Distribution and Abundanve of Winter Shorebirds on Tomales Bay

Home , Bodega Harbor, Olema Valley, Point Reyes, Tomales Bay

WESTEt BIRDS

Volume 32, Number 3, 2001

DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY, CALIFORNIA: IMPLICATIONS FOR CONSERVATION

JOHN P. KELLY,Cypress Grove Research Center, Audubon Canyon Ranch, Marshall, California 94940

ABSTRACT: I analyzedthe distributionand abundanceof winteringshorebirds (Scolopacidae,Charadriidae, and Recurvirostridae) in Tomales Bay, California, on the basisof 57 baywidecounts conducted over 10 years,from 1989-90 to 1998-99. TomalesBay supportsup to 20,689 shorebirdsin earlywinter, thus qualifying as a wetlandof "regional"importance in the Western HemisphereShorebird Reserve Network. Minimum overall shorebird abundance fell as low as 1291 in late winter. TomalesBay supported approximately a thirdof the winteringshorebirds in the Point Reyes/Bodegaarea in early winter.Observations of tidallystructured flock move- mentsof severalspecies suggested that the northernand southernends of Tomales Bay are occupiedby differentwintering groups. In associationwith cumulative seasonalrainfall, most species declined in abundancesignificantly in midwinter.The Sanderlingand MarbledGodwit increased with cumulativerainfall in the north and southbay, respectively, suggesting weather-related influxes from outercoastal beaches. After accountingfor the effectsof cumulativeseasonal rainfall and a 10-yeartrend in annualrainfall, I detectedno trendsin species'abundances. Foraging and roosting shorebirdsat the northernend of the baywere vulnerable to directdisturbance from concentratedrecreational use. Long water-residence times in southernTomales Bay suggestthat shorebirdsthere may be particularlyvulnerable to toxicspills or anthro- pogeniceutrophication. The closenessof San FranciscoBay impliesa highpotential for invasionof nonnativeorganisms established there, which could alter the availability of benthicprey to shorebirdsin TomalesBay. Shorebirdfeeding habitat at the deltasof Walker and Lagunitascreeks may be adverselyaffected by heavyrainfall leadingto the depositionof sediment.Daily influxesof roostinggulls from a local landfill were associated with reduced shorebird use of tide fiats. Shorebirds' use of open tide fiatsdeveloped for maricultureis reduced,although floating oyster bags provide roostingareas during high tides. Breachinglevees that isolate historic wetlandsmay increaseshorebird use in some areas.The likelihoodof regularor episodicintraseasonal movements among Point Reyes/Bodegaarea wetlandssug- gestsTomales Bay and other nearbywetlands are worthyof broadprotection.

WesternBirds 32:145-166, 2001 145 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

TomalesBay is one of California'slargest and leastdisturbed estuaries. It hasbeen recognized as a wetlandof "regionalimportance" on the basisof its supportingup to 20,000 shorebirds,a criterionfor inclusionin the Western Hemisphere Shorebird Reserve Network (WHSRN; Harrington and Perry 1995, Page and Shuford2000). An analysisof shorebird abundance and distribution on the Pacific coast of the United States identifiedTomales Bay as a keywetland for shorebirdconservation because it supportsin at leastone seasonat least1% of the totalpopulation of 8 of 13 shorebirdspecies that concentratein estuariesand brackishwetlands (Pageet al. 1999). In spiteof thisrecognition, little has been published on the spatialor temporalvariations in shorebirdabundance on TomalesBay, or the birds'vulnerability to environmentalthreats. Kelly and Tappen (1998) reportedpreviously on the valueof TomalesBay to otherwinter waterbirds. Shufordet al. (1989) analyzedthe resultsof up to 10 consecutiveyears of shorebirdcounts in the adjacentPoint Reyesarea between1965 and 1982 but providedlittle information on shorebirduse of TomalesBay. As part of a statewidesurvey, Jurek (1974) reportedon fiveyears of monthlyshorebird countsat the Walker Creek delta, in the northernpart of the TomalesBay, but did not surveyabundances baywide. Over recentdecades, Tomales Bay has been suggestedfor protectionas habitatfor shorebirds(Smail 1972, National Oceanicand AtmosphericAdministration 1987, Neubacheret al. 1995, Pageand Shuford2000). However,with one exceptiondealing with the effectsof aquacultureon shorebirds(Kelly et al. 1996), evaluationsof conservationissues in TomalesBay havenot directlyaddressed patterns of shorebird use. In this paper, I (1) presentresults from 10 years of baywidewinter shorebirdcensuses on TomalesBay; (2) comparethese results with other studiesto providea historicaland geographicalperspective on the importanceof TomalesBay to shorebirds;(3) evaluatespecies' distributions within the baywith regard to the importanceof particularareas used by shorebirds; (4) examineprocesses that influenced abundance trends over the 10 yearsof study,and (5) identifyneeds for conservationof winteringshorebirds on TomalesBay. I addressthese objectiveswith regardto all Scolopacidae, Charadriidae,and Recurvirostridaeassociated with TomalesBay and imme- diatelyadjacent seasonal wetlands (Figure 1).

STUDY AREA

TomalesBay floodsthe lower20 km of the fault-riftOlema Valley on the centralCalifornia coast, about 45 km northwestof San Francisco(Figure 1; Galloway1977). Approximately18% of the bay's28.5-km z areais intertidal,providing primarily sand or mudfiats suitable for foragingshorebirds and cobblestonebeaches along the eastshore. In general,sediments grade from primarilyfine to coarsesand in the northernreaches of the bay to muddiersubstrates in southernportions of the bay (Daetwyler1966). Two primarypoints of freshwaterinflow, LagunitasCreek at the south end of the bay and Walker Creek near the north end of the bay, are associatedwith largetidal deltas suitable for foragingshorebirds (Figure 1). Numeroussmaller delta marshes and tidal fiatsoccur where smallperennial

146 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

Dillon Beach

8.7 35.4

Creek 110.3

BlakesLanding 31.9 Cypress Point Pacific Marshall Ocean

55.7

37.5 215.4 Inverness 10.0 0 2 4 km Lagunita• Creek

Figure 1. Shorebirdcount areas on Tomales Bay, California. 1, White Gulch; 2, Lawsoh'sMeadow; 3, SandPoint; 4, Tom's Point;5, WalkerCreek delta; 6, North Marshall;7, South Marshall;8, LagunitasCreek delta; 9, GiacominiPasture; 10, Invernessshoreline. Arrows indicatelength of shorelinein each count area; labels indicateextent (ha) of exposedtidal flat at MLLW. and ephemeralstreams enter the bay. Adjacentseasonal wetlands suitable for shorebirdsare normallylimited to about15 ha of a 200-ha dikedpasture at the south end of the bay and approximately20 ha of wet meadow surroundedby sanddunes at the north end of the bay. Additionalseasonal wetlands,used by shorebirdsduring periods of heavy flooding,occur in agriculturalareas northeast of the bay. Most(95%) of the annualrain fallswhen wintering shorebirds are present, from October throughApril, with 55% falling from Decemberthrough February(Audubon Canyon Ranch, unpublished data). Constraints on tidal exchangewith the ocean,imposed by the linearshape of the bay, interact with winter runoffto createcontrasting habitats at the two endsof the bay. In the southernhalf of the bay, variablyhigh levelsof freshwaterinflow duringwinter, low flowsin summer,and long water-residence times result in highlyvariable salinities (Hollibaugh et al. 1988). Salinitiesrange from nearly

147 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY freshafter heavywinter runoff to slightlyhypersaline in late fall (Smithand Hollibaugh1997). In northernTomales Bay, regulartidal mixing maintains salinitiesthat consistentlyreflect those of nearshorewaters along the outer coast.The differencebetween mean high and mean low tides is about 1.1 m, with an averageannual maximum tide swingof about2.5 m (U.S. National Oceanicand AtmosphericAdministration harmonics and correctiontables; Tides and Currents,Nautical Software, Inc.).

METHODS

! dividedTomales Bay into 10 subareaswithin which all shorebirdscould be countedin 60 to 90 minutes(Figure 1). Theseareas include almost all of the intertidalflats in the bay,with the exceptionof a few smallareas at creek mouths along the west shore. Teams of qualifiedobservers counted all shorebirdsby speciesin all subareassimultaneously. Observerscounted during risingtides, at tide levelsbetween 0.76 and 1.22 m abovemean lower low water(MLLW) at Blake'sLanding (Figure 1). Count-areaprotocols were coordinated so that adjacent areas were surveyed simultaneously.Abundances generally represented counts of foragingindi- viduals,with estimatesof largemobile flocks made only rarely. During three workshopsheld to examineand reduce observer bias in estimatingflock size, biaseswere inconsistentand withouta clear centraltendency. Therefore observer bias was assumed not to affect overall estimates of abundance. The time and directionof all flock movements,departures, and arrivalsduring count periodswere recordedand examinedlater to minimize chancesof birds being doublecounted. Counts were conductedonly on days with weathersuitable for usingtelescopes to identifyshorebirds. Each year from 1989-90 to 1998-99, we completedapproximately three countsin each of two intraseasonalperiods (n -- 57): early winter (mean 2.8, range 1-4; 1 November-19 December)and late winter (mean 2.9, range 2-3; 15 January-4 March). I summarizedresults within intraseasonalgroups to compareearly- and late-winterpopulation levels. Mostindividual shorebirds were identifiedto species.Occasionally, individu- alswere not identified to speciesand were recorded in pooledspecies groups suchLeast/Western sandpiper. Individuals recorded in pooledspecies groups wereallocated to singlespecies groups in directproportion to the numberof identifiedbirds of eachspecies in eachcount area on eachday (Pageet al. 1999). Such adjustmentswere made, however, only if the number of identified individuals exceeded the number of undifferentiated individuals and was >50 for groupsof two speciesor >100 for groupsof threespecies. Observations of shorebird flocks revealed restricted movements in the middlesection of the bay.Of 19 flocksof severalshorebird species observed departingas the tide inundatedthe feeding area at Walker Creek delta, 100% wereoriented to the northwestand only one flockflew by fromfarther south.In southernTomales Bay, 56 flocksof severalspecies were observed departingnorth from feedingareas during rising tides, but only one flock (2%)was seen flying north past Cypress Point in Marshall(Figure 1). These observationssuggest that severalspecies winter in discretegroups. In addition, a reciprocal translocationof color-bandedDunlins, the most

148 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY abundantspecies in the bay, revealedthat winteringpopulations at the two endsof the baywere separate (Kelly 2000). Therefore,I analyzedthe results separatelyfor subregionsof TomalesBay north (areas 1-6) andsouth (areas 7-10) of CypressPoint, Marshall,in additionto analyzingthem for the bay as a whole (Figure1). I transformed abundance data into densities,based on the extent of exposedmudflat in eachcount area, estimatedat MLLW with a planimeter on a bathymetricchart (Figure1; U.S. NationalOceanic and Atmospheric AdministrationChart 18643, 16th ed., Dec 1995). Estimatesof shorebird preferencesbased on densitiesamong different habitat types may be sensitiveto different measuresof availability(Warnock and Takekawa 1995), particularlyif birdsconcentrate along the edge of the risingtide (Recher1966, Burgeret al. 1977) andthe linearextent of the tideline is not closelycorrelated with arealextent of the habitat.However, suitable shore- birdhabitat on TomalesBay is limitedprimarily to tidalflats, and eachcount arearepresented a similarproportion of the areaavailable for foragingalong the tide line at any particulartide level. Consequently,densities based on differentwidths of tidal exposuredid not significantlyalter estimatesof relativeshorebird use by countarea. To examinethe effectsof count-arealocation, intraseasonal period, and yearon patternsof shorebirduse, I usedmixed-model analysis of covariance of shorebirddensities, controlling for days sincethe beginningof each intraseasonal(early or late winter)period and treatingyear as a random effect.Shorebird densities were log-transformedto improvenormality and stabilizethe varianceof residuals.Significant count-area effects were fol- lowedby multiplepairwise comparisons within the northern and southern subregionsof the bay. I inspectedbivariate scatterplots of shorebirdabundance on cumulativeseasonal rainfall, using Cleveland'srobust locally weightedregression algorithm (LOWESS; Cleveland 1979, Chamberset al. 1983), to look for nonlinearthresholds and trends.I selectedthis method becauseit useslocally weighted least squares and a robustfitting procedure to definesmoothed points that are relativelyinsensitive to outlyingvalues, and it allowsa flexibledegree of smoothingby adjustingthe proportionof data scanned(f) for each fitted value.To improvethe normalityof abun- dancesused in the analysis,I basedtrends for the SemipalmatedPlover, BlackTurnstone, and LeastSandpiper on the naturallogarithms (In) of the counts;distributions of other species'abundances were approximately normal without this transformation. Annual and cumulative seasonal rainfall were derived from daily rainfall measuredat CypressPoint (Figure 1; AudubonCanyon Ranch,unpubl. data). I estimatedannual trends in species abundancewithin subregionsas partial regressioncoefficients on year, independentof the lineareffect of cumulativeseasonal rainfall. I compared these results with correlated trends in annual rainfall.

RESULTS

TomalesBay supportedas many as 20,689 shorebirdsin early winter, with minimumnumbers falling as low as 1291 in late winter. The most abundantwinter shorebird in TomalesBay was the Dunlin, accountingfor

149 •' o' 6 o o 6 6 6 oo 6 6 o c•c•v • • DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY over half (53%) of all shorebirdsin earlywinter and nearlya third (29%) in late winter.The secondmost abundant species was the WesternSandpiper, representing13ø-/o in earlywinter and 20ø-/oin late winter(Table 1). The total numberof shorebirdsdeclined significantly from early to late winter in northern (t -- 4.67, df -- 56, P < 0.001) and southern(t: 4.80, df -- 54, P < 0.001) TomalesBay, asdid several individual species (Table 1). In contrast to thispattern, the MarbledGodwit increased significantly during winter in southernTomales Bay, and the Sanderlingincreased significantly in the northern bay and baywide(Table 1). Maximumannual abundance varied by a factorof 8.9 in the Dunlin,8.0 in the MarbledGodwit, 6.7 in the Killdeer,6.2 in the GreaterYellowlegs, 4.4 in the Sanderling,4.2 in the Western Sandpiper,3.5 in the Spotted Sandpiper,3.4 in the Willet, 3.2 in the BlackTurnstone, 2.3 in the Black- belliedPlover, and 4.1 in all speciescombined. Maximum annual abundance varied substantiallymore in some species,by a factor of 17.8 in the dowitchers,16.7 in the SemipalmatedPlover, 15.5 in the LeastSandpiper, and 12.0 in the PacificGolden-Plover, reflecting proportional changes in smallerpopulations and/or difficultiesin detectinginconspicuous birds. Densitiesin mostspecies varied significantly from year to year (Table2). Winter shorebirddensities differed significantly from countarea to count area withinboth subregionsof TomalesBay (Table2). Most speciesshowed significantannual and intraseasonalshifts in distributionof densitiesamong countareas, indicated by significantinteraction terms in analysesof covariance (Table2). Althoughaverage midwinter declines in count-areadensity, weightedequally for large and smallareas, were significantonly for the Dunlin and dowitchers(Table 2), severalspecies decreased significantly in overallabundance on the basisof northbay, southbay, and baywidetotals (Table 1). NorthernTomales Bay supportedmore shorebirds,overall, than southern TomalesBay in early winter (t--2.92, dr--55, P<0.05) and late winter (t -- 4.57, df -- 55, P < 0.001). The WalkerCreek (area 5) and LagunitasCreek (area8) deltasnot onlysupported the greatestconcentrations of shorebirds but alsothe highestdensities for more shorebirdspecies than other areasin TomalesBay (Table2, Figure2). The northeastshoreline from SandPoint (area3) to Vincent'sLanding south of Tom'sPoint (area4) alsosupported relativelyhigh abundancesof severalspecies (Table 2, Figure2). Shorebird use of seasonalwetlands at Lawson's Meadow (area 2) and Giacomini Pasture(area 9) washighly variable; consequently, mean shorebird densities in theseareas could not be distinguishedclearly from thosein other areas (Table2). GreaterYellowlegs concentrated primarily in the extremesouth end of TomalesBay, whileSanderlings and MarbledGodwits concentrated primarilyat the north end of the bay (Figure2). The PacificGolden-Plover, SnowyPlover, and Red Knotoccurred exclusively in northernTomales Bay. Cumulativeseasonal rainfall was associatedwith significantdecreases in the useof TomalesBay by mostshorebird species but significant increases by the MarbledGodwit and Sanderling (Figure 3). Bivariateplots suggested that 25-30 cm of rain mustfall in a seasonto triggerthe declinesof the Western Sandpiper,Least Sandpiper, and dowitchers(Figure 3). Alternatively,rainfall correlationsmight be the resultof intraseasonalchanges in abundance

152 153 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

lOO 1 Black-bellied Plover Pacific Golden-Plover o=8o

,- 60 169 i 107 "• 4O

m 20

0 12345678910 12345678910 15 2O oe SnowyPlover SemipalmatedPlover 15 • lO lOO

11 0 0 0 . 0 0 0 0 0 0 0 5o ?i 49, 1 2 3 4 5 6 7 8 910 12345678910 10 Killdeer Greater Yellowlegs 31

5 6 57 7 _i__i., , , 2345678910 1 2 3 4 5 6 7 8 910 Count Location Count Location Figure 2. Mean winter abundanceof shorebirds,by count area, in northern (solid bars) and southern(striped bars) Tomales Bay, California, 1989-90 to 1998-99. Error bars indicate standarderrors; numbers above bars indicate maximum abun- danceobserved during the studyperiod.

unrelatedto rainfall.When countdate and year were includedas covariates, residual(uncorrelated) effects of cumulativeseasonal rainfall in the northbay weresignificant for the Black-belliedPlover (b -- -1.31, P < 0.05), Willet(b = 2.56, P < 0.05), Dunlin(b = -27.30, P < 0.05), and (In)Least Sandpiper (b = -0.06, P < 0.001); residualeffects in the southbay were significantfor the (In) LeastSandpiper (b -- -0.06, P < 0.05). Conversely,intraseasonal trendsindependent of cumulativeseasonal rainfall and year weresignificant for the Willet (northbay: b = -1.74, P < 0.01; southbay: b -- 1.01, P < 0.05), Dunlin(north bay: b -- -17.96, P < 0.01; southbay: b = -22.25; P < 0.01), and (In)Black Turnstone (south bay: b = -0.03; P -- 0.01).

154 DISTRIBUTIONAND ABUNDANCEOF WINTERSHOREBIRDS ON TOMALESBAY

120140I Willet 528 422 1 SpottedSandpiper

80 60 170 7 40 11 221 20 136øill 7 • ii4 1 2 3 4 5 6 7 8 910 1 2 3 4 5 6 7 8 9 10 350 1770Marbled Godwit BlackTurnstone 92 300 •5 • 200 •o 80 60 5 40 i/i 45 1002• 11211•• 0 ...... 11 1234567891O 1234567891O 5 Red Knot 300I Sanderling alO

200 .• 19 150 369640 50 110 iii 12..•.2103 175r o o o. . .o ,o .o .o ,o .o 0 6 o 1 2345678910 1 234567891o

Count Location Count Location

Figure 2 (continued).

After the linear effects of cumulativeseasonal rainfall and date are accountedfor, residual annual trends were not significant (P> 0.05),except in fivecases. In the southbay, significant linear trends independent of cumulativeseasonal rainfall and date suggested10-year declines in the Black-belliedPlover (b = -5.62; P < 0.001),(In) Semipalmated Plover (b -- -0.32; P<0.05),(In) Least Sandpiper (b= -0.32; P<0.05),and Dunlin (b -- -223.21;P < 0.001),and increases inthe Willet (b = 14.43,P < 0.01)and MarbledGodwit (b -- 10.68,P < 0.05).In northern Tomales Bay, significant 10-yeardeclines independent of cumulativerainfall and date were evident in theWillet (b = -13.10, P < 0.05)and Dunlin (b = -330.26, P < 0.01). Althoughthese changes in shorebirduse were independent of cumulative seasonalrainfall, they might have reflected escalating effects of successive increasesin annualrainfall (b -- 5.3 cm/year,P < 0.05; Figure4). The 10- yearduration of thisstudy was too short to distinguishbetween the influence of annualrainfall trend and possibleunderlying (correlated) population

155 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

1000800l[Western Sandpiper 33•0•, 450500]Least Sandpiper 2493 (•400600 •.• .[] 2700[• 400•.350 •6,•• .

500 •107 60 300180• o 57 0 3• 58• I 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6 7 8 9 10 2500Dunlin 6500 40tDøwitcher species. 284T 2000 T 8062 30/•. F•

150 900 100 5 49 6

500!2,323,,•0$$$ 29924r_•1,lillr/,/t•/• I6,7 27ß 0 1,5"='T='• "•-' i 4 0 12345678910 12345678910 4000t All 99_?910451 Count Location I.Shorebirdsm= T 3000/•/ 3041 8oo':[ -T-1716/ I•/] 400 Iii 755

o 1 2345678910 Count Location Figure2 (continued). trends.Annual trends in maximum(fall) abundance were nonsignificantfor all species(n = 10, P > 0.05), after differencesassociated with cumulative rainfallthrough November and in the previousyear were accountedfor.

DISCUSSION

Winter Abundance and Distribution My resultsconfirm Tomales Bay as a wetlandof "regional"importance on the basisof its supportingmore than 20,000 shorebirds,a criterionfor inclusionin the WHSRN (Harringtonand Perry 1995, Page and Shuford 2000). Relativeabundances of winteringshorebird species were similarto thosereported for other Point Reyesarea wetlands(Page et al. 1983, Page et al. 1992). Concurrentcounts conducted in 1989-90 in other nearby weftandsindicated that TomalesBay supportedapproximately a thirdof the

156 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

Black-bellied Plover SemipalmatedPlover Killdeer ,, ß b= -0.023 b = -0.026** ß 'E' ': "ß ß

ß i ß 8J,' ,' E - .. ß .r"•..•**t ,' . ,

ß o 1'o:•o •o,•o •o •o •o 80 o 'io•o •o,•o•o •o •o80 70 Willet 2500Marbled Godwi•t •50BlackTurnstone

[. • :.* _•--"•. ßI •ø15oo-I'.• . ß I: I '. * o__•,.•,• ,__•:,•.- I • I •* '*• _• • / * ß

- .. ] ß -- -• • •.• .

501 , ...... • u u, ..... -, , ' 0 1020304050607080 0 1020304050607080 0 1020304050607080 8Sanderling 9WesternSandpip•r 1oLeast Sandpiper b=O.01•***/• I . b=-0.012'• / b=-0.065'** • 7- .... ß • •'.;• ß • ••. ,: •• '1 • z•'...•. • a• '•,•.

ß ' ß I -- L• ß ' -- I

4 • 4• o, ...... 0 •'o•o•o•o•o•o½oao 0 • 0 20•0 4050 60 70 a0 0 •020•040s06070a0 Dunlin 7Dowitcher specie,' ) Totalshorebirds lO [• , b=-0.022'** _• 5•J. •_.•..ß ß b: -0.033'*

-3l', ,',%',',% J , , , , , , , o •'o•o •o,•o•o do•o 80 0 10 20 30 40 50 60 70 80 0 10 20 30 40 50 60 70 80 Cumulative Cumulative Cumulative seasonalrainfall (cm) seasonal rainfall(cm) seasonal rainfall(cm)

Figure3. Bivariateplots of baywideshorebird abundance (In) on cumulativeseasonal rainfallin TomalesBay. Lines represent LOWESS trends, with smoothing parameter f -- 0.6. Slopeof linearregression (b) is indicatedin eachplot: *P < 0.05, **P < 0.01, ***P < 0.001. Betweennorthern and southernTomales Bay linearslopes did not differ significantly(P < 0.05). Abundancesof the Marbled Godwit and Black Turnstone are not In-transformed.

157 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

11o loo 9o ,•,• 80 I '•% II 7o ...... I- ...... • ..... J...... 6o 50 4o 3o 2o lo

o I I I I 1989-90 1991-92 1993-94 1995-96 1997-98

Year Figure4. Means and standarderrors of cumulativeseasonal rainfall measured on shorebirdcount days on TomalesBay, from 1989-90 to 1998-99. Dashedline indicatestotal annualrainfall (July-June) across years. Horizontaldashed line indi- cateslong-term average annual rainfall. winteringshorebirds in the greaterPoint Reyes area, which includes Bolinas Lagoon,Drake's and Limantouresteros, Estero Americano, and Bodega Harbor (thisstudy and unpublisheddata from P. Connorsand G. Page). The only other baywidecounts of shorebirdsin TomalesBay are aerial surveysconducted from August 1968 to April 1970 (Smail 1972). Un- known differencesin detectability,however, preclude comparisons with theseaerial counts, which detected far fewershorebirds than my study.The aerialcounts did indicatethat TomalesBay supportedmore shorebirds than other Point Reyesarea wetlands,including Drake's and Limantouresteros and BolinasLagoon (Smail 1972). Jurek (1974) conductedcounts at the Walker Creek delta from 1969-70 to 1973-74 (area 5, Figure 1) and reportedabundances that do not differ significantlyfrom thoseI report; however,population trends could be obscuredby substantialvariability in shorebirduse from year to year and countarea to countarea (Table2). Amongseveral species, shorebirds wintering at the northernand southern endsof TomalesBay appearto representdifferent groups (this study, Kelly 2000). The apparentseparation of shorebirdsinto localwintering groups in the northernand southernportions of TomalesBay is consistentwith the spatialscale of wintersite fidelity demonstrated by other studiesalong the Californiacoast (Kelly and Cogswell1979, Warnockand Takekawa 1996). Although the compositionof shorebirdspecies in the northern and southernportions of the baywas similar, habitat conditions differed substan-

158 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY tially.Habitat differencesresulted from spatialand temporalhydrographic gradientsin the estuarythat affectsalinity (lower and more variablein the south),sediment size (finer in the south),turbidity (greater and more variable in the south),and flood disturbance (more frequent and intense in the south; •Daetwyler1966, Hollibaughet al. 1988). Suchdifferences are likelyto influencethe composition,abundance, or availabilityof invertebrateprey (Johnson1971, Wolff 1969, 1983)and, consequently,may be associated with spatialor temporalpatterns of shorebirduse. For example,Marbled Godwits concentratedin northern Tomales Bay, where substratesare composedpredominantly of well-sortedmedium and fine sand,but not in the southernportion of the bay, wheresubstrates are composedof finer particles(Daetwyler 1966). Within the Point Reyes/Bodegaarea, godwits also concentratein nearby Bodega Harbor (Page et al. 1983), where sedimentsare similarto substratesin northernbut not southernTomales Bay (Standinget al. 1975). Midwinterinfluxes of the Sanderlingand MarbledGodwit were signifi- cantlyassociated with cumulativeseasonal rainfall. These speciesoccur commonlyon nearbybeaches along the outercoast and are knownto switch habitatswith the tides (Shufordet al. 1989, Connorset al. 1981, Colwell and Sundeen2000). My studysuggested an intraseasonalinflux of these speciesinto estuarinetide flats, correlatedwith cumulativerainfall and independentof tides. Connorset al. (1981)linked tide-associatedmove- mentsbetween habitats with enhancedforaging profitability and foundthat variationin the timing of thesemovements varies seasonally to enhance profitability.Thus midwinterinfluxes of Sanderlingsand Marbled Godwits into TomalesBay mightreflect an intraseasonalincrease in the profitability of foragingin tidefiats over foraging on beaches.Alternatively, movements into TomalesBay may resultfrom a thermal advantageof refugefrom harsherweather along the outer coast. The significantmidwinter declines in severalspecies using Tomales Bay suggestlocal or regionalmovements to alternativewintering areas. At HumboldtBay Colwell(1994) alsoreported midwinter declines in several shorebirds.Nonmigratory movements of winteringshorebirds, ranging from use of localalternative habitats to interestuarinemovements to large-scale flightsfrom the coastinland, have been documentedat severalCalifornia estuaries,but in differentareas intraseasonal patterns are not necessarily similar(Page et al. 1979, Ruizet al 1989, Shufordet al. 1989, Warnocket al. 1995). Four of six principal wetlandsin the Point Reyes/Bodegaarea are considered,independently, to have potentialregional importance by the WHSRN (Harringtonand Perry 1995, Pageet al. 1999). Theseare Bolinas Lagoon(Page et al. 1979), Drake's and Limantouresteros (Page et al. 1983, Shufordet al. 1989), TomalesBay (thisstudy), and BodegaHarbor (Pageet al. 1983, Ruiz 1987). The closenessof theseareas, along with other locallyimportant wetlands, within approximately60 km of coastline indicatessubstantial importance of this area to wintering and migrating shorebirds.Movements of winteringshorebirds among wetlands in the Point Reyes/Bodegaarea have been suggestedby my resultsand by those of others(Page et al. 1979, Myerset al. 1980, Shufordet al. 1989, Ruizet al.

159 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

1989). Althoughstrong site fidelity often distinguishes wintering groups of coastalshorebirds (Warnock and Takekawa 1996, Kelly2000), the abilityto respondwith intraseasonalor episodicmovements among wetlands may be crucialto maximizingwinter survival in the faceof unpredictablechanges in foragingconditions (Connors et al. 1981, Myers 1980, Ruiz et al. 1989, Colwell1993, Warnocket al. 1995). Thus,regional complexes of wetlands suchas thosein the Point Reyes/Bodegaarea may needbroad protection and cooperationby the numerousagencies responsible for their management (Laubhanand Fredrickson1993). Page et al. (1992) reportedon the use of TomalesBay by migrating shorebirdsin the context of other sites in the PacificFlyway. Further informationon shorebirduse during spring and fall is neededfor the importanceof TomalesBay to migratingshorebirds to be understoodfully. The spatial scale and flexibilityof stopover-siteselection by migrating shorebirdswithin systems of coastalwetlands is poorlyunderstood but might involveshort-flight hopping (Iverson et al. 1996) or localmovements related to changinghabitat conditions, as found at inlandwetlands (Skagen and Knopf 1993, 1994). If so, PointReyes/Bodega area wetlands may provide extendedstopover support for migratingshorebirds. ConservationImplications Shorebirdstend to concentratewhere the density(Goss-Custard et al. 1977, Bryant 1979) or availability(Recher 1966, Goss-Custard1984) of invertebrateprey is greatest. Midwinter declines in severalshorebird species observedin thisstudy may be associatedwith the adverseeffects of heavy freshwaterrunoff and associatedsediment deposition on prey(Wolff 1983, Holland 1985, Anima et al. 1988, Nordbyand Zedler1991). Becauseof water-residencetimes as long as four months, prey in southernTomales Bay maybe particularlyvulnerable to the effectsof toxicspills or anthropogenic eutrophication(Smith and Hollibaugh1989, 1997). Midwinterchanges in shorebirduse of key feedingareas at Lagunitasand Walker creeksare consistentwith a hypothesisthat depositionof sandysediment during heavy runoff reducesthe suitabilityof some areasto foragingshorebirds. Core samplescollected in these areas 4-7 weeks after a major flood in 1982 revealeda surfacelayer of sandymaterial that was several centimeters thick and virtuallydevoid of invertebrates(Anima et al. 1988). Gerstenberg (1979) reportedreduced shorebird use in HumboldtBay after siltation of tide flatsduring heavy rains. Quammen (1982) foundreduced feeding success and abandonmentof preferredfeeding areas by Dunlins,Western Sandpip- ers,and dowitchers when tidal feeding areas were treated with a thin layerof sand.Periods of increasedsedimentation in TomalesBay havebeen associatedwith logging and intensive farming in the watershed(Rooney and Smith 1999) and could also result from development,overgrazing, or other disturbanceof uplandsoils (Storm 1972). Daily, tidal,or intraseasonalmovements of shorebirdsto alternativesites in seasonalwetlands and uplandfields have been reportedin severalareas alongthe centraland northern coast of California(Gerstenberg 1979, Ruiz et al. 1989, Rameret al. 1991, Colwell1993). Near TomalesBay, however, alternativeforaging areas are relativelylimited, and use of alternative

160 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY habitatswas generally limited to seasonalwetlands at Lawson'sMeadow and GiacominiPasture (Figure 1). Breachinglevees that isolatethe Giacomini Pasturewould restore approximately 150 ha of tidal marshand at least36 ha of tidal flats(elevation less than 1.2 m above MLLW) for shorebirdsat the southernend of the bay (PhilipWilliams and Associateset al. 1993). The protectionof routinelyused seasonal wetlands at Lawson'sMeadow adjacent to the north end of the bay may be especiallyimportant. During heavy rainfalland flooding,some shorebirds flew to seasonalwetlands 5-15 km northeastof the bay (pers.obs.). Directdisturbance by peopleor dogscan havedetrimental effects on the continuinguse of foraging(Burger 1986, Burgerand Gochfield1991) and roosting(Burger 1981, Kirby et al. 1993) areasby winteringshorebirds. In TomalesBay, aggregationsof roostingand feedingshorebirds along the shorelinebelow sanddunes at Brazil Beach, and on offshoretide flatsnorth of Tom'sPoint (areas 3 and4, Figure1), werefrequently disturbed by people from an adjacent1000-site campground (Shannon and Associates1998), who come for clammingor other activities(pers. obs.; Page and Shuford 2000). Dune beachesnear the mouth of TomalesBay may providecritical winter habitat for the threatenedWestern Snowy Plover (Charadrius alexandrinus nivosus;U.S. Fish and Wildlife Service 1995). Direct disturbanceof roostingand feedingSnowy Plovers by recreationalusers in this area wasevident during winter; during spring and summersuch disturbance may preventnesting by this speciesin otherwisesuitable habitat (Page and Shuford 2000). Gullsmay interferewith shorebirdforaging (Thompson and Lendrem 1985, Warnock1989, Amat andAguilera 1990). Duringwinter low tides at the WalkerCreek delta (area 5, Figure1), largeconcentrations of gullswere inverselycorrelated with shorebirduse of exposedtidal flats, suggesting that gullsdisplaced shorebirds from otherwisesuitable foraging habitat (Kelly et al. 1996). The gullsarrived daily from•a garbage dump approximately 20 km to the northeast. Possible indirect adverse effects of local landfills on shorebirduse of estuariesshould be investigatedfurther, and the useof such landfillsby winteringgulls could be reduced. Currently,leases for commercialoyster culture occupy 208 ha of Tomales Bay, but the limit placedon maricultureleases by the Marin County local coastalplan is 365 ha. (Tom Moore, Calif. Dept. Fish and Game, pers. comm.). Shorebirds'use of open tide flats decreaseswhen these are developedfor mariculture(Kelly et al. 1996). The two most abundant shorebirdsin TomalesBay, the Dunlin and Western Sandpiper,avoided areasdevoted to mariculturesignificantly, although Willets were attracted to oysterplots. Expansionof mariculture,or its redistributionto be concen- tratedmore heavilyon particulartidal strata,could further reduce foraging opportunitiesand possiblyabundances of winteringshorebirds. The occa- sionaluse by roostingDunlin and Western Sandpipersof floating oyster culturebags in subtidalareas during high tides(pers. obs.) suggests possible benefits to shorebirds. The invasivenonindigenous European green crab (Carcinus rnaenas) has becomeestablished in TomalesBay and canalter the abundancesof benthic invertebrateprey dramatically,although the consequencesfor shorebirds

161 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY are unknown (Grosholzand Ruiz 1995, Grosholzet al. 2000). The closeness of San FranciscoBay impliesa highpotential for invasionby nonindigenous cordgrass(Spartina alterniflora), which is establishedthere and known to affect shorebirdsadversely (Goss-Custard and Moser 1988, Daehler and Strong 1996). These concerns,along with those related to watershed effects,limited availability of alternativehabitats, and human disturbance associatedwith recreation,mariculture, and garbagedumps, indicate broad managementneeds for protectingshorebird foraging areas in TomalesBay.

ACKNOWLEDGMENTS

The followingobservers provided expertise in the field:Russ Agnew, Sarah Allen, Bob Baez, Norah & Hugh Bain, GordonBennett, Gay Bishop,Ellen Blustein, John W. Boyd,Brian Bullick, Ken Burton,Yvonne Chan, LisaCohen, Rigdon Curtie, Ann & Eric Davis,Mark Dean, Linda Devere, Jim Devore, John Dillon, CarolineDutton, Lew & Marilyn Edmondson,Steve Engel, Gayanne Enquist,Katie Etienne, Jules Evens,Gary A. Falxa, Katie Fehring,Grant & Ginny Fletcher,Carol Fraker,Dan Froehlich,Nicole Gallagher, Robert Gleason, Quinton Goodrich, Gayle Greeley, Sue Gueres, Catherine M. Hickey, Terry Horrigan, Lisa Hug, Maggie Hynes, Jeri Jacobson,Lynnette Kahn, Mary Ellen King, RichardKirschman, Carol Kuelper, Darlene Lam, Sarah Lynch, Flora Maclise,Aspen Mayers,John Mc Donagh, Dan Murphy,Terry Nordbye, David Press, Jim Rappold,Ellen Sabine, Dave Schurr,Craig Scott, Anne Spencer,Rich Stallcup,Judy Temko, Stephen Thal, Janet Thiessen, WayneThompson, Gil Thomson,Forest Tomlinson, Tanis Walters, Diane Williams, DavidWimpfheimer, and Chris Wood. Susan Kelly, Sarah Tappen, and Katie Etienne providedvaluable assistance in the coordinationof shorebirdcounts and management of data. Peter Connors,Gary Page, David Shuford,and Rich Stallcupprovided valuableadvice in designingthe monitoringprogram. Thomas R. Famula,Robert E. Gill, William J. Hamilton I||, Lewis W. Oring, and WesleyW. Weathersprovided valuablereview of the manuscript.This paper is a contributionof AudubonCanyon Ranch.

LITERATURE CITED

Amat, J. A., andAguilera, E. 1990. Tacticsof Black-headedGulls robbing egrets and waders. Anim. Behav. 39:70-77. Anima,R. J., Bick,J. L., and Clifton,H. E. 1988. Sedimentologicconsequences of the stormin TomalesBay, in Landslides,Floods, and Marine Effects of the Storm of January3-5, 1982, in the San FranciscoBay Region,California (S. D. Ellen and G. F. Waczorek, eds.). U.S. Geol. Surv., Menlo Park, CA. Bryant,D. M. 1979. Effectsof prey densityand sitecharacter on estuaryusage by overwinteringwaders (Charadrii). Estuarine and CoastalMar. Sci. 9:369-384. Burger,J. 1981. The effectof humanactivity on birdsat a coastalbay. Biol. Conserv. 21:231-241. Burger,J. 1986. The effectof humanactivity on shorebirdsin two coastalbays in northeastern United States. Env. Conserv. 13:123-130. Burger,J., and Gochfeld,M. 1991. Human activityinfluence and diurnal and nocturnalforaging of Sanderlings(Calidris alba). Condor93:259-265. Burger,J., Howe, M., Hahn, A., Caldwell,D., and Chase,J. 1977. Effectsof tide cycleson habitatselection and habitatpartitioning by migratingshorebirds. Auk 94:743-758.

162 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

Chambers,J. M., Cleveland,W. S., Kleiner,B., and Tukey,P. A. 1983. Graphical Methodsfor Data Analysis.Wadsworth & Brooks/Cole,Pacific Grove, CA. Cleveland, W. S. 1979. Robust locally weighted regressionsand smoothing scatterplots.J. Am. Stat. Assoc.74:829-836. Colwell,M. A. 1993. Shorebirdcommunity patterns in a seasonallydynamic estuary. Condor 95:104-114. Colwell,M. A. 1994. Shorebirdsof HumboldtBay, California: Abundance estimates and conservationimplications. W. Birds25:137-145. Colwell, M. A., and Sundeen, K. D. 2000. Shorebirddistribution on ocean beaches of northern California. J. Field Ornithol. 71:1-15. Connors,P. G., and Maron, J. L. 1989. EsteroAmericano bird populationstudy. Long-termDetailed Wastewater Reclamation Studies, Santa RosaSubregional Water ReclamationSystem. Tech. memorandum to the city of SantaRosa, 100 Santa RosaAve., Santa Rosa, CA 95403. Connors,P. G., Myers,J.P., Connors,C. S. W., andPitelka, F. A. 1981. Interhabitat movementsby Sanderlingsin relationto foragingprofitability and the tidalcycle. Auk 98:49-64. Daehler, C. C., and Strong, D. R. 1996. Status, predictionand preventionof introducedcordgrass $partina spp. invasionsin Pacificestuaries, USA. Biol. Cons. 78:51-58. Daetwyler,C. C. 1966. Marinegeology of TomalesBay, central California. Pac. Mar. Sta. Res. Rep. 6:1-169. Galloway,A. J. 1977. Geologyof the Point ReyesPeninsula, Marin County.Calif. Div. Mines Geol. Bull. 202. Gerstenberg,R. H. 1979. Habitatutilization by wintering and migrating shorebirds on HumboldtBay, California.Studies Avian Biol. 2:33-40. Goss-Custard,J. D. 1977. Optimalforaging and sizeselection by wormsin Red- shank,Tringa totanus,in the field.Anim. Behav.25:10-29. Goss-Custard,J. D. 1984. Intakerates and food supplyin migratingand wintering shorebirds,in Behaviorof Marine Animals,vol. 6, Shorebirds:Migration and ForagingBehavior (J. Burgerand B. L. Olla, eds.),pp. 233-270. Plenum,New York. Goss-Custard,J. D., and Moser, M. E. 1988. Ratesof changein the numbersof Dunlin,Calidris alpina, winteringin Britishestuaries in relationto the spreadof Spartina anglica. J. Appl. Ecol. 25:95-109. Grosholz,E. D., and Ruiz,G. M. 1995. Spreadand potentialimpact of the recently introducedEuropean green crab, Carcinusmaenas, in centralCalifornia. Mar. Biol. (Berlin) 122:239-247. Grosholz,E. D., Ruiz,G. M., Dean, C. A., Shirley,K. A., Maron,J. L., andConnors, P. G. 2000. The impactsof a nonindigenousmarine predator in a Californiabay. Ecology81:1206-1224. Harrington, B., and Perry, E. 1995. Important shorebirdstaging sites meeting WesternShorebird Reserve Network criteria in the UnitedStates. Report of the WesternHemisphere Shorebird Reserve Network, Manomet Observatory, P.O. Box 1770, Manomet, MA 02345. Holland,A. F. 1985. Long-termvariation of macrobenthosin a rnesohalineregion of ChesapeakeBay Estuaries8:93-113.

163 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

Hollibaugh,J. T., Cole, B. E., Dollar,S. J., Hager,S. W., Vink, S. M., Kimmerer,W. J., Obrebski,S., Smith, S. V., Valentino,M., and Walsh,W. W. 1988. Tomales Bay, California:A macrocosmfor examiningbiogeochemical coupling at the land-sea interface. Eos 69:843-845. Holway,D. A. 1990. Patternsof wintershorebird occurrence in a SanFrancisco Bay marsh. W. Birds 21:51-64. Iverson,G. C., Warnock,S. E., Butler,R. W., Bishop,M. A., andWarnock, N. 1996. Spring migration of Western Sandpipersalong the Pacific coast of North America:A telemetrystudy. Condor 98:10-2I. Johnson,R. G. 1971. Animal-sedimentrelations in shallow-waterbenthic communi- ties. Mar. Geol. 11:93-104. Jurek, R. M. 1974. Californiashorebird survey 1969-1974. Spec. WildlifeInvest. Rep. Pro. W-54-R, Job III-1. Calif. Dept. Fish and Game, 1416 Ninth St., Sacramento, CA 95814. Kelly,J.P. 2000. Foragingdistribution and energybalance in winteringDunlin. Ph. D. dissertation,Univ. of Calif., Davis. Kelly, J.P., Evens,J. G., Stallcup,R. W., and Wimpfheimer,D. 1996. Effectsof aquacultureon habitatuse by winteringshorebirds in TomalesBay, California. Calif. Fish & Game 82:160-174. Kelly,J.P., and Tappen, S. L. 1998. Distribution,abundance, and implicationsfor conservationof winterwaterbirds on TomalesBay, California.W. Birds29:103- 120. Kelly, P. R., and Cogswell,H. L. 1979. Movementsand habitatuse by wintering populationsof Willetsand MarbledGodwits. Studies Avian Biol. 2:69-82. Kirby, J. S., Clee, C., and Seager,V. 1993. Impact and extent of recreational disturbanceto waderroosts on the Dee estuary:Some preliminary results. Wader StudyGroup Bull. 68:53-58. Laubhan,M. K., and Fredrickson,L. H. 1993. Integratedwetland management: Conceptsand opportunities, in Trans.N. Am. Wildlifeand Nafi. ResourcesConf. 58 (R. E. McCabeand K. A. Glidden,eds.), pp. 323-334. WildlifeMgmt. Inst., Washington,D.C. 20005. Myers,J.P. 1980. SanderlingsCalidris alba at BodegaBay: Facts,inferences and shamelessspeculations. Wader Study Group Bull. 30:26-32. NationalOceanic and AtmosphericAdministration. 1987. Gulf of the Farallones NationalMarine Sanctuarymanagement plan (J. Dobbin Associates,Inc., eds.). U. S. Dept. Commerce,NOAA, Marineand EstuarineMgmt. Div., Washington, D.C. 20235. Neubacher,D., Dell'Osso,J., Livingston,D., and Murray, R. 1995. TomalesBay/ BodegaBay watershedboundary study. U.S. Govt. PrintingOffice 1995-685- 430, Point ReyesNational Seashore, Point Reyes,CA 94956. Nordby, C. S., and Zedler, J. B. 1991. Responsesof fish and macrobenthic assemblagesto hydrologicdisturbances in Tijuanaestuary and Los Pefiasquitos Lagoon,California. Estuaries 14:80-93. Page,G. W., andShuford, D. 2000. SouthernPacific Coast Regional Shorebird Plan, version1.0. U.S. ShorebirdPlanning Council, U.S. ShorebirdConservation Plan. PointReyes Bird Observatory, 4990 ShorelineHwy., StinsonBeach, CA 94970. Page, G. W., Stenzel, L. E., and Kjelmyr, J. E. 1999. Overview of shorebird abundanceand distributionin wetlandsof the Pacificcoast of the contiguous United States. Condor 101:461-471.

164 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

Page, G. W., Shuford,W. D., Evens,J. G., and Stenzel,L. E. 1983. The distribution and abundanceof aquaticbirds in wetlandsof the Point Reyesto Bodegaarea. Report to the Point Reyes-FarallonesNational Marine Sanctuary.Point Reyes Bird Observatory,4990 ShorelineHwy., StinsonBeach, CA 94970 Page, G. W., Shuford,W. D., Kjelmyr,J. E., and Stenzel,L. E. 1992. Shorebird numbersin wetlandsof the PacificFlyway: A summaryof countsfrom April 1988 to January1992. PointReyes Bird Observatory, 4990 ShorelineHwy., Stinson Beach, CA 94970. Page,G. W., Stenzel,L. E., and Wolfe, C. M. 1979. Aspectsof the occurrenceof shorebirdson a centralCalifornia estuary. Studies Avian Biol. 2:15-32. PhilipWilliams & Associates,Ltd., WetlandsResearch Associates, Strong Associates, and Butler,L. J. 1993. An evaluationof the feasibilityof wetlandrestoration on the GiacominiRanch, Marin County.Philip Williams & Associates,Pier 35, The Embarcadero,San Francisco,CA 94113. Quammen, M. L. 1982. Influenceof subtlesubstrate differences on feedingby shorebirdson intertidal mudfiats. Mar. Biol. (Berlin) 71:339-343. Ramer,B. A., Page,G. W., and Yoklavitch,M. M. 1991. Seasonalabundance, habitat use,and diet of shorebirdsin ElkhornSlough, California. W. Birds22:157-174. Recher,H. F. 1966. Some aspectsof the ecologyof migrantshorebirds. Ecology 47:393-407. Rooney,J. J., and Smith, S. V. 1999. Watershedland use and bay sedimentation.J. Coastal Res. 15:478-485. Ruiz, G. M. 1987. Interactionsamong shorebird,crab, and their invertebrateprey populations.Ph.D. dissertation,Univ. of Calif., Berkeley. Ruiz, G. M., Connors,P. G., Griffin, S. E., and Pitelka, F. A. 1989. Structureof a winteringDunlin population. Condor 91:562-570. Shannon and Associates.1998. Lawson'sLanding Master Plan. Shannon and Associates,P.O. Box 322, BodegaBay, CA 94923. Shuford,W. D., Page, G. W., Evens,J. G., and Stenzel, L. E. 1989. Seasonal abundanceof waterbirdsat Point Reyes:A coastalCalifornia perspective. W. Birds 20:137-265. Skagen, S. K., and Knopf, F. L. 1993. Toward conservationof midcontinental shorebirdmigrations. Cons. Biol. 7:533-541. Skagen,S. K., andKnopf, F. L. 1994. Migratingshorebirds and habitat dynamics at a prairiewetland complex. Wilson Bull. 106:91-105. Smail,J. 1972. The birdsof TomalesBay, in TomalesBay EnvironmentalStudy: Compendiumof Reports(R. Corwin,ed.), pp. 124-134. ConservationFounda- tion, Washington,D. C. Smith, S. V., and Hollibaugh,J. T. 1989. Carbon-controllednitrogen cycling in a marine"macrocosm": and ecosystem-scale model for managingcultural eutrophication. Mar. Ecol. Prog. Ser. 52:103-109. Smith, S. V., and Hollibaugh,J. T. 1997. Annualcycle and interannualvariability of ecosystemmetabolism in a temperate climate embayment.Ecol. Monogr. 67:509-533. Standing,J., Browning,B., and Speth,J. W. 1975. The naturalresources of Bodega Harbor.Calif. Dept. Fishand Game CoastalWetland Ser. 11. Storm, D. R. 1972. Hydrologyand water qualityaspects of the TomalesBay Study, in TomalesBay EnvironmentalStudy: Compendium of Reports(R. Corwin,ed.), pp. 84-101. ConservationFoundation, Washington, D.C.

165 DISTRIBUTION AND ABUNDANCE OF WINTER SHOREBIRDS ON TOMALES BAY

Thompson,D. B. A., and Lendrem,D. W. 1985. Gullsand plovers:Host vigilance. Behaviour 33:1318-1324. U.S. Fishand Wildlife Service. 1995. Proposeddesignation of criticalhabitat for the Pacificcoast population of the WesternSnowy Plover.Fed. Reg. 60:11767- 11809. Warnock,N. 1989. Piracyby Ring-billedGulls on Dunlin.Wilson Bull. 101:96-97. Warnock,N., Page, G. W., and Stenzel,L. E. 1995. Non-migratorymovements of Dunlinson their Californiawintering grounds. Wilson Bull. 107:131-139. Warnock, S. E., and Takekawa, J. Y. 1995. Habitat preferencesof wintering shorebirdsin a temporallychanging environment: Western Sandpipersin the San FranciscoBay estuary.Auk 112:920-930. Warnock, S. E., and Takekawa,J. Y. 1996. Winteringsite fidelityand movement patterns of Western SandpipersCalidris rnauri in the San FranciscoBay estuary.Ibis 138:160-167. Wolff, W. J. 1969. Distributionof non-breedingwaders in an estuarinearea in relation to the distributionof their food organisms.Ardea 57:1-27. Wolff, W. J. 1983. Estuarinebenthos, in Ecosystemsof the World 26: Estuariesand EnclosedSeas (B. H. Ketchum, ed.), pp. 151-182. Elsevier Scientific, Amsterdam.

Accepted 2 June 2001

WesternSandpipers Sketch by George C. West

166