The Mascarene White-Eye Zosterops Borbonica
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INTRA-ISLAND VARIATION IN THE MASCARENE WHITE-EYE ZOSTEROPS BORBONICA FRANK B. GILL ORNITHOLOGICAL MONOGRAPHS NO. 12 PUBLISHED BY THE AMERICAN ORNITHOLOGISTS' UNION INTRA-ISLAND VARIATION IN THE MASCARENE WHITE-EYE ZOSTEROPS BORBONICA BY FRANK B. GILL ORNITHOLOGICAL MONOGRAPHS NO. 12 PUBLISHED BY THE AMERICAN ORNITHOLOGISTS' UNION •,A.L k Color forms of Zosterops borbonica on Reunion Island (top to bottom): gray morph; highland brown-headed brown morph; lowland brown- headed brown morph; gray-headed brown morph. ORNITHOLOGICAL MONOGRAPHS This series,published by the American Ornithologists'Union, has been establishedfor major paperstoo long for inclusionin the Union's journal, The Auk. Publicationhas been made possiblethrough the generosityof Mrs. Carll Tucker and the Marcia Brady Tucker Foundation,Inc. Correspondenceconcerning manuscripts for publication in the series shouldbe addressedto the incomingEditor, Dr. John W. Hardy, Moore Laboratoryof Zoology,Occidental College, Los Angeles,California 90041. Copiesof OrnithologicalMonographs may be orderedfrom the Treasurer of the AOU, Burt L. Monroe, Jr., Box 23447, Anchorage,Kentucky 40223. (See price list on insideback cover.) OrnithologicalMonographs, No. 12, vi + 66 pp. Editor, Robert M. Mengel SpecialEditor for OrnithologicalMonographs No. 12, Marion Anne Jenkinson Issued May 4, 1973 Price $2.00 prepaid ($1.60 to AOU Members) Library of CongressCatalogue Card Number 73-78162 Printedby the Allen Press,Inc., Lawrence,Kansas 66044 CONTENTS ACKNOWLEDGMENTS .......................................................................................................... vi INTRODUCTION .................................................................................................................... 1 REUNION ISLAND ............................................................................................................. 1 FIELD STUDIES ........ •...................................................................................................... 7 BIOLOGY OF ZOSTEROPS BORBONICA ............................................................................... 8 COLOR V•U•L•T•ON IN ZOSr•ROeS sORSOWICA ............................................................. 12 Spec•en exm•ation ............................................................................................. 12 Basis of the color variation ........................................................................ 12 Back coloration ..................................................................................................... 14 Head colorationin brown morphs...................................................................... 21 Underart coloration ........................................................................................... 30 MtNS•L V•T•ON ................................................................................................... 39 S•c•en measurements........................................................................................... 39 Statistics ........................................................................................................... 40 •alysis .......................................................................................................... 41 DmCUSS•ON .................................................................................................................... 44 V•iafion in cont•ental white-eyes.......................................................................... 44 T•onomy of Zosterops borbonica ......................................................................... 48 Relation of Zosterops borbonica mauritiana to •e Re•ion Isl•d forms .............................................................................. 49 Evolutiona• history of Zosterops borbonica ...................................................... 49 Ch•acter variation and adaptation ........................................................... 51 Character divergence • natural populations .................................................... 52 SUMM•Y ........................................................................................................................ 56 L•Ts• •so ......................................................................................................... 59 Arrs•g I ....................................................................................................................... 62 ArrsNDg II ................................................................................................................... 65 ACKNOWLEDGMENTS This studywas made possibleprimarily throughthe cooperationof the governmentand citizensof France,Department de la R•union. In particular, I am gratefulto V. Robin, Directeurdes Affairs G•n•rales,for handlingthe many administrativeformalities with maximum dispatch,and to Armand Barau, ConseilerG•n•ral de la R•union, Harry Gruchet, Conservateurdu Museum d'I-IistoireNaturelie de Saint Denis, and ChristianJouanin of the Museum National d'Histoire Naturelie in Paris, who were a sourceof advice, encouragement,and assistance.Several private landownerson Reunion Island, includingMessrs. A. Barau (Bols Rouge), G. Barau (Beaufonds, Ste.Marie), I. Boyer (La Possession),P. Lougnon(La PetiteFrance), J. de Villineuve (Grand Fond), and the Banque de la R6union,permitted me to studyand collectwhite-eyes on their properties.The frequentcooperation of the Servicedes Eaux et For•ts de la R•union, and in particular Messrs. Moulin, Miguet, and Soroquereis also gratefully acknowledged.Messrs. Malick and Laidet of the Service M•t•orologique de la R•nnion placed climatologicaldata from R6unionat my disposal,and M. ThereseanCadet generouslyprovided most of the plant identificationsmentioned in this study. In Ann Arbor, Michigan,the membersof my doctoralcommittee, Morris Foster,Robert W. Storer,Harrison B. Tordoff, and Warren H. Wagner,pro- vided many a stimulatingsuggestion. I am indebtedalso to R. B. Bartels of the Universityof MichiganComputing Center for making availablethose facilitiesand to William Lnnk who paintedthe color illustration. I have bene- fitted from discussionswith J. Alan Feduccia, Theodore H. Fleming, John P. Hubbard, William C. Preston,and Donald R. Tinkle. Acknowledgment and thanksare due especiallyto my wife, Frances,for her many contributions to this study. This studywas supportedby a National ScienceFoundation Predoctoral Fellowshipin 1967 and 1968 and a National ScienceFoundation Grant (GB 3366) to T. H. Hubbellat The Universityof Michiganfor researchin System- atic and EvolutionaryBiology. vi INTRODUCTION Geographicvariation reflects the geneticdivergence of conspecificpopula- tionsthrough natural selection;it therebyserves as primary evidenceof the evolutionaryprocess. Although every local populationdiffers from adjacent populationswith respectto the frequencyof somecharacteristics, conspicuous phenotypicdifferentiation on a microgeographicscale is evident primarily in terrestrialand relativelysedentary animal groups. Geographicvariation in highly mobile organismssuch as birds, however, tends to involve great distanceso.r else conspicuousisolation. The characteristicuniformity of most localbird populationsseems to reflectgenetic unity that is the combinedresult of gene flow between adjacentdemes and the resistanceof coadaptedgene poolsto fragmentationby disruptiveselection (Mayr, 1963:296, 361). Yet, the precisenessof avian adaptationto climaticgradients (James, 1970) and the surprisingrapidity of populationcharacter divergence in Passerdomesticus (Johnstonand Selander,1964) continue to emphasizethe overridingim- portanceof natural selectionas the primary determinantof patterns of geographicvariation. A birdpopulation on a smalloceanic island is characteristicallyhomogeneous in color and size. Althoughinter-island differentiation is common,intra-island variation is exceptional. Until the discoveryof four apparent races of a white-eye,Zosterops borbonica, on Reunion Island (Storer and Gill, 1966), Jamaicawas the smallestisland (4,540 squaremiles) known to have geo- graphicallyseparated bird populationssufficiently distinct to be considered subspecies.Reunion Island is only about1,000 squaremiles in area. No other islandwith an area of lessthan 10,000 squaremiles is known to have a poly- typic speciesin its avifauna. On the other hand, many larger islandshave differentiatedbird populations,the number of which tends to increasewith island size. But even on islandsas large as Madagascar(240,000 square miles) and Borneo (290,000 squaremiles), only one or two specieshave as many as four races. Thus island size seemsto impose some constraintson geographicvariation. This studyexamines the patternsof phenotypicvariation in Zosteropsborbonica on Reunion Island and considerstheir evolutionary origin and maintenance. REUNION ISLAND Reunion Island is one of the MascareneIslands, a group of three oceanic islandsof volcanicorigin in the westernIndian Ocean. Reunion is situated approximately500 mileseast of Madagascar(Figure 1) at latitude21ø Off S and longitude55 ø 30' E, is ellipticalin shape,39 mileslong by 29 mileswide, and encompassesabout 1,000 squaremiles. It is both the largestof the three MascareneIslands and the highest,reaching an elevationof 10,068 feet. 1 2 ORNITHOLOGICAL MONOGRAPHS NO. 12 Mauritius -20 $ R•union -25 $ INDIAN OCEAN FretroE 1. Location of the Mascarene islands of Reunion and Mauritius in the western Indian Ocean. The third Mascarene island, Rodriguez, is located 250 miles to the east. Reunion'sfaunal