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INTERNATIONAL STUDY GROUP BULLETIN

Nest fate and vegetation characteristicsfor Snowy and in Colorado, USA

Todd J. Mabee & Veronica B. Estelle

Mabee,T.J. & Estelle,V.B. Estelle2000. Nest fate andvegetation characteristics for snowyplover andkilldeer in Colorado,USA. WaderStudy Group Bull. 93: 67-72.

We quantifiedvegetation characteristics at nestsites of SnowyPlovers ( alexandrinus)and (C. vociferus)in south-easternColorado during 1995 to assess interspecificdifferences in nestinghabitat and determine whether characteristics influenced nestfate. Killdeersnested in areaswith significantlymore grass, litter, and vegetation within 10 cm of the groundthan Snowy . We foundno significantdifferences between any of the coverageor structuralhabitat characteristics and successful or failednests of SnowyPlovers and Killdeers. Small-scalemanagement of nestinghabitat of shorebirdsshould be undertakenonly if reproductivesuccess varies among habitat types.

ToddJ. Mabee (correspondingauthor), Department of Biology,Colorado State University,Fort Collins, Colorado 80523. Present address:ABR Inc. PO. Box 249, Forest Grove, Oregon USA 97116. Phone: (503) 359-7525. Emaih tmabee•abrinc.com VeronicaB. Estelle, ColoradoBird Observatory,13401 Piccadilly Road, Brighton, Colorado 80601. Presentaddress.' Department of Fisheryand WildlifeBiology, Colorado State University, Fort Collins, Colorado 80523

INTRODUCTION productivity,it mustbe demonstrated.that successful Habitatdegradation has been implicated as a principal nestsoccur in vegetationwith differentcharacteristics causeof populationdecline for federally-listed than unsuccessfulnests for the speciesof concern. shorebirdssuch as the Piping Plover (Charadrius Vegetationdensity of nestinghabitat has oftenbeen melodus)and Snowy Plover (C. alexandrinus)(USFWS implicatedas an importantinfluence on the ability of 1994,Page et al. 1995). To reversethis trend, habitat predatorsto destroynests (for review,see Martin 1992). managementon the breedinggrounds has been For example,vegetation structure could affect nesting recommendedto maximizenesting success and survival successby reducingthe detectabilityof the incubating of endangeredshorebirds (Prindiville, Gaines & Ryan adultby predators(Bergerud & Gratson1988) or leadto 1988,USFWS 1994,Koenen et al. 1996). Habitat differentialrates of nestpredation (Pampush and managementmay includeseveral techniques that arenot Anthony1993). Regardlessof themechanism, if mutuallyexclusive, including, creation of nestinghabitat, reproductivesuccess does not vary in disparatenesting modificationof substrate,restoration of damagedsites, habitat,then attempts to modify the existinghabitat to or controlof vegetationby chemicalmethods or grazing. increasereproductive success is likely to be ineffective. By no meansare the issuesof habitatdegradation and To date,studies examining the relationshipbetween nest habitatmanagement limited to shorebirds.For example, fate and habitat characteristics for shorebirds have severalspecies of ground-nestinggrassland face documenteda significantrelationship for SnowyPlovers similarthreats of habitatdegradation on the breeding (Pageet al. 1985),a temporallyvariable relationship for grounds;loss of prairiehabitat and reduced complexity PipingPlovers (Prindiville, Gaines & Ryan 1988),or no of grasslandsdue to uniformgrazing pressure and relationshipfor Wilson'sPhalarope (Phalaropus disturbance(Knopf 1996). Giventhat one-thirdof tricolor;Colwell 1992). prairiebird speciesin North Americaare decliningat statisticallysignificant rates (Knopf 1996), Our objectiveswere to (1) quantifyinterspecific will likely be managedin the futureto increasenesting differencesin nestinghabitat between Snowy Plovers success. andKilldeers (C. vociferus);and (2) determinewhether successfulnests occur in vegetationwith different However,before one canmanage for increased characteristics than unsuccessful nests. 67

Bulletin93 December2000 INTERNATIONAL WADER STUDY GROUP BULLE TIN

STUDY AREA AND METHODS the generalhabitat available for nestingplovers. A Approximately25,000 playa lakes are scattered largerscale of measurementwould have incorporated throughoutsouth-eastern Colorado and adjoiningareas additionalhabitat types that would not havebeen of Kansas,Oklahoma, Texas, and New Mexico (Bolenet representativeof plovernesting areas (e.g., denseforbs al. 1989). Playalakes are ephemeral (i.e., at thejunction of lakeshoresand upland prairie). Within frequentdrying and flooding cycles) characterized by eachquadrat, we measuredcoverage by visually RandallClay soilsin theplaya interior that grade into estimatingthe percentof grass,sedge, forb, shrub,and moreporous soils in adjacentuplands (Davis and Smith litter to the nearest10% interval. All observerspracticed 1998). Our studysites encompassed several impounded visualestimation of coverageto minimizeinterobserver andnatural playa lakes in Kiowa andProwers counties, variability. We measuredstructural attributes of andJohn Martin Reservoirin Bent County,Colorado. vegetationby passinga 4.8 mm diameterrod vertically SnowyPlovers and Killdeers are ground-nesting throughthe vegetationat four standardizedlocations shorebirdsthat incubate2-4 eggsfor -26-27 d (Warriner within the quadrat.At eachpoint we countedthe et al. 1986,Powers 1978). Bothspecies frequently numberof timeseach plant life form contactedthe rod initiatednests along lakeshores (open or within each10 cm intervalup to 1 m. We collected alkalineflats) surrounding impounded and natural playa habitatmeasurements at the nestmicro-site only when lakes. We searchedall habitattypes (i.e., opento we initiallyfound a nest(to minimizedisturbance) denselyvegetated) repeatedly for nests. and at both scales when a nest became inactive.

Commonvegetation at reservoirsand altered playa lakes Habitat derived variables includedgrasses (Bouteloua gracilis, B. hirsuta, We selectedand derivedseveral variables describing Buchloedactyloides, Aristida longiseta,Hordeurn horizontal and vertical structure and horizontal jubatum) and forbs(Plantago patagonica, Oenothera heterogeneityfollowing researchers studying habitat albicaulis,Sphaeralcea coccinea, Zinnia acerosa, relationshipswith ground-nestingpasserines and Conyzacanadensis, Helianthus annuus, Dalea candida, shorebirds(Wiens 1974, Rotenberry and Wiens 1980, Psoralea tenuifiora,Chamaesyce glyptosperma, Colwelland Oring 1990). Thesevariables were useful for Reverchoniaarenaria, Heliotropium convolvulaceum, investigatingthe relationshipbetween nest successand Chenopodiumglaucum). Commonvegetation at natural habitatfeatures because they characterizedsalient playalakes included grasses (Distichlis spicata), forbs habitatfeatures around playa lakesin our studyarea. (Kochia americana,K. scoparia,Salsola iberica, We indexedhorizontal structure by calculatingthe Sesuviumverrucosum), and shrubs(Suaeda torreyana). averagenumber of contactswithin the first 10 cm interval (HIT 10). We measuredvertical structure by calculating We recorded habitat characteristics at nest sites of the averagetotal numberof contactsover the entirerod SnowyPlovers and Killdeers between 30 June-6 August, (TOTHITS), andthe average maximum 10 cm interval at 1995and 19April-7 August,1995, respectively, as part whichvegetation contacted the rod (MAXHGT). We of a studyevaluating the effectivenessof predator assessedhorizontal heterogeneity of vegetationby exclosuresfor nestingplovers in Colorado(Mabee and calculatingthe coefficientof variationof HIT 10 Estelle2000). Becauseexclosures were ineffectual (CVHIT 10)and MAXHGT (CVMAXHGT). Becausewe [i.e., we did not find a significantdifference in the daily distributedour samplingpoints over horizontalspace, survivalrate betweenprotected (0.977; SE -- 0.009; the between-pointvariation in thesevariables measured n -- 14) or unprotected(0.973; SE -- 0.011; n = 14) horizontalheterogeneity (Rotenberry and Wiens 1980). SnowyPlover nests (Z -- 0.20; P = 0.42)] andthey were allocatedrandomly with respectto habitat,we assumed Nestsselected for habitat analyses that nestsuccess was independentof the presenceof a We floatedeggs from all SnowyPlover (n = 31) and predatorexclosure and used both typesof nestsin the Killdeer (n = 49) clutchesin tap water to estimatethe age habitatanalyses. We did not protectKilldeer nests with of a clutch(Hill 1985,Alberico 1995, Sandercock 1998) predatorexclosures during 1995. andto determinewhich nests were early in incubation (i.e., a maximumof 7 d old). We usedthis subsetof Habitat measurements SnowyPlover (n = 23) andKilldeer (n = 33) neststo We sampledhabitat features at two scalesusing a square determineif therewere interspecific differences in habitat quadrat(0.25 m2);the nest micro-site (one quadrat when birds initiated a nest. We also collected habitat centeredover the nest)and within a 5-m radiusof the dataon inactivenests (either hatched or failed)to nest(five quadratsplaced randomly within a 5-m radius determineif nestsuccess (> 1 egghatch per clutch)was of the nest). We selectedthese scales because they independentof habitatfeatures for eachspecies. We characterized habitat features of nest sites and the included nests that were either successful or failed due generalplant communitysurrounding a nest, to predationor abandonment(Snowy Plover; n = 27, 68 respectively.Together, these local scalesencompassed Killdeer; n = 25). Bulletin93December2000 INTERNATIONAL WADER STUDY GROUP BULLETIN

Statisticalanalyses.--We calculated a Spearman we usedthe nonparametricMann-Whitney U-test to test correlationcoefficient for all possiblepairs of variables for differencesbetween initial habitats, and between nest (SAS 1990)and decided apriori to removeone member fate and individualhabitat variables (SAS 1990). We of anypair of intercorrelatedvariables that had an r• usedthe midpointsof the coverageand MAXHGT value of> 0.85 at either the nest or 5-m scale. The intervalsin the analyses(e.g., an intervalof 1-10% = 5%, degreeof correlationamong retained variables ranged l 1-20% = 15%, etc.) and roundedthe valuesto the betweenr• = 0.20-0.79.To insure adequate samples, we nearest5% (coverage)and 5 cm (MAXHGT) in the omittedcoverage categories that occurredat <15% of tables. We calculated mean values of habitat variables nests. Removal of redundant or minor variables reduces from the five quadratsused in the 5-m scaleto compare the numberof comparisonsand increasesthe power of to the value obtained at the nest microsite. We used the detectingdifferences when making multiple comparisons nonparametric,sequential Bonferroni test to controlthe (Rice1989). experiment-wiseerror rate at alpha= 0.10 for all individual comparisons(Rice 1989)of SnowyPlover and Killdeer Becausemost variables were not normallydistributed, nestsites. We selectedthis alphalevel a priori due to

Table 1. Vegetationcharacteristics of nestsites of SnowyPlovers and Killdeers in Colorado,1995. Habitatvariables denotemedian (range). P valuesindicate significance of individualMann-Whitney U-tests.

Habitatvariable Snowypl'Over ..,Killdeer (n,...=23): (n.=:33)

Coverage(%) Grass 0 (__a) 0 (0-85) -3.8 (<0.01)* rorb 5 (0-85) 5 (0-85) 0.8 (0.43) Litter 5 (__a) 5 (0-95) -3.5 (<0.01)* Structural HIT 10b 0 (0-2.8) 1.3(0-3.3) -4.0(<0.01)* Heterogeneity(CV) CVHIT10 0(0-200) 61 (0-200) -1.6 (0.11) a Indicatesno rangeof values. bHIT 10 equalsaverage number of vegetationcontacts within 10 cm of the ground. * Significant(P œ0.10) whenexperimentwise error rate was controlled at alpha= 0.10 by thesequential Bonferroni test

Table 2. Vegetationcharacteristics of nestsites of SnowyPlovers in Colorado,1995. Habitatvariables denote median(range). P valuesindicate significance of individualMann-Whitney U-tests.

Nest site 5 m radius Successful Failed Successful Failed Habitat variable n = 14 n = 13 Z(P) n = 14 n = 13 Z(P)

Coverage(%) Forb 5(0-25) 5(0-75) 0.9(0.37) 5(0-25) 0(0-20) 0(0.98) Shrub __a •a a 0(•) 0(0-5) -1.5(0.13) Litter 5 (•) 5 (---•) 0(0.99) 5 (0-5) 5 (0-5) -0.4(0.70) Structural HIT10' 0.1 (0-1) O.3(0-3.3) O(O.99) 0.1 (0-1.2) 0.1 (0-0.95) -1.2(0.24) Heterogeneity CVHIT10d 64(0-200) 92(0-200) 0.2(0.85) 309(0-447) 255(0-447) -1.2(0.22)

Missingvalues were not calculatedbecause of low frequencyof occurrence. Indicatesno rangeof values. HIT 10 equalsaverage number of vegetationcontacts within 10 cm of the ground. CVHIT 10equals coefficient of variationof HIT 10.

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Bulletin93 December2000 INTERNATIONAL WADER STUDY GROUP BULLETIN

Table 3. Vegetationcharacteristics of nestsites of Killdeersin Colorado,1995 Habitatvariables denote median (range). P valuesindicate significance of individualMann-Whitney U-tests.

Nest site 5 m radius Successful Failed Successful Froled Habitatvariable n = 9 n = 16 Z (P) n = 9 n = 16 Z (P)

Coverage(%) Grass 5 (0q55) 5 (0-55) 1.6(0.10) 10(0q55) 5 (0-50) 1.2(0.23) Forb 5 (0-15) 5 (0-25) -0.9(0.36) 5 (5-15) 5 (0-45) 0.3(0.80) Shrub __a a •a 0 (0-15) 0(0-15) 1.2(0.22) Litter 5 (5-25) 5 (5-55) 0.21(0.83) 5 (5-15) 5 (5-15) 0.8(0.45) Structural HIT10b 2(0-5) 1.4(0.3-3.5) 0.8(0.41) 1.8(0.8-4.8) 1.3(0.1-4.6) 1.3(0.19) MAXHGT c 5(0-10) 5(0-10) 0(0.98) 5(1-15) 5(1-10) 0.7(0.51) Heterogeneityd CVHIT10 71 (0-155) 82(29-200) -0.9(0.38) 93(56-309) 112(37-308) -0.7(0.48) CVMAXHGT 0(0-115) 84(0-200) -1.7(0.10) 84(34-215) 97 (34-308) -1.0(0.33)

Missingvalues were not calculatedbecause of low frequencyof occurrence. HIT10 equalsaverage number of vegetationcontacts within 10 cm of the ground. MAXHGT equalsaverage maximum height of vegetationin cm. Heterogeneityexpressed as coefficient of variation.

concernof low expectedsample sizes and to reducethe DISCUSSION riskof a typeII error.We presentedmedian, minimum, Documentingvariation in nestinghabitat within and and maximum values for all habitat variables in the tables betweenspecies is importantwhen attempting to manage becausedata were not normallydistributed. or createnesting habitat for a particularspecies. In severalgeographic locations Snowy Plovers have been RESULTS characterizedas nesting in openareas, often on alkaline Interspecifichabitat analysis.--At the nestmicro-site flats surroundinglakes (Page et al. 1995). Resultsfrom scale,Snowy Plovers initiated nests in areas this studywere consistentwith thisgeneral description, characterizedby significantlylesser amounts of grass, as newly-initiated SnowyPlover nests were located litter, andvegetation within 10 cm of the groundthan aroundplaya lakes in openareas with limitedlitter and Killdeers,whereas there were no significantdifferences sparsevegetation. Killdeers nest in a wide varietyof in the amountof forbs surroundingnests of these openand vegetated habitats throughout the U.S. (Nol species(Table 1). SnowyPlover nest sites contained no & Lambert 1984, Colwell& Oring 1990). In Colorado, grass,variable amounts of forbs,and sparselitter, Killdeersappear to be flexiblein their nestinglocations whereasKilldeers nested in areaswith highlyvariable as they nest along gravel roadsides,throughout the amountsof grass,forbs, and litter (rangingfrom 0-95%). shortgrassprairie, and around playa lakes. Therewere no significantdifferences in thehorizontal heterogeneityof vegetationbetween Snowy Plover and Nest successof Snowy Ploversand Killdeerswas Killdeer nests(Table l). independentof habitatattributes that we measuredand their degreeof horizontalheterogeneity (patchiness) at Analysisof nestfate by habitatattributes.--There were boththe nest-siteand 5-m scales.Most SnowyPlover no significantrelationships between nest fate and any of andKilldeer nests during 1995 failed due to predationby the coverageor structuralhabitat variables at eitherthe unknownpredators, deer mice (Peromyscus nestor 5-m scalefor SnowyPlovers (Table 2) or maniculatus),and bullsnakes(Pituophis melanoleucus) Killdeers(Table 3) afterwe controlledthe experimentwise or coachwhips(Masticophisfiagellum) (Mabee and errorrate. Measuresof horizontalheterogeneity at both Estelle2000). The absenceof a discernablerelationship scalesdid not differ significantlybetween successful betweennest successand habitatfeatures in our study andfailed nestsfor SnowyPlovers (Table 2) or Killdeers may havebeen due to severalfactors, including, the (Table 3). Median andrange of coverage,structural, and occurrenceof thesepredators in a wide varietyof habitat heterogeneityvariables were similarfor successfuland types,the methodof locatingprey (e.g., olfactory vs. failed nestsof SnowyPlovers (Table 2) and Killdeers visualcues), or the smallsample sizes in our study. (Table3). 70 Bulletin9$l)ecember2000 INTERNATIONAL WADER STUDY GROUP BULLETIN

Previousstudies examining the relationshipbetween nest thank J.A. Wiens, B. Van Horne, G. Rodda, and M. fate and habitat characteristics for shorebirds have Colwell,B. Sandercock,and four anonymous reviewers shownmixed results.Page et al. (1985) establishedthat for reviewingthis manuscript. This study was Snowy Plover clutcheshad highernest successwhen conducted under the U.S. Fish and Wildlife Service locatedunder objects or in the opencompared to nests FederalEndangered and permit besideobjects. Prindiville Gaines and Ryan (1988) found number PRT-704930 and the Colorado Division of PipingPlovers had highernest success in areaswith Wildlifescientific collection permit number 95-0119. little vegetativecover during one year and in areaswith highlyclumped vegetation during the followingyear. •CES Colwell(1992) reportednest success of Wilson's Alberico,J. A.R. 1995. Floatingeggs to estimate Phalaropeswas not correlatedwith vegetationattributes. incubationstage does not affecthatchability. Koenenet al. (1996) usedartificial nests to simulate gqMlifeSociety Bulletin 23:212-216. SnowyPlover and LeastTern (Sternaantillarum) nests. They documentedthat nestsnear vegetationhad higher Bergerud,A. T & M. W. Gratson. 1988. Survivaland lossesto canidpredators but lower lossesto flooding breedingstrategies of grouse.In Adaptivestrategies duringone year,whereas this relationship was absent andpopulation of northerngrouse. Pp. 473- duringanother year. Dissimilaritiesin theresults among 577. (A. T. Bergerud& M. W. Gratson,editors.) thesestudies may be attributedto differentpredators in Universityof MinnesotaPress, Minneapolis, Minnesota. eachlocation and uniquebehavioral responses of each shorebirdspecies to variouspredators. Bolen,E.G., L. M. Smith,& H. L. Schramm,Jr. 1989. Playalakes: prairie wetlands of the southernhigh plains. We recommendthat future studiesshould identify the BioScience 39:615-623. predatorcommunity surrounding plover nest sites to predictif vegetationfeatures surrounding nest sites Colwell,M.A. 1992. Wilson'sPhalarope nest success is would influencepredation rates. If so,then studies not influencedby vegetationconcealment. Condor couldfocus on appropriatescales around a nestsite and 94:767-772. measurevegetation characteristics to identifywhether nestfate is relatedto vegetationattributes at those Colwell,M. A. & L. W. Oring. 1990. Nest-site scales.This informationis essentialfor managersto characteristicsof prairieshorebirds. Canadian Journal decidewhether vegetation should be managedin nesting of Zoology68:297-302. areasand at what scale,particularly for threatenedand endangeredspecies. If nestsuccess varies significantly Davis,C. A. &L. M. Smith. 1998. Ecologyand amonghabitat types, then one shoulddetermine whether managementof migrantshorebirds in theplaya lakes habitatfeatures influence nest success indirectly (e.g., regionof Texas. WildlifeMonographs 140. by attractingpredators to nestinghabitat) or directly (e.g.,by alteringthe nest microclimate). Ifpredation is Hill, L. 1985. Breedingecology of Interior LeastTerns, the dominant causeof nest failure, it is essentialto SnowyPlovers, and AmericanAvocets at Salt Plains understandthe method(s)predators use to locatenests National WildlifeRefuge, Oklahoma. M.S. Thesis. andhow birdsrespond to predatorsin orderto under- OklahomaState University, Stillwater, Oklahoma. standthe importanceof vegetationattributes around nestsites. An experimentalapproach may be necessary Knopf,E L. 1996. Prairielegacies-birds. In Prairie to identifythe mechanism(s) explaining the relationship Conservation,p. 135-148(F. B. Samsonand F. L. Knopf, between habitat attributesand nest success,and editors.).Island Press, Washington, D.C. ultimately,to evaluatewhether habitat management will increasereproductive success of shorebirds. Koenen,M. T, D. M. LeslieJr., & M. Gregory.1996. Habitat changesand successof artificial nestson an alkaline flat. Wilson Bulletin 108:292-301. ACKNOWI ,EI•MENTS We thankS. Earsom,J. Lynn, and J. Bishop for their Mabee,T. J.,& V. B. Estelle. 2000. Assessingthe valuable work in the field, and R. Rondeaufor advice on effectivenessof predatorexclosures for plovers. gqlson vegetationmeasurements. We thank C. Loeffier and J. Bulletin 112:14-20. Slater(Colorado Division of Wildlife)for logisticsupport. Financialsupport was provided by the ColoradoBird Martin, T. E. 1992. Breedingproductivity considerations: Observatory,Colorado Natural Areas Small Grants what are the appropriatehabitat features for Program,Colorado Division of Wildlife, FrankM. measurement?In Ecologyand conservationof ChapmanMemorial Fund, U.S. Fish and Wildlife Service neotropicalmigrant landbirds, p. 455-473 (J. M. Hagan (Golden,CO), andBureau of LandManagement. We III andD. W. Johnson,editors.). Smithsonian Institution 71

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Press,Washington, D.C. Rotenberry,J. T & J.A. Wiens. 1980. Habitatstructure, patchiness,and avian communities in North American Nol, E. & A. Lambert. 1984. Comparisonof Killdeers, steppevegetation: a multivariateanalysis. Ecology Charadriusvociferus, breeding in mainlandand 61:1228-1250. peninsularsites in southernOntario. CanadianField Naturalist 98:7-11. Sandercock,B. K. 1998. Chronologyof nestingevents in Page,G. W., L. E. Stenzel,& C. A. Ribic. 1985. Nest site Westernand Semipalmated Sandpipers selectionand clutchpredation in the SnowyPlover. Auk nearthe Arctic circle. Journalof FieM Ornithology 102:347-353. 69:235-243.

Page,G. W., J. S. Warriner,& P.W. C. Paton. 1995. StatisticalAnalysis Institute. 1990. SASprocedures SnowyPlover (Charadrius alexandrinus). In Thebirds guide.Version 6, 3rd edition.SAS InstituteCary, North ofNorth America, No. 154 (A. Pooleand F. Gill, editors.). Carolina. TheAcademy of NaturalSciences, Philadelphia, PA, and The AmericanOrnithologists' Union, Washington, D.C. U.S. Fish and Wildlife Service. 1994. Draft revised recoveryplan for PipingPlovers, Charadrius melodus, Pampush,G. J.& R. G. Anthony. 1993. Nest success, breedingon the GreatLakes and Northern of habitatutilization and nest-site selection of Long-billed the .U.S. Fish andWildlife Service,Twin Curlewsin theColumbia Basin, Oregon. Condor 95:957- Cities, Minnesota. 967. Warriner,J. S., J.C. Warriner,G. W. Page,&L. E. Stenzel. Powers,L. 1978. Recordof prolongedincubation by a 1986. Mating systemand reproductive success of a Killdeer. Auk 95:428. smallpopulation of polygamoussnowy plovers. •'lson Bulletin 98:15-37. PrindivilleGaines, E. & M. R. Ryan. 1988. PipingPlover habitatuse and reproductivesuccess in . Wiens,J.A. 1974. Habitatheterogeneity and avian JournalofI, lqldlife Management 52:266-273. communitystructure in North Americangrasslands. American Midland Naturalist 91:195-213. Rice,W. R. 1989. Analyzingtables of statisticaltests. Evolution 43:223-225.

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