The Auk 117(3):748-759, 2000 MOVEMENT PATTERNS OF WINTERING GRASSLAND 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 (Ammodramus 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 habitat 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 birds 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 grasslands, 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