J. Field Ornithol., 68(2):183-199 VARIATION IN THE EXTENT OF THE FIRST PREBASIC WING MOLT OF DARK-EYED JUNCOS ROBERT S. MULXqHILL ()arnegieMuseum of Natural History Powdermill Nature Reserve HC 64, Box 453 Rector,Pennsylvania 15677-9605 USA RAY L. WINSTEAD Departmentof Biology Indiana Universityof Pennsylvania Indiana, Pennsylvania15705 USA Abstract.--We studied geographicaland sexualvariation in the extent of the first prebasic wing molt of Dark-eyedJuncos (Junto hyemalis)using data from migrant "Northern" juncos (J. h. hyemalis;n = 679) and residentjuncos from "Pennsylvania"and "Southern"Appala- chian populations(J. h. carolinensis;n = 150). In addition, we used skull pneumatization data to assesspossible effects of hatchingdate on molt extent and migration timing of North- ern juncos caught at Powdermill Nature Reservein southwesternPennsylvania. At a maxi- mum, immaturejuncos replacedthe lesser,median, greater secondaryand carpal coverts, proximal and middle feathersof the alula, and proximal three secondaries(i.e., tertials) during their first prebasicwing molt. There were no differencesin averagemolt extent be- tween the Pennsylvaniaand Southern samples,but juncos in both these samplesmolted significantlymore feathersthan Northern juncos. Modal pattern of wing molt in the North- ern sample (lesser,median, greater secondaryand carpal coverts) was approximatelythe minimum obselwedtbr the Pennsylvaniaand Southern samples.Within all three samples malesaccomplished slightly more molt than females,although the differencewas not sta- tisticallysignificant for the Southernsample. Juncos in the Northern samplecaught during the secondhalf of the springand fall migrationseasons had molted significantlyfewer feath- ers than juncos caughtduring the first half of thoseseasons. Because there waslittle or no relationshipbetween relative hatching date (estimatedfrom skull data) and molt extent or migration timing, we attribute the obsexxedintraseasonal variation to geographicalvaria- tion within the Northern sample.Within the latitudinal range representedby this sample, juncos from more northerly areaslikely molt lessand migrate through Powdermill later in both the springand fall comparedto juncosfrom farther south. VARIACIONES EN LA EXTENSION DE LA PRIMERA MUDA PREBfi.SICA DEL ALA EN JUNCO HYEMAHS Sinopsis.--Estudiamosvariaciones geogrfificas y sexualesen la extensi6nde la primera muda prebfisicadel ala en Junto hyemalisusando datos de los migrantes"nortefios" (J. h. hyemalis; n = 679) y de las poblacionesresidentes de "Pennsylvania"y del sur de los Apalaches(/. h. Carolinensis;n = 150). Ademfis,usamos datos de pneumatizaci6ncraneal para riotar posibles efectosde la fecha de eclosi6nen la extensi6nde la muda y del tiempo de la migraci6nde la poblaci6nmigrante tornados en la ReservaNatural de Powdermillen el suroestede Penn- sylvania.A lo mfiximo, los inroadufosreemplazaron las secundariasmenores, medias y su- perioresy las cubiertascarpales, plumas proximales y mediasde la alula, y las tres secunda- rias proximales(i.e., terciarias)durante su primera muda prebfisicadel ala. No hubo dife- renciasen la extensi6npromedio de la muda entre las muestrasde Pennsylvaniay del sur, pero hubo en ambaspoblaciones aves que mudaron significativamentemas plumasque las avesnortefias. E1 patr6n modal de la muda en la muestranortefia (secundariasmenor, me- dianay mayory cubiertascarpales) se aproxim6al mlnimoobsexxado en lasmuestras de Pennsylvaniay del sur. En las tres muestraslos machosmudaron ligeramente mils que las 183 184] R. S. Mulvihill and R. L. Winstead J.Field Ornithol. Spring 1997 hembras,aunque la diferenciano fu6 estadisticamentesignificativa para la muestradel sur. Avesde la poblaci6nnortefia capturadosdurante la segundamitad de las estacionesmigra- toriasde primaveray de otofiohab[an mudado significativamente menos plumas que aves capturadasdurante la primera mitad de estasestaciones. Debido a la poca o ninguna rela- ci6n entre la fecha de eclosi6n (estimadade datoscraneales) y la extensi6nde la muda o el tiempode la migraci6n,atribu•mos las variaciones intraestacionales a la variaci6ngrogr/t- fica dentro de la muestranortefia. En la extensi6nlatitudinal representadapot estamues- tra, avesde las poblacionesm/rs nortefias probablemente mudan menos y migran a trav6s de Powdermill m/rs tarde durante ambasmigraciones primaveral y otofial en comparaci6n con aves de areas m/rs al sur. The first prebasicmolt of most passerinesis incomplete (Dwight 1900, Ginn and Melville 1983,Jenni and Winkler 1994, Mulvihill 1993, Pyle et al. 1987, Stresemannand Stresemann1966, Svensson1992). Within spe- cies the extent of this molt frequently varies with sex, hatching date, and/or geographicalorigin. Although this variation has been well docu- mented for many European passerines(Jenni and Winkler 1994), detailed quantitativestudies of individual variation in the extent of this molt cur- rently are limited to a handful of North American species(Pyle 1995). This is unfortunate becauseknowledge of intraspecificvariation in the pattern and extent of the first prebasicmolt facilitatesrecognition of adult and immature age classes(Jenni and Winkler 1994, Mulvihill 1993, Pyle 1995), enabling researchersto account for an important variable in many studies of migration, population dynamics, reproductive and foraging ecology,and morphology (Mulvihill 1993). The purpose of our study was to describe individual variation in the first prebasicmolt of feathers in the upper alar (wing) tract of the Dark- eyedJunco (Juncohyemalis). Data were analyzed for possibledifferences in the extent of this molt related to sex, geographic origin, and hatching date ofjuncos in our sample. MATERIALS AND METHODS We recorded the pattern and extent of the first prebasicmolt of the major feathers (Fig. 1) of the upper alar tract of 829 immature Dark-eyed Juncos,mostly live birds (n = 785) trapped for banding, but also study skins in the collections of Carnegie Museum of Natural History (CMNH; n = 17) and U.S. National Museum (USNM; n = 27). The followingcom- prised our geographicsubsamples: (1) 679 individualsof the migrant northern subspecies,J. h. hyemalis,caught during the course of year- round banding operations from 1986-1988 at the field research station of CMNH (Powdermill Nature Reserve [PNR]) in the mountains of south- western Pennsylvania(for details of the PNR banding program, see Leb- ermanand Wood 1983); (2) 35 livebirds banded in 1989and 27 studyskins of southernJ. h. carolinensisfrom the mountains of Virginia, Tennessee, and North Carolina; and (3) 71 live birds banded from 1983-1989 and 17 studyskins from a breeding populationin the mountainsof southwest- ern Pennsylvania.Juncos from this population also are ascribableto caro- linensis (Mulvihill and Chandler 1991, Mulvihill 1992), which differs from Vol.68, No. 2 Molt in Juntos [185 FIGURE1. Major feathersand feather groupsof the upper alar tract of Dark-eyedJuncos: LC (lesser coverts); MC (median coverts);AC (alula covert); A1 (middle alula feather); A2 (distal alula feather); CC (carpal covert); GPC (greater primary coverts);GSC (greater secondarycoverts); Pl-P9 (primaries one through nine); S1-S9 (secondaries one through nine; S7-S9 are the "tertials"). nominate hyemalisin size, plumage color, and bill coloration (Miller 1941). These three groups were identified as Northern, Southern, and Pennsylvaniasamples, respectively. Both the Southern and Pennsylvania samplescame from populationsthat are largelysedentary (Hostetter 1961, Mulvihill and Chandler 1991, Rabenold and Rabenold 1985). One problem in conductinga studysuch as this is the difficultyof dis- tinguishing between retained juvenal and molted first basic feathers. These two feather generationsare similar in appearancein many small emberizids,includingjuncos (Mulvihill 1993). However,under goodlight- ing conditionsdifferences between these feather generationsin juncos are discernable,making possiblethe accurateidentification of immature birds (hatchingyear [HY] or secondyear [SY] birdsin bandingterminol- ogy) beyond the completion of skull pneumatization,until the latter stagesof the second (i.e., definitive) prebasicmolt. First basicfeathers were recognized (in approximate order of importance) by color, luster, wear, and size (seeMulvihill 1993). Retainedjuvenal featherswere always browner (with narrow tan edging), lessshiny, and noticeablymore worn than comparablemolted feathers,which were usuallylustrous dark gray, with broad light gray edges. In addition, juvenal feathers were often smallerthan their first basiccounterparts. They were particularlyeasy to distinguishwhen a given feather group (e.g., greater secondarycoverts, alula, or tertials) wascomposed of both juvenal and recentlygrown first basicfeathers. Rarely, adult juncos may retain somefeathers following 186] te. s. Mulvihill and R. L. Winstead j. FieldOrnithol. Spring 1997 their usuallycomplete prebasic molt (Yunick 1976). Four adult females caught during our studyshowed asymmetrical retention of one or more alular feathers, one or two greater primary coverts,and up to two second- aries. Such birds cannot be confused with immatures, however, because the retained, worn, brownishfeathers are isolatedin an otherwiseentirely fresh plumage. Juncosin our studywere sexedby the presencein breeding (SY) indi- vidualsof a brood patch (female) or cloacal protuberance (male), or by a combination of wing length and plumage characters (summarized in Kettersonand Nolan 1976). Birds that could not be sexedby these