Abundance of Shorebirds at Willapa Bay, Washington
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ABUNDANCE OF SHOREBIRDS AT WILLAPA BAY, WASHINGTON JOSEPH B. BUCHANAN and JOSEPH R. EVENSON, Cascadia Research Collective,218•/• W. FourthAvenue, Olympia, Washington 98501 There is a growingbody of literaturedocumenting large concentrations of shorebirdsat winteringsites or migratorystaging areas along the Pacificcoast of North America. In the PacificNorthwest, such areas include the Fraser River delta (Butler 1994, Vermeer et al. 1994), Puget Sound (Buchanan 1988a, Evensonand Buchanan1995), GraysHarbor (Herman and Bulger 1981), WillapaBay (Widrig 1979), andbeaches of the outercoast (Buchanan 1992). Someof thesesites support numbers of shorebirdsthat qualify them as regionallyor internationallyimportant sites in the Western Hemisphere ShorebirdReserve Network (I. Davidsonpers. comm.). Others comprise clustersof smallersites that togethersupport substantial numbers of birds (Evensonand Buchanan1995). Identifyingthe role these sites play in supportingwintering and migrantshorebirds is an importantstep in develop- ing conservationstrategies to protectshorebirds and their habitats. Prior researchon shorebirduse of Willapa Bay has been limited.At LeadbetterPoint, Widrig (1979) conductedmultiple shorebird surveys but only over a singleyear. Becauseshorebird abundance can vary both from year to year (Buchanan1988a) and within a year (Evensonand Buchanan 1995), both intensiveand long-termstudies are essentialfor assessing populationtrends. The objectiveof thisstudy was to estimatethe abundance of shorebirdsat WillapaBay, Washington,to identifyimportant candidate siteswithin Willapa Bay for future monitoringefforts. Here we present resultsof ground-and aerial-basedcounts of shorebirdsduring winter, spring,and autumnfrom 1991 to 1995 that documentthis site as among the mostimportant to shorebirdson the westcoast of North America. STUDY AREA WillapaBay is on the outer coast of Washingtonstate between Grays Harbor andthe Columbia River mouth. It coversabout 420 kmz and is fed by the Cedar, North,Willapa, and Palixrivers in the northand the Nemah,Naselle, and Bear riversin the south(Figure 1). A long,narrow peninsula (North Beach Peninsula) dominatedby lowdunes, pastures, and woodlands separates the mainbody of thebay from the Pacific Ocean. Of theseveral salt marshes around the bay, the largestare at the mouthsof the WillapaRiver to the northand the Bear River to thesouth. Low tide (0.0 m)exposes about 200 kmz of mudfiats. Numerous sandshoals are exposedduring all but the highesttides; there are no dredge- spoilislands in the bay.Two areas,Leadbetter Point and the BearRiver/Lewis Unit fiat, are withinWillapa National Wildlife Refuge. METHODS Our countsinvolved a combinationof aerial and ground-basedefforts. BecauseWillapa Bay is largeand accessto manyareas is limited,surveying 158 WesternBirds 28:158-168, 1997 ABUNDANCE OF SHOREBIRDS AT WILLAPA BAY, WASHINGTON + •+ 0 1 2 3 4 5 6 7 Pacific Kilometers Ocean Figure1 WillapaBay, showingthe locationof sitescovered by groundand aerial surveysfor shorebirdsbetween 1991 and 1995. Lightstippling, intertidal foraging and roostinghabitats. shorebirdsadequately from the groundis difficult.For thisreason, on each surveywe madecounts from a smallairplane flying at a heightof about60 to 70 m. These aerial countswere the basisof our primaryestimates of shorebirdabundance in the estuary.We also made aerial countsof birds (primarilySanderlings) on outerNorth Beachon five occasions;the beach coveredby the countswas 37 km long. 159 ABUNDANCE OF SHOREBIRDSAT WILLAPA BAY,WASHINGTON Groundcounts were madeby a total of 18 observers(median 5 per seasonalcount) who countedshorebirds at eightprimary sites: Tokeland, CedarRiver, North River, Willapa River, Palix River, Bear River/Lewis Unit, East North Beach Peninsula,and LeadbetterPoint. Ground countswere usedto estimatethe proportionsof speciesnot identifiedfrom the air.Thus, dependingon theseason, ground observers determined the species compo- sition of groupsof look-alikespecies, including the Greater and Lesser yellowlegs;Whimbrel, Marbled Godwit, and Long-billedCurlew; Western Sandpiper,Least Sandpiper,and Dunlin; and Red Knot and dowitchers. Proportionsderived from ground counts of particularsites or regionsof the bay were appliedto the correspondingunidentified species groups from aerialcounts of theseregions, then regional totals were summed to provide baywideestimates. Aerial counts alone were used to estimatepopulations of the Black-bellied Plover. Ground counts alone were used for all other species,which consequently were underestimated because of partialcover- age of the bay by this method. Becausethe mainobjective of thesecounts was to identifyimportant sites forfuture monitoring efforts, they were usually made only once each season (Table1). Countswere made during rising or fallingtides generally within four hoursof hightide. The logisticalconstraints noted above necessitated conductingthe groundcounts at siteswhere shorebirds aggregated immedi- atelyprior to and/or afterhigh tide. Consequently, aerial counts covered the entireshoreline whereas ground counts covered only the areasof major concentrationsof birds.Our scheduledflights were occasionallypostponed Table 1 Dates of Shorebird Counts at WillapaBay, Washington,1991-1995 Aerial Ground Winreft 28 Nov 1992 21-22 Nov 1992 4 Dec 1993 22 Nov-4 Dec 1993 31 Dec 1993 28-29 Dec 1993 21 Dec 1994 21-30 Dec 1994 9 Feb 1995 28 Jan-9 Feb 1995 Spring• 27 Apr 1991 27 Apr 1991 27, 30 Apr 1993 25 Apr-1 May 1993 26 Apr, 1 May 1994 28, 30 Apr 1994 Autumn 30 Aug 1993 15-20 Aug 1993 •Two complete(aerial and ground)counts were con- ducted in the winters of 1993-94 and 1994-95. •Twoflights were made in the springsof I993 and I994; each pair of flightscoincided with a single groundcount. 160 ABUNDANCE OF SHOREBIRDS AT WlLLAPA BAY, WASHINGTON becauseof adverseweather conditions, and thereforesome ground counts duringwinter and springwere not made on the sameday as the aerial counts.Nonetheless, we believethat the estimatesof the proportionsof speciesderived from these counts are reliablebecause the numberof species duringwinter was low and did not vary appreciably within the bay,and most groundcounts in springwere made on or withintwo daysof the aerial counts. RESULTS AND DISCUSSION SpeciesAccounts Black-belliedPlover (Pluvialis squatarola). This was the secondmost numerous speciesin winterin all threeyears and generallythe fourthmost numerous species duringmigration (Tables 2 and3). The totalof 4049 countedon 21 December1994 included3848 roostingon outerNorth Beach at hightide, the highestknown winter countat a singlesite in Washingtonstate (Paulson 1993). SemipalmatedPlover (Charadrius semipalmatus). The irregularityof thisspecies in Washingtonin winter(Buchanan 1992) is reflectedin our results(Table 3). The SemipalmatedPlover is locallydistributed in all seasons. Killdeer(Charadrius vociferus). The Killdeerwas observedin low numbersin all seasons(Table 3). Widrig(1979) sawvery few Killdeersat LeadbetterPoint during any season. GreaterYellowlegs (Tringa melanoleuca).The GreaterYellowlegs is a regularly occurringwinter residentin estuarinehabitats in the PacificNorthwest (Buchanan Table 2 SeasonalHigh Countsof Black-belliedPlovers, Dunlins, Western Sandpipers,and Short-billedDowitchers at PrimaryCensus Sites in Willapa Bay, 1991-1995 Winter Spring Black- Black- Short- bellied bellled Western billed Site Plover Dunlin Plover Dunlin Sandpiper Dowitcher Tokeland 415 2,320 35 832 3,603 300 Cedar River 67 450 8 756 4,844 1,780 North River 240 3,915 100 1,890 12,110 5,190 WillapaRiver 5 13,510 100 2,970 16,830 12,460 BetweenWillapa R. and Palix R. 6 3,120 180 2,481 14,059 2,345 PalixRiver 35 8,800 84 1,191 6,749 4,380 Between Palix R. and Nemah R. 15 2,160 85 2,586 10,344 1,465 NemahRiver 205 8,150 300 2,400 9,600 4,000 BearRiver 210 27,260 300 12,300 36,900 5,000 E. North Beach peninsula 50 5,115 65 2,554 3,831 680 LeadbetterPoint 54 3,935 125 7,930 5,070 4,425 161 ABUNDANCE OF SHOREBIRDS AT WILLAPA BAY, WASHINGTON Table •3 Numbersof ShorebirdsCounted or Estimatedat WillapaBay, Washington, 1991-1995 Winter Spring Autumn Species 1992-93 1993-94 1994-95 1991 1993 1994 1993 Black-belliedPlover • 415 881 4,049 918 653 245 205 SemipalmatedPlover b 12 0 0 27 122 264 98 KilldeeP 0 6 1 0 0 2 4 GreaterYellowlegs • 84 42 97 40 160 275 65 LesserYellowlegs a 0 0 0 0 1 0 2 Willetb 0 0 0 0 0 0 5 SpottedSandpiper • 0 0 3 0 0 1 1 Whimbrela 4 9 3 0 82 27 229 Long-billedCurlew d 0 67 70 0 8 1 0 Marbled Godwitd 0 303 462 0 115 51 70 Largesandpiper spp. 0 0 0 150 0 0 0 RuddyTurnstone • 0 2 0 0 2 2 0 Black Turnstone• 28 26 29 1 0 7 1 Red Knot• 0 0 0 0 177 1 0 Sanderlingd 372 135 16 0 0 6 2 WesternSandpiper • 80 e 258 82,575 55,195 49,615 8,976 LeastSandpiper • 0 22 235 1 e 150 374 Dunlina 47,017 69,850 27,120 7,525 29,720 41,640 0 Long-billedDowitcher • 50 94 61 0 0 0 0 Short-billedDowitcher • 0 0 0 23,865 8,900 16,595 153 Dowitcherspp? 0 0 0 0 0 0 31 CommonSnipe b 10 2 0 0 1 1 0 øDirectaerial count of the entirebay. bNumbercounted from the groundonly; total for somespecies may under-represent actual number of birds present. CDirectaerial count of the entire bay unless ground count revealed Lesser Yellowlegs, in which case proportionalcounts were used. dNumberprojected from proportions of similarspecies counted during surveys on theground (see Methods). ePresent but not counted. 1988b). The countfrom winter 1994-95 appearsto be the highestwinter count yet made in the region(Table 3; Paulson1993, Buchananunpubl. data). LesserYellowlegs (Tringa