Geographic Variation and Sexual Dimorphism in the Tremblers (Cinclocerthia) and White-Breasted Thrasher (Ramphocinclus)
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GEOGRAPHIC VARIATION AND SEXUAL DIMORPHISM IN THE TREMBLERS (CINCLOCERTHIA) AND WHITE-BREASTED THRASHER (RAMPHOCINCLUS) ROBERT W. STORER Museumof Zoologyand Departmentof Biology,University of Michigan,Ann Arbor,Michigan 48109 USA ABSTRACT.--Icompared interisland variation in color and in length of wing, tarsus,and bill of the LesserAntillean thrashers,Cinclocerthia and Ramphocinclus(Mimidae). Statistical analysesshow that someearly-taken specimens were missexed.I recognizetwo speciesof trembler,Cinclocerthia ruficauda (Brown Trembler) and C. gutturalis(Gray Trembler), and one of Ramphocinclus(White-breasted Thrasher). Brown Tremblersfrom Sabato Monserrat ("C. r. pavida")are not consideredseparable from those of Guadeloupe(C. r. tremula). I summarizeforaging methods and give possibleexplanations for the smaller amount of sexualdimorphism in bill length in Cinclocerthiaon islands where Ramphocinclusoccurs. Received6 June1988, accepted1 December1988. THE avifauna of the West Indies is rich and land of Antigua (Steadmanet al. 1984, Pregill well-known. Yet it has been underused as a et al. 1988). sourceof information for zoogeographicand The two forms of the White-breasted Thrash- evolutionarystudies. The LesserAntilles, which er have been consideredseparate species (Ridg- extend in an arc south from the Virgin Islands way 1907), but they are currently considered toward Trinidad, are home to several endemic conspecific(Hellmayr 1934, Davis and Miller genera,including the tremblers(Cinclocerthia) 1960). They inhabit the adjacentislands of Mar- and the White-breasted Thrasher (Ramphocin- tinique (Ramphocinclusbrachyurus brachyurus) and clusbrachyurus). St. Lucia(R. b.sanctaeluciae). Thus Ramphocinclus Tremblersare long-billed, reddish brown or is everywhere sympatricwith Cinclocerthiagut- brownish gray thrashersfound from the islands turaliswhereas C. ruficaudaoccurs alone both to of Saba and St. Eustatius south to St. Vincent. the north and to the south of the islands on At present, they are found primarily in wet, which Ramphocinclusand Cinclocerthiaare sym- upland forestsand less frequently in second pattic. growth (Bond 1936, American Ornithologists' Union 1983).In earlierclassifications (e.g. Ridg- way 1907),the grayishforms on Martinique and MATERIALS AND METHODS St. Luciawere both consideredseparate species. I noticed both long- and short-billed birds in most Current arrangements(Hellmayr 1934, Davis of the populationsof tremblers in the collectionsof and Miller 1960) classifyall the tremblersas a the AmericanMuseum of Natural History.This dif- single specieswith six subspecies.From north ference was at least in part correlatedwith sex; the to south,these subspecies are Cinclocerthiaruff- long-billed birds were usuallysexed as females,and caudapavida (Saba, St. Eustatius,St. Kitts, Nevis, the short-billed birds as males. Data from this and and Monserrat), C. r. tremula(Guadeloupe), C. other museumsshowed that there were apparentex- r. ruficauda(Dominica), C. r. gutturalis(Marti- ceptions,especially among the specimensfrom Domi- nique), C. r. macrorhyncha(St. Lucia), and C. r. nica whence seven short-billed birds were sexed as tenebrosa(St. Vincent). For reasons given below, femalesand four long-billed ones as males (nearly the grayishtremblers of Martinique and St. Lu- one-fourthof the early-taken,sexed specimens from that island). Other preliminary data suggestedthat cia are here considereda separatespecies, Cin- sexualdimorphism was reducedon Martinique and clocerthiagutturalis (the Gray Trembler), and the especiallySt. Lucia, where Ramphocinclusoccurs with reddish brown birds of the other islands are Cinclocerthia, so I examined and measured most avail- consideredto be C. ruficauda,(the BrownTrem- able specimensof thesegenera. bler). A fossil trembler is also known from de- Of the 391 specimensexamined, 359 adults(280 of posits between 2,500 and 4,500 ¾.B.P.on the is- Cinclocerthiaand 79 of Ramphocinclus)were usedin the 249 The Auk 106: 249-258. April 1989 250 ROBERTW.STORER [Auk,Vol. 106 analyses.This is not a largesample by modernstan- I believe the assumptionthat birds were missexed dards,but it isso much larger than the seriesavailable is valid.First, there is a decidedgap between the data to earlier reviewers(Ridgway [1907] reported on 61 for malesand femalesin Schwartz'ssample, and sec- specimens,and Hellmayr [1934],on 50) that, for the ond, 10 of the 14 presumablymissexed specimens first time,variation within severalof the populations weretaken by two collectors,A. Hyatt Verrill (7) and can be treatedstatistically. SelwynBranch (3). (Two birds taken in 1905,presum- Measurementstaken included wing length (chord), ablyby Verrill,came to the Universityof Michigan tail length, tarsuslength, length of bill from the an- Museumof Zoologyby way of thecollection of Henry terior borderof the nostril,and culmenfrom the junc- K. Coale,who removedthe original labelsand sub- tion of the bill with the skull. Of these,wing length stitutedhis own. It is possiblethat Coalemay have and length of bill from nostrilproved the mostuseful; switchedthe labels,assuming that the longer-billed tail lengthwas so greatly affected by wearthat many bird wasthe male.)On the other hand,Bond's (1952) measurementsof it wereuseless; and bill lengthfrom questioningor discreditingmany of Verrill's bird rec- skull proveddifficult to duplicate.The ratioof wing ordsfrom Dominicasuggests that he wasnot a careful length to length of bill from nostril was calculated worker. for each bird. Thepreliminary results were tested further by ob- BeforeI attemptedan analysis of thevariation among tainingdiscriminant functions for wing length, tarsus the populations,it was necessaryto establishthe ex- length,and bill fromnostril for the Schwartzsample tent of sexualdimorphism in Cinclocerthia.Although andusing these to testthe remaining birds from Dom- Ridgway (1907) reported sexualdimorphism in bill inica.In all cases,the resultsfrom the preliminary length, I found long-billed birds sexedas malesand analysiswere confirmedby discriminant-function short-billedbirds as females. This raisedthe question analysis.A similar analysiswas performedon the of whether these birds were missexed or whether birds from Guadeloupe.The resultsindicated that a individual variation in this genusis unusuallylarge. single female (MCZ 66485) was missexed.However, Missexedbirds are probablymore numerous in col- the bill lengthwas within the rangeof femalesand lectionsthan is generally realized. They can be de- outsidethe rangeof malesfor thatpopulation, where- tectedmost easily in specieswith little or no overlap asthe measurementsfor wing and tarsuswere within in one or more measurements(e.g. grebes;Storer and the rangeof bothsexes. Because dimorphism is great- Getty 1985, Storer 1987). Becausethe males of most est in bill length and becausethere were no other bird specieshave longer bills than the females,col- individualsshowing overlap, this bird was consid- lectorsprobably missexmore individuals of species ered correctlysexed as a female. like the tremblers in which the reverse is true. I alsotested the sexingof tremblersfrom St. Lucia I usedthe largestsample, 99 adult specimensfrom by using discriminant functions. A single bird Dominica,for preliminarytests. Thirty-six birds from (BM[NH] 94.1.2.242,collected in February1893) sexed that island collectedby Albert Schwartzin 1961and asa femalewas classifiedas a male by discriminant- 1962 yielded the following data: bill from nostril-- function analysisand was, therefore, consideredmis- 24 males, 20.1-22.3 mm; 10 females, 23.7-27.5 mm; sexed. The sexes of other birds I assumed to have been wing length/bill from nostril--23 males,4.16-4.86; 9 missexed or that I sexed on the basis of measurements females, 3.25-4.01. were verified by this test. Assuming that these birds were correctly sexed,I A preliminary discriminant-functiontest for the comparedthese data with the measurementsof all the Dominica sampleon birds from St. Vincent showed earlier-takenbirds. The bill lengthsof four olderspec- that two Schwartzbirds from the latterisland might imens (22.4, 22.4, 22.6, 23.3 ram) fell above those for have been missexed. Discriminant functions calcu- males and below those for females in the Schwartz lated from the St. Vincent birds not collectedby series. Of these, all but the third were well within Schwartzwere usedto checkthe Schwartzsample, the range of wing/bill ratios (4.32, 4.35, 4.12, 4.23, and the likelihoodof missexingwas confirmed. The respectively) for males in the Schwartz series. All bill measurementsof these two birds, sexedas males, exceptthe first bird (which was unsexed)were sexed fell within the range of females.These birds were as malesby the collectors.The third bird is nearerto taken in February,when gonadsare small and the Schwartz'smales than to hisfemales in bothbill length chanceof missexingmost likely, so they were con- and wing/bill ratio. Therefore, I consideredall four sideredmissexed. The sexesassigned to birdsunsexed to be males. Four birds sexed as males were within by the collectorsand to birds believed to have been the range of femalesin both charactersand nine "fe- missexedare listed in Appendix 1. males" fell within the range of males.A tenth bird After sexingwas checkedand corrected,I calculat- sexed as a female had a bill for which bill-from-nostril ed means and standard deviations for each sex of the measurement could not be taken but had a total bill populationof trembler on each island. Descriptive length