The Auk A Quarterly Journalof Ornithology Vol. 115 No. 2 April 1998

The Auk 115(2):281-283, 1998

OVERVIEWS

HYBRIDIZATION IN

FRANK B. GILL • NationalAudubon Society, 700 Broadway,New •brk, New York10003, USA

HYBRID.The word itself immediatelyevokes hybrid increasesthe scorefor intragenerichy- the powerful conceptsof novelty,strength, and brids,but it addsto the pile of evidenceof odd sterility. Hybrids symbolize both inferiority hybrid viability. Parkes'Paradox points to is- and superiority. Hybrids violate racial purity sues beyond wood warblers,specifically the and clan identity. Hybrids challengebreeders, widespreadgenomic compatibility and poten- defy taxonomists,and fascinateornithologists. tial for hybridizationamong strikingly differ- Three papersin this issueof TheAuk feature ent birds (Grant and Grant 1992).Retained ge- hybrid birds--testimony to their continued nomiccompatibility permits successful hybrid- centralrole in modernornithology. Novel phe- izationbetween unlikely parents. The ideology notypes,for example,instantly capture our at- thathybridizing must be closelyrelated tention. Recall Audubon's Cincinnati Warbler is dead. (Helminthophagacincinnatiensis), only recently Why, then, don't birds hybridize more often diagnosedto be a hybrid betweenKentucky than they apparentlydo? Why don't we find Warbler (Oporornisformosus) and Blue-winged "wonder warblers" more often, if intergeneric Warbler (Vermivorapinus; Graves 1988). Recall hybridsare viable?The answerlies in thepow- alsoSutton's Warbler (Dendroica potomac), a tan- ers of avian socialrecognition. Early imprint- talizing hybrid betweenNorthern Parulas(Pa- ing startsa processof socialpreferences that rula americana)and advancingYellow-throated rigorously segregatescoexisting individuals Warblers (Dendroicadominica; Dunn and Gar- into distinct cultural clusters.Assortative pair- rett 1997).Now, TheAuk reportsa new "wonder ing follows,with few mistakes.Projected to its warbler," which SteveLatta and JosephWun- full conclusion,this line of thinking suggests derle found wintering on Puerto Rico. Ken that speciationin birds is as much a cultural Parkesdiagnosed it to be the first known Mag- phenomenonas it is a geneticphenomenon. nolia Warbler (Dendroicamagnolia) x "Myrtle" Beyondnovelties lies continuedinterest in Warbler (D.c. coronata)hybrid (Latta et al. hybridzones. The biologicalspecies concept fo- 1998). cusedattention appropriately on hybrid zones This latestdiscovery adds to the list of odd to evaluate whether certain taxa interbred free- wood-warblerhybrids, the sourceof Parkes' ly or assortatively.Now, hybrid zonesinvite Paradox:"Why are wood warblersin different probesof increasingsophistication into the dy- generamore prone to occasionalhybridization namicsof secondarycontact between divergent than are wood warblers classified in the same taxa. ?"(Parkes 1978). The Magnolia x Myrtle Field study of hybridizationbetween Lazuli Buntings (Passerinaamoena) and invading In- digoBuntings (P. cyanea) has lagged behind in- E-mail: [email protected] terest in flickers and other well-known Great

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Plains hybrid zones.In this issueof TheAuk, winged Warblers (Vermivorachrysoptera) after Myron Bakerand MichaelJohnson (1998) start invasionand hybridizationwith Blue-winged to catchup. They report that hybridizingmale Warblers (V pinus) seemsto be driven by fe- Lazuli Buntings and Indigo Buntings show male Blue-wings with their marker mtDNA greater distinction in plumage color than in (Gill 1997),rather than by socialdominance of size characteristics,suggesting differential in- territorial males (Confer and Larkin 1998). trogressionof characters.Data on geneticintro- The papers on hybrids in this issueof The gressionin buntingsremain limited; Bakerand Aukprovide testimony to the progressof orni- Johnson'ssurvey of allozymesproved essen- thology.Contrary to Rohweret al.'s provoca- tially noninformative.Yet, their results lend tive first line, viz. "The predominanceof thebi- modestsupport to previousstudies of North- ologicalspecies concept trivialized the studyof ern Flickers(Colaptes auratus), which alsoshow hybrid zonesduring muchof the lasthalf-cen- steeperclines in plumage charactersthan in tury," we bear witnesshere to maturingorni- othercharacters. Bill Moorehypothesized that thologicalstudies of hybridsthat mirror ma- social preferencesfor parental phenotypes turing theoryand technology.This progressis steepenedthe characterclines and limited the timely. Ongoing transformations of natural width of the hybrid zone (Moore 1995). landscapesguarantee more casesof population The lead contribution in this issue of The Auk expansion,confrontation, and transienthybrid- is an analysisof newly discoveredzones of hy- ization3all of which provide opportunitiesfor bridization between Hermit Warblers (Dendroi- new insights. ca occidentalis)and Townsend's Warblers (D. Thehistory of Blue-wingedWarblers, Golden- townsendi)in the PacificNorthwest. This article wingedWarblers, and theirhybrids offers a con- by Sievert Rohwer and ChristopherWood cludingmetaphor of the constructivelychang- (1998) offerslessons of ornithologicalhistory. ing ornithologicallandscape. The descriptionin Two factors contributed to the belated discov- 1874 of the first Brewster'sWarbler (Helmintho- ery. First was new accessibility to key sites phagaleucobronchialis), and alsoof the first Law- openedby loggingroads that violate remnant rence'sWarbler (Helminthophaga lawrencei), pro- old-growthforests. Second was the disguising vokeda 40-yeardebate whether these "wonder of hybridsby the geneticdominance of theface warblers"were distinctspecies, color morphs, pattern of the Hermit Warbler.Detailed and hybrids,or agevariants. Then followed an eraof creative analysesof plumage differencesand inquiry promptedby interestin the biological age changeswere requiredto separatethe ef- speciesconcept, inquiries that revealedlimited fectsof hybridizationfrom intraspecificvaria- interbreedingwith localextinction and replace- tions. The dramatic differences in facial color ment. The processof replacementdominates pattern controlledby simple Mendelian genetic currentresearch. Now we probethe dynamics of traits reinforce the view that the distinctions of ecologicalcompetition, genetic invasion, and cy- plumageby whichornithologists tend to define tonucleardisequilibria to gain insightsinto ge- speciesmay reflectonly minor geneticchanges. neticand socialarchitectures. Only the studyof Parkes'Paradox echoes again backstage. hybridzones affords these research opportuni- More than discoveryof new hybrid zones, ties (Harrison 1993). the confrontation between Hermit and Town- LITERATURE CITED send'swarblers strongly heightens our grow- ing awarenessof dynamic, transient hybrid BAKER,M. C., ANDM. S. JOHNSON.1998. Allozymic zonesthat may drive the extinctionof onespe- and morphometriccomparisons among Indigo ciesand replacementby a competitivesuperior and Lazuli buntingsand their hybrids.Auk 115: (Rhymer and Simberloff 1996). These Pacific 537-542. Northwesthybrid zonesare narrow,apparent- CONFER,J. L., AND J. L. LARKIN. 1998. Behavioral in- ly becauseof (social?)selection against hy- teractionsbetween Golden-wingedand Blue- brids. These hybrid zones also are moving winged warblers.Auk 115:209-214. DUNN, J. L., ANDK. L. GARRETT.1997. A field guide southward, apparently because territorial to the warblers of North America. Houghton Townsend's Warblers are dominant and com- Mifflin, New York. petitively superiorto Hermit Warblers.In con- G•LL,E B. 1997. Local cytonuclearextinction of the trast,the predictablelocal extinction of Golden- Golden-wingedWarbler. Evolution 51:519-525. April 1998] Overviews 283

GRANT,P. R., ANDB. R. GRANT.1992. Hybridization Pooleand F. Gill, Eds.).Academy of Natural Sci- of species.Science 256:193-197. ences,Philadelphia, and American Ornitholo- GRAVES,G. R. 1988. Evaluationof Vermivorax Opo- gists'Union, Washington,D.C. rornis hybrid wood-warblers. Wilson Bulletin PARKES,K. C. 1978. Still another parulid interge- 100:285-289. neric hybrid (Mniotilta x Dendroica)and its tax- HARRISON,R. G. (Ed.). 1993. Hybrid zones and the onomic and evolutionaryimplications. Auk 95: evolutionaryprocess. Oxford University Press, 682-690. New York. RHYMER, J. M., AND D. SIMBERLOFF.1996. Extinction LATTA, S.C., K. C. PARKES,AND J. M. WUNDERLE,JR. by hybridizationand introgression.Annual Re- 1998. A new intragenericDendroica hybrid from view of Ecologyand Systematics27:83-109. Hispaniola. Auk 115:533-537. ROHWER,S., AND C. WOOD. 1998. Three hybrid MOORE,W. S. 1995. (Colaptesaura- zones between Hermit and Townsend's warblers tus).In The birds of North America, no. 166 (A. in Washingtonand Oregon.Auk 115:284-310.