NOTES
INCOMPIE FIRST P!C MOLT IN THE WRENTff
MAUREEN E. FLANNERY and THOMAS GARDALI, PointReyes Bird Observatory, 4990 ShorelineHighway, Stinson Beach, California 94970
Flight-feathermolt in most passerinesbegins with the innermostprimary and proceedsdistally, while the replacementof secondariesbegins at the outermost featherand proceeds proximally (Ginn and Melville 1983, Pyle1997a, 1998). During their first year, most passerinesdo not replaceany flight feathers(i.e., the molt is partial),while a few replacesome (i.e., the molt is incomplete)or all remiges(i.e., the molt is complete)(Jenni and Winkler 1994, Pyle 1997a). In a few species,incomplete first-year molts follow different patterns, which involve a sequenceof replacementsimilar to that foundduring complete molts but beginat differentpoints along the wing (Jenniand Winkler 1994). Such incompleteremex molt, termed "eccentric,"has been documentedfor some North Americanpasse- rines,including the LoggerheadShrike (Lanius ludovicianu$),Yellow-breasted Chat (Icteria viren$),and certain tyrant flycatchers (Miller 1928, Phillips1974, Thompson and Leu 1994, Pyle 1998). BetweenNovember 1999 and March 2000 PointReyes Bird Observatory(PRBO) biologistsdocumented several examples of incompletemolt of flightfeathers on first- yearWrentits (Chamaea fa$ciata) captured at three mist-nettingsites operated as part of PRBO'slandbird-monitoring program. The studysites were locatedapproximately 28 km northwestof SanFrancisco, within and just outside of the PointReyes National Seashore,Marin County, California.These are the first recordedobservations of incompletefirst prebasic molt in the Wrentit,which was previously considered to have a completefirst prebasic molt (Pyle1997a, b). We examined101 studyskins of Wrentitsat the CaliforniaAcademy of Sciences (CAS) to determinewhether or not the mist-nettedbirds were anomalous.To ensure that molt had been completedand feather wear was minimal, we examinedonly specimenscollected between September and February. We examinedboth wings and notedthe locationof retainedfeathers. The age of the bird, if known,was recorded fromthe tag data.Because of the difficultyof viewingthe closedwings of studyskins, both of us examinedthe sameskins independently to ensureaccurate recording of presenceor absenceof retainedfeathers. On the 23 mist-nettedWrentits, we examinedthe primariesand secondariesof eachwing carefully and noted the locationand symmetry of retainedfeathers between wings. We determinedthe age of each live bird either by the degree of skull pneumatization(Pyle 1997a) or, if the individualhad been bandedpreviously, we referencedPRBO capturedata. The differencein flight-featherage was obviouson livebirds in the hand,retained feathers being noticeably worn andlight brown, newly replacedfeathers fresh and dark brown. Of the 16 first-year(hatched in 1999) Wrentitsexamined, nine (56 %) had retainedsome juvenal flight featherswhile the other 44% had undergonea completefirst prebasicmolt. All of the older birds (hatchedprior to 1999) had undergonea completemolt as expected,with all flight feathershaving been replaced (n: 7). Of the 101 museumspecimens examined, 11 showedincomplete wing molt. The collectorsidentified only three of the 101 studyspecimens to ageand none of the 11 with incompletemolt was aged. The locationsof specimenswith incompletemolt includedMarin, San Mateo, Alameda,Placer, and LosAngeles counties. On both live birdsand studyskins the patternsof replacementvaried. The most commonpattern (10 of 20 birds)was the bird'sreplacing all primariesand retaining onlya few secondaries,usually secondaries 1-3. Anotherpattern seen in threeof 20
Western Birds 31:249-251, 2000 249 NOTES birdsincluded retention of primaries1-2 and secondaries1-4. We observeda few examplesof asymmetricalreplacement, with somefeathers retained on onewing and replacedon the other.This wasnot unexpected,as eccentric sequences of molt were asymmetricalin 60% of casesreported by Jenni and Winkler (1994). In the life historyof a bird,molt is a fundamentalhigh-energy expenditure not fully understoodby biologists.There are numeroustheories that may explain eccentric molt observedin somebirds. One possibleexplanation may be poor environmental conditions.If weatherwere bad and/or foodscarce, an incompletemolt would enable the bird to conserveenergy by replacingonly the most criticalfeathers. In species whosefirst prebasicmolt includessome to all flight feathers,hatching date and its effect on the timing of molt may influencethe extent of feathersrenewed. Birds hatchedlate in the seasonexperience more of a time constrainton molt andthus may molt morequickly after fiedging, finish molting in a shortertime period,and replace fewer feathersthan those hatchedearlier in the season(Jenni and Winkler 1994). Also,in the Wrentit,which spends its entire life in densescrub or chaparral,enduring extensivefeather wear throughcontact with harshvegetation, incomplete molt may be a strategyto molt the minimalnumber of feathersnecessary to protect and preservethe retainedfeathers. Incomplete remex molt is alsoknown in suchdiverse North Americanspecies as the Verdin(Auripart•s fiaviceps), White-eyed Vireo (Vireo griseus),Yellow-breasted Chat, FieldSparrow ($pizella pusilia), and severalspecies of wrens,thrashers, sparrows, and orioles (George 1973, Willoughby1991, Thomp- son and Leu 1994, Pyle 1997b, 1998). Retainedfeathers are a helpfulcriterion used to age passefines in the hand. Most adult North Americanpasserines have a completeprebasic molt (Pyle 1997a), so incompletemolt indicatesa firstwearbird, at leastuntil the firstprealternate molt or secondprebasic molt (Mulvihill1993, Jenni and Winkler 1994, Pyle 1997b). The occurrenceof incompletemolt in the Wrentitmakes it possibleto agesome first-year birds after skull pneumatizationis complete. We caution that over time retained juvenalfeathers may be moredifficult to discernas renewed feathers age andbecome more worn and faded.We promotefurther study of incompletemolt and encourage museumcurators to considerspread-wing specimens to facilitatefuture studies. We thankDouglas J. Longand the CaliforniaAcademy of Sciencesfor permission to examinestudy specimens. We are gratefulto RyanDiGaudio, Steve N. G. Howell, Diana Humple, and Peter Pylefor commentson the manuscript.We are indebtedto GeoffreyR. Geupelfor his continuedcommitment to the mist-netprogram at the Palomafin FieldStation. We offer additionalgratitude to the manyfield biologists at PRBO for their careful examination of Wrentits. This is contribution 950 of the Point ReyesBird Observatory.
LITERATURE CITED
George,W. G. 1973. Molt of juvenileWhite-eyed Vireos. Wilson Bull. 85:327-300. Ginn, H. B., and Melville,D. S. 1983. Moult in Birds.Guide 19. Br. TrustOrnithol., Tring, England. Jenni, L., and Winkler, R. 1994. Moult and Ageing of European Passerines. AcademicPress, London. Miller,A. H. 1928. The moltsof the LoggerheadShrike Lanit•s It•doviciant•s.Univ. Calif. Publ. Zool. 30:393-417. Mulvihill,R. S. 1993. Usingwing molt to agepasserines. N. Am. BirdBander I8:1-10. Phillips,A. R. 1974. The firstprebasic molt of the Yellow-breastedChat. WilsonBull. 86:12-15.
25O NOTES
Pyle,P. 1997a. IdentificationGuide to NorthAmerican Passerines. Slate Creek Press, Bolinas, CA. Pyle, P. 1997b. Molt limits in North American passerines.N. Am. Bird Bander 22:49-90. Pyle, P. 1998. Eccentricfirst-year molt patternsin certaintyrannid flycatchers. W. Birds 29:29-35. Thompson,C. W., andLeu, M. 1994. Determininghomology of moltsand plumages to addressevolutionary questions: A rejoinderregarding emberizid finches. Condor 96: 769-782. Willoughby,E. J. 1991. Molt of the genus$pizella (Passeriformes,Emberizidae) in relationto ecologicalfactors affecting plumage wear. Proc. W. Found.Vert. Zool. 4:247-286.
Accepted 24 November 2000
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