FEATURED PHOTO

IDENTIFICATION OF ADULT PACIFIC AND AMERICAN GOLDEN PLOVERS IN THEIR SOUTHBOUND MIGRATION

ALVARO JARAMILLO, San FranciscoBay Bird Observatory,P.O. Box 247, Alviso, California 95002

Connors (1983) and Connors et al. (1993) confirmed that American (Pluvialis dominica) and Pacific(P. fulva) Golden-Ploversbreeding in sympatryin Alaskaare distinctspecies, a conclusionadopted by the A.O.U. (1993). Distinguishingthese two speciesrequires that the bird'sage or plumageclass be determinedfirst beforethe speciescan be identified.At anystage the identificationrequires close study, but adults in molt are the mostdifficult. In both speciessome prebasic molt takesplace during fall migration.Because some field marks useful in juvenalor basicplumage, such as the colorof the supercilium,do not applyto moltingadults, many purported sightings of American Golden-Plovershave been questioned,and the distributionof adultsin fall migrationis uncertain.All or nearlyall AmericanGolden-Plovers occurring along the Pacificcoast south of Alaskain fall migrationare juveniles.In fact, theremay be no certainrecords of adultsin thisregion at thisseason (Paulson 1993). Becauseof uncertaintyover publishedrecords, the California Bird RecordsCommittee added the American Golden-Ploverto its review list (Cole and McCaskie 2004), seeking documentation for all American Golden-Plovers in California from 2004 onward. This note is an attempt to focusattention on this issuein field identification. The PacificGolden-Plover breeds largely in Siberiaeast to westernmostAlaska and wintersalong the coastsof Asia and Australia,on Pacificislands, and in small numbers in California. The American Golden-Plover breeds from Alaska east to BarfinIsland and wintersin southernSouth America. Althoughsome populationsof the Pacifichave migrationsas long as thoseof the American,some Pacific Golden- Ploversbreeding in Alaskaperform a comparativelyshort but nonstopmigration to the HawaiianIslands. American Golden-Plovers winter in the southernhemisphere, almostall in the pampasregion of easternArgentina. Any golden-ploverin winterin Californiais almostcertainly a PacificGolden-Plover. When golden-ploversare in full alternateplumage, the male Americanis distin- guishedby bulgingteardrop-shaped white patcheson the sidesof the breast,black flanks,and blackundertail coverts. At this stagethe male Pacifichas a white stripof more or lessuniform width extendingfrom the sidesof the breastthrough the flanks and partlywhite undertailcoverts. In both speciesfemales have a variablenumber of white feathersscattered among the blackones on the underparts,but they maintain a pattern similarto that of the males. While the birdsare in their breedingrange, the alternateplumage of both species beginsto changethrough slow molting during the incubationperiod, when a less conspicuousplumage is more advantageousthan during courtship.Jukema et al. (2003) proposedthat thismolt is distinctfrom the prebasicmolt, althoughit maybe part of a protractedprebasic molt. Once bodymolt beginsin earnest,it affectsthe headand neckfirst. The upperparts,central belly, and breastappear to be someof the lastbody feathers to be molted.Even well into their prebasicmolt someindividu- als, especiallymale AmericanGolden-Plovers, can be identifiedby their remaining alternateplumage. The top photo on this issue'sback covershows one suchbird. Althougha great proportionof the bodyis in basicplumage, we cansee several features that identify this individualas an AmericanGolden-Plover. Molt of the flanksis nearlycomplete, but a

120 WesternBirds 35:120-123, 2004 FEATURED PHOTO fewtelltale black feathers remain, suggesting the blackflank pattern of theAmerican. Thereis a ghostpattern of the teardrop-shapedwhite patch on the sidesof the breast typicalof the AmericanGolden-Plover; this contrastingpatch remains well into the prebasicmolt but is not presenton the PacificGolden-Plover. Finally, the undertail covertsshow a substantialblack, again suggesting the AmericanGolden-Plover. The lower photo is of a PacificGolden-Plover in a similarstage of molt. This bird showsno ghostof a whiteteardrop on the sidesof the breast,no blackon the flanks, and no blackon the undertailcoverts. On the upperpartsthe more worn feathers of the alternateplumage can be distinguishedfrom the freshfeathers of the basic plumage;the latter plumageshows bright golden colors, unlike the more subdued andgrayish feathers of the AmericanGolden-Plover. At thisstage of moltthe whitish supercilium,typical of the AmericanGolden-Plover in basicplumage, is of no usein distinguishingthese two species,as the white superciliumof the alternateplumage may stillremain on a PacificGolden-Plover, as seenin this lower photo. Structurally,the upper bird is long-winged,with a long primaryand wing exten- sion,as is typicalof an AmericanGolden-Plover. The primaryprojection (distance the primariesproject past the tertials)is longeron the AmericanGolden-Plover, with four or five primarytips visiblepast the tertials.On the PacificGolden-Plover only two or three are visible(Dunn et al. 1987, Johnsonand Johnson2004). Similarly, wingprojection (distance primaries extend past the endof the tail)is lessthan 1 cm in the Pacific Golden-Ploverbut 1-2 cm in the American Golden-Plover(Johnson and Johnson2004). The differencesin primaryextension may be accentuatedby apparentlylonger tertials in the PacificGolden-Plover (pers. obs. and examinationof photos),although tertial lengths have not beenquantified. There is overlap between the speciesin billlength and thickness and in leg length.The billand legsaverage longer in the PacificGolden-Plover, although the AmericanGolden-Plover is larger in overall size,as reflectedin othermeasurements. Although not diagnostic,the longer-billed and longer-leggedlook of the PacificGolden-Plover in the lower photo is a useful supplementaryfeature. Males and femalesare aboutthe samesize in eachspecies. The timingand extentof moltare correlatedwith migrationdistance, age, and the bird'sreproductive and nutritional status. Molt in golden-ploversvaries substantially not onlyby speciesbut by ageand migratory status. Understanding these differences may help in fieldidentification. Juveniles of bothspecies arrive on the winteringgrounds in juvenalplumage. Pacific Golden-Plovers may remainin the winterrange during their firstand sometimessecond summer; these over-summering birds tend to molt into a dull alternateplumage resembling the basicplumage. But many one-year-old birdsmove north to the breedinggrounds in their firstspring, and thesetend to as- sumea brighteralternate plumage. Surprisingly, Pacific Golden-Plovers never molt primariesduring their first winter.Immatures summering in the winter range molt theirwings during their first summer (July-November), whereas immatures migrating to the breedingrange do so in their secondautumn (August-January) (Johnson and Johnson1983). YoungPacific Golden-Plovers may perform three migrationson juvenalprimaries! According to Johnsonand Johnson(1983), American Golden- Plovers,on the other hand, molt their primariesduring their firstwinter. Therefore, in their firstspring, they showprimary wear similarto that of adults,while in Pacific Golden-Ploversthe firstspring birds' primaries are noticeablymore worn than those of the adults.In the golden-ploversas in the Black-belliedPlover (P. squatarola),it is unclearif there are one or two bodymolts in the firstwinter. The brightplumage colorsacquired by somefirst-spring birds may not be due to a molt differentfrom that of birdsretaining a subduedplumage; rather, the plumagedifferences may be due to differencesin hormonelevels controlling the colorsof the feathersas they grow (Howelland Pyle 2002). The molt scheduleof adultPacific Golden-Plovers has been well documented,but the same cannot be said for the American Golden-Plover. The available evidence is that

121 FEATURED PHOTO the molt of the Americanis significantlylater than that of the Pacific,this difference beingmost clear in wingmolt. The scheduleof bodymolt is morepoorly known. Most adultPacific Golden-Plovers return to winteringareas in Augustwith one quarteror lessof the alternateplumage remaining; fewer than 15% showa largerpercentage of alternateplumage upon return(Johnson and Johnson1983). Bodymolt is there- fore performedlargely on the breedinggrounds, or at a migrationstop-over site, if any. Thereforefall migrantadult Pacific Golden-Plovers should show a mix of basic and alternatebody plumagewhen they arrive in California.Primary molt of adult PacificGolden-Plovers tends to occuron the winteringgrounds (August-December), althougha few individualsbegin primary molt on the breedinggrounds (Johnson and Johnson1983). AmericanGolden-Plovers molt little of the body on the breeding grounds,and no primariesare moltedthere (Cramp and Simmons1983). Bodymolt appearsto occurat migratorystop-over sites, as lessthan 50% of AmericanGolden- Ploversarriving in Argentina show traces of alternate plumage(German Pugnali pers. comm.).Primary molt in the AmericanGolden-Plover takes place in the non- breedingrange, and adultsare in the earlystage of wing molt in mid-October(pers. obs.).In comparison,Pacific Golden-Plovers are at a similarstage of primary molt from Augustto mid September(Johnson and Johnson1983), suggestingthat the Americanmolts its wingsone to two monthslater than the Pacific.Thus an autumn golden-ploverin wing molt in California,and probablyanywhere in North America, is almostcertainly a Pacific. To identify a southboundadult golden-ploverconcentrate on its structure,par- ticularlyprimary projection,and the remnantsof alternateplumage. Molt timing and extent may help in making an identification;in particular,a golden-ploverin wing molt in North Americais muchmore likelyto be a Pacific,and a first-summer individualwith well-wornprimaries again points to a likelyPacific. A carefuluse of structure,molt timing,and remnantsof alternateplumage should be enoughto iden- tify the majorityof moltingadult golden-plovers. Observers are encouragedto fill in the blanksof what is knownon the timingof bodymolt in theseplovers, particularly with respectto the AmericanGolden-Plover. A criticalquestion still to be answered is when the tertialsare dropped. Thanks to George Armistead,Chris Benesh,and Jon L. Dunn for revisingan earlier version of this note.

LITERATURE CITED

Connors,P. G. 1983. Taxonomy,distribution, and evolutionof goldenplovers (Plu- vialis dominica and Pluvialis fulva). Auk 100:607-620. Connors, P. G, McCaffery, B. J., and Maron, J. L. 1993. Speciationin Golden- Plovers,Pluvialis dominicaand P. fulva: Evidencefrom the breedinggrounds. Auk 110:9-20. Cramp, S., andSimmons, K. E. L. (eds.)1983. Handbookof the Birdsof Europe,the MiddleEast, and North Africa,vol. 3. Oxford Univ. Press,Oxford, England. Dunn, J. L., Morlan,J., andWilds, C. P. 1987. Fieldidentification of formsof Lesser Golden-Plover,in Proc.4 thInt. IdentificationMtg. (1986), pp. 28-33. Int. Bird- watchingCenter, Eilat, Israel. Howell, S. N. G., and Pyle, P. 2002. Ageingand molt in nonbreedingBlack-bellied Plovers. W. Birds 33:268-270. Johnson,O. W., andJohnson, P.M. 1983. Plumage-molt-agerelationships in "over- summering"and migratoryLesser Golden-Plovers. Condor 85: 406-419. Johnson, O. W., and Johnson,P.M. 2004. Morphometricfeatures of Pacificand American Golden-Ploverswith commentson field identification.Wader Study Group Bull. 103:42-49.

122 FEATURED PHOTO

Joop,J., Tulp, I., and Bruinzee,L. 2003. Differentialmolt patternsin relationto antipredatorbehaviour during incubation in fourtundra plovers. Ibis 145:270. Paulson,D. R. 1993. Shorebirdsof the PacificNorthwest. Univ. of Wash.Press, Seattle. Prater,A. J., Marhcant,J. H., and Vuorinen,J. 1977. Guide to the Identification and Ageingof HolarcticWaders. BTO Guide17. Br. TrustOrnithol., Tring, England.

American Golden-Plover

Sketch by Jamie M. Chavez

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