JOURNAL OF FIELD ORNITHOLOGY Publishedby Associationo{ Field Ornithologists VOL. 60, NO. 1 W•NTF.P. 1989 P^GF.S1--139. J. Field Ornithol., 60(1):1-11 SOURCES OF VARIATION IN MEASUREMENTS OF BIRDS IN A PUERTO RICAN DRY FOREST WAYNE J. ARENDT USDA Forest Service SouthernForest Experiment Station Instituteof TropicalForestry Call Box 25000 Rio Pie&as, Puerto Rico 00928-2500 USA Jo•N F^^BoRc Divisionof BiologicalSciences I lO Tucker Hall Universityof Missouri-Columbia Columbia, Missouri 65211 USA Abstract.--Basedon more than 2000 total capturesof 40 speciesof birds in 14 avian families, resultingfrom 15 yr of bandingat a singlenet line in southwesternPuerto Rico, bodymass and wing length measurementsof 246 individualsof 25 speciesof birds that were captured and recaptured612 timeswere usedto showthat: (1) Amongand within differentindividuals, temporalvariation in selectedavian morphological characters is minimal when measurements are taken at the sametime of year. (2) Measuring instrumentswidely usedin the field are precise.(3) Samplingtechniques are comparable.(4) Investigatorinfluence, for singleand cooperativebanders, is minimal evenwhen small samplesare compared.Seasonal and daily variation within the data did appear, however, and were attributed to seasonalfluctuations of food resources,feather wear, breedingactivities, and the added massof ingestedfood. These data emphasizethat the time of measurementis a critical factor when comparing morphologicalmeasures among different studies. Comparing measures from one seasonwith thosefrom another, or comparingmeasures taken during a whole year with thosetaken during a brief period,may show differencesthat are not biologicallysignificant. FUENTES DE VARIACI•)N EN LAS MEDIDAS DE AVES ESTUDIADAS EN UN BOSQ;UEXEROFiTICO DE PUERTORICO Resumen.--Durante 15 aftosse ban anillado avesen una 1ocalidadparticular del suroeste de Puerto Rico. Datos sobreel pesoy largo del ala de 25 especiesde avesrepresentados pot 246 individuosque rueton capturadosy recapturadosen 612 ocasionesson utilizadospara demostrarque: (1) entrediferentes individuos la variaci6ntemporal de caracterlsticasmor- fo16gicasson mlnimas,cuando las medidasson tomadasdurante la misma 6pocadel afio; (2) los instrumentosusualmente utilizados en el campopara tomar medidasson precisos; (3) las t6cnicasde muestreoson cornparables;(4) la influencia del investigadorsobre ani- 11adoresparticulates o en grupos, es minima cuando se comparanmuestras pequefias. Variaciones diarias o estacionales fueron encontradas. Estas son atribuidas a factores tales comorespuestas alas fluctuacionesen recursosalimenticios, desgaste de la pluma, actividades 2] w.j. Arendtand J. Faaborg j. FieldOrnithol. Winter 1989 reproductivasy al incrementoen pesopor ingesti6nde alimento.Los datos enfatizan que el tiempoen que setoman las medidases critico cuando se comparan pargtmetros morfo16gicos tomadosen diferentesestudios. E1 compararparfimetros tomados en una estaci6ndel afio con otrostornados en otra o el compararmedidas tomadas durante todo un afio con otros de un breveperiodo, pueden mostrar diferencias que no sonbio16gicamente significativas. Many studiesin avian morphologyaddress the variation of skeletal and plumagecharacters in an evolutionary(Selander 1971) or ecological context(Hespenheide 1973). When taking measurementsof avian mor- phologicalcharacters in museums,exacting techniques can be used.How- ever,measuring birds under the rigorsof field conditionsis more difficult, and biologistsoften questionthe accuracyof the data generated.Such uncertaintyis increasedwhen data are gatheredby morethan oneperson (i.e., investigatorinfluence). In laboratorystudies, Berger (1968), Collins and Atwood (1981) and Nisbet et al. (1970) showed that field scales, spring balances,and calipersused widely to measureavian body mass and wing chord are accurate and that investigatorinfluence does not appear when working with large sample sizes. However, less is known about measurementaccuracy under field conditions,especially among many workers dealing with small samples.In this study, morphological variation in Puerto Rican dry forest birds was examined over a 15-yr period that included severedrought conditions(Faaborg et al. 1984). Becausenumerous banders, often using dissimilar instrumentation, gath- ered thesedata at different times, the effectsof investigatorinfluence and banding techniqueswere analyzedto determineif they affectedmea- surementsof body massand wing length in sampledbirds. STUDY AREA AND METHODS Mensural data were collected from birds (vernacular and scientific namesin the Appendix) capturedover a 15-yr period (1973-1988) at our net line within the Guanica Forest located in southwestern Puerto Rico (seeTerborgh and Faaborg 1973 for a descriptionof netting tech- niques and the area). Most of the samplingwas done during the dry season,January or early February of 1973-1976, 1978, 1980-1988. Some samplingwas done during the wet season,June of 1973, and July of 1981, for comparison.All birds were weighedwith Pesolaspring scales, either in bags(WJA) or by their leg bands(JRF and his students).Wing chord (from the bend of the wing at rest to the longestprimary) was measuredwith dial calipers (WJA) or with a wing rule (JRF and stu- dents).Although JRF et al. sharedin taking body measurementsduring the first half of the study (1973-1980), and in 1988, WJA aloneweighed and measuredall birds during the secondhalf (1981-1987) of the study, excluding1988. Investigatorinfluence involving biologists other than JRF and WJA is assessedfor the first half of the study (1973-1980). Both parametric and nonparametricstatistical tests were usedto ana- lyze the data. An F-test showedhomogeneous variance of both bodymass and wing chord measurementsbetween capturesand recapturesin all speciestested, justifying the assumptionthat the probability densityfunc- 60,No. 1 AvianMeasurements in Puerto Rican Forest [3 tions F(x) and G(x) take the same form. Becauseinterspecific morpho- logical characterswere compared,the coefficientof variation (CV) for eachspecies was usedto standardizeour measuresamong variform species. Interspecificcoefficients of variation were comparedusing methodsde- scribedin Lewontin (1966). Analysisof variance(ANOVA) and $tudent's t-tests were used in most body mass and wing chord comparisons.A repeatedmeasures ANOVA test was used to comparewing chord and body massdifferences between captures and recapturesof the samein- dividuals banded by a single bander using the same instrumentsand techniquesat each measurement.A nonparametricprocedure, the Wil- coxonsigned-rank test (testingmedian differencesin paired samples), was substitutedwhen paired sample sizeswere small or when the form of the probability densityfunctions was in doubt. Significancewas as- sumed at the 5% level in all statistical tests. RESULTS Summaryof captures and recaptures.--Morethan 2000 individualbirds comprising40 speciesin 14 avian familieswere capturedbeen 1973 and 1988 at our Guanica netline. Of the 40 different speciescaptured, 25 specieswere recapturedat leastonce. JRF, his colleagues,and his students capturedbirds over 7 seasons(1973-1976, 1978, 1980, 1988). Of 887 total capturesduring theseyears, 63 individualsof 17 avian speciesin 8 avian families were capturedand recaptured133 times, with each indi- vidual being capturedan averageof 2.3 times (SD = 0.54, range = 2-4, CV = 0.2). Interestingly,744 (almost84%) were neverrecaptured. During a similar 7-seasonsampling period (1981-1987), more than 1000 total captureswere recordedby WJA and his groups,who captured 138 in- dividualsof 24 avian speciesin 10 familiesa total of 329 times,with each individualbeing capturedan averageof 2.4 times (SD = 0.83, range = 2-6, CV = 0.3). Similarly, some700 individualbirds (about70%) were neverrecaptured during this period.During the entire 16-seasonsampling period, somebirds were capturedand recapturedone or more times by JRF et al. (1973-1980) and then later recapturedone or more times by WJA et al. (1981-1987), or viceversa (1988). This resultedin a total of 45 individualsof 13 speciesin 7 families that were captureda total of 140 times,with an averagecapture rate of 3.1 per individual(SD -- 1.26, range = 2-7, CV -- 0.4). Bananaquits,Puerto Rican Bullfinches,and Puerto Rican Flycatchersmade up about 50% of JRF et al.'s captures and recaptures,and about the samepercentage in JRF et al.'s captures andWJA et al.'srecaptures, whereas bananaquits, bullfinches, and Pearly- eyed Thrashers constitutedabout 50% of WJA et al.'s capturesand recaptures. Temporalvariation.--In adult birds, intraspecificbody massand size, thoughgeographically variable (Baker 1980), are fairly constantlocally. Howeveryearly, seasonal,and evendaily differenceshave been reported for speciesand individualsin restrictedgeographic areas; see Clark (1979) for a review, and Biermannand Sealy (1985). To compareintraspecific 4] W.J. Arendtand J. Faaborg J.Field Ornithol. Winter 1989 variation in size, measurementsof body massand wing (chord) length were taken on the 11 most commonspecies captured at Guanica (see Appendix).Intraspecific coefficients of variation (CV's) for wing chord ranged from 2% in the Black-and-white Warbler to 6% in the Puerto Rican Tody. CV's for body massranged from 5% in the Red-legged Thrush to 12% in the bullfinch.Larger CV's for bodymass may reflect the inclusionof somejuvenile individualsin the more monochromatic speciesand daily massfluctuations. Fluctuation in bodymass on a daily and hourly basisis due, at