a comparison of interval and continuous sampling methods for

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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.
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