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 (Cinclocerthzaruficauda), one of the endemicMimidae of the LesserAntilles, has a conspicuousand frequent habit of tremblingits wings,the behavior for which it is named.Trembling consistsof a perchedbird droopingboth wings 1-2 cm, then quicklyraising them back to a normalposition several times in rapid succession.Each "tremble"takes approximately 1 s. In this study,Brown Tremblerswere observedand frequencyof tremblingwas measuredin March 1991, on Dominica.The rate of trembling was significantlydifferent in differentsocial contexts. Solitary, singing Brown Tremblers stoppedsinging and trembledat a meanrate of 1.74/min. SolitaryBrown Tremblersthat were foragingtrembled at a meanrate of 6.8/min. Brown Tremblersforaging or traveling within 2 m of a conspecifictrembled at a mean rate of 28.6/min, and Brown Tremblers involvedin a rapid chase,<0.5 m apart, trembledat a mean rate of 50.0/min. Solitary Brown Tremblers (n = 31) show a significantlyslower rate of tremblingthan socialbirds (two Brown Tremblers present,n = 13, P < 0.001, Mann-Whitney U-test, U = 43). The higherfrequency of tremblingin socialcontexts supports the hypothesisthat tremblingis a visualintra-specific signal. This interpretationis reinforcedby data showingthat Brown Tremblersalmost always trembled immediately upon arriving at a new perch,and by the absenceof a contactnote used by Brown Tremblers.

LPORQU• TIEMBLACINCLOCERTHIA RUFICAUDA? Sinopsis.--Eltemblador pardo Cinclocerthia ruficauda), uno de los Mimidaeend(•micos de las Antillas Menores, tiene el h/tbito de temblar de forma conspicuay frecuentesus alas, conductaa la cual debe su nombre. E1 temblar consisteen bajar ambasalas 1-2 cm, para luegoelevarlas sfibitamente a una posici6nnormal, conducta que se repite r/tpidamente en varias ocasiones.Cada episodiode "temblar" roma aproximadamenteun segundo.Este estudio,que se 11ev6a cabodurante el mes de marzo de 1991 en Dominica, tuvo como objetivoestudiar la frecuenciade temblaren estasaves. La tasade temblarresult6 signifi- cativamentediferente en diversoscontextos sociales. Individuos solitariosque cantaban dejaronde cantary temblarona raz6n de 1.74/minuto.Aves solitarias que forrajeaban temblarona una raz6n de 6.8/min. Mientras que conespecificosque forrajeabano viajaban juntosa dosmetros de distanciael uno del otrotemblaron a raz6npromedio de 28.6/min; avesenvueltas en persecuciones,a < 0.5 m de distanciauna de la otra, temblarona una proporci6npromedio de 50/min. Individuossolitarios (n = 31) mostraronuna proporci6n de temblarsignificativamente menor que la de avessociales (dos tembladores presentes, n

247 248] J. K. Markowskyetal. J.Field Ornithol. Spring 1994

= 13, P < 0.001, Prueba U Mann-Whitney, U= 43). La mayor frecuenciade temblar, en un contextosocial, apoya la 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 conspicuousand frequent habit of trembling its wings,the behaviorfor which it is named.Trembling consists of a perchedbird droopingboth wings 1-2 cm, then quickly raising them back to a "normal" positionseveral times 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 behaviorhave included: a camouflagemovement making the bird resemble a bunchof dry leaves(Noble 1916), a "warning" (Noble 1916), a behavior making the bird more conspicuous(Bond 1928), and socialsignal with aggressivecontent (Zusi 1969). The purposeof this investigationwas to assessthe function of trembling behavior in the Brown Trembler. After 7 d of qualitativeobservations of Brown Tremblers,we quantified and analyzed the frequencyof trembling in various socialcontexts. 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, resting or travelling);two Brown Tremblers foragingor travellingtogether within 2 m of eachother; and two Brown Tremblersin a rapid chase,< 0.5 m apart. Mean frequenciesof trembling were as follows:solitary, singingBrown Tremblers stoppedsinging and 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). These differences amongcontexts were significant(P < 0.001, Kruskal-Wallis Analysisof Variance, H = 64.3) if all contextsare usedin the analysis.Times between successivetrembles, however, may not be independentwithin an observation period. If only mean trembling rates of socialvs. solitary birds are analyzed, solitary birds (n = 31) show a significantly slower rate of trembling than 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 noted trembling 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•rozJJrtTremblers [249

These results,showing a fasterrate of tremblingassociated with social contexts,support the hypothesisthat trembling is usedfor intraspecific communication(Zusi 1969). More specifically,we suggestthat trembling may functionas a visual intra-specificcontact signal, analogous to contact notesvocalized by many speciesthat foragein flocks.Our observations of trembling after arrival on a new perch support this interpretation, as does the absenceof a contact note used by Brown Tremblers (Storer 1989). Trembling appearsto differ both in form and function from wing- flashingobserved in other Mimidae, in which wings are raisedup from the body, and in which the wing-flashingbehavior is associatedwith foraging behavior (Hailman 1960) or with the presenceof predators (Horwich 1965, Michael 1970). Trembling occurredduring foragingand non-foragingactivity; significantvariation in its rate of occurrencewas related to socialcontext, not to foraging or 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 ExperimentStation Publication No. 1748.

LITERATURE CITED

BOND,J. 1928. On the birds of 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-flashing behavior and its as- sociationwith foraging. Wilson Bull. 72:346-357. HORWmH,R. H. 1965. An ontogenyof wing-flashingin the Mockingbird with reference to other behaviors. Wilson Bull. 77:264-281. MICH^t•L, E. D. 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. Received 11 May 1993; accepted1 Oct. 1993.