Effects of Habitat Structure on Patch Use by ...

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Effects of Habitat Structure on Patch Use by Loggerhead Shrikes Wintering in a Natural Grassland Author(s): Felipe Chavez-Ramirez, Dale E. Gawlik, Felipe G. Prieto, R. Douglas Slack Source: The Condor, Vol. 96, No. 1 (Feb., 1994), pp. 228-231 Published by: University of California Press on behalf of the Cooper Ornithological Society Stable URL: http://www.jstor.org/stable/1369086 Accessed: 25/01/2010 10:03 Your use of the JSTOR archive indicates your acceptance of JSTOR's Terms and Conditions of Use, available at http://www.jstor.org/page/info/about/policies/terms.jsp. JSTOR's Terms and Conditions of Use provides, in part, that unless you have obtained prior permission, you may not download an entire issue of a journal or multiple copies of articles, and you may use content in the JSTOR archive only for your personal, non-commercial use. Please contact the publisher regarding any further use of this work. Publisher contact information may be obtained at http://www.jstor.org/action/showPublisher?publisherCode=ucal. Each copy of any part of a JSTOR transmission must contain the same copyright notice that appears on the screen or printed page of such transmission. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected].

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at the Mangaurcu,Ecuador,locality had grasshopper remainsin its stomach.

rope the MiddleEastand North Africa.The birds of the WesternPalearctic.Vol. IV. OxfordUniv. Press, Oxford,England. ANDB. B. COFFEY. HARDY,J. W., G. B. REYNARD, 1989. Voices of the New World nightjarsand their allies. ARA Records,Gainesville,FL.

We thank Miguel Moreno, Director of the Museo Ecuatorianode CienciasNaturales(MECN),Quito,and the Ministeriode Agricultura,Quito, especiallySergio Figueroa,for supportand authorizationof our ongoing S. 1958. The birds of the Santa Elena researchprogramin Ecuador.ANSP fieldworkduring MARCHANT, Peninsula,S.W. Ecuador.Ibis 100:349-387. 1990-1992 was supportedby the MacArthurFounS. 1959. The breedingseason in S.W. dation.ThomasSchulenbergkindlyprovidedthe spec- MARCHANTr, Ecuador.Ibis 101:138-152. comVan Remsen made he and and helpful trogram R. M. 1976. Rapid tail molt and tempomentson the manuscript.JohnArvin,Steve Hilty,and MENGEL, impairedflight in the Chuck-will's-widow. rarily Niels Krabbegenerouslysharedtheir unpublishedobWilson Bull. 88:351-353. servations with us, and Jos6 Santistebanidentified MEYERDE SCHAUENSEE,R. 1966. The species of birds stomachcontents. of South Americaand their distribution.Livingston Publishing,Narbeth,PA. LITERATURECITED PETERS,J. L. 1940. Check-list of the birds of the M. J. 1937. Note sur quelques oiseaux rares BERLIOZ, world, Vol. 4. HarvardUniv. Press, Cambridge, ou peu connus de l'Equateur.Bull. du Mus6um MA. Nationald'HistoireNaturelle,2nd serie,tome IX, ROHWER,S. A. 1971. Molt and the annual cycle of no. 2. the Chuck-will's-widow,CaprimulguscarolinenF. M. 1923. New South Americanbirds. CHAPMAN, sis. Auk 88:485-519. Amer. Mus. Nov. no. 67. SCHWARTZ,P. 1968. Notes on two Neotropical nightF. M. 1926. The distributionof bird-life CHAPMAN, jars, Caprimulgusanthonyiand C. parvulus.Conin Ecuador.Bull. Am. Mus. Nat. Hist. 55:1-784. dor 70:223-227. CRAMP,S. [ED.]. 1985. Handbook of the birds of Eu-

The Condor 96:228-231 ? The Cooper Ornithological Society 1994

EFFECTS OF HABITAT STRUCTURE ON PATCH USE BY LOGGERHEAD SHRIKES WINTERING IN A NATURAL GRASSLAND' FELIPECHAVEZ-RAMIREZAND DALE E. GAWLIK

Departmentof Wildlifeand FisheriesSciences, Texas A&M University, CollegeStation, TX 77843-2258 FELIPEG. PRIETO

UnitedStates Fish and WildlifeService,MatagordaIsland National WildlifeRefuge,P.O. Box 100, Austwell,TX 77950 R. DOUGLAS SLACK

Departmentof Wildlifeand FisheriesSciences, Texas A&M University, CollegeStation, TX 77843-2258 with the differentialdeclinesof shrikepopulationsobserved in various regionsof the United States. Populations in the intensive agriculturalareas of the midRecent attemptsto explain the decline of many Log- west, south, and southeastaredecliningmore severely gerheadShrike(Laniusludovicianus)populations(Bys- than those in the westernUnited Statesdominatedby trakand Robbins 1977,Geisslerand Noon 1981,Mor- grasslands(Arbib 1977, Morrison 1981). Althougha rison 1981)have focusedon habitatloss due to modern considerableamount of informationexists on shrikes agriculturalpractices(BrooksandTemple 1990;Smith in agriculturalsystems,none is availableregardinghaband Kruse 1992;Yosefand Grubb1992, 1993;Gawlik itat changesand the mechanismsaffectingshrikesin and Bildstein1993).Degreeof habitatloss is consistent naturalgrasslands.Understandingshrikeuse of natural Key words: Conservation;habitat use; Lanius ludovicianus;LoggerheadShrike;management;perch.

grasslands can lead to a better understanding of shrike

responseto land-usechanges. Recent emphasis on conservation

'Received 21 September 1993. Accepted 23 September 1993.

of wintering

groundsand migrationcorridors(Terborgh1992)have underscoredthe significanceof identifying essential

SHORT COMMUNICATIONS habitatcomponentsin all ecosystemsin which a species occurs which are critical for conservationof migratorybirds.Grasslandsarethe primaryhabitatsused by shrikes on the Texas coastal region where shrikes are common during winter (Rappole and Blacklock 1985, Root 1988). The importanceof open areaswith short vegetation, particularlyimproved pasturesand grasslands,as habitatfor shrikeshas been previously noted (Bohall-Wood1987, Brooksand Temple 1990, Smith and Kruse 1992, Gawlik and Bildstein 1993). In agriculturalsystems, shrikesforagein grassyareas from elevated perches(i.e., above canopy level) (Bildstein and Grubb 1980, Scott and Morrison 1990) and exhibit high use of foragingsubstratewithin 10 m of elevated perches (Morrison 1980, Yosef and Grubb 1992). Indeed, perches are a necessaryhabitat componentfor shrikesto utilizeforagingsubstrateanddensity of elevatedperchesreflectsterritoryquality(Brooks and Temple 1990, Yosef and Grubb 1992). Dependingon height and density of groundcover, shrikeswill increasetheir perch height, to enlargethe utilizableforagingarea and allow for greaterprey encounterrates(Mills 1979). However,in the absenceof elevatedperches,shrikesin a grasslandfrequentlyhunt from lower herbaceousperches(Chavez-Ramirezand Gawlik, pers. observ.). Mills (1979) argued that by perchingcloserto the ground,shrikesreducedhandling time of prey, increasedcapturesuccess,and decreased energyexpenditurewhen returningto perches,factors thatareinfluencedgreatlyby vegetationstructure.Natural grasslandvegetation provides greaterstructural heterogeneity,both vertical and horizontal,than agriculturallands.Thus, we predictedthat in a grassland, reduceddensity of elevated percheswould not result in a concomitantreductionin the use of the area by shrikes. This study representsthe first experimental study of perchuse by LoggerheadShrikeconductedin a grasslandpresentlymaintainedonly by naturalprocesses. STUDYAREAANDMETHODS This study was conducted on MatagordaIsland National Wildlife Refiugeand State NaturalArea in Calhoun County,Texas, USA. This 22,934-ha barrierisland, includes saltmarshes,sand dunes, and upland barrierflats. Upland flats, the primaryareas used by shrikes,arecoastalgrasslandsdominatedby marshhay cordgrass(Spartinapatens), gulfdunepaspalum(Paspalum monostachyum),and seacoast bluestem (Schizachyriumscoparium).Woody vegetation, primarily mesquite(Prosopisglandulosa)and false willow (Baccharis helmenifolia),is sparseand scatteredthroughout. For the purposesof this study, we classifyall shrike habitatinto the broad categoriesof agriculturallands and grasslands.We define agriculturallands as those lands maintainedeither throughcrop cultivation, intensive grazing,or regularmowing (i.e., lawns, roadsides, and hay fields).Grasslandsarethose landsmaintained throughnaturalprocessessuch as fireand lowintensitygrazing. From 14 to 17 October 1992, beforemost wintering shrikesarrived,we selectedfor manipulationsix fences approximately0.8 km in length and at least 0.8 km apart,with one fence containinga 900 bend. All wire above the vegetationcanopywas removedso thateach

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fenceappearedas a line offence posts, thuseliminating the confoundingeffectof barbedwireas a perchingand impalingsite. To reducevariabilityamong fences, we removedall elevatedpercheswithin 20 m and mowed the vegetationwithin 10 m of fences to a heightof 18 cm. Thus, each of our six habitatpatchesconsistedof similar 800 x 20 m mowed strips of vegetationcentered lengthwiseon a line of fence posts and including 10 m of surroundingnaturalgrasslandvegetation. To determineif shrikesreduceduse of the habitat patchesin responseto reducedperchdensity, we used a completelyrandomizedblockdesignwith two blocks and three treatmentsper block. Each habitat patch receivedonly one treatment.Treatmentswereapplied on 10 and 11 January1993, and consistedof reducing the densityof fence posts in each patchfrom an initial densityof approximately158 posts/hato either0 posts/ ha (100%reduction),16 posts/ha (90%reduction),or 158 posts/ha (0% reduction)as a control. The 90% reduction (distance between posts >20 m) provided foraginghabitatgreaterthan 10 m away from any elevatedperch,the distancewherethe majorityof shrike foragingattacksoccur(Morrison1980,YosefandGrubb 1992).Any reductionless than90%wouldstill provide elevated perches within 10 m of some portion of a habitatpatch. Initiallywe intendedto use a statistical test accountingfor blocks in data analysis. However, because our a priori expectation was no difference amongtreatments,we weremoreconcernedwith committing a Type II errorratherthan a Type I (Freund and Wilson 1993). Therefore,we chose a more powerful test (Kruskall-Wallistest; Conover 1980). To control for seasonal and annual variation, we conductedour experimentwithin one winterseason,a periodduringwhich we did not expect movementsor changesin shrikenumbers.We determinedpatch use from 30-min observation periods conducted weekly from 8 November 1992 to 7 February1993. Observations were conductedduringthe morninghours on three habitat patches/day for two consecutive days. Duringobservationperiodswe instantaneouslyscanned the entirepatcheach minuteand recordedthe number of shrikes present. The dependent variable was calculated as total number of shrike-minutes(e.g., two shrikes seen on one scan and three shrikes seen on anotherscan result in five shrike-minutes)per observation period, averagedover four pre-treatmentand four post-treatmentobservationperiods. In additionto reducingthe variabilityamongpatches, mowing reducedvegetation height and may have improved habitat quality for shrikes (Bohall-Wood 1987, Brooks and Temple, 1990, Smith and Kruse 1992, Gawlik and Bildstein 1993). To determine if mowing increasedpatch use by shrikes,we compared two additionalhabitatpatchesthat receivedno mowing or perchremovalwith two unmowedcontrolpatches in the perch manipulationexperiment. The unmowed patchesalso lackedwire above the vegetation canopyand possessedsimilardensitiesof posts as our experimentalhabitatpatches.We surveyedeach habitat patchfor 30 min weeklyduringthe morninghours on two habitat patches per day for two consecutive days, followingthe same proceduredescribedabove. We used a WilcoxonSignedRanktest (Conover1980) to determinedifferencesin shrikeuse betweenmowed and unmowedpatches.

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TABLE 1. Habitatpatch use by LoggerheadShrikes pre-treatment(n = 4 surveys per habitat patch) and post-treatment(n = 4 surveys).Treatmentswere0 post/ ha (100%reduction),16 posts/ha(90%reduction),and 158 posts/ha (0%reduction)as a control.

Unfortunately,the authorsdid not reportinformation on perch use. The most strikingdifferencebetweenagricultural and grasslandsystemsis the scaleor size of habitatpatches. In agriculturalsystems, vegetation is uniform within fieldsrelativeto vegetationamongfieldsand therefore, short grassy patches occur at the scale of individual Shrike-minutes fields much largerthan shrikeforagingareas.Hunting Pre-treatment Post-treatment % Treatment Block mean(SD) mean(SD) Change perchesused by shrikesare primarilyfenceposts, utility lines, and woody vegetation (Bohall-Wood 1987, 1 0%removal 7.3 (7.9) 8.7 (8.9) +20 Yosef and Grubb 1992, Gawlik and Bildstein 1993) 2 5.0 (4.6) 2.0 (2.4) -60 usuallyarrangedin linearstripsalongthe edgesof fields. 90%removal 1 18.8 (14.2) 0.8 (1.4) -96 Thus, the resultinglandscapeconsists of largemono2 5.8 (6.0) 0.0 (0.0) -100 typic habitat patches adjacentto linear strips of ele7.5 (7.5) -56 100%removal 1 17.0 (9.9) vated perches.Yosef and Grubb(1992) suggestedthat 2 4.0 (2.7) 0.0 (0.0) -100 a reduceddensity of perches in agriculturalhabitats diminished the amount of available foraginghabitat becauseshrikesrestrictedtheiruse of foragingsubstrate to within 10 m of elevatedperches.Thus, muchof the As an independentmeasureof perchuse by shrikes potentialforagingsubstratein areasof low perchdenon unmanipulatedareasof MatagordaIslandwe con- sity was unusable. ducted road surveysfor raptorsand shrikeswherewe In contrast to agriculturalareas, the landscapeon notedspecificsubstrateson whichshrikeswereperched. MatagordaIsland consists of relativelysmall habitat We used a Chi-SquareGoodness-of-Fittest (Conover patches that occur at the scale of several meterswith 1980)to determinedifferencesamongperchsubstrates. nonwoodyperchesdistributedevenly throughout(see also Prescott and Collister 1993). The birds in our RESULTS study utilizedthe abundantnonwoodyperchessignifPatch use by shrikesdid not changefollowingthe re- icantly more than other substrates,unlike shrikes in = duction in fence posts (H = 2.28, df = 2, P 0.32, agriculturalsystems where elevated woody substrates Table 1). It appearedthat shrikes perchedmore fre- arethe primaryhuntingperches.Thus,the diversevegquently on lower nonwoody vegetation.Naturalveg- etation structurepresenton MatagordaIslandallowed etation surroundingthe mowed strips approximated shrikes to use a high proportionof the vegetationas 100%groundcover and occurredin clumps up to 1.1 potentialforaginghabitattherebycompensatingforany m in height,and providedadequatesupportfor perch- habitatmanipulationsat the scale we provided. ing shrikes. This was reflectedin the wide range of Ourresultssuggestthat shrikepopulationsin natural naturalperchesthat shrikesin unmanipulatedareasof grasslandsdo not behavelike those in agriculturalsysMatagordaIsland used. Shrikes used abundantnon- tems, with respectto foraginghabitat and perch use, woody perch structuressignificantlymore (x2 = 24.6, as previouslyreported(Bohall-Wood1987,Brooksand df = 3, P < 0.05) than other substrates.Of 51 shrikes Temple 1990,SmithandKruse1992,GawlikandBildobserved perchedduring surveys, 57%were on non- stein 1993, Yosef and Grubb 1993). We found no evwoody plant perches(sunflowerHelianthusspp. 33/%,o,idencethatmowingvegetationor manipulatingdensity partridgepea Cassia spp. and sesbaniaSesbaniaspp., of man-made perches affects shrike use of habitat 14%,and grass 10%),22%perchedon woody vegeta- patches in a naturalgrassland. tion (mesquite 16%and false willow 6%), 10%were Our results pertain directly to the conservationof observed on posts, and 1 1%perchedon other struc- shrikesandotherdecliningspeciesbecausetheysuggest tures (e.g., wood piles, signs, and concretestructures). that management strategies should not be applied Also, shrikesin our experimentdid not utilize mowed universally across all ecosystems. Although habitat patches (x = 6.75, SD = 9.4, and x = 3.5, SD = 4.24) manipulationmay increaseshrikepopulationsin agrisignificantlymore (z = 0.73, df= 12, P > 0.05) than culturalecosystems, such alterationsmay not be apunmowed patches (x = 3.1, SD = 6.6 and x = 2.9, SD propriatein less disturbed,more naturalsettings. In = 3.7). addition, we must considerthe effects of short vegetation and increasedperch density on the entire bird DISCUSSION community.Forexample,increasingperchdensitymay The lack of differencesin shrikeuse between mowed attracthighernumbersofraptors(potentialpredators), and unmowed patches and nonsignificantdecline in which could ultimately have a negative impact on use to reducedperchdensity observedin our study is shrikes. In our study site 65% of Northern Harriers not consistent with studies of shrike habitatuse con- (Circuscyaneus)and White-tailedHawks (Buteo alducted in agriculturalsystems (Bohall-Wood 1987, bicaudatus)observedduringbiweekly raptorsurveys Brooks and Temple 1990, Smith and Kruse 1992, wereperchedon fenceposts,whereasonly 35%perched Gawlik and Bildstein 1993, Yosef and Grubb 1993), on naturalvegetation.Althoughwe recognizethe urgent where shrikesexhibit high use of short grassyvegeta- need to implementmanagementstrategiesfor shrikes tion. However,resultsobtainedon a naturalgrassland on a largescale, we urgecaution in applyingmanagein Albertasuggestthat, like our study, shrikesdid not ment strategiesfor shrikes developed in agricultural prefershortgrassyareas(Prescottand Collister1993). systems directlyon naturalgrasslandsuntil more in-

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