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Molt of the Gray Vireo ’ The Condor 102:610-618 0 The Cooper Ornithological Society 2000 MOLT OF THE GRAY VIREO ’ GARY VOELKER Barrick Museum of Natural History, Box 454012, Universityof Nevada Las Vegas, Las Vegas,NV 891.54, e-mail: [email protected] Abstract. Using museum specimens, I document the molt cycles and molting grounds of the Gray Vireo (Vireo vicinior). During prebasic molts, adult female Gray Vireos replace their primaries in 57 days, whereas adult males take 70 days; all body plumage is replaced during this molt. Prebasic molts occur almost exclusively on the breeding grounds;just 3 of 41 specimensreplacing primaries were collected away from breeding grounds.No molting specimens were collected from wintering areas. Prealternatemolt occurs on the wintering grounds,and appearslimited to the replacementof innermost secondariesand a limited molt of body plumage. By performing prebasicmolt on breeding grounds,the Gray Vireo differs from several other western breeding passerinespecies that use desert regions of the south- western United States and northwesternMexico to exploit late summer food resources.The areasof these southwesterndesert regions used by other speciesform a small portion of the breeding grounds, and encompassthe entire wintering grounds,of Gray Vireos. I hypothe- size that this contrast in molting regions is not due to differences in the general timing of prebasic molts among these species;rather, the contrastmay be due to constraintsimposed on Gray Vireos by a dietary shift to fruit during winter and the need to defend winter territories. Key words: Gray Vireo, molt, plumage, Vireo vicinior. INTRODUCTION era1 species, these desert regions do not form Molt is an integral part of the avian annual cy- part of either the breeding or wintering range. It cle, and recent studies have documented inter- has been argued that this stopover is performed esting life-history tradeoffs in the scheduling of in order to take advantage of food flushes pro- annual molts (Thompson 1991, Voelker and duced by late summer rains (Rohwer and Man- Rohwer 1998). In passerines, these tradeoffs ning 1990, Young 1991, Voelker and Rohwer generally involve the scheduling of fall molts 1998). relative to fall migration. For instance, many This molt-migration stopover raises an inter- eastern North American breeding species re- esting question with regard to food resourcepro- place flight feathers while still on their breeding ductivity on the breeding grounds versus non- grounds, perhapsbecause winter ranges are typ- breeding grounds: do birds that breed in the xe- ically much smaller than are breeding ranges, ric western/southwestern U.S., but that winter which could increase competition for the re- farther south, also take advantage of this late- sources necessary to support the high energetic summer increase in resources?If so, we should expect to find that molting birds are predomi- costs of molt (Rohwer 1971, Voelker and Roh- nately found in the molt-migration stopover re- wer 1998). gion, in specieswith disjunct breeding and win- In contrast to eastern birds, adults of many tering ranges that do not include this area. Or, western breeding species of North American we should find that molting birds are found pre- passerines begin fall migration prior to com- dominately in one portion of either the breeding mencing fall molts. This would allow these spe- or wintering range, specifically the molt-migra- cies to avoid the late-summer droughts typical tion area, for specieswhose breeding or winter- for much of western North America (Baldwin ing range includes this area. 1973), and an associatedlack of food resources. In this paper, I begin to addressthis question A taxonomically diverse set of these western by describing the molts and documenting the breeding species appear to stop during fall mi- molting grounds of the Gray Vireo (Vireo vici- gration to commence molt in the Sonoran and nior). This species breeds throughout arid re- Chihuahuan desert regions of the southwestern gions of the western/southwesternU.S., includ- United States and northwestern Mexico; in sev- ing part of the molt-migration area described for other species, and winters primarily along the ’ ’ Received9 July 1999. Accepted 28 February 2000. Gulf of California. 16101 GRAY VIREO MOLT 611 METHODS 1988), and I use it here. A more accuratemethod apparently exists for estimating molt duration SPECIMENS (Underhill and Zucchini 1988), but is not readily I examined 178 specimens from 28 museums available. Furthermore, it is unclear whether a (see Acknowledgments). I requested from each basic assumption of this method, that observa- museum any specimen collected between 1 July tions constitute a random sample from the rele- and 30 April, up to 10 adult specimens from vant population, could be appropriately applied May and up to 10 from June, and any juvenile to specimen-baseddata, or for that matter to any specimen from May and June. I followed Pyle data set where a “relevant population” can not (1997) by referring to birds as either Hatching be unambiguously defined. Year (HY, birds in their first calendar year) or I followed the molt terminology of Humphrey After Hatching Year (AHY, birds in at least their and Parkes (1959). Following Langston and second calendar year). Sex was establishedfrom Rohwer (1996), I define a molt series as a set of museum labels. flight feathers that are replaced by a single set of rules. Yuri and Rohwer (1997) should be con- SCORING MOLT sulted for a detailed explanation of the rules of I generally followed Rohwer (1986) to deter- feather replacement and the identification of mine whether molt was adventitious. I did, how- molt series. Briefly, to identify the feathers of a ever, score asymmetrical primary feather molt if series and the rules by which they are replaced, one to several contiguous primaries on one each growing (focal) feather is placed in one of wing, but not the other, had been dropped, so three categories based on its stage of replace- long as primary one was included in this set of ment and the stage of replacement of adjacent feathers. I did this because primary feather molt feathers. Nodal feathers are those feathers re- is often not synchronized between wings, and placed first in a molt series and these are always thus it is possible for one wing to start replacing more fully grown than adjacent feathers. Ter- feathers slightly ahead of the other wing. minal feathers are those feathers replaced last in In examining specimensfor molt, I used a 3X a molt series and these are always less fully magnifying lamp lighted with a 22-W fluores- grown than adjacent feathers. Other growing cent bulb, and a small forceps to lift feathers and feathers provide directional information, show- quantify molt. For body molt, I estimated the ing whether replacement proceedsproximally to percentage of feathers growing in each of five distally, or distally to proximally. In the simplest regions: chin and throat, breast, belly, head, and possible scenario for primary replacement, Pl back (definitions in Rohwer 1986). I used Roh- would be nodal (dropped/replaced first), subse- wer’s (1986) scale of 0, 10, 30, 50, 70, and 90% quent primaries would be dropped in order with of feathers in development. An overall body P2 longer than P3, P3 longer than P4, and so on molt score was calculated for each specimen by until PlO was reached. This sequencewould in- averaging the scores of the five body regions. dicate proximal to distal replacement of prima- For scoring flight feather molt, I generally fol- ries, and, because PlO was dropped last, would lowed Rohwer’s (1986) practice of estimating indicate that PlO was the terminal feather in the the fraction of full length (by 0.1 intervals) at- series. More complex scenarios would involve tained by each growing feather, but used N to multiple series in a single feather tract, and designate new feathers, which allowed a score would be evidenced by multiple nodal feathers, of zero to be used for missing feathers (Voelker multiple directional waves, and multiple termi- and Rohwer 1998). Gray Vireos have a total of nal feathers (Langston and Rohwer 1995). 20 primaries, 18 secondaries,and 12 rectrices. I scored both sides of each specimen for flight BREEDING AND WINTERING RANGES feather condition. Gray Vireos breed from southern California, All regression models for estimating molt du- southern Nevada, southern Utah, western and ration are flawed. However, Pimm’s (1976) re- southeastern Colorado, and northwestern and gression method to estimate average duration of central New Mexico south to northwestern Baja molt in individual birds is less flawed because California, central and southeastern Arizona, the method overcomes the problem of heteros- southern New Mexico, western Texas, and cedasticity in molt data (Underhill and Zucchini northwestern to central Coahuila (Phillips 1991, 612 GARYVOELKER AOU 1998). The winter range is central and southern Baja California, southwesternArizona, Sonora, the Big Bend region of western Texas (AOU 1998), and perhaps southwesternCoahui- la (Phillips 1991). In Sonora, the winter range is strongly associatedwith the distribution of ele- phant trees (Bursera microphylla; Bates 1992a) in coastal and lowland desert scrub areas (Rus- sell and Monson 1998). MOLT AND MIGRATION To determine where Gray Vireos molt and to determine when they molt in relation to fall mi- gration, I plotted the collecting localities of molting specimens on a map of southwestern North America. RESULTS FIRST PREBASIC MOLT Based on six HY birds that were undergoing first prebasic molt, this molt appearsto be limited to an intense, complete body molt and the replace- ment of at least some flight feather coverts. In juvenile plumage, wing bars on the greater sec- ondary coverts are brownish and relatively in- distinct; several specimens undergoing first pre- basic molt had some new greater coverts edged white as in AHY birds.
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