RELATIONSHIPS BETWEEN BLACK-EARED AND PLAIN-EARED FORMS OF BUSHTITS (PSALTRIPARUS) RALPH J. RAITT REWEW of literature pertaining to relationshipsamong bushtits (Psal- triparus) of the southwesternUnited States and northern Mexico reveals a long-standingdivergence of opinionas to specieslimits. The point at issueis whether the populationscharacterized by black-facedmales, oc- curring in Mexico north to approximatelythe internationalboundary, are specificallydistinct from the northerly onesof the plumbeustype, which generallylack black. Oberholser(1903), in a synopticreview of Psaltriparus,allocated black- eared populationsto a distinct species,P. melanotis,and describedbriefly the basisof separatingspecimens of its northernrace, P. melanotislloydi, from the specimensof the partially sympatricplain-eared form, P. plumbeus (presentlyconsidered as a subspeciesof P. minimus). Other than presence or absenceof black colorationon the head, he mentionedonly the greater contrastbetween the colorsof head and back in young males of lloydi. Ridgway (1904: 429-431) admitted difficulty in separatingP.m. lloydi and P. plumbeusand found evidenceof intergradation,but he retained the speciesseparation between P. melanotis and P. plumbeus. Apparently he relied upon the opinion of Oberholser,"who has had excel- lent opportunitiesof studyingboth forms in life [and] assuresme that they not only both breed in the same localities in southwesternTexas, but that they eachhave distinctivepeculiarities of voice, etc." Swarth (1913, 1914), although aware of indicationsof conspecificity, appears to have been influencedby Ridgway's and, more particularly, Oberholser'sopinions and did not recommendnomenclatural merger. The prevalenceof this view of specific distinctivenesswas terminated in the early 1930's. In the fourth editionof the A.O.U. Check-list(1931), all forms of bushtits were listed as geographicalraces of P. minimus. Shortly thereafter, Hellmayr (1934: 88-92) used a similar treatment but admitted unfamiliarity with lloydi and pointed out the need for a "thorough investigation"of the relationship between this form and the plain-earedplumbeus and cecaumenorum. In the report of the first study of this problem which was.actually conductedin the zone of contact between black-earedand plain-eared forms,Van Tyne and Sutton (1937: 64-67) adducedevidence of species divergencebetween lloydi and plumbeus.Their interpretationwas based, among other things, on the occurrencein the Chisos Mountains and other rangesof west Texas of two distinct populations,partially separatedby altitudinal preferences. Thus, they treated the populations as members 503 The Auk, 8z•: 503-528. October, 1967 504 R^•r, Relationshipsof Bushtits [ Auk I_ Vol. 84 of two species,P. melanotisand P. minimus. However,they did call for additionalstudy of relationships. In the absenceof such additional study, most subsequenttreatments have followedVan Tyne and Suttonin retainingblack-eared and plain- earedgroups as separatespecies--notable examples are the fifth edition of the A.O.U. check-list (1957) and the check-list of the birds of Mexico (Miller et al., 1957). Agreementhas not been unanimous,how- ever; van Rossem(1945: 178-180), Blake (1953: 388), Phillips (1958), and Phillips e.t al. (1964: 111-112) are among those who have held contrary views. A number of recent authorshave emphasizedthe un- certainty as to relationships(e.g., Miller et al., 1957; Davis, 1959; and Eisenmann, 1955). In considerationof theseconflicting views and expressionsof uncertainty, it was obviousthat only a thoroughstudy, includingadditional field work and examinationof pertinentspecimens already available, could hope to solvethe questionof specieslimits and elucidaterelationships within this group of populations.It also appearedprobable that such a study would add to knowledgeof the processesof speciation.Accordingly, I beganan investigationof theseproblems in the spring of 1960. ACI<NOWI•EDGMENTS A large number of persons and institutions aided in this research in a variety of ways. Grateful acknowledgment is extended to all of them. Financial support was provided by National Science Foundation Research Grant G-19340 and by the Research Center of New Mexico State University. The U.S. Bureau of Sport Fisheries and Wildlife, the game departments of Arizona, New Mexico, and Texas, and Dr. Rodolfo Hernandez Corzo, Director General de la Fauna Silvestre of Mexico, gave permission for the necessary collecting. Study in Big Bend National Park was facilitated by many officials of the Park, especially by Stanley C. Joseph, Harold J. Broderick, Douglas B. Evans, and by Juan Aguilar. Mr. Carl L. Appel of Ozona, Texas, Mr. A. R. Eppenauer of Matra, Texas, St. Armando Varela of Agua Prieta, Sonora, and St. Carlos Castrej6n of Juan Mata Ortiz, Chihuahua, permitted me to work on their land. St. Rene Morales and, especially, Ing. Josg Frisby, both of Agua Prieta, Sonora, assistedme in Sonora. My colleagues in the Department of Biology, W. A. Dick-Peddle and James R. Zimmerman, and former students,Terence R. Best, George I. Child, Carl W. Hender- son, James D. Lane, Kenneth L. McWilliams, and David R. Moore, aided me in the field. Advice and assistancewere also given by Robert K. Selander, Allan R. Phillips, the late Alden H. Miller, Ned K. Johnson, and, especially,Joe T. Marshall. Soundspectrographs were made with facilitiesat the Moore Laboratory of Zoology, Occidental College, Los Angeles (Director, John W. Hardy) and at the Southwestern Research Station of the American Museum of Natural History, Portal, Arizona (Director, Vincent D. Roth). Specimenswere borrowed from the following collections: The American Museum of Natural History (through Dean Amadon), University of Arizona (Joe T. Marshall), 1967Oct. ] RA•TT,Relationships o!Bushtits 505 Carnegie Museum (Kenneth C. Parkes), Cornell University (Charles G. Sibley), Chicago Natural History Museum (E. R. Blake), Dickey Collection (Thomas R. Howell), Moore Laboratory of Zoology (John W. Hardy), Museum of Comparative Zoology (Raymond A. Paynter, Jr.), Museum of Natural History, University of Kansas (Richard F. Johnston), Museum of Vertebrate Zoology, University of Cali- fornia (Frank A. Pitelka), Museum of Zoology, Louisiana State University (George H. Lowery, Jr.), Museum of Zoology, University of Michigan, (Harrison B. Tordoff and John Hubbard), Peabody Museum, Yale University (S. Dillon Ripley and Mary A. Heimerdinger), San Diego Natural History Museum (Richard C. Banks), Texas Cooperative Wildlife Museum (William B. Davis), University of New Mexico (James S. Findley), University of Texas (Robert K. Selander), U.S. National Museum (Herbert G. Deignan and Phillip S. Humphrey), and the private collections of Allan R. Phillips, Warren M. Pulich, William J. Shefiler, and George M. Sutton. My gratitude is extended to these institutions and individuals. I am very grateful to Gene M. Christman for drawing the figure illustrating head coloration. Finally, a special debt is owed to my former student, Robert D. Ohmart, who participated in nearly all of the field work. His energy and ability were outstanding. METI-IOD.S Field work.--From the outset I assumed that field work should be concentratedin the critical "islands" of bushtit habitat in west Texas, especiallythe ChisosMountains, the area that Van Tyne and Suttonhad reported as a region of syruparty. Results of early work confirmed this assumption,and 12 trips were made there. Another critical region, em- bracing portions of Arizona, Chihuahua, New Mexico, and Sonora,was visited 11 times. Other field work was carried out both north and south of these areas of possiblesympatry or intergradationso that information on "typical" P. melanotisand P. minimuscould be obtained for compara- tive purposes. Field work generallyincluded collection of specimensand observations on ecologicalfactors and on behavioralphenomena such as socialgroupings, nest construction,feeding of young, territoriality, and vocalizations. The usualprocedure was to locatea socialgroup (suchas a flock,mated pair, or family), to documentthe behaviorof the birds, and then to at- tempt to collect the entire group. Collecting efforts yielded 427 specimens,357 from areas especially pertinent to this problem (see Figure 1). Tape recordingsof various types of vocalizationswere made in nine different localities,with a Nagra III H B tape recorderat 15 i.p.s. Sound spectrogramswere made on a Kay Electric Company Sonagraph. Specimenstudy.--Over 1,000 specimenswere examined,including those taken during the study and those in collections. Plumage coloration, especiallyof head, back, and flanks, was noted for each specimen.The bill and tarsus on all adult specimenswere measuredwith dial calipers. 506 •RAITT,Relationships of Bushtits [ Vol.Auk 84 Tail lengthand wing chord (from bendof wing to tip of longestprimary) were measuredon all specimenscollected in late autumn,winter, and early spring(by late April, rectricesand longerprimaries are very badly worn). SEX ANn AOE DETERMINATION AND MOLTS Sex and age groups vary in presenceand amount of black facial colorationand in other morphologicalcharacters that also vary between populationsof Psaltriparus. As pointed out by Miller (1955: 169), Phillips(1958), and Phillipset al. (1964: 111-112), failureto appreciate the nature of this normal intrapopulationvariation has contributedsig- nificantly to the confusionin understandingof interpopulation relation- ships. For this reason,I paid particular attention to sexual dimorphism and to