j. Field Ornithol., 56(3):258-264 A COMPARISON SAMPLING OF INTERVAL AND CONTINUOUS METHODS FOR BEHAVIORAL OBSERVATIONS BY THOMAS C. TAGHA, PAUL A. VOHS, AND GEORGE C. IVERSON Behavioralanalysisis an important componentof modern field ornithology.Frequencyof occurrence,duration,andpercentof time spent exhibitingbehaviorhavespecificvaluesin ethologicalstudies(Altmann 1974).The typeof behavioralanalysis dependson the studyobjectives. Avian behavioralstudiesoften relate percent allocationsof time exhibiting specificbehavioralpatternsto habitats(e.g.,Weins 1969, Seigfried 1974);time of dayor year (e.g.,Verner 1965, Schartzand Zimmerman 1971); age, sex, and socialor reproductivestatus(e.g., Dwyer 1975, Tacha 1981a);or energy expenditures(e.g., King 1974, Kendeighet al. 1977, Fredrick and Klaas 1982). Most avian behavioral studiesuse someform of interval or continuoussamplingmethod. Our purposeis to quantitativelydemonstrateimportant differencesamongthesemore commonlyusedbehavioralsamplingmethods,and to providesomebasic guidelinesto considerwhen selectingmethods. Many different methodsare availableto record, describe,quantify, andanalyzeanimalbehavior(seeHutt and Hutt 1970, Weinset al. 1970, Altmann 1974, Hazlett 1977, Lehner 1979). For example,a studymay sampleall occurrencesof specificaction patterns,or sampleall or selectedbehaviorof individualsor groups.Samplingmay begin and end with a specificbehavior, a specifictime of day, or at random. Duration of observationsmay be defined by length of a specificbehaviorof interest,the time a focal individualor group is available,or by predetermining length of observations.Within an observationperiod, behavior maybe recordedcontinuously,or at specifiedintervals.Our studycomparescontinuousand interval samplingmethods,usingfixed-lengthbehavioral observationsof individual Sandhill Cranes (Grus canadensis). Specifically,we documentthe effectsof interval length and samplesize (number of observationperiods)on bias and precisionof estimatesof the percentageof time spentexhibitingbehavioralpatternsof varying frequencyof occurrenceand averageduration. METHODS A repertoire of 34 mutuallyexclusivebehavioralpatternsprovideda basisfor time budgetsof SandhillCranes(Tacha 1981a). These included 4 formsof agonism,8 of courtship,1 alert, 2 preflight, 5 preening,3 stretching,2 sleeping,2 loafing, 3 locomotion,and 4 foraging. The percentageof time devoted to each pattern was calculatedfor each observationperiod.Behaviorwasrecordedcontinuouslyfrom individual SandhillCranesduring 327 20-min observationperiodsin March and early April 1980 in the Platte River Valley near Hershey,Nebraska(see 258 Vol.56,No.• Behavior Sampling Methods [259 Tacha 1981a). Distributionof observationperiodsincludedall major habitattypesusedby cranesduringall daylighthours;stratificationby agegroupsensuredadequatesamplingof young-of-the-year. A subsample of 97 observationperiodswaslater selectedat randomto evaluate the effect of samplesize. Behavioralpatterns were recorded to the nearest full second when continuous observations were transcribed from tapesto codingsheetsfor subsequent computeranalyses. The 34 behavioralpatternswere categorizedby frequencyand averageduration. Patternsobservedin lessthan 10% of observationperiodswere consideredrare, 11-50% were consideredmoderatelyfrequent, and >50% were consideredcommon.Patternswith an average duration of less than 10 s were considered short, 11-60 s were moderate duration, and >60 s were consideredlong. Interval observations were obtained from each of the 327 observation periodsby usinga computerprogram to tabulatebehavioroccurringat specifiedintervals(e.g., 10 s) within each 20-min observationperiod. Use of 10-s intervals resulted in 120 observations from each 20-min (1200-s)period. The percentageoccurrenceof eachbehaviorwithin a period wasthen calculatedand usedas an estimateof the percentage of time spentexhibitingeachbehavior(afterAltmann 1974).Completed compilationsfor each observationperiod provided 7 estimatesof the percent of time spent exhibiting each of 34 behavioral patterns;one estimatefrom the original continuousobservations,and one for each of 6 different interval lengths (i.e., 10-, 12-, 15-, 20-, 30-, and 60-s intervals). Intervals of 10, 12, 15, 20, 30, and 60 s were the most common found in avianbehavioralliterature. For example,Weinset al. (1970) sampled bird behaviorat 10-sintervals,but intervalsof up to 60-s have been used(e.g., Verner 1965, Tacha 1981b). The StatisticalAnalysisSystem(Helwig and Council1979) wasused for dataanalyses. Frequencydistributions alloweddetectionof behavior recorded by continuousobservationsbut not recordedby intervals. Paired t-tests were used to test for differences between interval and continuousobservationsin the mean percentageof time calculatedfor each behavior. StandardF-testscomparedvarianceestimatesfrom interval and continuous observations. RESULTS AND DISCUSSION Unrecorded behavior.--Between1 and 9 of the 34 behavioralpatterns observed in continuous observations were not detected in interval ob- servations(Table 1). Patterns not detected in interval observationswere generallyrare and of short duration. Longer intervalsincreasedthe number of behavioralpatternsmissed. Reducingthe numberof observationperiodsfrom 327 to 97 doubled the numberof patternsmissedby intervalobservations (despitetheir presencein the 97 continuousobservationperiods),and patternsof moderatefrequencyand duration were missedwhen 30- and 60-s in- 260] T•,BLE T. C. Tacha et al. j. FieldOrnithol. Summer 1. 1985 Number of categoriesof behaviorpresentusingcontinuousobservations of SandhillCranesthat were not recordedby interval observations. No. of cat- egories available No. of observation periods Frequencyand usingconduration of behavior • tinuous observations Interval length (s) 10 12 15 20 30 60 2 0 0 2 0 0 0 0 0 3 0 0 4 0 4 1 0 0 18 9 1 0 2 0 2 0 1 0 3 0 4 0 7 0 0 0 0 0 0 Frequency Rare Moderate Common 21 9 0 327 Duration Short Moderate Long Frequency Rare Moderate Common 21 2 2 4 4 7 8 9 4 0 0 0 0 0 0 0 0 0 0 1 0 8 97 Duration 18 2 2 4 4 6 Moderate Short 9 0 0 0 0 1 1 Long 7 0 0 0 0 0 0 aFrequencyof occurrenceof behaviorusingcontinuousobservations where rare -• 10%, moderate-- 11-50%, and commonY 50%. Mean durationof behaviorusingcontinuousobservationswas classifiedas short _• 10 s, moderate -- 11-60 s, and long y 60 s. tervalswere used(Table 1). Nine behavioralpatternswere missedwhen n--97 and 60-s intervals were used. Although the 9 patterns represented26% of the behavioralrepertoire and 44% of the rare and short duration patterns,they accountedfor only 0.1% of the total time. If the purposeof behavioralobservationswas (for example)to cal- culateenergyexpenditures,the 9 missingpatternswould make little differencebecausethey cumulativelyaccountedfor solittle time. However, if the purposewasto trace socialrelationshipsthen 60-s intervals and 97 observationperiodswere inadequatebecauseimportant social behaviorwasmissed.The 4 agonisticpatternsand 8 courtshippatterns of SandhillCraneswere recordedusingcontinuousobservationswith n -- 327, but all 4 agonisticand 7 of 8 courtshippatternswere missed with n = 97 and 15-, 30-, or 60-s intervals(9 of the 11 were missedwith 60-s intervals).The 7 courtshippatternsundetectedwith smallsample sizes and interval observations were rare and of short to moderate du- ration, but were essentialto pair formation. Biasofintervalobservations.--Interval observations bothoverestimated Vol.•6,I•o.• Behavior Sampling Methods [261 T^BI.E 2. Bias associatedwith interval observationswhen comparedwith continuous observations of SandhillCranes.Table entriesare the numberof behavioralcategories where intervalobservations significantly (pairedt-tests,P • .05) overestimated (+) or underestimated (-) the meanpercentageof time obtainedfrom continuousobservations. No. of oboFrequencyand servation periods duration Interval length (s) of 10 behavior' 12 15 20 30 60 Frequency Rare 1- 1- 0 1- 1- 1- Moderate Common 0 0 0 0 0 0 0 0 10 2+ 0 Duration Short Moderate 10 10 0 1- 10 11- 10 Long 0 0 0 0 0 2+ 1- 327 Frequency Rare 1 + 1- Moderate Common 2- 0 0 0 0 0 1+ 2 + 11- 1+ 0 1+ 0 1- 1+ 1- 1+ 0 1 + 21- 11- 0 0 20 0 0 1+ 1+ 1+ 1 + 10 97 Duration Short Moderate Long See footnote to Table 1+ 1. and underestimated(P < .05) the percentageof time spentin exhibition of variousbehavioralpatterns(Table 2). Significant(P = .05) differences can be expectedfor 1 of every 20 patternsdue to random chance.The frequencyof significantdifferences(P < .05) in Table 2 approximates random chance. However, the direction of bias was associated with the frequencyand duration of behavior,and twice the number of biased estimates accrued when n was reduced from 327 to 97. Interval obser- vationstended to underestimatethe percentageof time for rare and short and moderate duration behavior, and overestimate behavior of moderatefrequencyandlongduration.For example,preeningthewings was a rare behavior of moderate duration that interval observations underestimatedby half. Sleepingwhile standingwasa moderatelyfrequentbehaviorof longdurationthat intervalobservations (n = 327, 60s interval) overestimatedby about 5%. Frequencyof occurrenceand duration of behaviorshouldbe consideredwhen combiningcategories for use with interval observations.Use of behavioralcategoriesthat containonly rare and short duration behaviorcouldbiasresults. Precision ofintervalobservations. mEstimatesfrom interval observations of the percenttime spentexhibitingdifferent behavioralpatternswere consistentlylessprecise than estimatesfrom continuousobservations 262] T. C. Tacha etal. J.Field Ornithol. Summer 1985 TAB•.E3. Fraction'of behavioralcategorieswhere varianceestimatesfrom intervalobservations were significantly(F-tests,P < .05) higherthan varianceestimatesfrom continuous observations of Sandhill No. of ob- Frequencyand servation periods duration Interval lengths(s) of 10 12 15 20 30 60 12/20 2/9 0/4 13/19 1/9 0/4 14/19 2/9 0/4 15/20 2/9 0/4 12/18 2/9 0/4 13/17 2/9 0/4 13/17 1/9 13/16 1/9 14/16 2/9 16/17 1/9 12/15 2/9 12/14 3/9 0/7 0/7 0/7 0/7 0/7 0/7 8/19 2/9 0/4 11/19 2/9 0/4 8/17 2/9 0/4 8/17 2/9 0/4 9/14 2/9 0/4 9/13 1/8 0/4 10/16 0/9 12/16 1/9 8/14 2/9 10/14 0/9 10/12 1/8 8/10 2/8 0/7 0/7 0/7 0/7 0/7 0/7 behaviorb Frequency Rare Moderate Common Cranes. 327 Duration Short Moderate Long Frequency Rare Moderate Common 97 Duration Short Moderate Long Numberof categories/number available. See footnote for Table 1. (Table 3). Variance estimatesfrom interval observationswere signifi- cantly(P < .05) higherfor 60-75% of rare and 11-33% of moderately frequent behavioral patterns, and for 62-82% of short and 0-33% of moderate duration patterns. Variance estimatesfrom interval and continuousobservationswere similar (P > .60) for commonand long duration patterns.Generally,as interval length increased,the percentof patternswith inflated varianceestimatesincreased.The reductionfrom n = 327 to 97 had little effect on precisionof estimatesfrom interval observations. The high variance estimatesof interval observationsreduced our ability to detectdifferencesin meanpercentof time spentexhibitinga behavior betweenages,sexes,habitats,or other variablesof interest. For example,thepercentof timespentexhibitingeachof the 7 courtship patternswassignificantlydifferent (P < .05) betweenadult andjuvenile Sandhill Cranesusingcontinuousobservations,but no difference(P > ß15) between adults and juveniles was found for any of the courtship patternswheneven 10-sintervals(n = 327) wereused.Inability to detect differencesin time allocationscould severelyrestrict interpretationof time budget data. Vol.56,•o.• Behavior Sampling Methods SUMMARY AND [263 CONCLUSIONS Continuousand intervalsamplingmethodswerecomparedusing327 anda subsample of 97 twentymin observation periodsof SandhillCranes. Interval observationsdid not record up to 26% of rare and brief behavioralpatterns,andtendedto underestimate the percentof time spent exhibiting the remaining rare and brief patternsand to overestimate patternsof moderatefrequencyandlongduration.Interval observations alsoprovidedestimatesof percentageof time that wereconsistently less precisethan continuousobservations. Asintervallengthincreased,more rare and short duration behavior was missed,and the proportion of patternswith inflatedvarianceestimatesincreased.Reductionin sample sizefrom 327 to 97 interval observationperiodsresultedin doubling the numberof rare and brief behavioralpatternsmissed. Biologistsshouldselectbetweencontinuousor interval observations basedon studyobjectives.Continuousobservationsare required for studieswherecomparisons of absolutefrequencyoccurrenceor duration of behavior are important. Continuousobservationsare more valuable than intervalobservations for studiesthat involvebehavioralcategories that are of an averagedurationlessthan twicethe intervallengthused or are rare in frequencyof occurrence.Continuousobservationsare alsomore valuablefor studieswhere any comparisonamongvariables of the percentageof time spentexhibitingspecificbehavioralpatterns ismade.However,our experiencesuggests that continuous observations require more complicateddecodingand analysisthan interval observations. ACKNOWLEDGMENTS We thank M. E. Heitmeyerfor reviewingthis manuscriptand W. D. Warde for assistance with statisticalanalyses. This studywasfundedby Contract 14-16-0008-2133, AcceleratedResearchProgram for Migratory Shoreand Upland GameBirds,administeredby the Central ManagementUnit TechnicalCommitteeandthe MigratoryBird andHabitat ResearchLaboratory, U.S. Fish and Wildlife Service.Additional funding wasacquiredfrom the MigratoryBird Habitat ResearchLaboratory (USFWS), the Oklahoma CooperativeWildlife ResearchUnit, OklahomaStateUniversity,andthe NationalWildlifeFederation.The OklahomaCooperativeWildlife ResearchUnit hasOklahomaState University, Oklahoma Department of Wildlife Conservation,U.S. Fish and Wildlife Service,and Wildlife ManagementInstitute cooperating. LITERATURE CITED AI•TM^•, J. 1974. Observationalstudyof behavior:samplingmethods.Behavior49: 227-265. DWYER, T.J. 1975. Time budgetof breedingGadwalls.WilsonBull. 87:335-343. FREO•½K,R. B., ^NOE. E. KI•^^s. 1982. Resourceuseand behaviorof migratingSnow Geese.J. Wildl. Manage.46:601-614. 264] T. C. Tachaetal. J.Field Ornithol. Summer 1985 HAZLETT,B. 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Received 6 June 1984; accepted17 July 1985.