The Auk 115(3):591-601, 1998
WINGBEAT FREQUENCY OF TWO CATHARUS THRUSHES DURING
NOCTURNAL
MIGRATION
ROBERT H. DIEHL • AND RONALD
P. LARKIN
IllinoisNaturalHistorySurvey,607 EastPeabody
Drive,Champaign,
Illinois61820, USA
ABSTRACT.--Radio
transmittersemitting continuoussignalswere mountedon Swainsoffs
Thrushes(Catharus
ustulatus)
andVeerys(C.fuscescens)
duringtwospringmigrationseasons.
Wingbeatsmodulatedthe continuoussignals,and signal-processing
techniques
permitted
examinationof wingbeatfrequency(WBF)duringnocturnalmigration.Distincttakeoffand
cruising-flightphaseswere evidentin both species.As birds climbedduring the takeoff
phase(whichusuallylastedabout15 min), WBFtypicallydeclinedexponentiallyby at least
4 Hz. Median cruisingWBF from sevenflightsvariedfrom 9.2 to 10.9Hz, valuessimilarto
thosecalculatedfrompublishedequationspredictingWBFbasedonflightmechanics.
CruisingWBFtypicallyvariedslowlyandirregularlythroughout
all flightsby about1 Hz. During
takeoffand cruisingflights,all birdsflappedcontinuously,
showingno evidenceof theflapcoastingpatterncommonin passerine
migrantspreviouslystudiedwith othertechniques
suchasradar.All fivebirdsexhibiteddeparturetimes,coursedirections,
andflightdistances
typicalfor migratingthrushes.Received
23 May 1997,accepted
11 November
1997.
EXCEPT FOR BRIEF PERIODS,such as when includeusingbroken-light
beam(Mooreet al.
birds cross the face of the moon, researchers 1986), microphone(Payne 1981), and high-
rely mainly on indirectmeansto studynoctur- speedvideobothoutsidewind tunnels(Butler
nal flight. Empiricalstudy of wingbeatfre- and Woakes 1980, Norberg 1991, Tobalske
quency(WBF) in birds intensifiedwhen re- 1996) and inside (Butler et al. 1977,Rayner and
searchersconsideredusing trackingradar to Thomas 1991). Of these, only wind-tunnel
identify speciesby their wingbeatcharacteris- methodsallow monitoringWBF for durations
tics.AlthoughWBFin insectsvariesinterspe- comparable to nocturnal migration (Pennycifically(Moore et al. 1986),intraspecificvari- cuick et al. 1996). However, data from windation in WBF in birds has been too great for tunnel studiesmay not be directly comparable
speciesidentification(Emlen 1974, Vaughn to data from free-flyingmigrants(Raynerand
1974).A moreenduringbodyof theoretical
and Thomas1991,Pennycuicket al. 1996).
empiricalworkconsiders
WBFin thecontextof
The pioneeringwork of W. W. Cochranprounderstanding powered flight (Permycuick vided an opportunityto measureWBF when
1968, 1989; Tucker 1973; Rayner 1979). Better radio transmitters emitting continuoussignals
data on WBF may increaseour understanding yielded serendipitousdata on respirationand
of bird flightperformance
by providinga more wingbeats of Mallards (Anas platyrhynchos;
completetheoreticalpicture of mechanical Lord et al. 1962). Taking advantageof this
poweroutputof flight muscle(Pennycuick
and property of radio transmittersemitting continRezende1984),helpingexplainflightstrategies uoussignals,we studiedSwainson's
Thrushes
in flappingversussoaringflight (Hedenstr6m (Catharusustulatus
) and Veerys(C.fuscescens
) to
1993),and improvingestimatesof WBFduring obtain long recordsof WBF during nocturnal
cruising flight (Hedenstr6mand Alerstam migration.
1992).
Most radar tracksof WBF are short,typically
METHODS
lessthan a few seconds(Williams et al. 1972,
Vaughn1974,Larkin unpubl. data), and they
Overtwo seasons
in May,two Swainson's
Thrushseldom include birds taking off and landing es and threeVeeryswere capturedin mist netsin a
(but see Hedenstr6m and Alerstam 1994). In
forest fragment near Champaign-Urbana,Illinois.
additionto radar,methodsof measuringWBF Wemeasuredwing chordandmassfor eachbird and
attached
modified
302-MHz
radio
transmitters
weighingabout1.5g (4 to 5% of bodymass)dorsally
near the centerof gravity (Raim 1978).Most birds
E-mail: [email protected]
591
592
DIEHLANDLARKIN
werecapturedin themorning,heldfor about45min
duringtransmitterattachment,
andreleased
longbe-
[Auk, Vol. 115
2570
fore the onsetof nocturnalmigration.Transmitters
emittedcontinuous
signals.Theyhad shortenedlife
o 2535
comparedwith pulsedtransmittersbut operatedfor
the few daysrequiredto obtainWBFdataand radiated sufficientpower to maintain radio line-of-site
contact(Cochran1980)at distancesoftenexceeding
10 km. The continuoussignal varied in frequency u. 2500
and amplitudewhenthe bird movedabout;during
flight,themodulationsreflectedwingbeats.Welater
•
,
0.5
•
1.0
estimatedWBFfrom recordsof signalmodulations.
We followed
the birds with a vehicle-mounted
di-
rectionalYagi antennaduring nocturnalmigration
while recordingbearingsrelativeto the vehicleor,
when possible,with a compass.From bearingsof
birds'positions,migratoryrouteswereestimatedby
0
1
the method of Cochran (1972). We recorded contin-
uous receiversignal (beat-frequencyoscillatoroutput) on analogreel-to-reeltape(RRT)in 1981andon
digital audiotape (DAT) in 1994.Becausewow and
flutter on RRT introduced
unwanted
variation
in an-
alogrecordings,we provideda referencefor subsequent correctionof receiver signal by playing a
1,000-Hztiming signalinto a secondchannelwhile
recordingon RRT.
Receiversignal was filtered (for DAT: B and K
band-pass
filtertype1621at3%band-pass)andthen
playedthrougha Schmitttriggerto obtainreceiversignalperiod. The mean of every 10 receiver-signal
periods was computed,correctedfor variation in
tape speed (for RRT), and saved to a hard disk.
Changesin receiver-signal
perioddue to RRTspeed
variation were correctedduring playback,and receiver-signalfrequencywas reconstructed
digitally
by:
Time (s)
FIC.1. Bottom:Threeseconds
of digitizedreceiver signalof a Swainson's
Thrush5 min aftertakeoff.
Top:One-halfsecondis enlargedand the fitted sine
wave superimposed
over the data. During this epoch, WBF = 10.8 _+SE of 0.14 Hz, amplitude (&) =
19.2_+2.50Hz, phase(4))= -48.5 _+14.8ø.Suchcontinuousflappingwastypicalof all theflights.
plitude,and phaseof wingbeatmodulationfrom a
sinewavefittedby leastsquares
to eachepochofdigitized receiversignal:
i=1 .....
N, 0 -< t• -< 0.5,
f,. = 6t ßsin(2•vßWBF. t, + 4)),
1
(2)
1
whereN is numberof digitizedsignalsamples
within an epoch(N is ->30and usually>>50,comprising
Fig. 1),• is the digitizedrewheref is receiver-signal
frequency(Hz), •c is cor- aroundfive wingbeats;
rectedreceiver-signal
period (s),ß is uncorrectedre- ceiverfrequency(Hz), & is the estimatedamplitude
ceiver-signalperiod (s), and •t is timing-signal of thefittedsinewave(Hz), t•is sampletime (s)withperiod (s). Cumulativeperiodsprovidedan elapsed in eachepoch,and 4) is the estimatedphaseof the
time base,and from the inverseperiod we recon- sine wave. The sine-fittingalgorithm was constructedreceiver-signal
frequency(Fig.1).Datafrom strained to 7 < WBF < 17 Hz, becausepreliminary
RRTwerefilteredvia softwarewherefrequency
sam- exploratorydata analysisshowedWBF for these
ples >3 standarddeviationsfrom the median fre- thrushes to be well within this bandwidth. WBF esquencywereomitted.Thefilteringof DAT andRRT timatesfrom the Pascalprogramwerevalidatedby
data was done to remove environmental
interference
the factsthat:(1) fittedsinewaveswerevisuallycom(seebelow)andignitionnoise.Thesedatawerethen parableto the digitized receiversignal(Fig. 1); (2)
dividedinto 0.5-sepochs,whicharelongenoughto Pascalprogramresultswere nearlyidenticalto SAS
estimateWBF,yetshortenoughto permita compute- PROCNLIN (SASInstitute1990)programresults(a
intensivemethodof estimatingWBFfor eachepoch. completelydifferentbut muchslowermethodof calTo removesuddenchangesin receiversignalcaused culation);and (3) Fouriertransformsof digitized reby tuning, we normalizedthe databy fitting linear ceiversignalagreedwith Pascaland NLIN results
regressions
to eachepochandsubtractingresiduals, (Fourier transformswere not used for the calculabecausetheylackthe precisionof
leaving a sinusoid-likewaveformwith a mean of tionsthemselves
zero.
sine-wavefits in estimatinga singlefrequency).To
Using a Pascalprogram,we estimatedWBE am- determinethe accuracyof WBFestimates,NLIN was
f •c- •.0.001/•,'
(1)
July1998]
Wingbeat
Frequency
ofThrushes
593
used on every20th epochto generatethe standard the recordto removea possiblelandingphase(see
error of the sine-wave fit.
Results). Three morphologicalmeasuresare reOf 12 thrush flights,four were short,lastingless quired for WBF prediction:(1) body plus transmitter
than 15 min (preciselandingtime wasdifficultto de- mass,(2) wing span,and (3) wing area.We measured
termine).The eight long flightswere eithergreater wing spanand wing areaof non-radiotaggedCathathan 100 min, or showed takeoff behavior similar to rus thrushesas outlinedin Pennycuick
(1989:8-13;
thatprecedingotherlongflights(i.e.veer4b;Table1). see also http://detritus.inhs.uiuc.edu/-rdiehl/
Sevenlongflightswereselectedto examinecruising- wingdata.html)and used log correlationsbetween
flight WBF during migration. We used LOWESS eachvariableand wing chordto estimatewing span
smoothing(locallyweightedregression;
Cleveland and areafor radiotaggedbirds.
1979)to highlighttrendsin WBFovertime.LOWESS
incorporates
a smoothing
parameterthat is theproRESULTS
portionof thetotalsampleof pointsusedin eachregressionwithin theLOWESSfunction.Highervalues
Modulations of the continuoussignals reof the parameterincreasethe amountof smoothing.
Smoothingconsistently
acrossdifferentsamplesizes flected the activity of birds as they moved
wasachievedby varyingthesmoothingparameteras aboutthe habitatprior to and after migration.
the ratio of 550 over the number
of observations
in
The signalsfluctuatedirregularlyduringday-
the dataset.Although550wassomewhat
arbitrary, time. A period of steady,intensesignalthat
this numberservedLOWESSwell in reflectingover- varied from 4 to 41 min, presumably correall trends in WBE
DecliningWBFsattributableto a takeoffphase(n
= 7) wereseparatedfrom the longWBFrecordsfor
spondingto a quiescentperiod, occurredimmediatelybeforetakeoff(Hebrard1971).
The extensiveroadsystemand openhabitat
furtheranalysis.Thepointof transitionfromtakeoff
phaseto cruisingflight wasdesignatedfrom LOW- typical of the centralUnited Statessimplified
maintainingradiocontactwhile followingmigrating birds on coursesthrough five states
(Fig. 2). Nonetheless,
four WBFrecordsended
of transition.In thesetwo cases,thebeginningof the becausebirds were lost.For example,a SwainESS fits to WBF data to be the time when LOWESS
valuesfirst ceasedto declineafter takeoff,exceptin
two flightswheredataweremissingduringthepoint
datagap definedthe end of the takeoffphase(Table son'sThrush (swthl; Table1) was lost in north1). BecauseWBF during takeoff appearedto decline
exponentially,takeoffphaseswere comparedby fitting negativeexponentials
to the data:
WBF• = WBF0. t k,
(3)
where t is time (s) of eachWBF estimate(in Hz),
WBF0 is WBF at t = 0, and k is the estimated rate of
WBFdecline.Thisexponentialdescriptionwasvalidatedby comparingmeansquarederror(MSE)of exponentialand LOWESSfits to data from the takeoff
phase.SimilarMSEssuggestthattheexponential
accuratelydescribes
thetakeoff.Unlikeanexponential
LOWESSassumesno parametricform in fitting the
data, but it permits later WBF valuesto influence
westernIndianaasit appearedto headoutover
Lake Michigan. Other migrants evaded us
whentheyencountered
strongtail windsor assumed
courses difficult
to follow
over the
ground.
Evenwhenthe trackingvehiclewas closeto
the migrant,radio contactfrequentlywas interrupted by environmentalinterference(i.e.
garagedoor openersor alarm systems;seeCudak et al. 1991) or obstructedby trees and
buildings.Signalstrengthwas diminishedif
the receivingantennawasorientedeitherfore
or aft of the bird and its transmittingantenna
WBF values at takeoff; therefore, the LOWESS esti(in a heador tail null; CochranandKjos1985).
mates at t • 0 will be biased low.
Of severalpublishedequationspredicting WBF Poor signal-to-noiseratios and difficultiesin
(Greenewalt1975;Pennycuick1978,1990,1996;Ray- use of RRT explainedoccasionalgapsin WBF
ner 1988,1995),the mostrecenttwo were selectedfor records(Fig. 3), althoughuseof DAT in place
comparisonwith observedWBFs. Both equations of RRTgreatlyimproved
thecontinuityofthese
representa compromisebetweensimplicityin applicationand precisionin explainingvariation,and
their similarity in form demonstrates
someconvergenceof theory. BecausePennycuick's(1996;and we
assumeRayner's[1995]) equation predicts a bird's
WBF for "steadycruisingflight," mediancruising
WBF was determinedfrom data remaining after removingboththetakeoffphasefromthebeginningof
a WBFrecordand an equaldurationfromthe endof
records. Overall use of continuous-signal
transmittersproved effectivefor gathering
WBFdata on migratingthrushes.Oftentimes
signalswere receivedfrom morethan 10 km
away when we were in line-of-sightcontact.
Wingbeat modulationswere more sinusoidal
than the spikedor picket-fence
waveformsoften seenin trackingradarrecordsof bird wing-
594
DIEHLANDLARKIN
A
A
A
[Auk, Vol. 115
July1998]
Wingbeat
Frequency
ofThrushes
595
MINNESOTA
IOWA
Chic,
INDIANA
5b
0 km 100
Champaign
FIc. 2. Approximatemigratorytracksof threeVeerysandtwo Swainsoh's
ThrushesthroughthenorthcentralUnitedStates.Smallcirclesalongeachtrack'spath marklandings.Dashesindicatewhereradiosignalswerelostduringtracking.Flightslastinglongerthanabout15 min arelabeledandcorrespond
to flight
numbers
in Table 1.
beats (Flock 1974:433).Although wingbeats
were sometimesdetectedin amplitudemodulationsof the receiversignal,they were more
often detected in its frequency modulation.
Most recordedreceiversignalwas sufficiently
free of noise to allow preciseWBF estimation
by sine-fitting(range of median WBF SEs of
eightlongflightswas0.09to 0.45Hz).
Fourof the five migrantsexhibitedtwo distinct phasesof WBF change,a takeoffphase
characterizedby rapid exponentialdeclinein
WBF,and a subsequent
cruising-flightphase
where WBF varied gradually(Figs.3 and 4).
Three long flights were trackedto landing
(veer3, veer4a, veer5a; Table 1), and each
showeda generalincreasein WBFtowardthe
end of the flight (Figs.3 and 4). SuchWBFincreasesprior to landing may representa landing phase(butseePennycuick
et al. 1996).During cruisingflight,long-termvariations
in WBF
dominatedtrendsin the data(Fig. 4), whereas
epoch-to-epoch
variationsuperimposedover
theselong-termvariationstendedto vary over
a narrow range (interquartilerangefrom 0.45
to 1.13 Hz). We attribute long-term variations
to the birds' behavior(seebelow),whereasthe
epoch-to-epoch
variationsappearto be attrib-
utable to a combinationof samplingerror, an
artifact of the method of data analysis,and
probablysomeshort-term
variationin WBE
Receiversignal was visually inspectedfor
wingbeatpattern.Regardlessof the phaseof
flight,the examinedreceiversignalshowedno
indicationof flap-coasting(Larkin et al. 1979)
orpausing(Blochet al. 1981)wingbeatpatterns
commonto many nocturnalmigrant passerines
(Vaughn1974, Williams and Williams 1980,
Larkinand Frase1988).Exceptwhencluttered
by noise, only epochsshowing continuous
wingbeatingsimilar to that in Figure 1 were
apparent.
Mediantime of departurefor the firstflight
of an evening(n = 9) was 1958CST;sevenof
thesedeparturesoccurred40 to 56 min after
sunset (Table 1; Moore and Aborn 1996). Im-
mediatelyafter takeoff,WBF declinedexponentially(seeFig. 3); this rate of declinewas
greaterforVeerysthanforSwainson's
Thrushes
(Table1). All of the ratiosof MSE (LOWESSfit)
to MSE (exponentialfit) werebetween0.97and
1.01, substantiatingan exponentialdescription
of decliningWBFduringthetakeoffphase.Mi-
grantsappearedto assumecruisingflight
about15 min after takeoff(Table1, Fig. 3), and
596
DIEHLANDLARKIN
[Auk,Vol. 115
12ti swthl
12-J
,, swth2a
03 J : veer3
11t/!
veer4a
! •: veer5b
3•0'
'41•0'
'
ElapsedTime (min)
FIC.3. WBFduringsixlongflightsof Swainson's
Thrushes
andVeerys,
estimated
byLOWESS.
Individual
datapointsarenot shown,but theirpatternsresemblethat in Figure4. Dottedlinesconnectgapsin these
WBF recordscausedby tape-stoppingand low signal-to-noiseratios.The verticaldotted line 15 min after
takeoffapproximates
thetransitionpointfrom takeoffto cruising-flight
WBE
althoughfor veer5awe reporta takeoffphase bird took off in a WNW wind, circled once,and
lasting44 min, this bird alsoexhibiteda brief landedin thesameforestpatchabout6 minlatlevelingof theLOWESS
curveafter15min(Fig. er.
4). Althoughlong,theflightof veerl offeredinSevenof 12 flightswere long enoughto exsufficient takeoff data for this evaluation. Four
amine cruising-flightWBF trends.LOWESS
shorterflightswerenot subjectto thoseanal- smoothing
showedthatcruisingWBFvariediryses.For example,veer4flew 5.5 km and land- regularlyovertimeby asmuchas1.6Hz (e.g.
edin a smallforestpatch;about30minlaterthe swth2a;Fig. 3) and was seldomtruly steady.
July1998]
Wingbeat
Frequency
ofThrushes
597
Median
cruising-flight
WBF
estimates
differed
from thosepredictedby Pennycuick's
(1996)
and Rayner's(1995)generalWBF equationsby
an averageof +1.6 Hz and +1.9 Hz, respec-
tively(Table
1).Slowvariations
inWBFevident
during
cruising
flight
(Fig.
3)didnotresolve
intopatterns
ofchanging
WBFs
onshorter
time
scales.
Seven
100-ssamples
ofWBFshowed
no
significant
autocorrelations
(outtolag32s)that
werenotreadilyaccounted
forbynonlinear
artifacts.
Noisysignals,
asopposed
to signalsdomi-
nated
byvariation
inactual
WBEshowed
over1958
2031
2104
2138
2211
2245
all poor sine-wave fits and simultaneously
large goodness-of-fit
estimatesfor all parameTime (hh:mm)
tersduringnoisyepochs.
Thisobservation
was
FIC. 4. WBF of veer5a (Table1) on 17 May 1994.
borneout by positivecorrelations
amongthe
Each
point marksone 0.5-sestimateof WBE The
standard errors of WBE &, and (b for the sinedensecloudof pointsvariesduetobothactualvariwave fits (r = 0.31 to 0.94, all Ps < 0.001).
ation in WBF (estimated by the LOWESS-fitted
curve) and error in WBF estimation (median SE of
DISCUSSION
0.18Hz). Pointscatteraboveand belowthis cloudis
mainly an artifactand is prominentwhen the re-
Most monitoredbirds tookoff shortlyafter ceivedsignalwasweak.The LOWESScurveunderWBF immediatelyafter takeoff.The bird
sunset,as previouslyreportedfor thrushes estimates
departedfrom Champaign-Urbana,
Illinois at 1958
trackedwith pulsedtransmitters
throughcen- CST and landed 160 min later near Mt. Pulaski, Illi-
tral Illinois (Cochranet al. 1967, Cochranand nois,about85 km WSWof Champaign-Urbana
(Fig.
Kjos 1985) and for nocturnal passerinemi- 2). After takeoff, WBF declined exponentially,and
grantsin general(Alerstam1990:307).During the bird assumeda WNW heading in moderate
the quiet period prior to takeoff (Hebrard winds from the NNE.
1971),when thrushespresumablycould observeorientationcuessuchas thoseprovided
by the settingsun (Vleugel1979,Moore1980,
1987),the signalbecamealmostentirelysteady
in frequencyand amplitude for minutesat a
time exceptfor warblesof lessthan2 s thatwe
interpretas the bird changingpositionwhile
perchedor flyingto a nearbyperch.At themoment of takeoff, periodic modulation began
suddenly,thereceivedsignalgrewin strength
quicklyasthebird clearedthetreetops,theazimuth of the signalbeganto turn towardthe
departurebearing,and the periodicmodulation remained
continuous from the moment of
takeoffuntil landing, wheneverthe receiver
couldpickup a clearsignal.The frequencyof
theperiodicmodulations
duringcruisingflight
(post-takeoff,pre-landing)was closeto WBF
predictedfrom morphologically
basedequa-
technique,as indicatedby recordingsmade
from a similar transmitter mounted by W.W.
Cochranon a Downy Woodpecker(Picoides
pubescens;Diehi and Larkin unpubl. data) in
whichburstsof modulatedsignaldirectly cor-
respondedto the characteristic
undulating
flightof woodpeckers
(Ritchison1994).Forinstance,whenwe flushedthewoodpeckerby approachingtoo closely,we saw directly and
heard on the radio receiverthe undulating
flight to a moredistantperch.
Several other lines of evidence, in addition to
the thrushes' takeoff behavior and observations
of the woodpecker,
pointto wingbeatsasthe
sourceof periodicmodulationsin signal frequencyand amplitudedescribed
here.Transmitterson thrushesheldin our handsprior to
tions (Rayner1995, Pennycuick1996).
release
gavesteadysignals,
butwhenreleased
Monitoredthrushesdid not showflap-coast birds flew into nearby vegetation,periodic
patterns typically seen in tracking-radarre- modulationscointidedwith their flying and
cordsof passerinemigrants(Blochet al. 1981). alighting.In daytimebeforetakeoffandduring
However, this result was not an artifact of the stopovers,
asthebirdsmovedaboutthehabitat,
598
DIEHLANDLARKIN
irregularwarblesand shortburstsof periodic
modulationinterspersedwith steadysignals
werecharacteristic
of birdsengagedin feeding
and otherbehaviors(Kjosand Cochran1970).
WBFs observed for the thrushes matched
thoseof similar-sizedbirds trackedby Vaughn
(1974).We are awareof no publisheddatathat
documentwhethernocturnallymigratingCatharusthrushesshow a continuousflapping
pattern (but see Hedenstrtim and Alerstam
1992),althoughPennycuick
et al. (1996)reported continuous
flappingin ThrushNightingales
(Luscinia
luscinia)during 16-hflightsin a wind
tunnel. Twenty-two per cent of unidentified
North Americanmigratingbirds trackedat
night by 3-cmradar (Larkin 1991)and having
similar WBFsto the thrushes(9.4 to 11.0 Hz)
showedno pausesin flapping(Larkinunpubl.
data); however, these results were based on
samplesof shortduration(2 s), andsomeof the
birdsundoubtedlywerenonpasserines.
We identified distinct WBF phasesduring
takeoffand cruisingflight;WBFchanges
characteristicof landingalsomay exist.Although
thefunctionof theseWBFchanges
remainsunclear,thechanges
arenotartifactsgenerated
by
the system.Epoch-to-epoch
variationin WBF
was small and comparableto previouslypublished measuresof WBF variation (Griffiths
1969,Emlen 1974),suggestingthat both the
measurement
techniqueand a bird'scontrolof
WBF often were precise.Data collectedusing
both digital and analogrecordingequipment
[Auk, Vol. 115
over,duringtakeoffsandperhapsduringlandings,WBFchangedin a directionoppositethat
expectedfrom an influenceof air densityalone
(Pennycuick
1996:equation9). Changesin rate
of climb more likely are responsiblefor decreasesin WBF during takeoff (Emlen 1974,
WilliamsandWilliams1980),probablyswamping the effectof changingair density.WBFestimated during landings generallyincreased;
however, because terrain and structures usu-
ally occludethesignalfroma descending
bird,
an effect compoundedby our usually being
several kilometers from the bird, we had diffi-
culty obtainingcleardata during landingsegments.
Predicted cruising WBF from Pennycuick
(1996:equation9) and Rayner (1995:equation
8) generallyagreedwith our empiricalestimations(Table 1). Measuringmorphological
variables(seePennycuick1989)directly on radiotaggedmigrants rather than predicting
themfrom regressions
on otherthrushesmay
haveimprovedourestimates
of predictedWBE
Pennycuick
(1978)predictsthata steadilyflappingbird will consume
someflight-muscle
tissueand thatWBFwill decline(subjectto simplifying assumptions)proportionalto the
square root of total mass decreaseduring
flight.Althoughthispredictionwasborneout
in wind-tunnel experimentswith a Thrush
Nightingale (Pennycuicket al. 1996),we observedno overall decreasesin cruising-flight
WBE Presumably,
suchlong-termflight redemonstrated
similar slow variation in cruiscordswill permit more exacttest of theories
ing WBF (Fig. 3), providing strong evidence concerning
bird flight.
that the variation is not an artifact.
Emlen (1974) reported high intraspecific
Pennycuick(1996:equation9) predictsthat variationin WBF of sparrowsdroppedfrom elWBF varies as an inverse function of air denevatedcagesand then trackedwith radar (see
sity. Becauseair density decreaseswith in- also Vaughn 1974). Veerys and Swainson's
creasingaltitude,we may expecta nominal1- Thrushesmonitoredin our study overlapped
Hz increase in WBF for a 2,000-m increase in
substantiallyin WBE Indeed, biologicallydealtitude (lateral air-density variations from termined,intraindividualcruising-flight
WBF
weathersystems
arenegligiblein comparison). variation(1.6 Hz; Fig. 3, Table1 for swth2a)
Althoughwe did notobtaingoodestimates
of overlappedby more than one-halfthe entire
altitudeduringmigration(andthereforedonot range of cruisingflight WBFs for all birds
know how altitude varied during cruising tracked(2.9Hz). Suchintraspecific
andintrainflight),we knowthataltitudeincreased
attake- dividualvariationin WBFagreeswith previous
off and decreasedupon landing. Becausethe findingsthat WBFin passerines
is not speciesheightabovegroundlevelof migratingthrush- specific(Emlen 1974).
es seldomexceeds2,000m (Cochranand Kjos
The meansby whichwingbeatsmodulatea
1985),we concludethat continualchangesin radio signalare unknown.The transmitter,its
air density are not solelyresponsiblefor the crystaland antenna,and the bird comprisea
slow, 1-Hz variationsin cruisingWBE More- complex resonant entity. We suspectthat
July1998]
Wingbeat
Frequency
ofThrushes
599
changesin a bird'sshapeby the respiratory noisDepartmentof Conservation,UnitedStatesFish
system,muscles,and flapping wings; inertiadriven small movements of the transmitter
and
andWildlifeService,IllinoisNaturalHistorySurvey,
and Universityof Illinois cooperating.
its attachments
asthey"ride" the flyingbird;
LITERATURE CITED
and flapping of the trailing antennatoward
and awayfrom thebody andtail all contribute ALERSTAM,
t. 1990.Bird migration.CambridgeUnito wingbeat-driven
modulationof thesignal.
versityPress,Cambridge,United Kingdom.
The dragandmassof thetransmitters
prob- BLOCH, R., B. BRUDERER,AND P. STEINER.1981. Fluablyinfluenced
theflightsof migratingthrushgverhalten
nachtlich
ziehender
V6gelradardaten
es to someextent,perhapsminor Caccamise
fiberden Zug verschiedener
Vogeltypen
auf einemAlpenpass.Vogelwarte31:119-149.
and Hedin (1985)suggestedthat frontal-area
drag of externallymounted transmittersinflu-
encespowerrequirements
onlyslightlyrelative
to addedmass.Indeed,thetransmitter
largely
rested under the back feathers,so the effect of
BUTLER,P. J., N.H. WEST,AND D. R. JONES.1977. Re-
spiratoryand cardiovascular
responsesof the
pigeonto sustained
levelflightin awind-tunnel.
Journalof ExperimentalBiology71:7-26.
BUTLER,I• J., AND A. J. WOAKES.1980. Heart rate, re-
frontal-areadrag on aerodynamics
likely was
spiratoryfrequencyand wing beatfrequencyof
negligible.It is unclearhow transmittermass
free flying BarnacleGeeseBrantaleucopsis.
Jourmay haveinfluencedflightbehaviorand WBF
nal of ExperimentalBiology85:213-226.
in particular Pennycuick(1996) and Rayner CACCAMISE,
D., ANDR. HEDIN.1985.An aerodynam(1995)predict an increaseof about0.15 Hz in
ic basisfor selectingtransmitterloads in birds.
Wilson Bulletin 97:306-318.
WBFdue to the addedmassof 1.5 g. In addiW. S. 1979.Robustlocallyweightedretion, addedmasslikely increases
wingbeatam- CLEVELAND,
gressionand smoothing
scatterplots.
Journalof
plitude and the proportionof upstroketo
the American Statistical Association 74:829-836.
downstroke(Ruppel and Knop 1985,Hughes
COCHRAN,W. W. 1972. Long-distancetracking of
and Rayner 1991) and decreasesrate of climb
birds. Pages39-59 in Animal orientationand
(Pennycuick
et al. 1989,Hedenstr6m1992,Hednavigation(K. Schmidt-Koenig,G. Jacobs,S.
enstr6m and Alerstam 1992), variation in WBF
Galler, and R. Belleville, Eds.). United States
(Ruppel and Knop 1985), and maximum susGovernment
PrintingOffice,Washington,
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tained airspeed (Gessamanand Nagy 1988, COCHRAN,
W. W. 1980.Wildlife telemetry.Pages507Hedenstr6m1992).Added massmay alsohave
520in Wildlifemanagement
techniques
manual
increased
WBFduringtheinitialclimbingportionof theflight(Table1, Fig. 3). It is possible
that the transmitters
caused the observed
steadyflappingpattern,if the addedmassreduced the coastingportion of a "normal"
thrushflap-coastcycleto zero (Rayner1985).
Nevertheless,
the long, oriented,apparently
normalflightsof theseandotherthrushescarrying similar transmitters (Cochran 1972,
CochranandKjos1985)indicatethattheeffects
of the transmitter on these birds were minor
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