WHY DO BROWN TREMBLERS TREMBLE?

j. Field Ornithol., 65(2):247-249
WHY
DO
BROWN
TREMBLERS
TREMBLE?
JULIET KELLOGGMARKOWSKY
Departmentof Wddhfe
5755 Nutting Hall
Universityof Marne
Orono, Maine 0dd69-5755 USA
WILLIAM
Cl LANZ
ZoologyDepartment
5757 Murray Hall
Umversityof Marne
Orono, Marne 0dd69-5757 USA
MALCOLM
HUNTER
Departmentof Wildlife
5755 Nutting Hall
Universityof Maine
Orono, Maine 04469-5755 USA
Abstract.--The Brown Trembler (Cinclocerthza
ruficauda),one of the endemicMimidae of
the LesserAntilles,hasa conspicuous
andfrequenthabitof tremblingitswings,thebehavior
for which it is named.Trembling consists
of a perchedbird droopingboth wings 1-2 cm,
thenquicklyraisingthembackto a normalpositionseveraltimesin rapid succession.
Each
"tremble"takesapproximately1 s. In this study,Brown Tremblerswere observedand
frequencyof tremblingwas measuredin March 1991,on Dominica.The rate of trembling
was significantly
differentin differentsocialcontexts.Solitary,singingBrown Tremblers
stoppedsingingand trembledat a meanrate of 1.74/min. SolitaryBrown Tremblersthat
were foragingtrembledat a meanrate of 6.8/min. BrownTremblersforagingor traveling
within 2 m of a conspecific
trembledat a meanrate of 28.6/min, and Brown Tremblers
involvedin a rapid chase,<0.5 m apart, trembledat a meanrate of 50.0/min. Solitary
Brown Tremblers (n = 31) showa significantlyslowerrate of tremblingthan socialbirds
(two Brown Tremblerspresent,n = 13, P < 0.001, Mann-Whitney U-test, U = 43). The
higherfrequencyof tremblingin socialcontextssupportsthe hypothesis
that tremblingis
a visualintra-specificsignal.This interpretationis reinforcedby data showingthat Brown
Tremblersalmostalwaystrembledimmediatelyuponarrivingat a new perch,and by the
absenceof a contactnote usedby Brown Tremblers.
LPORQU• TIEMBLACINCLOCERTHIA
RUFICAUDA?
Sinopsis.--EltembladorpardoCinclocerthia
ruficauda),
unode losMimidaeend(•micos
de
las Antillas Menores, tiene el h/tbito de temblar de forma conspicuay frecuentesusalas,
conductaa la cual debesu nombre.E1 temblar consisteen bajar ambasalas 1-2 cm, para
luegoelevarlas
sfibitamente
a una posici6n
normal,conducta
queserepiter/tpidamente
en
varias ocasiones.
Cada episodiode "temblar" roma aproximadamente
un segundo.Este
estudio,que se 11ev6a caboduranteel mes de marzo de 1991 en Dominica,tuvo como
objetivoestudiarla frecuencia
de temblaren estasaves.La tasade temblarresult6significativamentediferente en diversoscontextossociales.Individuos solitariosque cantaban
dejaronde cantary temblarona raz6n de 1.74/minuto.Avessolitariasque forrajeaban
temblarona una raz6nde 6.8/min. Mientrasque conespecificos
que forrajeabano viajaban
juntosa dosmetrosde distanciael unodel otrotemblarona raz6npromediode 28.6/min;
avesenvueltasen persecuciones,
a < 0.5 m de distanciauna de la otra, temblarona una
proporci6n
promedio
de 50/min. Individuos
solitarios
(n = 31) mostraron
una proporci6n
de temblarsignificativamente
menorque la de avessociales(dostembladores
presentes,
n
247
248]
J. K. Markowsky
etal.
J.Field
Ornithol.
Spring 1994
= 13, P < 0.001, Prueba U Mann-Whitney, U= 43). La mayor frecuenciade temblar, en
un contextosocial,apoyala hip6tesisde que el temblar es un signovisual intraespecifico.
Esta interpretaci6ntoma pesopot la ausenciade una nora de contactoen la especiesy por
datosque muestranque el ave siempretiembla inmediatamenteal posarseen una percha
nueva.
The Brown Trembler (Cinclocerthiaruficauda), one of the endemic
Mimidae of the LesserAntilles, has a conspicuous
and frequent habit of
tremblingits wings,the behaviorfor which it is named.Trembling consists
of a perchedbird droopingboth wings 1-2 cm, then quickly raisingthem
back to a "normal" positionseveraltimes in rapid succession.
Wings are
not raised from the body. Both males and females tremble, and each
"tremble" lasts about 1 s. Speculationsabout the function of trembling
behaviorhaveincluded:a camouflagemovementmaking the bird resemble
a bunchof dry leaves(Noble 1916), a "warning" (Noble 1916), a behavior
making the bird more conspicuous
(Bond 1928), and socialsignalwith
aggressivecontent(Zusi 1969). The purposeof this investigationwas to
assessthe function of trembling behavior in the Brown Trembler.
After 7 d of qualitativeobservations
of BrownTremblers,we quantified
and analyzedthe frequencyof trembling in varioussocialcontexts.We
observedBrown Tremblers and timed trembling in 44 observationperiods
ranging from 2 s to 6 min in duration. Observationsand measurements
occurredover 4 d in March 1991, at six study sites within 1.5 km of
Springfield Plantation (380 m elevation), Dominica, West Indies. Each
observationperiod was classifiedinto one of four categories:bird solitary
(no other Brown Trembler visibleto observer)and singing;bird solitary
and not singing(foraging,restingor travelling);two Brown Tremblers
foragingor travellingtogetherwithin 2 m of eachother;and two Brown
Tremblersin a rapid chase,< 0.5 m apart. Mean frequencies
of trembling
were as follows:solitary, singingBrown Tremblers stoppedsingingand
trembled at a mean rate of 1.74/min (eight trembles,four observation
periods). Solitary Brown Tremblers that were foraging trembled at a
mean rate of 6.8/min (115 trembles, 27 observationperiods). Brown
Tremblers foraging or travelling within 2 m of a conspecifictrembled at
a meanrate of 28.6/min (52 trembles,11 observationperiods),and Brown
Tremblers involvedin a rapid chase,<0.5 m apart, trembledat a mean
rate of 50.0/min (11 trembles,two observationperiods).Thesedifferences
amongcontextswere significant(P < 0.001, Kruskal-Wallis Analysisof
Variance, H = 64.3) if all contextsare usedin the analysis.Times between
successive
trembles,however,may not be independentwithin an observation period. If only mean trembling rates of socialvs. solitarybirds are
analyzed, solitary birds (n = 31) show a significantlyslower rate of
tremblingthan socialbirds (two Brown Tremblers present,n = 13, P <
0.001, Mann-Whitney U-test, U = 43).
In addition, trembling was strongly associatedwith changesof perch
in all contexts.Among 106 perch changesobserved,we notedtrembling
within 1 s of landing on a new perch 104 times, with only two instances
of a Brown Trembler landing on a new perch without trembling.
Vol.65,No.2
J•rozJJrt
Tremblers
[249
Theseresults,showinga fasterrate of tremblingassociated
with social
contexts,supportthe hypothesisthat trembling is usedfor intraspecific
communication(Zusi 1969). More specifically,we suggest
that trembling
may functionas a visualintra-specificcontactsignal,analogousto contact
notesvocalizedby many speciesthat foragein flocks.Our observations
of trembling after arrival on a new perch supportthis interpretation, as
doesthe absenceof a contactnote used by Brown Tremblers (Storer
1989).
Trembling appearsto differ both in form and function from wingflashingobservedin other Mimidae, in which wings are raisedup from
the body, and in which the wing-flashingbehavioris associatedwith
foraging behavior (Hailman 1960) or with the presenceof predators
(Horwich 1965, Michael 1970). Trembling occurredduringforagingand
non-foragingactivity;significantvariation in its rate of occurrencewas
related to socialcontext,not to foragingor to the presenceof predators.
ACKNOWLEDGMENTS
We thank Edward H. Burtt, Jr. and an anonymousreviewer for their helpful comments
and references,and the ArchboldTropical ResearchCenter for the useof their facilitiesat
the SpringfieldField Station,Dominica.Maine Agricultural ExperimentStationPublication
No.
1748.
LITERATURE
CITED
BOND,J. 1928. On the birdsof Dominica,St. Lucia, St. Vincent, and Barbados,B.W.I.
Proc. Acad. Nat. Sci. Philadelphia 80:523-545.
HAILMAN,J.P.
1960. Field studyof Mockingbird'swing-flashingbehaviorand its as-
sociationwith foraging. Wilson Bull. 72:346-357.
HORWmH,R. H. 1965. An ontogenyof wing-flashingin the Mockingbird with reference
to other behaviors.
MICH^t•L, E. D.
Wilson
Bull.
77:264-281.
1970. Wing flashingin a Brown Thrasher and Catbird. Wilson Bull.
82:330-331.
NOBLE,G.K.
1916. The residentbirds of Guadaloupe. Bull. Mus. Comp. Zool. 60:359-
396.
STORER,R. W. 1989. Geographicvariation and sexual dimorphismin the Tremblers
(Cinclocerthia)and White-Breasted Thrasher (Ramphocmclus).
Auk 106:249-258.
ZusI, R. L. 1969. Ecologyand adaptationsof the Trembler on the island of Dominica.
Living Bird 8:137-164.
Received11 May 1993; accepted1 Oct. 1993.