WESTERN BIRDS Volume 33, Number 2, 2002 ABUNDANCE AND DISTRIBUTION OF SHOREBIRDS IN THE SAN FRANCISCO BAY AREA LYNNE E. STENZEL, CATHERINE M. HICKEY, JANET E. KJELMYR, and GARY W. PAGE, Point ReyesBird Observatory,4990 ShorelineHighway, Stinson Beach, California 94970 ABSTRACT: On 13 comprehensivecensuses of the San Francisco-SanPablo Bay estuaryand associatedwetlands we counted325,000-396,000 shorebirds (Charadrii)from mid-Augustto mid-September(fall) and in November(early winter), 225,000 from late Januaryto February(late winter); and 589,000-932,000 in late April (spring).Twenty-three of the 38 speciesoccurred on all fall, earlywinter, and springcounts. Median counts in one or moreseasons exceeded 10,000 for 10 of the 23 species,were 1,000-10,000 for 4 of the species,and were less than 1,000 for 9 of the species.On risingtides, while tidal fiats were exposed,those fiats held the majorityof individualsof 12 speciesgroups (encompassing 19 species);salt ponds usuallyheld the majorityof 5 speciesgroups (encompassing 7 species); 1 specieswas primarilyon tidal fiatsand in other wetlandtypes. Most speciesgroups tended to concentratein greaterproportion, relative to the extent of tidal fiat, either in the geographiccenter of the estuaryor in the southernregions of the bay. Shorebirds' densitiesvaried among 14 divisionsof the unvegetatedtidal fiats. Most species groups occurredconsistently in higherdensities in someareas than in others;however, most tidalfiats held relativelyhigh densitiesfor at leastone speciesgroup in at leastone season.Areas supportingthe highesttotal shorebirddensities were also the ones supportinghighest total shorebird biomass, another measure of overallshorebird use. Tidalfiats distinguished most frequenfiy by highdensities or biomasswere on the east sideof centralSan FranciscoBay andadjacent to the activesalt ponds on the eastand southshores of southSan FranciscoBay and alongthe Napa River,which flowsinto San Pablo Bay. The bay is criticalto large numbersof wintering,migrating, and breedingshorebirds, despite extensive loss of natural wetlands.Geographic limita- tions of species'distributions in the bay shouldbe consideredwhen wetlandrestora- tion is planned. The San Francisco-SanPablo Bay estuary and associatedwetlands (hereafter,the bay) are of hemisphericimportance (Harrington and Perry 1995) to winteringand migratingshorebirds. On the conterminousU.S. Pacificcoast, the bay holdsmore total shorebirdsthan any otherwetland in all seasons,and it holdsthe majorityof individualsof the 13 mostabundant WesternBirds 33:69-98, 2002 69 ABUNDANCE AND DISTRIBUTION OF SHOREBIRDS IN SAN FRANCISCO BAY shorebirdsin one or more seasons(Page et al. 1999). While a varietyof naturaland artificialhabitats support shorebirds in the bay, more detailed patternsof habitatuse are knownonly generally for all shorebirdsand locally for a few species. In 1964 and 1965, Bollmanet al. (1970) conducteda large-scalebut not comprehensivecensus of water birds in the bay, documentingsizable populationsand generaldistribution patterns. Otherwise, past studiesof shorebirdsusing the bay have focusedprimarily on seasonalabundance patterns,species composition at individualsites (Storer 1951, Recher1966, Holway 1990), or occurrencein specificnontidal habitats (Anderson 1970, Swarth et al. 1982, R. Pratt unpubl.data). While studiesof shorebirds' habitat use in the bay have addressedthe Marbled Godwit, Willet, and WesternSandpiper (Luther 1968, Kellyand Cogswell1979, Warnockand Takekawa 1995), there hasbeen no overallassessment of the relativerole of differenthabitat types or areasin supportingshorebirds. Detailed knowledge of habitatuse is criticalfor recognizingthe importanceof differentparts of the bayto shorebirds,for maintainingthe integrityof habitatsystems used by individualbirds, and for maximizingthe qualityof weftandscreated, restored, or managedin the bay. It is alsoimportant to understandingthe sourcesand significanceof environmentalcontaminants found in shorebirds,especially in urbanizedenvironments such as San FranciscoBay (Ohlendorf and Fleming 1988). In thispaper we reportthe abundanceand spatialdistribution of the bay's mostcommon shorebirds and comparetheir densitiesby 14 divisionsof the tidal fiat. STUDY AREA Our studyarea was the intertidalportion of the San Francisco-SanPablo Bay estuaryand associatednontidal wetlands (Figure 1). Habitats covered during surveysincluded intertidal (fiats, sloughs,and marshes),actively managed salt ponds, and "other" wetlands (sewageand other water- treatmentponds, salt ponds in disuse,and a widevariety of dikedwetlands). Prior to the firstcensus, we reconnoiteredthe bay'sshore, salt ponds, and other wetlandsextensively. From these visitswe outlinedand described censusareas and devisedprotocols for coverageintended to minimize undetectedflock movements that wouldintroduce count errors (Stenzel and Page 1988). Prior to subsequentcensuses we madea reconnaissancefor the availabilityof saltponds and otherwetlands to keep our census-sitedescrip- tionscurrent with changesin habitatconditions and accessibility. We dividedthe bayinto fourmajor regions: San PabloBay (SPB) between the CarquinezBridge and points San Pedro and San Pablo, North San FranciscoBay (SFN)between points San Pedroand San Pabloand the San Francisco-OaklandBay Bridge,Central San FranciscoBay (SFC)between the San Francisco-OaklandBay and San Mateo bridges,and South San FranciscoBay (SFS) south of the San MateoBridge (Figure 1). SFN and SFC combinedwere called Central San FranciscoBay by Stenzeland Page (1988) and the Goals Project(1999). We further subdividedthe intertidal fiatsof the four regionsinto 14 tracts(Figure 1). 7O ABUNDANCE AND DISTRIBUTION OF SHOREBIRDS IN SAN FRANCISCO BAY San Pablo 10 0 10 YJIom•er• '•. "Central• •• BaY'c31 w•s• 'South Bay'•• Figure1. San Francisco-SanPablo Bay estuary,showing San PabloBay and three regionsof San FranciscoBay. Tidal tractsin San PabloBay Regioncomprise P1, the west shore between Point San Pedro (referencepoint a) and the Petaluma River mouth (b);P2, the northwestshore between point b and Sonoma Creek (c);P3, the northeastshore between point c and the Napa Rivermouth (d); P4, Napa Riverfiats northof pointd; P5, the eastshore between the CarquinezBridge (e) and Point Pinole (f); and the southeastshore between point f and Point San Pablo(g). Tidal tractsin north San FranciscoBay compriseN1, the west shore betweenpoint a and the GoldenGate (h); and N2, the eastshore between point g andthe BayBridge (i). Tidal tractsin centralSan FranciscoBay compriseC i, the west shorebetween i and the Hayward-SanMateo Bridge0); C2, San LeandroBay (k) and the southshore of Alameda;C3, Haywardshoreline between Bay Farm Island(1) and j. Tidal tractsin southSan FranciscoBay includeS1, the west shorebetween j and the Dumbarton Bridge(m); S2, the eastshore between j and m; and S3, the shoresouth of m. Base map from the EcoAfias,version 1.5003, July 1998, San FranciscoEstuary Institute. 71 ABUNDANCE AND DISTRIBUTION OF SHOREBIRDS IN SAN FRANCISCO BAY The westernand northern shoresof San Pablo Bay are surroundedby extensiveagricultural and undevelopeddiked baylands, the largestexpanses of tidalmarsh in the bay,and actively managed salt-evaporation ponds. The easternshore of SPB is partiallydeveloped for residencesand industry. San PabloBay itselfis relativelyshallow; a broadswath of tidalfiat rimsthe west and north shores,and abouthalf the bay is underless than 2 m of waterat low tide. SPB comprisessix tidal tracts (Figure 1), whichtogether were 34- 36% of the total tidalfiat surveyedon our three fall and five springcounts. North San FranciscoBay is distinguishedby a heavilyurbanized shoreline, with most of the rockyintertidal and bluff-backedshore in the bay, and a smallamount of supralittoralwetland habitat. SFN is mosfiydeep water with relativelylittle tidal fiat, whichwe allocatedinto two tidal tracts(Figure 1). Only 8-9% of the total tidal fiat surveyedon our three fall and five spring counts was in SFN. The westernshoreline of centralSan FranciscoBay is developedmostly for industrybut the easternside is a mixtureof developedand undeveloped uplandsand restoredwetlands. A wideswath of tidaland shallowsubtidal fiat dominatesthe southeastshore, but tidal fiat is patchyelsewhere in this region(Figure 1). SFC comprisesthree tidal tracts(Figure 1), whichcom- binedwere I4-I6% of the tidalfiat surveyedon our threefall and fivespring counts. South San FranciscoBay is rimmed by a large systemof managedand disusedsalt ponds, with residential|ydeveloped shoreline limited to the northwestportion of the region.Tidal and shallowsubtidal fiats lie outside the outerlevees of the saltponds. SFS comprises three tidal tracts (Figure 1), togethermaking up 41-44% of the tidal fiat surveyedon our three fall and five springcounts. There wereapproximately 11,400 ha of tidalflats above 0.0 meanlower low water(MLLW) and 14,000 ha of activelymanaged salt ponds in the bay duringour study.We covered83-91% of the tidalfiat on the surveys.North of San PabloBay were 2890 ha of saltponds; reconnaissance trips revealed few shorebirdsand littleshallow habitat, so we coveredonly 7.2-13% of the saltponds in thisarea. At the southend of the baythere were 11,100 ha of salt ponds,of whichwe covered26-87% on the surveys. METHODS We conducted13 censusesbetween April 1988 and April 1993 to estimatetotal numbersof shorebirdsin the bay. We madethree fall counts betweenmid-August and mid-September,1988-1990, three "earlywinter" countsin earlyNovember