WESTEP BIRDS Volume 34, Number 1, 2003 BREEDING STATUS AND HABITAT USE OF BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SOUTH SAN FRANCISCO BAY CHRIS RINTOUL, NILS WARNOCK, and GARY W. PAGE, PRBO Conservation Science,4990 ShorelineHighway, Stinson Beach, California 94970 JANET T. HANSON, San FranciscoBay Bird Observatory,P.O. Box 247, Alviso, California 95002 ABSTRACT: In light of recentand proposedrestoration projects that will affect bird numbersin San FranciscoBay, California,we assessedthe statusof breeding populationsof the Black-neckedStilt (Himantopus mexicanus) and American Avocet (Recurvirostraamericana) in SouthSan FranciscoBay in May 2001. We counted 1184 stiltsand 2765 avocets.Considering only birdsobserved exhibiting breeding behaviors,our low estimatesof breedingbirds in the southbay were 270 stiltsand 880 avocets,but the true numbersare probablycloser to the numberof stiltsand avocets we actuallycounted. Our estimatesof the breedingpopulation fall within the rangeof similarestimates from the southbay 20-30 yearsago. We know of no othersites on the Pacificcoast of the UnitedStates with breedingpopulations of stiltsand avocets whosesizes approach those of the SouthSan FranciscoBay. The greatestnumbers of stiltsand avocetsbred on salt ponds in the south bay; lessernumbers bred in a combinationof fresh and salt marshes.The observeduse by stiltsand avocetsof availablehabitat differed significanfiy from expected use. Stilts used tidal marshes and saltponds approximately in orderof availability,whereas avocets made greater use of salt ponds. Within marshes,stilts most often used vegetatedareas followed by mudfiat/openwater habitat, whereas for avocetsthe patternwas reversed. Within salt ponds,both specieswere most often observedon islands,but their order of useof other microhabitatsin salt pondsdiffered. We observedlittle use of tidal fiats by breedingstilts and avocets. The San FranciscoBay estuary(hereafter, the bay)is recognizedas a site of hemisphericimportance to shorebirds(Harrington and Perry 1995) becauseit holdsover 500,000 shorebirds(Page et al. 1999). Over 90% of the bay'swetlands, especially tidal marsh, have been filled or dikedover the past I50 years to create agriculturallands and salt-evaporationponds (Harvey et al. 1988, Goals Project 1999). Salt ponds now cover over 12,000 ha aroundSan Franciscoand San Pablobays (Goals Project 1999), the majoritybeing in South San FranciscoBay (hereafter,the southbay). 2 WesternBirds 34:2-14, 2003 BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY The presenceof American Avocet bones in native American middens suggeststhat this species was in the bayprior to the firstpublished report in 1884 (Grinnellet al. 1918, Howard 1929). The firstbreeding record was an observationof downyyoung in 1926 (Gill 1977), a year prior to Grinnelland Wythe's(1927) listingthe bird as an irregularlycommon visitor to the bay. No additionaldocumentation of breedingby avocetsexists prior to Martin's (1939) discoveryof youngin SantaClara County in 1937. By 1952, Sibley (1952) consideredthe speciesto be a common residentbut listed only scatteredbreeding records for 1941 and 1950. In 1972, Gill (1972) estimated1800 breedingpairs in the southbay. Subsequent studies by Moss (1980) and Rigneyand Rigney(1981) estimated800 and 650 breeding pairs,respectively. Grinnellet al. (1918) notedthat Black-neckedStilts appeared sparingly in the bay, and Grinnelland Wythe(1927) reportedthe firstnesting there by this species. The Black-necked Stilt remained an uncommon summer residentand rare winter visitantin the southbay throughthe early 1950s (Sibley1952). Numbersof nestingstilts increased over the nexttwo decades with breedingpopulations in the southbay estimatedat 400-500 pairs in 1971 (Gill 1972) and 600-650 pairsin 1981 (Rigneyand Rigney1981). The creationof saltponds has beencredited with increasingbreeding and nonbreedingpopulations of Black-neckedStilts and AmericanAvocets in the bay (Gill 1977, Harvey et al. 1988), providingroosting, foraging, and nestinghabitat for both species. Here, we reporton a springsurvey in 2001 that examinedthe abundance, distribution,behavior, and habitat use of stiltsand avocets in the southbay. We compareour resultsto thosefrom prior surveys in the early1970s and 1980s anddiscuss our findingswith respectto variousactive or proposedrestoration projectsthat may affectfuture breeding populations of thesebirds. STUDY AREA AND METHODS We surveyedstilts and avocetsin San FranciscoBay southof the San Mateo Bridge(Figures 1 and 2) where other studieshave focused and where the majorityof the estuary'sstilts and avocets breed (Gill 1977, Harveyet al. 1992). Two surveyteams started on oppositesides of the bay and, in general,moved south to completecoverage of the studyarea. We searched saltponds and other wetlands in theirentirety for adultstilts and avocets. We alsosurveyed tidal fiats adjacent to saltponds and marshesthat borderthe bayas far out aswe couldsee. Although we triedto coverall wetlands,some privatesalt-crystallization ponds were not accessible(see Figure 1 or 2). In addition,outer Bair Island and its immediate vicinity, including adjacent tidal fiats,were not surveyedbecause of difficultiesin access.We surveyed9613 ha of saltponds, 4068 ha of tidal or dikedmarshes, 575 ha of other diked wetlands,and approximately4039 ha of tidalfiats. StudyPeriod and SurveyTechnique We surveyedfor 120 hoursfrom 15 to 25 May 2001, duringthe peak breedingperiod for stilt and avocetsin the southbay (PRBO unpubl.data; seealso Robinson et al. 1997, Robinsonet al. 1999). Two teamsof two or BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY •z BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY threeobservers drove or walkedall leveesand roads, counting from vehicles or exitingthe vehicleto countareas requiring a greaterfield of view, using spottingscopes and binoculars.Although locating nests was not the goalof the survey,we recordedall neststhat we found.We triedto avoiddisturbing nestingbirds, but in a few areasof high nestingactivity, observers walked leveesand countednests and eggs. Data collectedfor adult birds includednumber of individuals,behavior, habitat, microhabitat,and observedbreeding status. Adults' behavior was recorded as (1) alarm (alarm calling though not actively engaged in a distractiondisplay), (2) brooding (adultsattending young), (3) breeding display(copulation or postcopulatorydisplay), (4) distractiondisplay (mob- bing behavioror broken-wingdisplay), (5) feeding(eating or searchingfor food), (6) incubating(sitting on or standingat a nest),(7) roosting(sitting, standing,or preening),(8) alert (standingin an alert postureand not alarm calling), or (9) other. We did not use incomplete and/or questionable behaviorobservations for analysis(<2% of total observations).For most analyses,we groupedbehaviors as potential breeding,feeding, or other behaviors.Breeding activities included brooding, breeding display, distrac- tion display,and incubationbehaviors. Feeding and swim-feedingbehaviors made up the feeding category,while alarm, alert, roosting,and other behaviorsconstituted the other category. For analysis,we categorizedhabitats as.' marsh, salt pond, tidal fiat, or other wetland.Marshes included all tidal, freshwater,diked, and vegetated marshes.Other wetland habitat includedother diked wetlands,sewage ponds,and miscellaneoushabitats. Salt pondsand tidal mudfiatswere their own categories. We categorizedmicrohabitats as (1) channel(channel or sloughwithin a habitat),(2) dike(on side or top of dikeor leveebordering a habitat),(3) island (ofdry substrate that could not be coveredby water in a strongwind), (4) mud (dry or wet, includingshallow water <10 cm deep), (5) shore(wet or dry substratewithin 1 m of shoreline),(6) vegetatedmarsh, or (7) water (> 10 cm deep). We chose 10 cm as an approximatecut-off depth becausewater depthsof > 10 cm precludeuse by mostshorebirds except those that swim (Safranet al. 1997, Isola et al. 2000). We plottedlocations of observationson mapsvisually, then transferred themto a geographic-informationsystem by meansof ArcView3.2a (ESRI, Redlands,CA). Statistics We tested for differencesbetween speciesin frequenciesof behavior, habitat,and microhabitat use with Pearson's •z test(Snedecor and Cochran 1967), usingStata (version6.0; Stata Corp., CollegeStation, TX). For our analysiscomparing observedversus expected habitat use of stilts and avocets,we calculatedthe expectedfrequency of habitatuse based on the area of each habitat we surveyed(see Methods above). For instance, marshesconstituted 22.2 % of the habitatwe compared(Table 1); therefore, the expectedfrequency of the 401 observationsof Black-neckedStilts for that habitat was 89 observations.For all analysis,we recordedgroups of BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY Talkie 1 Habitat Use of Black-neckedStilts and AmericanAvocets During Surveysof South San FranciscoBay, California, May 2001 Black-neckedStilt Observed AmericanAvocet Observed Expecteda Marshb 29.2% 13.5% 22.2% Saltpond 55.4% 75.6% 52.6% Tidal mudfiat 1.2% 1.3% 22.2% Other weftandc 14.2% 9.6% 3.2% Total observations• 401 757 aExpecteduse basedon availablehabitat (seeMethods). bMarsh habitat includesboth fresh and salt marshes. cOtherwetlands include diked wetlands, sewage ponds, and other habitats. •ln all habitats combined. individualsengaged in the samebehavior at the sametime and placeas one observationto