POPULATION
DENSITIES
OF BIRDS
TRANSECT
COUNTS
DERIVED
FROM
JoH• T. EMLE•
GOODestimates
of absolutepopulationdensityas distinctfrom indicesof
relative abundancehave been virtually unavailablefor nonflockingland
birdsexceptin the breedingseasonwhensingingmales,representing
mated
pairs,restrictthemselves
to moreor lessfixed territorieswherethey or their
nestscan be counted. The lack of efficient and reasonablyaccuratecensus
methodsapplicableat any seasonhas seriouslyhamperedthe progressof
quantitativestudiesof avian populationecology.
This paper, after reviewingthe potentialitiesand limitationsof currently
availablemethods,describesa new methodthat is, 1) applicableat all seasons,2) moreefficientin termsof area coveredper unit of effort than the
nest or territory count methods,and 3) comparablein accuracy. The
methodusesthe lateral distributionpattern of all detectionpointsfor each
speciesto derivecoefficientsof detectabilitywith which trail countsmay be
converteddirectly to densityvaluesin units of birds per 100 acres.
The methodwas developedover a period of 3 years while the author was
gathering data on the ecologicaldistribution of birds in mesquitegrass-
lands in southernTexas, pine forestsin Florida and the Bahamas,and
mixed woodlandsin Wisconsinand Michigan.
AVAILABLE METHODS
Direct counts of birds in circumscribed and measured areas can often be
usedeffectivelyon flockingspeciesand variousnonflockingbirds living in
exposedsituations,but direct and completeenumerationover an area is
impossiblefor the many nonflockingland birds dispersedthrough the
obscuringvegetationof woodlands,fields, and marshes. Simple tallies of
individualsdetectedper unit of effort under closelystandardizedconditions
are valuable as indicesfrom year to year and are relatively easy to obtain
(Kendeigh,1944). Censuses
of absolutedensity,necessary
for determinationsof biomassand energyfunctions,requirecontrolledmeasurements
of
area as well as population.
Six currentlyavailablecensusmethods,plus the new onedescribedin this
report,are listedandcharacterized
in Figure 1 and Table 1. In the first of
these(A) a fractionof the residentpopulationof a selectedtract is captured, marked,and released,then balancedagainstthe coexistingunmarked
populationin a subsequenttally or tallies. The ratio of marked to total
amongthe observedbirds in this tally shouldequal the ratio of the total
numberof birds marked to the total numberin the population.
323
The Auk, 88: 323-342.
April 1971
324
Jo•
T. E3•/LEN
IDENTIFICATION
SKILL
OBSERVATION
SCREENING
CONDiTiONS
EFFECT
CONSPtCLIOLISNESS
BOUNDARY
OF
OF
OBSERVER
(WEATHER,
ETC.}
HABITAT
BIRDS
OVERLAP
NON-BREEDERS,
CHANGES
OF
[Auk, Vol. 88
TRANSIENTS
DURING
SEASON
BIRDS
OR
NESTS
DISTANCES
UNRELIABLE
ACTUAL
POPULATION
MISSED
MISJUDGED
CONVERSION
FACTOR
DENSITY
Figure 1. Factors limiting the accuracyand efficiency of censusmethods for nonflocking terrestrial birds.
In thenext fourmethods(B, C, D, and E) controldepends
on achieving
completeenumerationon small measuredplots or narrowstripsalonga
trail. The problemof accountingfor each of the elusiveresidentsof the
plot is resolvedin B by substitutingnestsfor the birds themselves.In C
thepopulationindicatorsare singingterritorialmales;songperchpositions
are spottedon maps,and clustersof suchpositionson composite
mapsare
considered
to representindividualterritorial males (Williams, 1936; Williamson,1964). For translationto populationdensityvalues,eachnest (in
]3) or singingmale (in C) may be regardedas roughlyequivalentto one
pair of residentbirds.
Completecoverageof all birdsis directlyattemptedin methodD, the
count of detectionsbeing restrictedto a narrow strip of designatedwidth
(and area) alongthe transecttrail (Merikallio, 1946, 1958) or to appro-
April 1971]
Transect Counts
325
326
Jous T. E•L•
[Auk, Vol. 88
priate strip widths assignedto the variousspecieson the basisof character-
isticdetectiondistances(Kendeigh,1956). Adjustmentsfor the inevitable
incompleteness
of singletransectsmay be made by adopting the largest
tally for eachspeciesin a seriesof sevenor moretransectreplications,a
processcalledsummationby its inventor(Palmgren,1930). Again,singing
malesmaybe interpretedasroughlyequivalentto onepair of residentbirds.
In method E all birds are tallied as encountered and their distances
The effective size of the
from the observer at detection are recorded.
censusarea is then definedin termsof the positionsof the recordedbirds
by adoptingthe mean distancefrom the observeras one half the width
of the censusstrip. This methodwas developedby R. King for gamebirds
that crouchand flush (Leopold, 1933; Hayne, 1949) but has been used
in an exploratoryway on songbirds (Stewart et al., 1952).
In the sixth method (F) and the new method (G) control rests on the
degreeof completeness
of coveragein wide strips. In F completeness
is
evaluatedby referenceto observerperformancelevelson tracts intensively
censusedfor completecoverageby a restricted plot method such as the
countof singingterritorial males (methodC) (Colquhoun,1940a; Howell,
1951; Enemar,1959), or by notingthe incidenceof occurrence
of particular
individualsin a seriesof traversesover a transectroute (Seierstadet al.,
1965). The per cent of completeness
indicatedfor a speciesby this test is
referredto as its e]jectivity (Enemar, 1959). In methodG, completeness
of coverageis calculateddirectly for each speciesfrom the characteristic
distributionpattern of detectionpointslaterally from the censustrail and
the implicationsof this distributionfor overalldetectabilitywithin a wide
strip (describedin the next sectionof this paper).
All methodshavetheir complications
and limitations. With the exception
of the first two (A and B), all are besetwith confusingobservationalvariables (centralblockin Figure 1) associated
with 1) the experience,acuity,
attentiveness,and speedof advance of the observer; 2) the observation
conditionsrelated to weather,time of day, etc.; 3) the screeningeffect of
the habitat; and 4) the conspicuousness
of the birds as related to. their
noisiness,movements,size, and color. The first two of these factors are
essentiallyindependentvariables,best controlledby restrictingfield work
to well-qualifiedobserversand prescribedobservationconditions.Graded
adjustments
for suboptimum
conditions,
e.g. afternoonhoursor coldwet
weather,couldconceivablyprovidean alternativeto rigid standardization
in the secondfactor. The screeningeffect of the habitat relatesprimarily
to vegetationdensityand cannotbe controlledby standardization
of procedure without seriouslyrestricting the scope of operations. Vegetation
typeanddensityshouldbedescribed
andmeasured
oneachcensus
tractand
the data incorporatedas a part of the surveyrecord. It may eventuallybe
April 1971]
Transect Counts
327
possibleto devisea scaleof screening
valuesfor the variousvegetationtypes
(seeTable 3). Conspicuousness
of the bird variesmarkedlyfrom speciesto
speciesand probablyreflectsdistinctivespecies-characteristic
attributesin
eachcase. Each speciesmust thereforebe dealt with as a separateentity.
The conspicuousness
of a speciesmay, of course,vary considerablywith
seasonalchangesin activity and, in somecases,with sex. But suchvariationsshouldbe predictable,and with the accumulationof data a characteristic value or set of values shouldbe assignableto each species. Several
workers(Colquhoun,1940b; Stewartet al., 1952; Enemar, 1959; Williamson, 1964) have recognizedthis principleand made preliminarymovesto
establish"coefficientsof conspicuousness"
for commonspecies.
The first threemethods(A, B, and C) whencarefullycarriedout provide
relatively good control of the general observationalvariables described
aboveand may, underfavorableconditions,offer relativelyhigh levelsof
accuracy. Method A posesthe difficult requirementsthat the marked and
unmarked componentsof the populationmust be equally trappable or
observablein the retally procedures,and that no ingressor egressinto or
from the study plot can occurduring the study period. The nest and territory mappingmethods,B and C, suffer in efficiency (area coveredper man
hours of work, Table 1) when conductedintensively,or in accuracywhen
appliedlessintensivelyin opensituations(Snow, 1966). The small sizeof
plotsnecessitated
by the excessivetime involvedin intensivecoverageraises
further problemsof evaluatingthe inclusionof a large proportionof fractional, boundary-lineterritories.
Completeness,
the criterionof accuracyin B, C, D, and E, is at bestan
elusivetarget. Singingmales,the critical unit of measurementfor methods
C, D, and F, changedrasticallyand often quite abruptly in their conspicuousness(song frequency) and local distribution as the breedingseason
advances.Large numbersof repeatruns are thereforenecessary;runs must
be spreadover many weeksto catch the singingperiod of all speciesand
individuals,yet concentratedenoughto permit valid interpretationsof the
distribution of recorded song perchesin terms of individual territories.
After the data are in, the problem of translating the number of singing
males or neststo the ultimate populationvalue of birds per unit of area
remains,complicatedby variablesassociatedwith songintensity (Colqu-
houn, 1940b), nonbreeding
residentmales,transients,polygamy,irregular
movementof individualsacrossterritory boundaries,and large territories
only partially incorporatedin the tract. The use of maximum counts
(summationprocedure)in the fixed-striptransectmethod(D) introduces
the possibility of seriouserror from temporary movementsof boundaryline birds into the relatively narrow censusstrip.
A further disadvantage
of the intensiveplot methods(B and C) and the
328
JoHs T. EMLES
15 A B
WIDTH
OF STRIP
(FT)
50
50
I00
212
C
[Auk, Vol. 88
D
412
ABCDTOT
•3•
PROJECTED
NO.
PRESENT
60 60 120250490
Figure2. Schematic
modelof a standof vegetationwith randomlyscatteredbirds
(dots) bisected
by a transectroute (medianline). Dots representing
birdsthat are
detectedby eyeor ear are encircledin the model. Linesparallelwith the transectroute
at 50-foot intervals define strips of coverage. Hypothetical counts of birds on a
2-mile traverseof the transectare tabulatedand graphedfor eachstrip or substrip
at the top right of the figure. A horizontalline at the top of the graphextrapolates
the level of the mean of the first five 10-footsubstrips(12 birds) horizontallyto a
boundaryline at 412 feet. Assuming
complete
coverage
in thesebasalsubstrips,
and
randomdistributionof birds,the areabelowthis line represents
the populationof a
2-milesegmentof the 824-footband (200 acres). This area (total projectedpopulation
of 490 birds) dividedinto the areaof the columns(155 birdsdetected)givesthe per
centof birdswithinthe bandthat are detected,
i.e. the coefficient
of detectability
for
April 1971]
Transect Counts
329
dependent
stripsurveymethod(F) is theirrestrictedapplicabilityto a few
monthsduring the breedingseason.Densitiesof transientpopulations
during migration seasonsand of mixed populationsof residentsand
migrantsduringthe winterare thereforeentirelybeyondthe reachof these
approaches.The strip transectmethodsD, E, and G escapethis restriction
by shiftingfrom fixed indicatorsof sedentarybirds to simpletalliesof all
birds regardlessof their residencestatus. The critical controlsin theselatter
methods,replacingthe segregationand enumerationof distinct individual
birds, is the determinationof distancesfrom observerto bird, neededfor
recognizingand rejecting outsidersin D, for establishingthe functional
strip boundariesin E, and as the basisfor evaluatingspeciesdetectability
in G. Instrumentalmeasurements
are not feasiblein practice,but skills
equivalentto a good6-inch range finder can, in the author'sexperience,be
quicklyacquired.An examinationof the analysisnear the bottomof Figure
1 suggests
that the grossof proceduralerrorsinherentin thesestrip methods
is probablyno greaterthan thoseinherentin the intensiveplot methods.
DESCRIPTION OF THE NEW METHOD
The new method,briefly introducedabove, involvesthe determination
for eachspeciesof a detectabilityvalue (or values) hereaftertermedthe
co.eJficient
of detectability(C.D.) of the species,and the applicationof
this value to the mean count for the speciesin a traverse or seriesof
traverses through a censustract.
Coefficientsof detectability.--Coefficients
of detectabilityrepresentthe
proportionof the populationof an area that is ordinarilydetectedby an
observer running a transect. It is a direct reflection of the theoretical
"effectiveradiusof the organism"in Yapp's (1956) modelof interactions
betweenan observerand the birds along a transect line. Each species
appearsto have its characteristicC.D. value, varying within limits and in a
predictablemannerwith the seasons
and the type of vegetation.
The C.D. value for a populationof birds within sight or hearing of a
transectroutedependson two variables: 1) the distanceof the individual
birds from the route and 2), their absolutedetectability,independentof
distance.In the proceduresdescribedbelow,correctionsfor the first vari-
able are madeby applyinga lateral distanceconversion
to the count,and
for the latter by applying a basal detectabilityadjustment.
Lateral distance conversion.--The lateral distance conversion factor for
a speciesmay be determineddirectly from data on the linear distribution
the birds of this stand (155/490• 0.316 in this hypothetical sample). Hypothetical
countsof birds on a 2-mile traverseof the transectare tabulated and graphedfor each
strip or substrip at the top of the figure.
330
Jons T. Ez•Lzze
BOBWHITE
t
(WINTER)
THICK-B.
VIREO-
200
PINE
FOR.
C.D.
4,2......•C.
D.
,,•2
:.138
• .....
0
[Auk, Vol. 88
:.:357
N: 118
412
PALM WARBLER
N=275
0
(WINTER}
200
CROW-
PINE
412
FOREST
i••'.'.' N:2150
C.D..12
:.248
0
200
:"•
412
0
200
FLUSHED
SEEN
CALLING
SINGING
412
Figure3. Typesand distributionof detections
laterallyfrom the transectroute in
four representative
populations.
The valuefor the basal(proximal)stripis plottedas
100 (full columnheight)in eachcase,and valuesfor the distalstripsare represented
proportionately.
The Bobwhitesampleis of a nonsinging(wintering) populationin
whichthe birdsweregenerallydetectedby flushingat closerange;very few detections
are made beyond 100-feet laterally, and C.D. values are therefore low. The Palm
Warblersampleis of a nonsinging
winteringpopulation;talliesbeyond100 feet were
nearlyall auditorydetections
of call notes;noneweredetectedbeyond200 feet. The
Thick-billed
Vireopopulation
on GrandBahamarepresents
a resident
breeding
species
with a moderatelyloudsongcarryingthroughthe forestvegetationfor 200 to 300 feet.
The Crow samplerepresentsa situation in which visual detectionfalls off in the first
100to 200 feetwhileauditorydetection
extends
laterallyfor 1000feet or moreproducing a high C.D. value.
of detectionpointslaterally from the transectroute. The rationaleof this
procedure
is, perhaps,
bestunderstood
by visualizing
a field (the stand)
(Figure2) withrandomly
scattered
objects
(thebirds)slowly
traversed
by
a recording
sensor(the observer)
scanning
a swatheor band(the areaof
coverage
alongthesurveyroute).The objects
in thismodelirregularly
emitvisualandauditory
signals
ofvarying
intensity,
andthefieldisirregularly conductive
for thesignals.Thesensor
thusdetects
onlya fractionof
the signals,
theproportion
beinghighcloseto.the routeand decreasing
laterally.
Plotting
thefrequency
of detection
values
forsuccessive
parallel
stripson
April 1971]
Transect Counts
331
either side of the route, a distributioncurve is obtainedas shownin the
graphat the top of Figure 2. In mostspeciesthis curvemaintainsa high
level for from 40-100 feet, then declinesrather precipitously,and finally
levelsoff to approachzeroat the limit of detectabilityof the species.Forms
of the curve for representativespeciesare shownin Figure 3.
If we assumefor the momentthat all birds are detectedin the few proximal strips,coefficientsof detectabilitymay be derivedfrom theselateral
distribution curvesby the followingsteps:
1. Determinethe height of the basal plateau by averagingthe number
of birdscountedin eachof the 10-footstripsproximalto the point of
inflexion of the curve.
2. Extrapolate this plateau level horizontallyto a prescribedlateral
boundaryline definingthe limits of the censusband (see below).
3. Divide the total countof birdswithin this boundaryby the projected
number below the horizontal extrapolationline. The basis for this
procedureis that if the countin the proximalstripsapproaches
completeness,it will, when extendedto the boundary,representthe projected count of the entire band. The value obtainedis the per cent
o.fthe total populationof the specieswithin the band that is detected,
i.e. the coefficientof detectability.
A finite limit mustobviouslybe set to the lateral extensionof the survey
band but there is no ultimate reasonwhy this limit cannotbe remoteenough
to includeall detections.Since,for mostspecies,detectionsbeyond300 or
400 feet rarely contributemore than an insignificantportion of the total
count,however,a moremanageableboundaryline can be considered.For
most speciesin the studiesperformedto date an arbitrary cutoff point at
412 feet was adopted, far enoughto include nearly all of the visual and
auditory observations,yet near enough to be realistic in terms of stand
limits. The value, 412 (126 meters), was chosenbecausea band of this
width alongeither sideof a trail and 1 mile in lengthincorporates
just 100
acres,a convenientand widely used base in populationdensity studies.
Similarly a lateral boundary distanceof 125 meters (410 feet) would
incorporatejust 100 hectaresin 4 km of transect. For loud-voicedspecies
that are commonlydetectedbeyond 412 feet (e.g. flickers, crows,jays,
wrens,Mockingbirds)a baseof 825 or even 1,650 feet is preferablegiving
valuesin termsof individualsper 200 or 400 acresfor eachmile of transect.
For smallstandsC.D. valuesfor any speciesmay be basedon the portion of
the lateral distancecurvelying proximalto the point representingthe mean
distancefrom trail to boundaryline (Figure 4).
The translationof a transectcount for a given speciesinto a population
estimateis effectedby dividingthe countby the appropriateC.D. value of
332
MN.
Jon•r T. E•.EN
DIST.
TO
BOUNDARY
CENSUS
[Auk, Vol. 88
OF
TRACT
AREA
LATERAL
(ACRES/MILE)
DISTRIBUTION
C.D.
50
FT.
12
103
F'r
26
.85
206
FT.
50
.50
412
FI
FOR THE
OF DETECTION
POINTS
SPECIES
1.00
I00
.:>7
0
50
103
:>06
41 :>
Figure4. Area and C.D. valuesfor census
tractsof variouswidthsfor a hypothetical
lateral distributioncurveof the winteringPalm Warbler type.
the species.Thus, if 40 Palm Warblersare tallied along5 milesof transect
througha Florida pine-palmettoforest in mid-February, and a C.D. 412
value of 0.25 has been establishedfor this speciesin many miles of winter
surveysthrough similar habitat, the derived estimateper mile, and hence
per 100 acres,wouldbe
40/5
0.25
= 32 birds.
Lateral distancepatternsof detectionsand C.D. valuesmay changewith
the seasonsand particularly with the advent of the breedingseasonwhen
malesadvertisetheir presenceloudly thoughintermittently with song,and
femalesretire to obscurity on their nests. At such times song and nonsongdetectionsshouldbe tallied and plotted separately.The basalplateau
of the lateral distance curve will obviously extend considerablyfarther
from the trail with songdetections,producinghigher detectability rates
and lower population estimates (Table 2). As these tallies and the derived estimateswill be restrictedto the male elementof the population,
they shouldbe multipliedby two (assumingan equal sex ratio). Theoretically an estimateof malesbasedon the songdetectionplateau should
match a directly obtainedestimateof the total populationbasedon the
nonsongdetectionplateau, but female inconspicuousness
at these times
may produceseriousincompletenesses
in the latter. In practice,I determine both valueswhen malesare singingand adopt the larger of the two
as the best estimate.Samplesof this methodare presentedin Table 6,
column
3.
Basal detectabilityadjustment.--Thepopulationestimatesderivedby
the procedures
described
abovewill obviously
be low in proportionto the
incompleteness
of detections
in the relativelynarrowstrip usedto establish the basal plateau of the lateral distributioncurve. Where birds are
awakeand activevery few will be missedin at least the proximal10 to
20 feetof thisstrip,andastheplottingof detection
pointscharacteristically
April 1971]
TransectCounts
TABLE
LATERAL DISTRIBUTION
333
2
OF DETECTIONS AND DERIVEr
VALUES IN A THEORETICAL
POPULATIONOF 90 BIRDS (45 MALES AND 45 FEMALES) BEFORE
AND AFTER TIlE
ONSET OF SINGING BY TIlE
Width
To
To
To
To
50'
100'
200'
400'
10
6
MALES
Total
of
to
pla-
pla-
Proj.
to
Est.
Total teau
teau
412'
popl.
Adj.
C.D. needed
Winter
All det.
1
0
17
50
10
82
82
0.21
1.1
Spring
Songsonly• 4
4
7
3
18
200
15
30
60
0.60
1.5
All det?
All det. s
4
4
7
7
3
3
23
19
50
50
9
5
75
41
75
41
0.31
0.46
1.2
2.2
9
5
• In this example females are not recorded and the projected population of males is incomplete
becauseof nonsingingindividuals. The coefficient of detectability is for males only.
s In this example females are detectable though total detectability is slightly less than in winter.
The plateau width is basedon nonsongdetectionsthough all detectionsare recorded.
a This examplerepresentsthe situationsin which femalesare largely undetectablewhile malesremain
conspicuousthrough movements,call notes, and song.
reveals a plateau out to 50 or more feet, the expectedlevel of completenesswill be high. But where birds are intermittently inactive, or where
the observeris obligedto dependon the singingof males,a wide basefor
projectionis required,within which a sizeablefaction of the population
will not be emitting detectablesignalswhile the observeris within range.
Under theseconditionsmany individualswill be bypassed,and a substantial adjustmentfor incompleteness
will be indicated.
The magnitudeof adjustmentsfor incompleteness
in the basal strip of
optimum coveragemay be estimatedby comparingthe convertedvalues
for a 412-foot transectstrip with populationvaluesobtainedin the same
area by other censusmethods. As no adequatecensusmethod has been
developed,our only recourseis to apply severalmethodsconcurrentlyto
the samepopulationand comparethe results. This has been done on two
areas (Table 5). The data in this table, preliminary observationson three
more areas, and a few observationsfrom the literature suggestthat adjustmentsfor countsof winteringbirds shouldrarely exceedX 1.1 or X 1.2
(Table 2). Breedingbirdpopulations,
particularlybecause
of theinactivity
of the femalesand the intermittent singingof the males, require adjustmentsrangingfrom X 1.1 to X 2.5 with an averageof perhapsabout X 1.5.
Poolingdatafor samplesize.--C.D.valuesshouldobviously
be obtained
underconditions
closelyresembling
thoseof the transectcountto be translated,and ideallyall the data for the conversion
shouldbe recorded
in the
same area and at the same time as the count. In the tests described in
thisreport,lateraldistance
measurements
were,in fact,talliedconcurrently
with the recordsof occurrence,
and the valuesapplieddirectly--the projectedbasalplateaudirectly definingthe populationvalue.
334
Jo•
T. EMLEN
[Auk, Vol. 88
Resultsfrom the best representedspeciesof this study stronglysuggest
that with the accumulation
of records,eachspecies
will be foundto have
one or a few characteristiclateral distributionpatterns,and henceC.D.
values,accurate
withintheratherbroadlimitsof acceptability,
andapplicable over a considerablerange of habitats and seasons.As already indicated,basaldetectabilityadjustments
for incompleteness
may alsobe
speciesand season-characteristic.
Suchbroadlybasedvalueswouldtheoreticallybe preferableto thoseobtainedfrom any singleclusterof surveys,
sincethe latter are bound to. vary meaninglesslyfrom week to week and
site to site becauseof smallsamplesize. Furthermoreif we can establish
and adopt a broadly basedC.D. value for a species,it will be unnecessary
to run large numbersof countsin eachcensustract; all that will be needed
is enoughmilesof transect,perhaps10, or even5, to providea reasonably
reliable mean of detectionsper mile (birds per 100 acres). Greater confidencecan be placed on such a density value than on one basedon a
statisticallyinadequatethoughsituationallycorrect conversionfactor.
C.D. values at monthly intervals from severallocalitiesare currently
being assembledfor selectedspeciesto test the authenticity of the concept
of species-characteristic
C.D. values and the variance to be expected. In
the meantimefiguresfor well-represented
speciesshouldbe basedon means
drawn from all available data and, for poorly representedspecies,on
interpolationsof valuesobtainedfor ecologicallyand behaviorallysimilar
species.
FIELD PROCEDURES
About 300 C.D. surveyshave beenconductedby the author in the past 3
years,and about 15,000detectionshavebeentallied and plotted for lateral
distance distribution curves. The field proceduresused in these tests are
briefly summarizedbelow. They apply equallyto the collectionof lateral
distance data and to transect tallies. In fact, the two operationswere
combinedin all surveysto date.
Siteswereselectedfor standsize (at least 50 acresexceptin a few special
cases)and for generaluniformity of physiographyand vegetation. Transect routesbisectedthe survey standsin fairly straight lines at various
angles,but existingtrailswereusedwhereavailableto reducedistractions
causedby obstacles
and to provideclearbaselinesfrom whichto estimate
lateral distances. Crisscrossingof routes and repeated traverses were used
to obtain adequatesamplingsin small tracts. Vegetationrows inducing
linear distributionpatterns of birds, as along bushy fence lines or broad
roadways,were crossedat right anglesrather than followed.
I did not start to tally data until I had becomefamiliar with the stand
and with the field marks and call notes of the local birds (on Grand
April 1971]
Transect Counts
335
Bahama). Where birds couldnot be identifiedquickly they were not
counted. I invariably worked alone in order to reduce distractionsfrom
the job in hand.
Progressalong the route was on foot and essentiallycontinuous,alter-
nating betweenslowwalking and brief pausesto look and listen. Long
stopswere avoidedto reducethe dangerof doublerecordings,and birds
aheadof the advancingobserverwerenot tallied until he had approached
to within 100 feet of their positionalong the trail. Net speedsaveraged
between1/.2and 1« mph in woodlandand between 1 and 2 mph in open
country. Interruptions to observeitems of specialinterest were postponed
until the formal surveywas completed.Detectionpointswere definedas
thosewhere a bird was first seenor heard; birds that entered the survey
tract after initial detection were not counted. Contrary to a common
recommendationfor bird-countingoperations,squeakingand pishingsounds
were used to lure hidden birds into view.
Efforts were made to record the
lateral distanceof the first responseof a lured bird, the point of origin
rather than the point of identification,and althoughthe effectiveness
of
the soundswanedwhenthey wereusedrepeatedlyon a trail (Emlen, 1969),
no correlatedchangein the pattern of lateral distributions (hence C.D.
values) was detected. The slightlyhighercountsobtainedon the first few
daysof a seriesas a resultof squeakinghad only a minor effecton the mean
of all counts along the route.
Surveyswere madeonly on dayswith favorableweather conditions,were
startedwithin 1/.2
hour after sunrise,and extendedfor about 2 hours. Every
natural meansof detection,visual and auditory, was usedand the type of
detectionrecordedwith each observation. The location of unseensinging
or calling birds was approximatedafter careful scanningor maneuvering.
Lateral distances(at right anglesto the route of progress)were estimated
in 10-footintervalsto 100 feet, then a singleinterval from 100 to 200 feet
(recordedas +), anotherfrom 200 to 400 (412) feet (recordedas ++) and
anotherbeyond412 feet (recordedas +++). After an initial periodof selftraining with a range finder, I found it best to carry no more than a series
of mental referencesto familiar linear situations,and to check these
repeatedlyby pacing. Estimatesof distancesgreater than 100 feet were
more difficult than short distances,but also less critical for determining
C.D.
values.
SOME PRELIMINARY
RESULTS
A few generalizationsand preliminary deductionscan be drawn from the
field testsrun to date. The methodappearsto be applicableto most nonflocking,temperatezonedoves,cuckoos,hummingbirds,woodpeckers,
and
passetines.It is poorlysuitedfor wide-rangingwater birds,shorebirds,and
336
Joan T. EMLEN
TABLE
[Auk, VO& 88
3
ATTENUATION OF DETECTABiLITY IN PINE FORESTS WITH A •VIoDERATE UNDERSTORY
ON GRAND BA•IA•V•A ISLAND1
Detection type
0-50'
50'-100'
100'-200'
200'-+-
All visible detections
Birds flushed
73
92
23
8
3
0
1
0
877
50
Birds seen moving
Still perching
All auditory detections
Birds calling
Birds singing
72
59
35
48
21
24
31
29
36
24
3
10
29
15
41
1
0
7
1
13
784
43
2108
1029
1060
Other sounds
All detections
N
79
16
5
0
19
46
27
22
5
2985
t Incidence of each type of visual and auditory detection is shown in four unequal distance intervals
from the trail. Figures represent per cent within each type for all speciescombined.
hawks,for nocturnalbirds,for treetopbirdsin tall denseforests,and for
swifts and swallowsthat cruise about above the vegetation. Flocking
speciespresentspecialproblemsand ordinarily are best handledby direct
counting within each flock.
Amongthe speciesthat meetthe basicrequirementsof the method,some
create specialproblemsby sneakingaway or, on the other hand, by approachingthe censustaker. The roadrunnerexemplifiesthe former group,
and hummingbirdsoften demonstratethe latter trait. Also many birds
closeto the trail, in additionto thoseactuallyflushed,are probablyaroused
to vocalizeor move,and thus to enhancetheir detectability. Compensating
or even overcompensating
for this effect is a tendencyfor birds of many
speciesto retreat laterally from the line projectedaheadof the observeras
he advances. Breckenridge(1935) noted this phenomenon,and in the
present study countswere commonlyhigher in the 20-29 and 30-39 foot
stripsthan in the 0-9 and 10-19 foot stripsalong the trail (top of Figure
2). Bias arising from thesebehaviorsis presumablynullified or at least
minimized by averagingthe countsout to 50 feet or more in determining
the basal plateau level of a lateral distribution curve.
It is too early to attempt to assigngeneralC.D. valuesor to analyzethe
seasonal
and habitat variationsfor any speciesas such,but data are available for a few species
in certainregionsthat seemto showcharacteristicand
suggestive
patterns. In thesebirds, valuesto the 412-foot lateral distance
(C.D. 412) rangedfrom 0.09 to 0.89. Low valuesindicatinglessthan 20
per cent coveragewereobtainedfor nonsinginggroundfeedersthat characteristically remainedundetecteduntil flushedat close range (wintering
BobwhiteQuail, SavannahSparrows,and Ovenbirds),quiet speciesfrequentingdensebrush(nonsinging
SongSparrowsand Yellowthroats),and
smallquiet arborealand subarborealbirds (Kinglets and Brown Creeper
April 1971]
Transect Counts
TABLE
337
4
C.D. VALIJES1 IN FIVE HABITAT TYPES ON GRANDBA•IAlVI:A
ISLAND
JANVARY-MARCH1969
Miles
C.D. 412
Vegetation type (% cover-high shrubs)
traversed
(mean)
Densehigh thickets (70-90%)
Pineswith high shrubs(50-60%)
Pines with sparsehigh shrubs(10-20%)
5.3
4.8
9.1
0.14
0.17
0.19
8.1
12.8
0.30
0.25
Pines with only low shrubs (none)
Open pines with low shrubs (none)
• All species combined.
in the fall and mostwinteringwarblers). Highestvaluesindicatinggreater
than 50 per cent detectionto the 412-foot line wereproducedby large noisy
arborealbirds (certainwoodpeckers,
crows,and jays in winter) and breeding birds with loud clear songs(Cardinal, CarolinaWren, Mockingbird,
etc.). These noisy birds may best be evaluatedto the 825- or even the
1,650-footline so that all detectionsare included.
Exceptin a few casessamplesizesare still inadequateto permit testsof
variancein singlepopulationsor to evaluateseasonalor regionalvariations
within a species.In the wintering populationof Palm Warblers on Grand
Bahama(N = 2430) six monthlyvalues(JanuarythroughMarch 1968and
1969) rangedfrom 0.196 to 0.240 and showeda meanand standarddeviation of 0.219 -+ 0.015. During thesesame6 months Gnatcatcherson Grand
Bahama (N = 321) showeda C.D. 412 meanvalueand standarddeviation
of 0.197 - 0.029. Winter and early springvaluesfor the Pine Warbler on
Grand Bahama and the Red-belliedWoodpeckerin Florida were 0.311 -+
0.059 (N -- 336) and 0.560 -+ 0.054 (N = 142) respectively.
Samplesizesare still too small to evaluate the seasonalchangeswithin
TABLE
5
COEFFICIENTS
OF DETECTABILITY
(C.D. 412) EORBREEDING
BIRD SPECIES
IN NORTHERN MINNESOTA AND NORTHERN MICHIGAN
Northern
Northern
Minnesota•
(1955-SCK)
Michigan2
(1969-JTE)
Great CrestedFlycatcher
0.93 (5) 2
0.80 (23) 2
Least Flycatcher
Eastern Wood Pewee
Red-eyed Vireo
Ovenbird
Scarlet Tanager
Rose-breastedGrosbeak
0.36 (73)
0.46 (35)
0.52 (73)
0.58 (28)
0.47 (10)
0.47 (14)
0.52 (13)
O.51 (65)
0.42 (65)
0.55 (73)
0.56 (14)
0.60 (7)
Kendeigh (1956).
This study.
Values in parenthesesare number of birds recorded.
Jo•IN T. EMLrN
338
TABLE
[Auk, Vol. 88
6
COMPARISON
OF BREEDINGPOPULATIONESTIMATES(BI•DS PER 100 ACRES)
OST^•ED •¾ T•RrE
METHODS•
Indicated
MethodC
MethodD
sum-
mapping mation
(terr.
(max.
Method G C.D.
conversion
(all detections)
(songs
X 2) a
adjustment
forincorepleteness
(col. 1/
X 2)
count)•
11
9
29
6
14
0.6
3
31
4
12.8
4.8
19.4
16.9
12.9
2.4
9.6
16.9
9.6
6.2
44.1
6.4
8.4
0.2
21.4
4.1
7.6
3.5
-
1.4
1.4
0.7
0.9
1.7
0.9
1.4
-
Bahaman Yellowthroat
Thick-billed Vireo
17
34
19.4
44.0
16.9
-
29.0
1.0
1.2
Olive-capped Warbler
14
19.5
15.5
-
0.9
Yellow-throated
Pine Warbler
Warbler
11
14
14.3
22.0
9.8
16.9
-
1.1
0.8
Bananaquit
29
22.0
21.2
-
1.4
Striped-headedTanager
40
34.0
41.2
-
1.0
Black faced Grassquit
11
12.0
8.4
-
1.3
278
292.3
Grand Bahama pine forest:
Ground Dove
Zenaida Dove
Cuban Emerald Hummingbird
Hairy Woodpecker
Greater Antillean Pewee
LoggerheadFlycatcher
Stolid Flycatcher
Blue-Gray Gnatcatcher
Red-leggedThrush
TOTAL ( 17 species)
Michigan deciduousforest:
Mourning Dove
Black-billed
Cuckoo
Hairy Woodpecker
Eastern Kingbird
Great Crested Flycatcher
Least Flycatcher
Eastern
Wood
Pewee
1.2
-
0.8
-
1
0
-
0.2
-
2
2
6
2
1.2
2.4
7.3
4.8
0.3
-
0.5
3.8
4.1
-
22.0
6
2.2
Crow
2
0
Black-capped Chickadee
4
1.2
White-breasted Nuthatch
Red-breasted Nuthatch
Brown Thrasher
Robin
Hermit Thrush
1
1
2
4
3
Veery
Common
Cedar Waxwing
Red-eyedVireo
Black-and-white
-
16.5
4.1
-
1.2
-
-
-
0.3
-
-
1.2
1.2
0
6.0
7.2
0.1
0.3
2.8
-
0.6
2.8
-
8
9.7
4.0
-
-
6
20
2.4
25.4
1.9
-
17.7
-
1.1
1
2.4
-
0.3
Black-throated Green Warbler
1
2.4
0.6
-
-
Pine Warbler
3
2.4
-
0.6
-
tData
Warbler
1.06
2
20
Blue Jay
263
col. 3) •
-
were taken on two census tracts: a 35-acre stand of plneland on Grand Bahama Island,
surveyedon 7 morningsbetween15 and 23 April 1969, and a 49-acre stand o[ aspen,oak, etc. in
northern Michigan, surveyedon 8 morningsbetween 10 and 14 June 1969.
• In method D the number of singing males within 200 feet of the trail was doubled and added to
the number of nonsongdetections,then multiplied by 2 to conformto the strip width of method G.
a In method G the highest o[ the two derived values was adopted as explained in the text.
4 In column 4 the values obtained by method G are related to those obtained by method C. There
is no final reference for completeness.
April 1971]
Transect
TABLE
Counts
339
6 (Continued)
Indicated
Method D
MethodC sum-
Michigan deciduous forest:
Myrtle Warbler
Ovenbird
Brown-headed Cowbird
mapping
(terr.
mation
(max.
X 2)
count)-•
4
22
10
3.6
24.0
11.0
Method G C.D.
conversion
(all detections)
1.6
-
(songs
X 2) 2
17.7
4.1
adjustment
for incompleteness
(col. 1/
col.3) •
1.2
2.4
ScarletTanager
6
4.8
-
2.5
-
Rose-breasted Grosbeak
6
4.8
-
1.3
-
Indigo Bunting
2
4.8
-
2.2
-
American Goldfinch
3
1.2
0.3
-
-
Vesper Sparrow
Chipping Sparrow
4
6
7.2
4.8
-
1.0
1.9
-
160
173.8
TOTAL (30 species)
95
1.69
any species,but in the BobwhiteQuail and Mockingbird,C.D. 412 values
roughlydoubledwith the adventof the singingseason.Ovenbirdson their
breeding groundsin Michigan had C.D. values about four times those
derived from nonsingingbirds on their wintering groundsin Florida and
Grand
Bahama.
Foliage,particularlyin the tall shrubstratum (eye and ear level of the
observer) rapidly attenuatesthe detectionof bird soundsand movements
laterally from the transectroute. Birds that can readily be detectedaurally
to 400 feet or more in an open situation may be undetectableat 200 feet in
densehigh brush. In a sampleof lateral distancedata from the Grand
Bahamapine forests(Table 3) visualdetectabilityfor all speciestogether
in the second50-foot strip was about 30 per cent o.f that in the basal 50
feet, and then dropped to about 2 per cent beyond the 100,-footline.
Auditory detectabilityin the samesampledeclinedto 83 per cent beyond
50 feet and to. 41 per cent beyond 10,0 feet, but for singing alone.there
was no appreciabledeclineinside the 200-foo.tline. In a comparisonof
resultsfrom variousforest types on Grand Bahama, overall C.D. values
(all speciescombined)were about twice as high in openpine forestswith
low palmettoshrubsas in dense,high shrubthickets (Table 4).
To date no other ornithologistshave followedmy field proceduresover
the sameor similarroutesto test for interobserver
variability,but a useful
comparison
canbe drawnfrom data presentedby Kendeigh(1956) on the
frequencyof detectionpointsat variousdistancesfrom his surveytrail in
northern Minnesota. C.D. values derived from these data correspond
reasonablywell with values I obtainedfor the same speciesin northern
Michigan in 1969 (Table 5).
340
Joan T. E•LE•
[Auk, Vol. 88
Direct testsof the accuracyof the C.D. conversionmethodare not possibleas thereis no knownway to obtain a completeand accuratecensusof
nonflockingland birds as a reference.Howell (1951) and Enemar (1959)
placedgreat reliancein the territory mappingmethod(C in Figure 1) in
derivingtheir effectivity valuesfor transectcounts. After conductingsew
eralsuchterritorymappingsurveys,notingthe wide-rangeof interpretations
that canbe extractedfrom composite
mapsof songperches,and evaluating
the problemsof nonbreeders,
transients,large partially incorporatedterritories,etc., I do not have sufficientfaith in the methodto acceptit as a
final reference.I have,however,madedirect comparisons
of the territory
mapping method, the maximum count in a fixed strip (summation)
method,and the C.D. conversion
method(C, D, and G in Figure 1) in a
pinelandstand on Grand Bahama and in a deciduousforest in northern
Michigan (Table 6). AlthoughI relate the valuesobtainedby the C.D.
method to thoseobtainedby territory mapping, I am inclined to think
that the former is more sensitive and balanced for most of the. uncommon
speciesand perhapsmore "foolproof" for most of the abundant species
in which territoriesare contiguous.This doesnot mean that adjustments
for completeness
are not needed.With appropriateadjustmentsthe final
valueswill apparentlyresemblethoseobtaineddirectlyby territory mapping in general order of magnitude.
AC KNOWLEDG1V•ENTS
Financialsupportfor field studieswasprovidedby the AmericanPhilosophical
Society
and the F. M. ChapmanMemorial Fund. Facilitiesand logisticsupportwere provided
by the Welder Wildlife Foundation of Sinton, Texas, the U.S. National Park Service,
and the Colonial ResearchInstitute of Freeport, Grand Bahama. Valuable assistance
in
the field and in analyzing data was given by Mrs. Virginia Emlen. Joseph J. Hickey,
J. Merritt Emlen, and Chandler S. Robbins offered helpful criticismsand suggestionson
the manuscript.
SUMMARY
Censusmethods for nonflocking land birds are reviewed and a new
methodis describedthat is applicableat all seasons,
is moreefficientthan
the intensiveplot methods,and is apparentlycomparablein accuracy. In
the newmethodfoot transectcountsare madein whichall detections,visual
and aural, out to the limit of detectabilityare tallied. The count for each
speciesis then multipliedby a conversion
factor (coefficientof detectability) representing
the per centof the populationthat is normallydetected
by theseprocedures.Conversion
valuesare deriveddirectlyfrom distribution curvesof detectionpointslaterally from the observer'strail. Conversionvaluesare finally adjustedfor incompleteness
in the strip.of optimum
coveragecloseto the transecttrail. Field proceduresused in testing the
April 1971]
Transect Counts
341
new methodare describedand samplesof preliminaryresultsare presented
and evaluated.
LITERATURE
CITED
BRECKENRIDGE,
W.J.
1935. A bird censusmethod. Wilson Bull., 47: 195-197.
COLQUaOUN,M.K.
1940a. The density of woodland birds determined by the sample
count method. J. Anim. Ecol., 9: 53-67.
COLQUaOUN,M.K.
1940b. Visual and auditory conspicuousness
in a woodland bird
community: a quantitative analysis. Proc. Zoo[ Soc. London, 110: 120-148.
E•LEN, J. T. 1969. The squeak lure and predator mobbing in wild birds. Anim.
Behar., 17: 515-516.
ENE•AR, A. 1959. On the determination of the size and compositionof a passerine
bird population during the breeding season. Vf•r Ff•g[lvarld, Suppl. 2: 1-114.
HAYNE,D.W.
1949. An examinationof the strip censusmethod for estimatinganimal populations. J. Wildl. Mgmt., 13: 145-157.
HOWELL,J. C. 1951. Roadside censusas a method of measuringbird populations.
Auk, 68: 334-357.
KEm)EIC•r,S.C. 1944. Measurement of bird populations. Ecol. Monogr., 14: 67-106.
KEm)Em•, S.C. 1956. A trail censusof birds at Itasca State Park, Minnesota. Flicker,
28:
90-104.
LEOeOLD,
A.
1933. Game management. New York, Chas. Scribner'sSons.
MERIXALLIO,
E. 1946. iJberregionale
Verbreitungund Anzahlder LandviSgel
in Sud
und Mittelfinnland,besonders
in dereniSstlichen
Teilen,im Lichte yon quantitativen
Untersuchungen.Ann. Zool-Soc. Vanarno, tom 12, No. 1.
MEmXA•LIO, E. 1958. Finnish birds, their distribution and numbers. Fauna Fennica
V. Helsingfors.
PAL•ORE•, P. 1930. Quantitative Untersuchungenuber die Vogelfauna in den W•ild-
ern Sudfinnlands.Acta Zool. Fennica,7: 1-218.
SEIERSTAD,
S., A. SEIERSTAD,
AND I. MYSTERUD. 1965. Statistical treatment of the
inconspicuousness
problem in animal population surveys. Nature, 206: 22-23.
S•ow, D.W.
1966. The relationshipbetween censusresultsand the breedingpopulation of birds on farmland. Bird Study, 13: 287-304.
STEWART,
R. E., J. B. COeE,C. S. ROBBINS,
Am) J. W. BRAINERD.1952. Seasonaldistribution of bird populationsat the Patuxent ResearchRefuge. Amer. Midi. Naturatist, 47: 257-363.
W•LL•S,
A. 13. 1936. The compositionand dynamicsof a beech-mapleclimax com-
munity. Ecol. Monogr., 6: 317-408.
W•LLn•SO•, K. 1964. Bird censuswork in woodland. Bird Study, 11: 1-22.
YAee, W.B. 1956. The theory of line transects.Bird Study, 3: 93-104.
Departmentof Zoolt•gy,Universityof Wisconsin,Madison,Wisconsin
53706. Accepted6 April 1970.
342
Jo•
T. E•xLE•
[Auk, Vol. 88
Appendix. Scientific names of birds referred to in text and tables
Bobwhite
Colinus virginianus
Mourning Dove
Zenaidura
Zenaida
Dove
Zenaida
Ground
Dove
Columbigallina passerina
Coccyzus erythrophthalmus
Black-billed
Cuckoo
Cuban Emerald Hummingbird
Hairy Woodpecker
Loggerhead Flycatcher
Great Crested Flycatcher
Stolid Flycatcher
Least Flycatcher
macroura
aurita
Chlorostilbon
ricordii
Black-capped Chickadee
Dendrocopus villosus
Tyrannus caudiJasciatus
Myiarchus crinitus
Myiarchus stolidus
Empidonax minimus
Contopus virens
Contopus caribaeus
C yanocltta cristata
Corvus brachyrhynchos
Parus atricapillus
White-breasted
Sitta
Eastern
Wood
Greater
Antillean
Pewee
Pewee
Blue Jay
Common
Crow
Nuthatch
Red-breasted
Brown
Nuthatch
Thrasher
carolinensis
Sitta canadensis
Veery
Blue-gray Gnatcatcher
Cedar Waxwing
Bananaquit
Toxostoma ruJum
Turdus migratorius
Mirnocichla plumbea
Hylocichla guttara
Hylocichla Juscescens
Polioptila caerulea
Bombycilla cedrorum
Coereba flaveola
Thick-billed
Vireo
crassirostris
Vireo
olivaceus
Robin
Red-legged Thrush
Hermit
Thrush
Vireo
Red-eyed Vireo
Black-and-white
Warbler
Mniotilta
varia
Myrtle Warbler
Dendroica
coronata
Black-throated
Dendroica
virens
Dendroica
dominica
Green Warbler
Yellow-throated
Warbler
Olive-capped Warbler
Palm Warbler
Pine Warbler
Ovenbird
Yellowthroat
Bahaman
Yellowthroat
Brown-headed
Cowbird
Dendroica pityophila
Dendroica palmarum
Dendroica pinus
Seiurus aurocapillus
Geothlypis trichas
Geothlypis costrata
Molothrus
ater
Scarlet Tanager
Striped-headed Tanager
Piranga olivacea
Spindalis zena
Rose-breasted
Pheucticus
Grosbeak
Indigo Bunting
American
Goldfinch
Black-faced Grassquit
Vesper Sparrow
Chipping Sparrow
ludovicianus
Passerina cyanea
Spinus tristis
Tiaris bicolor
Pooecetes gramineus
Spizella passerina