HYBRIDIZATION IN GROSBEAKS (PHEUCTICUS) OF THE GREAT PLAINS D^VlD A. WEST IN bisexual,cross-fertilizing organisms a speciesmay be definedas a groupof populationsthat showsintrinsic reproductive isolation (actual or potential) from othersuch groups of populations(Mayr, 1942). Sym- patticpopulations are easilyaccommodated by this definition,but in al]o- pattic groupsthe existenceof potentialreproductive isolation is moredif- ficult to determine.It is widely believedthat spatialisolation is necessary for speciesformation (Mayr, 1942; Dobzhansky,1951: 205). Once a populationis dividedand geneflow between the isolatesceases, differential selection and the occurrence of mutation and recombination lead to diver- gence.This is classicaltheory of speciationand needsno further elabora- tion. A later rejunctionof thesepopulations wi]] providea naturaltest of the degreeof intrinsic reproductiveisolation that they have acquired. For this reasonsuch rejunctions, or secondarycontacts, are of interest to the student of evolution. There are two basicoutcomes of a secondarycontact: the populations are reproductivelyisolated, or they hybridize. Actually a continuousseries of possibilitiesexists. Sincethe evolutionof speciesis a gradualprocess, its interruptionat any giventime wi]] resultin somethingbetween hybridi- zationand lack of it. In addition,the ecologicaleompatabi]ity of the two populationswill modify this simplified dichotomy. Hybridizatibnis of commonoccurrence in secondarycontacts. Among birds the studiesof Meise (1936), Sib]ey (1950, 1954, and 1958), Dixon (1955), and Mayr and Gi]]iard (1952) may be mentioned.A distinction shouldbe made betweenthe occasionalhybridization between sympatric species(e.g., ducksof the genusAnas), in whichreduced success of the hybridspreserves the integrityof the species,and hybridizationbetween a]]opatricpopulations in recentsecondary contact. In thispaper the latter case is considered. An exampleof a secondarycontact with hybridizationis foundin two North Americangrosbeaks, the Rose-breastedGrosbeak, Pheucticus ludo- vicianus,and the Black-headedGrosbeak, P. melanocephalus.These birds, hereinafterreferred to as ludovicianusand melanocephalus,are the North Americanrepresentatives of a smallgenus that alsoincludes P. chrysopeplus and P. aureoventrisof Central and South America. They are more closely related than either is to the Latin American species. They occupy com- plementaryranges, ludovicianus east of the GreatPlains, melanocephalus to the west. They are primarilywoodland birds and do not breedwidely in the Plains. They do occur in the riparian woodlandsof, the Plains 399 The Auk, 79: 399-424. July, 1962 400 W•sT, Hybridization in Grosbeaks [ Auk / Vol. 79 rivers,however, and there they interbreed.It has rarely been doubted that the two formsare goodspecies, despite the evidenceof hybridization reportedby Swenk (1936). The similaritiesof nests,eggs, and habits have often beennoted, however, and it appearsthat the differencesin maleplumage have convincedtaxonomists of their specificstatus. This paperis a reevaluationof the statusof the two grosbeaks.Speci- mensfrom the GreatPlains will be described,and evidencefrom hybridiza- tion will be consideredin relationto the specificstatus of the two forms. The paper describesthe biologicalsituation, but a taxonomicjudgment will be made. The investigationis part of a studyof avian hybridizationin the Great Plainsinitiated by CharlesG. Sibleyunder grant G-1832 of the National ScienceFoundation. Other studiesin this project includethe following: Indigo and Lazuli buntings,Passerina (Sibley and Short, 1960); Rufous- sidedTowhees, Pipilo (Sibleyand West, 1958); Flickers,Colaptes (Short, 1959); orioles,Icterus galbulaand bullockii(Short, in prep.). This paper is modifiedfrom a thesispresented to the GraduateFaculty of Cornell Universityin partial fulfillmentof the degreeof Doctor of Philosophy. METI-IODS AND MATERIALS Between1955 and 1957 field parties from Cornell Universitycollected 308 specimensof Pheucticusin Nebraska,South Dakota, and Colorado. Material was borrowed from other museums for additional information. Most specimenswere males, and little informationwas gatheredon females. All specimenswere taken between mid-May and late July. Standard measurementsare in millimeters. The length of the black throatof adultmales from the chinto the posterioredge of the blackarea was alsomeasured. In specimenswith an indefiniteposterior margin the measurementextended to the point where black and the breast color were equallymixed. The lengthsof terminalwhite spotson rectriceswere also taken. Hybrid index. Two color charactersare sufficientlydifferent in the two grosbeaks to be used in a hybrid index scheme of the sort commonly employed (Anderson, 1949). The characters are: 1. Breast, belly, and underwing color: rose-red in ludovicianus,yellow in melano- cephalus. 2. Extent of brown in plumage: present on underparts,rump, hind neck, and other areas of melanocephalus,absent and usually replaced by white in ludovicianus. Five gradations of each character could be derived from specimens,using only males. These are scored as follows: Belly color "0" rose, as in ludovicianus "1" slightly orange or salmon-pink "2" orange Vol.Auk79 ] WrsT,Hybridization in Grosbeal•s 401 yellow with slight salmon tinge rich yellow, as in melanocephalus Brown extent no brown in areas noted, as ludovicianus trace of hind neck collar; tinge of brown on rump and underparts collar obvious; other areas about half of maximum only slightly lessthan melanocephalus,especially in collar and underparts "4" brown as in melanocephalus Thus a male specimenis indexed on a scale of "0" to "8" when the two characters are summed. When given separately (as "l"q-"3"), the first number refers to belly color. Intermediacyin underwingcolor of malesis expressedeither as a uniform color or as a mixture of pinker and yellowet feathers. Becauseof the individual variation in brown head striping of melanocephalus,that area is not included in the scoring of brown. Correlation o] hybrid index characters. Ideally each character of a hybrid index should be independent. That is, it should not be linked with the other characters.If singlegenes, or genecomplexes, are the bases of the characters,then independentassortment is desired,although dif- ferentialsurvival of somecombinations might mask it. Lackingknowledge of the geneticsof the characters,a correlationcoefficient between the two characterswill give some indication of the degreeof linkage. If, in a schemewith three characters,two are closelylinked, any summedindex will be biasedin favor of the two linked characters.Using 67 specimens of obviouslyhybrid origin,a correlationcoefficient of 0.484 was calculated for the two charactersof the hybrid index. The hypothesisof no correla- tion is rejectedat the 0.001 level. Despite the dose correlationthere are two reasonsfor retaining the two characters. There are many specimens that do not fall into the categoryof perfect correlation.It would be dif- ficult to compressall the variation of the hybrids into a more simple scheme. In addition, since only two charactersare used, there is no danger of introducinga bias into the summedindex. Male plumage. Comparisonsof male plumage are given in Table 1. Descriptions are modified from Ridgway (1901), and they refer to typical specimens. There is considerablevariation, however, and several points concerning such variation should be noted. 1. Several adult male ludovicianusin nuptial plumage have partly obscuredsupra- and postocular stripes and capital stripes. These are identical in placement to the brown marks in melanocephalusalthough lacking the brown pigment of the latter. 2. All adult male ludovlclanus examined have a concealed hind neck collar of sub- terminal white bars on the otherwise black feathers. Melanocephalus commonly has several black-tipped feathers in its brown collar. 3. Many adult male ludovicianus have partly concealed,white subterminal bars on the black interscapulars. Melanocephalus feathers are brown-barred on rather lanceolate interscapulars. Some individuals of ludovicianus have similar lanceolate interscapularsthough lacking the brown pigment. 4. The extent of black on the throat of ludovicianus is variable. Some individuals 402 WEST,Hybridization in Grosbeaks [ Auk [ Vol. 79 TABLE 1 PLUMAGE OF ADULT MALES Plumage area ludovicianus melanocephalus Throat black, sometimesmixed with black for about 5 mm, rest rose (note 4) brown Breast rose brown, some with partly con- cealedyellow Belly center streak rose, sides white brown with yellow center streak Undertail coverts white pale brown Head and neck black (note 1) black with brown collar (some have head stripes, note 2) Back black (note 3) black and brown striped Rump white, with some black tips brown, with some black tips Underwing coverts and axillars rose yellow Tail black, outer three pairs of black with outer two pairs of rectrices with white tips rectrices white-tipped (note 5) (note 5) have the black restrictedto a narrow band at the chin, as is typical of melanocephalus, while others have the black extending onto the breast. Data are given in Table 2. 5. The number of white-tipped rectricesvaries in each form. Typical melanocephalus has two pairs of rectriceswith large white tips, while ludovicianususually has three such pairs. Table 3 summarizesvariations among "pure" samples of each form. Female plumage. Ludovicianus females usually have salmon-yellow underwing feathers,while in melanocephalusthis