Breeding Status and Habitat Use of Black-Necked Stilts and American Avocets in South San Francisco Bay

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 estimates of 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 knowof 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 avoidviolating assumptions of independenceof observations (Martin and Bateson1986). Statisticaltests were two-tailed,and differences were consideredsignificant at P = 0.05.

RESULTS

We recorded 1184 adultsand 71 chicksof the Black-neckedStilt, 2765 adults and 189 chicks of the American Avocet. Of 397 behavior observations of stilts,30.5% were breeding,42.3% were feeding,and 27.2% as other behaviors.Of 753 observationsof avocets,35.5% were breeding, 32.7% were feeding,and 31.9% were recordedas other behaviors. Salt ponds, the most extensivehabitat we surveyed,contained the greatestnumbers of stiltsand avocets (Table 1). Marshesheld the nextlargest numbers, followed by other wetlands and tidal mudflat. Both species' observedhabitat use differed significantlyfrom that expectedif use was randomuse (Black-necked Stilt, •:23 = 106.8, P = 0.000, if tidalfiats are excluded,•:2• = 22.6, P = 0.000; AmericanAvocet, •:•3 = 211.4, P = 0.000, if tidalflats are excluded, •:• = 54.1, P = 0.000; Table1). If analyses are restrictedto nesting birds, both species' habitat use also differed significantly(•:2• = 53.5, P = 0.000). Of the 137 Black-neckedStilt nests described,21% were in marshes,69% were aroundsalt ponds, and 9% were in other habitats,whereas of the 409 American Avocet nestsdescribed, 3% were in marshes,93% were around salt ponds, and 4% were in other habitats.There was no significantdifference in habitat use of stiltsand avocetswith broods (•:• = 2.21, P = 0.33). Of the 15 Black-neckedStilt broodsdescribed, 20% were in marshes,53% were aroundsalt ponds, and 27% were in other habitats,whereas of the 63 American Avocet broods described,11% were in marshes,73% were around salt ponds, and 16% were in other habitats.We observedfive groupsof stiltsand ten of avocets usingtidal fiats(Table 1). BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY

Table 2 Comparisonof ObservedMicrohabitat Use by Black-necked Stilts and American Avocets within Marshes and Salt Pondsof SouthSan FranciscoBay, California, May 2001

Marshes Salt Ponds

Microhabitat Stilt Avocet Stilt Avocet

Channel 2.6% 10.8% --• -- Levee -- -- 13.1% 18.7% Island 1.7% 2.9% 31.1% 39.5% Mudfiat/openwater 42.7% 51.0% 26.1% 24.7% Shoreline -- -- 29.7% 16.4% Vegetatedmarsh 47.9% 34.3% -- -- Other 5.1% 1.0% 0% 0.7% Totalgroups b 117 102 222 572

a__,microhabitat not presentin broaderhabitat category. ballmicrohabitats combined (see Methods).

Stiltsand avocets differed in microhabitatuse in bothmarshes (Z24 -- 12.3, P = 0.016) andsalt ponds (Z24 -- 21.8, P -- 0.000). In marshes,stilts were mostoften observedin vegetatedareas, followed by mudflat/openwater habitat, whereasfor avocets,the pattern was reversed(Table 2). In salt ponds,both specieswere most often observedon islands,but their order of useof other microhabitatsin saltponds differed (Table 2).

DISCUSSION

Only individualsexhibiting breeding behaviors considered, the minimum numberof breedingBlack-necked Stilts and AmericanAvocets in the south bay was 270 and 879 birds,respectively. Undoubtedly these are underesti- mates,given that in the breedingseason birds frequently engage in other behaviorsor are betweennesting attempts (Gibson 1978). If all stiltsand avocetswe countedwere breedingand the sex ratio is 1:1 (Robinsonet alo 1997, Robinsonet al. 1999), there were approximately590 pairsof stilts and 1380 pairsof avocetsin the southbay. An unknownproportion of our estimatedbreeding birds were likelynonbreeders because not all individuals of both speciesbreed in their first year and some nonbreedingavocets summerin nestingareas (Robinson et al. 1997, Robinsonet al. 1999). Our estimateof 590 pairsof the Black-neckedStilt in the southbay is withinthe rangeof previousestimates of 400-650 pairs(Gill 1972, Rigneyand Rigney 1981). Our estimatefor breedingavocets also falls within the broaderrange of 650-1800 pairsfrom prior studiesin the southbay (Gill 1972, Rigney and Rigney 1981). Differentcounting techniques and coverage areas may account for many of thedifferences in estimatesof stiltand avocet breeding populations in San BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY

FranciscoBay. These differencesare difficultto evaluatesince exact areas that were coveredon previoussurveys are unknown. Gill (1972) did not surveyall availablesouth bay habitat. He reliedon extrapolationsto estimate 1800 pairs of breedingavocets, using the product of average nearest- neighbornesting distances multiplied by miles of insularand noninsular levees.His estimateof 400-500 breedingstilt pairs was basedon "impres- sionsof adultsobserved throughout certain sections of the SouthBay" (Gill 1972). Rigneyand Rigney(1981) alsodid not surveyall availablehabitat in the southbay, but they employed a calculationthat combinedamount of the studyarea covered,percent of nestingmissed because of surveytiming, and number of birds observed. We too were unableto surveyparts of the southbay, and thismay affect our estimatesof breedingstilts and avocets.However, much of our unsur- veyed area consistedof salt-crystallizingponds and developedbay fill. Waterbirds'use of pondswhose salinity is over 250 partsper thousandis consistentlylow (Takekawaet al. 2000, Warnock et al. 2002), in part undoubtedlybecause of the lackof invertebrateprey at thesehigh salinities (Goals Project 2000). Potentiallygood habitat existson outer Bair and Grecoislands, which we did not cover.Gill (1971), however,found only two avocet scrapeson outer Bair Island, and our recent visitsto the island revealedno coloniesof breedingstilts or avocets(San FranciscoBay Bird Observatoryunpubl. data). In parts of the West, breedingand winteringavocet populationshave increased(Colwell et al. 2001), but that doesnot appear to be the casein San FranciscoBay. We know of no other siteson the Pacificcoast of the United States that have breeding-seasonnumbers of stilts and avocets approachingthose in the southbay (seealso Small 1994). Some regionsin the California interior, includingthe Klamath Basin (D. Shuford pers. comm., Small 1994), CentralValley (Small 1994), and SaltonSea (Shuford et al. 2000), may holdhigher breeding numbers than the southbay in some years. Sincethe 1980s, corebreeding sites for stiltsand avocetsin the southbay have changed somewhat.Gill (1972) and Rigney and Rigney (1981) reported concentrationsof breedingstilts and avocetsin the Alviso salt pondsat the southerntip of the bay. We alsofound concentrations in the Alviso region, especiallyaround New Chicago Marsh. Highest numbers, however,were on the eastside of the southbay, between the San Mateo and Dumbartonbridges and just southof the DumbartonBridge (Figures 1 and 2). We located77 avocetand 9 stiltnests in the Baumbergregion, whereas Rigneyand Rigney(1981) found 2 avocetand 5 stilt neststhere. Presum- ably,long-term habitat change during the past20 yearshas altered distribu- tionsof nestingstilts and avocetsin the southbay. Short-term alterations can affectthe breedingpopulations as well. Active managementof salt ponds can changethe ponds'depth and other features(Ver Planck1958) rapidly enoughto make foragingor breedinghabitat for stiltsand avocetssuitable or unsuitablein a matterof days. Our studypoints out differencesin the useof southbay habitatsby stilts and avocetsduring the breedingseason. Salt ponds containedmore than BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY half of all stilt and three-quartersof all avocet observations,surpassing expecteduse as a functionof habitatavailability. As well as being important breedinghabitat for bothspecies, salt ponds also serve as importantforaging and roostingareas in winter (Swarth et al. 1982, Takekawa et al. 2001, Warnock et al. 2002). The high use of marshesby stiltsin the southbay mirrorsthe pattern found in other areas (Hamilton 1975, Robinson et al. 1999). American Avocets(adults and nests)were found lessfrequently in marshesthan were stilts,and within this habitat most avocetswere observedin open areas of shallowwater. The New ChicagoMarsh, a managedtidal marsh in the Alviso region, was an exception,as it held high numbersof breedingavocets and stilts(Figures 1 and 2). The uneventopography of thishistorical tidal marsh providesa varietyof microhabitatsincluding shallow ponded water, vegeta- tion, and mudfiats(S. Maciaspers. comm.). The marsh is protectedfrom tidal influenceby a group of salt pondsbordering its northern edge. This closenessof the pickleweed-(Salicornia spp.) dominated marsh to salt pondsand the marsh'srelatively stable water levelsappeared to create a suitablerearing area for youngstilts and avocets.The pickleweedwas tall enoughto providethe youngcover from predators,and the nearbysalt pondsprovided an abundantsupply of insectssuch as brineflies (Ephydra spp.,W. A. Maffeiin GoalsProject 2000, pp. 179-182) for foragingbroods of stiltsand avocetsmoving between these habitats. Tidal fiatswere littleused by stiltsand avocetsduring the breedingseason. Similar observationsof low use of tidal flats by avocetswere reported by Boettcheret al. (1995), who found 3% and 6% of all observednonbreeding avocetsusing tidal fiats in coastalSouth Carolina in 1991 and 1992, respectively.Swarth et al. (1982) and Harvey (1988) also rarely observed stiltsusing tidal mudflatsin San FranciscoBay. Otherstudies have shown levees to be importantnesting sites for stiltsand avocets(Gill 1973, Hamilton 1975, Rigneyand Rigney1981, Robinsonet al. 1997, Robinsonet al. 1999). We foundfewer than 20% of the stiltsand avocetson leveesduring our breedingsurvey and greatestuse on islands.In the southbay, it is likely leveesare usedfor breedingless often than are islandsbecause of their accessibilityto humandisturbance (C. Rintoulpers. obs.)and to mammalianpredators, such as the introducedRed Fox (Vulpes vulpes)(E. K. Harding in Goals Project2000, pp. 252-252). Gill (pers. comm.)suggests that his 1971 studymay have foundmore useof leveesby stiltsand avocetsbecause it predatedthe arrivalof the Red Fox in the south bay. We foundislands, including old fragmentedlevees separated from the surroundingnetwork, heavilyused by stiltsand avocets,as noted in other studies(Gill 1973, Rigneyand Rigney1981, Swarthet al. 1982, Robinson et al. 1997, Robinsonet al. 1999). Our resultsreconfirm the importanceof South San FranciscoBay as a breedingarea for stiltsand avocets on the Pacificcoast of the UnitedStates. Annualfluctuations of nestingstilts and avocetsprobably exist, and monitor- ing is needed to understandthis variabilitybetter. Current conservation proposalsfor San FranciscoBay (i.e., Goals Project 1999) includethe conversionof existinghabitats, especially salt ponds, into tidal marsh.The

10 BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY effectsof the conversionof part of 6475-8500 ha of salt pondsto tidal marsheswill likelybe negativeon breedingavocets, whereas the effectson the stiltare more difficultto predict.For both species,modification of tidal marshesto includefeatures of saltponds like open bodiesof shallowwater for foragingand islandsfor breedingmay be beneficial.Additional research is necessaryto investigatethese species' differences in diet in the southbay marshesand salt ponds.

ACKNOWLEDGMENTS

We are indebted to the Gabilan Foundation and the Rintels Charitable Trust for generousfinancial support. Logistic support was provided by ClydeMorris of the Don EdwardsSan FranciscoBay NationalWildlife Refuge, Chuck Taylor of Cargill Salt Division,John Takekawa of the BiologicalResources Division, United States Geologi- calSurvey, Steve Quick of the EastBay RegionalParks, and Sunnyvale Baylands. We offer specialthanks to Sue Maciasfor helpingcollect data and to DianaStralberg for GIS help.Drafts of thispaper were improved by commentsfrom DavidShuford, Bob Gill, Tim Manolis,and an anonymousreviewer. This is contribution1082 of PRBO Conservation Science.

LITERATURE CITED

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Accepted15 December2002

13 BLACK-NECKED STILTS AND AMERICAN AVOCETS IN SAN FRANCISCO BAY

Black-necked Stilt $1•etchby ¸ •larni Fylling