalter this distribution of new patchescannot also be the bestcompet- significantlyduring the courseof mostwinters, itor within patches. becausethey consumeonly 20 to 30% of the In recent decades,the validity of equilibri- seed resourcesin many winters (PullJam and um-based, competition-structuredcoexistence Dunning 1987). Fixed home ranges during theorieshas been questioned(Connell 1975, midwintermay be an adaptationto exploitthis Connorand Simberloff1979, Strong 1983), par- within-winter constancyby remainingseden- ticularly in highly fluctuating environments tary and conservingenergy. such as grasslandand shrubsteppe(Wiens Plasticityof movementpatterns.--Although Sa- 1974, Rotenberryand Wiens 1980). The ratio- 756 CALEBE. GOl•OON [Auk, Vol. 117 nale for this argumentis that in communities that they should be able to make a living on wherebiological interactions never reach equi- patcheswith lower resourcedensities. librium and consumers do not saturate re- Significanceof movementstrategies in a frag- sources,competitive pressure should be low. mentedlandscape.--Recent models have shown Withoutcompetition, no reasonexists for nich- that sedentaryspecies, including competitive es to be competitively structured. Resource dominants, are more adversely affected by partitioning is not required for speciesto co- landscapefragmentation than are moremobile exist. The wide fluctuationsin sparrow abun- species(Dytham 1994;Tilman et al. 1994,1997; danceacross time and space(Table 1) suggest Moilanenand Hanski 1995).If suitablepatches that winter grasslandsparrow assemblages in- become widely separatedin space,sedentary deed are far from being in equilibrium. Fur- speciesare unableto find and colonizethem. thermore,sparrows are not believedto saturate Danielson (1991) and Pulliam and Danielson seed resourcesduring most winters (Pulliam (1991)modeled the effectof sedentarybehavior and Brand 1975, Pulliam and Parker 1979, Pul- on habitat selectionby varying the amountof liam and Dunning 1987).Therefore, it may not patchesthat individualssampled before they be appropriateto invokethe notionof resource chosea patchto inhabit. In their simulations, partitioningto explainthe significantdiffer- individuals that sampled a high number of encesin movement patterns among speciesre- patchesoccupied a higher percentageof the corded in this study. In short, the notion of betterhabitat than individualswith low patch competitive coexistencemay not apply to co- sampling. In landscapeswhere high-quality existingspecies of grasslandsparrows. habitat patcheswere rare, the ability to find Nonetheless,competitive structuring should goodpatches limited population growth, and a not be ruled out completely.Competition does high level of exploratorypatch sampling was not need to be intense or constant to affect the necessaryfor speciesto survive. distributionof nichesin a community(Connell This has an important implicationfor the managementof highly sedentaryspecies such 1980). PullJam(1985) suggestedthat competi- as GrasshopperSparrows, Baird's Sparrows, tion and resourcepartitioning among sparrows and Cassin'sSparrows. In additionto beingaf- winteringin Arizona grasslandsare important fectedby pure increasesor decreasesin their only in rare yearswhen seedproduction is ex- preferredhabitat types, sedentary species may tremely low and competitionfor seedsmay be suffer increased adverse effects relative to more especiallyintense. Furthermore, even if preda- mobile speciesif the distribution of suitable tors limit sparrowpopulations below their abil- habitat becomesfragmented. One important ity to saturatefood resources,niche partition- caveatis that in this study,the mobility of spar- ing may be required for coexistence(Holt rows was measured at the scale of several hect- 1977).Therefore, differences in movementpat- ares.This small scalemay not accuratelyreflect terns may function as an important coexistence regionalmovements, which perhapsare more mechanismfor ecologicallysimilar grassland appropriatelymatched to the spatiotemporal sparrowswintering in Arizona grasslands. scaleof environmentalvariability in this sys- Even in the absenceof nichepartitioning in tem. Figure3 suggeststhat theselocal, within- responseto competition,the tradeoff between year mobility differencesindeed reflectmove- within-patch efficiency and among-patch ment constraintsat the regionalscale as well. searchingmay posesignificant constraints for At the between-yearand betweenstudy site sparrows. Speciesthat sacrificedsome ability scales,abundances of the more locallyseden- to explorethe landscapemay have done so to tary specieswere more poorly correlatedwith gain efficiencyat the local scale.Indeed, the summerrainfall (a negativecorrelation in the three mostsedentary species in my study,Cas- caseof Baird'sSparrow) than were abundances sin's Sparrow, Grasshopper Sparrow, and of the moremobile species. Alternative hypoth- Baird'sSparrow, are short-wingedrelative to esescannot be ruled out, but a likely explana- othersparrows and are known to be weak fliers tion for this patternis that sedentarybehavior (Rising1996). This leads to the predictionthat constrainsthe ability of GrasshopperSpar- these sedentaryspecies are more locally effi- rows, Baird's Sparrows,and Cassin'sSparrows cient than are the more mobile sparrows,and to match their winter spatial distributionsto July2000] SparrowMovement Patterns 757

Baird's Sparrow Vesper Sparrow

40 R2=0.18 ø 2ooR 2 = 0.64 30 • 150

2010 •• 10050

0 0 03 0 100 200 300 0 100 200 300

GrasshopperSparrow Savannah Sparrow 4oo500R2 =0.21 , 420400R2 =0.38 • 200 40 100 20,o 300o , *, , 80o ••e , 0 100 200 300 0 100 200 300

o

Cassin's Sparrow Brewer's Sparrow 40 70

o 30 50 40 20 30 10 20 ,o10 0 i , , • . . 100 200 300 0 100 200 300

Total July-Septemberrainfall (mm) FIc. 3. Relationshipbetween summer rainfall and sparrowcaptures in subsequentwinter fromJanuary to March1996 to 1999.The threemost sedentary species at left are muchmore poorly (or negatively)cor- relatedwith rainfallthan the threemore mobile species at right, suggestingthat movementconstraints play an importantrole in large-scalehabitat selection. Each data point representsthe total numberof individuals capturedin sevenflush-netting sessions at a singlestudy site per winter.The 1997and 1998BANWR data setswere standardizedto sevenflush-netting dates by discardingdata from the eighthand ninth netting sessions.I extrapolated the 1999totals from threeto sevendates by dividing the three-weektotals by the averageproportion of birdsin a seven-weekdata set captured by the third week.To calculate these averages, I divided the total numberof birds caughtby the third weekby the total numberof birds that had been caughtby the seventhweek in all data setswith at leastseven netting dates. July to Septemberrainfall data are from rain gaugesat the headquartersof TRR and BANWR,1 to 5 km from all studyplots. R 2 values are givenonly for specieswith sevendata points. Brewer's Sparrows and Cassin'sSparrows do not occurreg- ularly at the higher-elevationTRR studysite; hence, only threedata points representing three flush-netting seasonsat BANWR are presented. the distributionof preferredconditions (high lands to shrublands in southeastern Arizona rainfall areas)in the landscape. overthe past 120 years(Bahre 1991). The sed- The grassland-obligateGrasshopper Spar- entarybehavior of thesespecies suggests that rows and Baird'sSparrows undoubtedly have managersneed to considerthe distributionof sufferedfrom widespreadconversion of grass- habitatpatches in the landscape,as well asthe 758 CALEBE. GORDON [Auk,Vol. 117 structuralcomposition of the patches.In other CLARK, M. E 1997. A review of studies of the breed- words, it's not just a questionof creatingthe ing biology of Australian birds from 1986-95:Bi- right conditionsin a particular patch:it also asesand consequences.Emu 97:283-289. may matterwhere the patchis relativeto other CONNELL,J. H. 1975. Somemechanisms producing structure in natural communities: A model and patcheswithin the range of existingpopula- evidencefrom field experiments.Pages 461-490 tions of sparrows.Managing for relatively sed- in Ecology and of communities(M. L. entaryspecies such as Cassin's Sparrow, Baird's Cody and J. M. Diamond, Eds.). Harvard Uni- Sparrow,and GrasshopperSparrow may re- versityPress, Cambridge, Massachusetts. quire maintaininga landscapewith aggregat- CONNELL,J. n. 1980.Diversity and the coevolution ed patchesof suitablegrassland, or coreareas, of competitors,or the ghostof competitionpast. as opposedto a landscapewith an equal area Oikos 35:131-138. of grasslandspread out amongwidely separat- CONNOR, E. E, AND D. SIMBERLOFF.1979. The assem- ed smallpatches. bly of speciescommunities: Chance or compe- tition? Ecology60:1132-1140.

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