The Bristle-Thighed Curlew Landfall of 1998

WESTEBN BIRDS Volume 30, Number 3, 1999 THE BR!STLE-THIGHED CUial_/LANDFALL OF 1998: CLIMATIC FACTORS AND NO'!T,S ON IDENTIFICATION STEVEN G. MLODINOW, 4819 GardnerAvenue, Everett, Washington 98203 STEVEN FELDSTEIN, Earth SystemScience Center, The PennsylvaniaState University,248 Deike Building,University Park, Pennsylvania16802 BILL TWEIT, P.O. Box 1271, Olympia,Washington 98507 The Bristle-thighedCurlew (Nu men ius tah it iensis)undertakes one of the longestknown transoceanic migrations of any shorebj•:d,commuting be- tweenits breedingrange in westernAlaska and itswintering grounds in the tropicalPacific. Its world population is likelyless than 10,000 (Marks1996), and its breedingpopulation consists of approximately3500 pairs(Gill and Redmond1992). Prior to 1998, there were only four North American recordsof the Bristle-thighedCurlew south of Alaska,which is not surprising giventhe species'small numbers and expectedmigratory route. DuringMay 1998, however,between 13 and 17 Bristle-thighedCurlews were recordedon the Pacificcoast between Point Reyes,California, and TatooshIsland, Washington. Notably, vagrants of three other trans-Pacific migrantswere alsofound along the samestretch of coastline that May, the Gray-tailedTattler (Heteroscelusbrevipes), Eurasian Whimbrel (Numenius phaeopusvariegatus or N. p. phaeopus),and Bar-tailedGodwit (Limosa lapponica).At the sametime, weatherpatterns over the North Pacificwere stronglyinfluenced by both the well-publicized1997/1998 E1Nifio and a muchless publicized but equallyinfluential positive phase of the WestPacific Oscillation(WPO). We hypothesizethat, in general,the occurrenceof a largenumber of long- distancetransoceanic vagrants is relatedto extremeweather, whereas the occurrenceof a singlevagrant may be causedby a number of factors, includingunusual weather, a defectiveinternal compass (Berthold 1993), and shipassistance. Consequently, we examinedwind and cloudpatterns in the North Pacificto searchfor anomaliesthat mighthave contributed to the Bristle-thighedCurlew landfall. We alsolooked at broaderclimatic phenom- ena (El Nifio and the WPO) for their part in any unusualconditions. WesternBirds 30:133-155, 1999 133 THE BRISTLE-THIGHED CURLEW LANDFALL OF 1998 Evaluationof the May 1998 Bristle-thighedCurlew reportsrequired a thoroughunderstanding of this species'identification. Therefore, we re- viewedpreviously published identification criteria and assessedthese further by examiningthe specimensin the BurkeMuseum, University of Washing- ton, Seattle. METHODS We locatedand assessedpast recordsof the Brisfie-thighedCurlew and other speciesby reviewingregional texts (see Literature Cited), reviewing AmericanBirds/Field Notes,and contacting state and provincial bird records committees.Sightings from the 1998 invasionwere first compiled by a review of MichaelPatterson's Bristle-thighed Curlew World Wide Web page and from discussionswith active birders. We assessedthe reportsby examiningdescrip- tions, photographs,and in one case, a wing of the bird involved.The photographswere gleanedmostly from Patterson'sWeb page, the descrip- tionswere obtainedfrom the observers,and the wing was inspectedat the Burke Museum (UWBM 59322). We evaluatedidentification criteria for the Bristle-thighedCurlew by reviewingpreviously published analyses and photographs(Johnsgard 1981, Hayman et al. 1986, Paulson1993, Rosairand Cottridge1995, Higginsand Davies1996), by studyingspecimens at the Burke Museum, and through discussionwith observersfamiliar with this species(David James, Dennis Paulson, Peter Pyle, and SievertRohwer). KevinAanerud and Mlodinowcompared the eightfull specimens and five spreadwings of the Bristle-thighedCurlew with 39 full specimensand 25 spreadwings of the Whimbrel,mostly N. p. hudsonicus, but including some variega tus from eastern Asia. Three of the Bristle-thighedswere in relatively freshplumage, two from April and one from August.The remainingfive whole specimenswere somewhatworn adultscollected in October.All Bristle-thighedwings were from fall. We measuredthickness of tarsi 2 cm belowthe tibiotarsaljoint. Our estimatesfor the total numberof Bristle-thighedCurlews making landfallsouth of Alaskawere basedon the numberof birdsseen, the amount of sandybeach habitatbetween Tatoosh Island and Point Reyes, and the extentto whichthis habitatthat was surveyed.Linear milesof sandybeach habitatwere estimatedfrom geologicalmaps for California,Oregon, and Washington.This habitatwas denotedas Quaternarysand or Quaternary beachand dune sands on thesemaps. Estimates of the amountof thishabitat searched were based on interviews with active birders. Estimates of total birds found were made on both most conservative and least conservative criteria. For the mostconservative estimate, all sightingswithin a 12-dayperiod and a 10-mileradius were consideredto be of the sameindividual(s). The 12-day periodwas basedon the stayof the birdsat the southjetty of the Columbia River,the longestwe know of. The leastconservative estimate supposed that all nonconsecutivesightings at least1 mileapart were of differentindividuals. We arrived at the most conservative estimate of total curlews involved in the 1998 eventby expandingthe mostconservative estimate of birdsby the most conservativeestimate of sandybeach habitat surveyed. Similarly, we derivedthe leastconservative estimate by expandingthe leastconservative 134 THE BRISTLE-THIGHED CURLEW LANDFALL OF 1998 estimateof birdsby the leastconservative estimate of habitat.We did not extrapolatebeyond the northernand southern edges of the observedarea of the curlew's occurrence. The monthlyaverage WPO index and the SouthernOscillation index (SOl)were both madeavailable by the NationalOceanic and Atmospheric Administration(NOAA)/Climate PredictionCenter in Washington,D.C. Theseindices are expressedby a positiveor negativenumber that measures the strengthof the WPO and E1Nifio/Southern Oscillation, respectively. To searchfor anomalouswinds between 15 April and 15 May 1998, we used dailyaveraged data from the NationalCenters for EnvironmentalPredic- tion/National Center for AtmosphericResearch Reanalysis (Kalnay et al. 1996), whichwere obtainedfrom the NOAA ClimateDiagnostics Center in Boulder, Colorado. We calculatedmodel trajectories to estimatethe pathtaken by the migrating Brisfie-thighedCurlews, using the windsat the 600-mb pressurelevel. This level,which approximately corresponds to an altitudeof 4 km abovesea level, is selectedbecause that is the typicalelevation for shorebirdsmigrating over water (Richardson1976, Williamset al. 1977, Elkins1988, Kerlingerand Moore 1989, Berthold1996). We calculatedthe trajectoriesfor sixconsecu- tive daysbeginning 26 April 1998. The initiallocation of the curlewsis specifiedas the southernmostend of the 20 m/s northerlywind contour (the dashedcontours in Figure 2) in the north-centralNorth Pacific.We then assumedthat the curlewsflew parallelto the localwind vector,i.e., drifted downwind,at an air speedof 25 m/s (similargeneral conclusions are obtained with otherrealistic air speeds).Mathematically, this can be writtenas V = Vw + ar, where the vector V is the bird's groundspeed, V•v is the observedwind vector,a = 25 m/s (the bird'sair speed,Marks and Redmond1994a), and r is a unit vector parallelto V•v. The equationsfor the longitudinaland latitudinaldisplacement of a birdundergoing downwind drift are dx = U(t)dt; dy = V(t)dt, where dx is the eastwarddisplacement and dy is the northwarddisplace- ment of a bird movingwith an eastwardground speed at time t of U(t) and a northwardground speed at time t of V(t) over a time intervaldt. The appropriatespherical representations are usedfor dx and dy (seeHolton 1992), andthe time stepdt is specifiedas one hour.With thisapproach, the temporalchanges to the observedwind patternthroughout downwind drift are taken into account.Also, temporal and spatial mathematicallinear interpolationis performedas the observedwind vectorsare obtainedfrom NOAA Climate DiagnosticsCenter as daily averageson a 2.5 ø latitude/ longitudegrid. RESULTS AND DISCUSSION Status and Distribution Bristle-thighedCurlews are knownto breed only in Alaska,where they havebeen found in the NulatoHills (the low mountainousregion just north and east of the lower Yukon River) and acrossthe northern half of the 135 THE BRISTLE-THIGHED CURLEW LANDFALL OF 1998 Seward Peninsula(Handel and Dau 1988, Kessel 1989). They typically arrive at their breedinggrounds in mid-May but have been found there as earlyas 5 May (Kesse11989);a recordfrom Attu Islandon 2 June1993 (AB 47:444) waslikely of a •atenorthbound migrant. After nesting, much of the populationstages in Alaska'sYukon-Kuskokwim delta before migrating south, with smallernumbers staging along the Seward Peninsulacoast (Hande•and Dau 1988, Kesse11989).The firstsouthbound adults appear at thesestaging areas as earlyas late June. A largerinflux of adultsoccurs in late July, followedby adultswith juvenilesin early August(Handel and Dau 1988). By mid-to-lateAugust most adultsand juveniles are gone. The latest date from Alaskais 18 October1994 on St. LawrenceIsland (FN 49:85). In both the up•andbreeding areas and the coastalstaging areas, dwarf shrub habitatis preferred(Kessel 1989). Little is known of Bristle-thighedCurlews during migration.The first possiblestop for southboundbirds is the northwesternHawaiian Islands, some4000 km away.These islands serve as the winteringground for 700 to 800 individuals,but few if any passagemigrants stop there duringeither fall or spring(Marks

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