Abundance and Distribution of Migratory Shorebirds at Mono Lake, California

WESTERN BIRDS

Volume 33, Number 4, 2002

ABUNDANCE AND DISTRIBUTION OF MIGRATORY SHOREBIRDS AT MONO LAKE, CALIFORNIA

EMILIE A. STRAUSS, StillwaterSciences, 2532 Durant Ave., Suite 201, Berkeley, California 94704 CHRISTINE A. RIBIC, U.S. GeologicalSurvey, Wisconsin Cooperative Wildlife ResearchUnit, Departmentof WfidlifeEcology, University of Wisconsin,Madison, Wisconsin 53706 W. DAVID SHUFORD, PRBO ConservationScience, 4990 ShorelineHighway, StinsonBeach, California94970

ABSTRACT:We countedmigratory shorebirds and documentedtheir patternsof shorelinedistribution at Mono Lake in the Great Basinof Californiavia annualspring and fall surveysfrom 1989 to 1995. We tallieda totalof 30 species,29 in fall, 25 in spring,and 24 in both seasons.Median countsof all shorebirdswere 31,432 (13,901-50,916) in falland 7792 (2683-25,616) in spring.Median counts exceeded 5000 eachfor the AmericanAvocet and Wilson'sand Red-neckedphalaropes in fall and 1000 for the Westernand Least sandpipersin spring.Of nine specieswith mediancounts of >50 individuals,medians were >50% higherin fall than springfor the AmericanAvocet, Wilson's Phalarope, and Red-neckedPhalarope, were >50% higher in springthan fall for the SemipalmatedPlover, Western Sandpiper, Least Sandpiper,and Dunlin, and similar in bothseasons for the SnowyPlover and Killdeer. The distributionaround the lakeof totalshorebirds and of mostindividual taxa, except for the SpottedSandpiper in fall,varied considerably from year to year.Shorebirds as a groupas well as most species considered separately concentrated on the south,east, or north shores;except for the Killdeerand SpottedSandpiper all taxa appearedto avoidthe westshore at both seasons.Mono Lake is one of the mostimportant sites for shorebirdsin the IntermountainWest, particularlyfor Wilson'sand Red-necked phalaropes.Mono Lake'slarge size,varied habitats, and abundantfood are very attractiveto shorebirds;habitat availability and food supplyare more reliablethere than at manysites where they fluctuatewidely with variationsin climate.A water- rightsdecision that raisesthe lake'slevel likely will improve shorebird habitat, though effects will be mixed.

Widespreaddeclines in shorebirdpopulations in westernNorth America (Pageand Gill 1994) emphasizethe needto identifyand protectimportant migratorystopovers for thisgroup of birds(Myers et al. 1987, Brownet al. 2000). Althoughrecent studies give broadoverviews of shorebirduse of

222 Western Birds 33:222-240, 2002 MIGRATORY SHOREBIRDS AT MONO LAKE wetlandsand agricultural habitats in westernNorth America (Shuford et al. 1998, Pageet al. 1999, Shufordet al. 2002), fewdata have been published on the patternsof shorebirduse at individualsites, where mostconservation and managementefforts will focus. Mono Lake is well recognizedfor its criticalimportance to breeding California Gulls (Larus californicus;Shuford and Ryan 2000), Snowy Plovers(Charadrius alexandrinus; Page and Stenzel 1981), migratory Eared Grebes(?odiceps nigricollis; Boyd and Jehl 1998), and Wilson's (?halaropus tricolor) and Red-necked(P. lobatus)phalaropes (Jehl 1986, 1988), yet little is known about the abundanceand distributionof most speciesof shorebirdsat the lake. Prior data on shorebirdmigration at Mono Lakecome primarily from five lakewide surveys of waterbirdsfrom 8 Julyto 14 September 1976 (Winkleret al. 1977) and from localizedcounts of shorebirdsat the Old Marinaand SouthTufa on 35 and 23 dates,respectively,from 1971 to 1973 (Jurek1973, 1974). Similarly,prior data on the distributionof shorebirdsaround Mono Lakeare restrictedmainly to detailed descriptionsfor the Red-neckedPhalarope (Jehl 1986) and anecdotalones for other species(Gaines 1992). To add to this knowledge,we counted shorebirdsat Mono Lake in springand fall from 1989 to 1995 as part of PRBO's PacificFlyway Project. Here we report the abundance,species composition, and patterns of shoreline distribution of shorebirds at Mono Lake. We also discuss the importance to shorebirdsof Mono Lake relative to other sites in the IntermountainWest, the implicationsto shorebirdpopulations of a 1994 water-boarddecision to raisethe lake levelsubstantially, and the need for additional shorebird research at the lake.

STUDY AREA AND METHODS

MonoLake is a hypersalineterminal lake in easternCalifornia located at the baseof the SierraNevada at the westernedge of the GreatBasin. In 1941, the city of Los Angelesdiverted streams flowing into Mono Lake. Consequently,the lakefell over40 verticalfeet anddoubled in salinity,from 50 gramsper liter in 1940 to 100 gramsper literby 1982 (Vorster1985). Increasingsalinity reduced the productivityof brineshrimp (A rternia monica) and alkaliflies (Ephydrahians) (Herbst 1988, 1992; Herbstand Bradley 1993), the primaryprey itemsof aquaticbirds using the lake. Duringour study,the elevationof MonoLake dropped from 6377.0 feet(1943.7 m) in April 1989 to 6374.1 feet(1942.8 m)in January1992, thenrose to 6377.6 feet (1943.9 m) by August1995 (LosAngeles Aqueduct Daily Reports). The lengthof the Mono Lake shoreline,exclusive of islands,is about38.0 miles(61.3 km) at a lakeelevation of 6378 feet (1944 m). For analysisof distribution,described below, we groupeddata from 25 subareasinto four largersegments of shoreline:south (13.9 km), east(10.24 km),north (18.99 km), and west(18.17 km) (Figure1). The southsegment was characterized by alkalimudfiats, with a relativelylong (1-2 km)reach of freshwatermarsh, springs,and tufa towers and shoals adjoining the beachat Simon'sSprings. The eastsegment of Mono Lake wasdominated by broadalkali mudfiats with a few springswhose fresh water flows down to Mono Lake as shallow

223 MIGRATORY SHOREBIRDS AT MONO LAKE

224 MIGRATORY SHOREBIRDS AT MONO LAKE laminarsheets and rills. During our study, some rills at Warm Springsran for 0.4 to 0.8 km before reachingMono Lake. The north segmentconsisted mostlyof rockyshoals of tufa-encrustedpumice boulders interspersed with patchesof mudfiat,freshwater marshes, and creek mouths;springs were concentratedat Mono Lake CountyPark, WilsonCreek delta, and Bridge- port Creek delta (east of DechambeauPonds). Man-made Dechambeau Pondscontained fresh water. The west segmentconsisted primarily of a narrowbeach dominated in manysections by sand,cobble, or tufa shoals ratherthan mud or alkalifiats, especially between the Old Marinaand Lee Vining Creek, where the steep drop-off providedrelatively little shallowwater habitat. Shorebird habitat became more limited on the western shore from 1994 to 1995 when the lake rose and inundatedmost beaches,which hadalready narrowed from encroaching vegetation during years of dropping lake level.The lake'stwo largestfreshwater sources, Lee Vining and Rush creeks, flowed into the west segment.A number of large springsalso dischargedon the westernshoreline, but some of thesewere either surroundedby dense marsh vegetation or didnot supportshallow-water habitat. Duringsome years, fresh water entering Mono Lakefloats on the surfaceof salinelake water, providinghypopycnal habitat important to phalaropes. Mostyears, berms of sandcreated narrow lagoons adjacent to the shoreline; thosein the northernportion of the eastsegment were hypersaline, whereas thosealong the southshore generally contained fresh or brackishwater. We conducted14 shorebirdcounts at Mono Lake, onceeach spring and fall from 1989 to 1995. Datesof springcounts ranged from 21 April to 6 May, thoseof fall countsfrom 13 to 26 August.We selectedthese dates becausethey represent peak migratory periods for mostshorebird species in the westernGreat Basin (seeWarnock et al. 1998). Our surveyswere not timed to coincidewith the breedingperiod of the six species,the Snowy Plover, Killdeer(Charadrius vociferus),American Avocet (Recurvirostra americana), Spotted Sandpiper (Actiris macularia), Wilson's Snipe (Gallinagodelicata), and Wilson'sPhalarope, that nestregularly at the lake in low to moderate numbers (Gaines 1992); the Black-necked Stilt (Himantopusmexicanus) is known to havenested only once (B. Millerpers. comm.).We did not conductwinter countsbecause no shorebirdsregularly overwinter,with the exceptionof verylow numbersof the Killdeer,Wilson's Snipe, and LeastSandpiper (Caliolris minutilla; Gaines 1992). To cover the long shorelineeffectively, we allowedtwo daysfor each count.To minimizedouble counting, we dividedobservers (generally 10 to 15 per census)into groupsof one to threeand assignedthem to contiguous stretchesof lakeshore.Observers, walking parallel to the shore, tallied shorebirdswithin 25 discretesubareas from 0.8 to 6.8 km in lengthbased on a lake level of 6378 feet (1944.0 m). In a few areas, observers occasionallycounted using a telescopefrom a fixed point. We also usually surveyedDechambeau Ponds but not the shorelineof any of the lake's islands. We used data from Dechambeau Ponds for tallies of shorebird abundancebut not for analysisof distributionpatterns around the lake. In most years, but not in fall 1989 and 1990 or spring 1992 and 1995, observerssurveyed all or a portionof the landbridge connecting Negit Island to the mainland.

225 MIGRATORY SHOREBIRDS AT MONO LAKE

Our surveysprovide minimal estimates of numbersof migratingshore- birds becauseof variouslimitations to our methods.Lacking adequate numbersof observers,we coveredthe entirelakeshore only in spring1994. On other counts,we assignedavailable observers to areasthat shorebirds typicallyuse heavily.Coverage ranged from about 75% to 100% of the shoreline,with the exceptionof only60% in spring1989 andfall 1993. We estimatewe probablycounted 70% to 90% of all shorebirdspresent. Access to someareas was restrictedby soft mud or quicksandor by massivealkali dust storms (spring 1995). In the falls of 1989 to 1991 and 1993, collaboratorscounted phalaropes by boat, which has proven the most reliablemethod for surveyingthese largely aquatic shorebirds. In theseyears we substitutedthe boat talliesof phalaropesfor land-basedcounts; boat counts were on the same weekend as land-based counts in 1990 and 1992 and 8 and 13 daysprior in 1989 and 1991, respectively.The protocolfor boat countswas describedby Jehl (1999). Becausethe FlywayProject emphasized the geographicextent of surveys at the expenseof their frequency,our twice-annualsurveys underestimated the numbersof someearly- or late-migratingspecies. In fall, we generally timedour surveystwo to threeweeks later than the peak occurrenceof the Wilson'sPhalarope (Jeh11988) and aboutsix weeks before the arrivalof the Dunlin(Calidris alpina; Gaines1992). In spring,most counts were too early to capturethe peak in abundanceof phalaropes,which occursin early to mid-May(Jehl 1986, 1988), or of the SpottedSandpiper, which is rare as a migrant or summer resident before early May (Gaines 1992). We also underestimatedthe abundanceof somecryptic species that extensivelyuse habitatsaway from the immediateshoreline of Mono Lake. These include the SnowyPlover and Wilson'sSnipe, which use broad alkali fiats and lake- fringingmarshes, respectively. Similarly, once-per-season surveys may miss peak passagebecause of naturalannual variation (Stenzel and Page 1988, Warnock et al. 1998), particularlyin spring when speciessuch as the WesternSandpiper (C. mauri), LeastSandpiper, and Long-billed Dowitcher (Limnodromusscolopaceus) have very short stopoverperiods (Warnock and Bishop1998). We instructedobservers to identifyall shorebirdsto specieswhen possible. Groupsof unidentifiedshorebirds fell mostly into four categories: yellowlegs, eitherGreater (Tringa melanoleuca) or Lesser(T.. flavipes); small sandpip- ersof the genusC'alidris, primarily Western Sandpipers, Least Sandpipers, or Dunlin; dowitchers,either Short-billed(Limnodromus griseus) or Long- billed;and phalaropes,either Wilson's or Red-necked.We assigneduniden- tifiedshorebirds to speciesusing methods described by Pageet al. (1999), leavingsome in unidentifiedcategories when the ratio of unidentifiedto identifiedwas high. For analyses,we groupedboth identifiedand unidenti- fied dowitchersas dowitcherspp. owingto the difficultyof identifyingmost individualsto specieson surveys.

Data Analysis To analyzedistribution, we selectedcounts with adequatecoverage of the lake'sshoreline. For each survey,we tabulatedeffort per segment(south, west, east, and north) and calculatedthe proportion of each segment

226 MIGRATORY SHOREBIRDS AT MONO LAKE surveyed.We consideredonly segmentsthat had at least 70% of their shorelinesurveyed as representative of the segmentand hence adequate for analysis.Coverage was adequate for allfour segments in bothspring and fall in 1991, 1992, and 1994, in springonly in 1990. Similarly,sample sizes were adequatefor analysisfor the SnowyPlover, Semipalmated Plover (Charadrius semipalrnatus),Killdeer, American Avocet, Spotted Sand- piper, Least/Westernsandpipers, Dunlin, and dowitcherspp. We analyzeddata for eachspecies or speciesgroup by season.Because not all segmentsreceived equal coverage, we adjustedtotals for individual speciesand all shorebirdscombined by proportionatelyreducing counts to the smallestarea surveyed in a segmentin anyyear (7.38 km in spring,8.41 km in fall).This adjustscoverage so it is equivalentacross all segmentsand yearsin the season,as suggestedby Haney (1986) and usedby Ribicet al. (1997). A priori, we excludedphalaropes from analysisbecause of varying surveymethods among counts and lackof breakdownby shorelinesegment on boat surveys. The null hypothesiswas that each speciesor speciesgroup had the same distributionaround Mono Lake regardlessof year.If there wasno evidence that a species'distribution differed from year to year,then datafrom allyears were combined and tested for a uniform distribution around Mono Lake. These hypotheseswere tested with log-linearmodels and standardized residualanalysis (Lloyd 1999). The nullhypothesis was tested by meansof the likelihood-ratiogoodness-of-fit statistic (G). If the null hypothesiswas rejected,standardized residuals showed where data were not fit well by the model.Large positiveresiduals indicate there are more birdsin a segment than expected,whereas large negativeresiduals indicate fewer birdsthan expected.Standardized residuals are distributedas normal (0,1) variables,so "large"is a valueof 2 or greaterand wouldbe consideredsignificant at a probabilityof 0.05. Residualsless than 2 but greaterthan 1.6 wouldbe significantat 0.10 and are consideredas trends or tendencies.Analyses were done with STATISTICA (StatSoft 1995).

RESULTS

SpeciesRichness and Abundance We tallied30 speciesof shorebirdson oursurveys of MonoLake (Table 1). Of these,we recorded29 in fall, 25 in spring,and 24 in bothseasons. We recorded6 speciesevery spring and fall and 11 otherson everycount in just one season(7 in fall, 4 in spring).Median countsof total shorebirdswere 31,432 (range13,901-50,916)in fall and 7792 (range2683-25,616)in spring. For individualspecies, median countswere >5000 each for the AmericanAvocet, Wilson's Phalarope, and Red-neckedPhalarope in fall and > 1000 for the Westernand Leastsandpipers in spring.Of nine species with mediancounts of >50 individuals,medians were >50% higherin fall than in spring for the AmericanAvocet, Wilson'sPhalarope, and Red- neckedPhalarope, were >50% higherin springthan in fallfor the Semipal- matedPlover, Western Sandpiper, Least Sandpiper, and Dunlin, and similar in bothseasons for the SnowyPlover and Killdeer.

227 MIGRATORY SHOREBIRDS AT MONO LAKE

Table 1 Numbersof Shorebirdsat Mono Lake, California, 1989-1995 ø

Fall•

Species 19 Aug 25 Aug 24 Aug 15 Aug 22 Aug 1989 1990 1991 1992 1993

Black-belliedPlover Pluvialis squatamla 5 8 0 4 0 SnowyPlover Charadrius alexandrinus 18 49 31 7 81 SemipalmatedPlover C. semipalmatus 25 30 8 0 4 KilldeerC. vociferus 147 202 32 66 30 Mountain Plover C. montanus 0 0 3 0 0 Black-neckedStilt Himantopus mexicanus 133 12 22 13 51 American Avocet Recurvimstra americana 8467 5398 7652 4520 8999 GreaterYellowlegs Tringa melanoleuca 4 9 16 7 4 LesserYellowlegs T.flavipes 9 11 15 1 3 Yellowlegsspp. 0 0 0 0 0 SolitarySandpiper T. solitaria 1 2 0 0 0 WilletCatoptrophorus semipalmatus 31 11 9 4 11 SpottedSandpiper Actiris macularia 43 53 24 15 0 WhimbrelNumenius phaeopus 0 0 0 0 0 Long-billedCurlew N. americanus 24 24 7 19 1 MarbledGodwit Limosa fedoa 3 124 16 2 1 RuddyTurnstone Arenaria interpres 2 0 1 0 0 Red Knot Calidris canutus 3 0 0 0 4 SandefiingC. alba 0 6 1 0 0 SemipalmatedSandpiper C. pusilia 0 0 1 0 0 WesternSandpiper C. mauri 639 3573 3171 490 272 LeastSandpiper C. minutilla 204 1244 527 24 410 Western/LeastSandpiper 226 634 10 0 0 Baird'sSandpiper C. bairdii 19 31 19 2 7 PectoralSandpiper C. melanotos 0 0 0 0 2 DunlinC. alpina 0 0 0 0 12 Westem/Least/Dunlin 0 0 0 0 0 DowitchersLimnodromus spp. c 28 46 27 4 20 Wilson'sSnipe Gallinago delicata 2 7 2 2 0 Wilson'sPhalarope Phalaropus tricolor 4500 478 10,000 1420 9000 Red-neckedPhalarope Phalaropus Iobatus 19,000 6320 18,000 5981 12,520 RedPhalarope Phalampus fulicarius 0 0 0 0 0 Phalaropespp. 0 1050 0 510 0 Totalshorebirds 33,533 19,322 39,594 13,091 31,432

228 MIGRATORY SHOREBIRDS AT MONO LAKE

Spring 13 Aug 12 Aug 22 Apr 21 Apr 27 Apr 25 Apr 1 May 5 May 29 Apr 1994 1995 1989 1990 1991 1992 1993 1994 1995

40 50 4 49 11 14 0 0 1 87 216 35 81 88 56 135 101 23 6 16 286 178 244 70 128 159 133 153 48 67 97 64 23 25 65 30 0 0 0 0 0 0 0 0 0 55 0 21 11 1 17 8 82 0 10,999 2191 1564 650 353 805 317 425 15 4 1 9 6 2 2 0 3 6 10 1 0 0 0 0 1 0 0 1 0 1 0 0 0 0 0 0 4 0 0 1 0 0 0 0 0 35 9 0 0 4 50 3 6 9 49 7 4 4 5 1 10 209 3 0 0 1 0 0 2 0 4 0 43 7 2 0 0 0 1 0 2 16 5 0 0 19 0 36 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 3 0 0 0 0 3 0 0 0 0 0 14 0 0 0 0 0 0 0 0 0 0 725 1129 14,217 18,640 1178 4939 1482 8921 1952 196 305 1298 4693 3243 1178 329 188 148 1 0 1810 584 1989 0 108 0 0 27 18 0 0 3 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1 183 430 313 263 88 725 325 0 0 540 0 0 0 0 0 27 30 297 22 39 100 8 11 22 1 7 5 3 22 5 4 7 7 5478 46,542 126 61 33 51 852 764 7 1113 75 64 0 179 4 9 129 0 0 5 0 0 0 0 0 0 0 0 250 0 106 2 4 35 675 0 19,071 50,916 20,534 25,616 7792 7587 3579 12,488 2683

øDatesrepresent the firstday of a two-daycount period. bPhalaropedata from boat counts on 11 August1989, 26 August1990, 11 August1991, and22 August 1993 and from land-basedcounts on all other surveys.These counts tended to greafiy underestimatenumbers of Wilson'sPhalaropes because they were two to three weekslater than the normalpeak passagefor that species. CLong-billed(L. scolopaceus)and Short-billed(L. griseus)dowitchers were recordedin both fall and spring.

229 MIGRATORY SHOREBIRDS AT MONO LAKE

Table I• Likelihood-Ratio Goodness-of-Fit Statistics Relative to the Null HypothesisThat Each ShorebirdTaxon Had the Same Distributionaround Mono Lake Regardlessof Year

Fall Spring Species G df p G df p

Total shorebirds 1522 6 <0.001 4642 9 <0.001 SnowyPlover 17.3 6 0.008 45.9 9 0.02 SemipalmatedPlover ------107 9 <0.001 Killdeer 3.3 6 0.8 45 9 <0.001 American Avocet 1959 6 <0.001 353 9 <0.001 SpottedSandpiper 0.5 6 0.9 ------Least/Westernsandpipers 787 6 <0.001 4642 9 <0.001 Dunlin ------15.4 6 0.02 Dowitcherspp. ------58 9 <0.001

Lakewide Distribution Patterns The distributionaround the lake for total shorebirds,American Avocets, andLeast/Western sandpipers varied considerably from year to yearin both springand fall, for the Snowy Plover,Semipalmated Plover, Dunlin, and dowitchersin spring,and for the Killdeerin springbut not fall (Table2). The SpottedSandpiper showed the mostpredictable pattern, as its distribution in fall did not vary from year to year. Analysesby year showedthat, in spring,shorebirds as a whole concentrated in numbersgreater than expectedin the southand east, as did the SemipalmatedPlover, Least/Western sandpipers, and Dunlin(Table 3). In fall, shorebirdsas a wholegenerally concentrated in numbershigher than expected in the east and north, as did the American Avocet and Least/ Westernsandpipers (Table 3). Shorebirdsas a group occurredin numbers lower than expectedin the west in both seasons(Table 3). Exceptfor the Killdeerand Spotted Sandpiper, all speciesoccurred in numberslower than expectedin the west in both seasons.The Killdeeroccurred in numbers higher than expectedin the west but generallyin numberslower than expectedin the north andeast. The exceptionwas in spring1991, whenthe Killdeer occurredin higher numbersin the east. In fall, the Spotted Sandpiperconsistently occurred in numbershigher than expectedin the west and in numberslower than expectedin the eastand north. Some individualspecies had additionaldistribution patterns of interest.In spring,the SnowyPlover consistently occurred in numbersgreater than expectedin the east and sometimesthe south, the SemipalmatedPlover mostlyin the southand sometimesin the east, Least/Westernsandpipers and Dunlinin the southand east, and dowitchers mostly in the east.Patterns for the American Avocet were among the most variablewith numbers greaterthan expectedmosfiy in the southin springand the north and east in fall.

230 MIGRATORY SHOREBIRDS AT MONO LAKE

DISCUSSION

Despitetheir limitations,our surveysgreatly expanded the knowledgeof the speciescomposition, abundance, and distributionof migrantshorebirds at Mono Lake. Our peak countsof AmericanAvocets and WesternSandpip- ers, for example,are aboutthree and nine timesgreater, respectively, than any prior counts(Jurek 1973, Winkler et al. 1977, Winter and Manolis 1978). We addedlittle information,however, on phalaropes.Jehl (1999) reported counts of Wilson's Phalaropesat Mono Lake averagingabout 55,000 (maximum70,000) in the earlyto mid-1980s,declining to 2100 by 1992, andincreasing again to about38,000 in 1997. In fallour countswere too late in the seasonfor peak numbersand hence are not comparableto Jehl's.In 1989, our land-basedcount on 12-13 Augustof 46,542 Wilson's Phalaropeswas anomalousin that it exceededJehl's (1999) boat countof 34,800 takenthat year duringthe typicalperiod of peak numbersfrom 25 Julyto 3 August.Jehl's (1986) peak countsof Red-neckedPhalaropes at Mono Lake from 1981 to 1984 rangedfrom 8000 to 14,000; on the basis of turnoverhe estimated52,000 to 65,000 passedthrough each fall. Our comparableboat counts for the Red-neckedon singledates in 1989, 1991, and 1993 rangedfrom 12,520 to 19,000. The highestestimate for this speciesat Mono Lakeon a singledate was >40,000 on 22 August1958 (H. Cogswellin Jehl 1986).

Importanceof Mono Lake A combinationof our data, for various shorebirds,and Jehl's (1986, 1988), for phalaropes,led to designationof Mono Lake as a site of internationalimportance within the Western HemisphereShorebird Re- serve Network (Harringtonand Perry 1995). Internationalsites annually supportat least100,000 shorebirdsor 15% of a species'flyway population. Withinthe entireIntermountain West, Mono Lakeis one of only 10 sitesthat hold> 10,000 shorebirdsin fall (Shufordet al. 2002) and, alongwith Great Salt Lake and Lake Abert, is one of the three most importantsites for Wilson's and Red-neckedphalaropes (Jehl 1986, 1988; Shuford et al. 2002). Mono Lake alsois one of the most importantsites in the interiorof Californiafor breedingSnowy Plovers (Page et al. 1991). Mono Lake is very importantto shorebirdsbecause of its largesize, long shorelinewith variedhabitats, and extremelyabundant prey baseof brine shrimp and alkali flies. Mono Lake is also a criticalhabitat becauseit is relativelydeep yet its shallowsprovide mudfiats and alkaliflats in both wet and dry periods.By contrast,many wetlandsin the arid Great Basinoffer shorebirdshabitat only in periodsof above-averageprecipitation (Reed et al. 1997, Warnock et al. 1998, Plissneret al. 2000). Conversely,other importantshorebird sites along the eastside of the Sierra Nevada,such as Crowley Lake and BridgeportReservoir, are not attractiveto most shorebirdsduring very wet yearswhen high water levelsinundate mudfiats (E. Strausspers. obs.).

231 MIGRATORY SHOREBIRDS AT MONO LAKE

Table 3 Standardized Residuals for Models of Distributions of Shorebirdsaround Mono Lake by Yeara

Segment Species South East North West

Total Shorebirds Spring 1990 52 -28.8 35.6 -58.7 1991 5.9 50 -22.2 -33.6 1992 31.0 8.5 -8.5 -30.9 1994 29.5 26.3 -13.1 -42.8 Fall 1991 10.6 36 -- -45 1992 -- 4.2 25 -28 1994 -17.1 7.6 46 -37 SnowyPlover Spring 1990 3.3 2.3 -- -4 1991 -2.2 11 -4.2 -4.4 1992 -- 4.5 -- -3.5 1994 3.0 3.2 -2 -4.2 SemipalmatedPlover Spring 1990 12 -3.2 -3.1 -5.7 1991 5.1 6.4 -5.4 -6.1 1992 6.5 -- -3.8 -3.8 1994 -2 11.4 -3.5 -5.9 Killdeer Spring 1990 -- -3.7 -3.2 6.8 1991 -- 3.2 -2.3 -- 1992 -- -- -2.0 4 1994 -- -2.8 -- 4.4 Fall 1991-94 b -- -5.5 -- 4.3 American Avocet Spring 1990 23 -4.8 -6.9 -11 1991 -- 2.9 3.8 -6.2 1992 7.1 -8.5 11.3 -9.9 1994 18.3 -7.5 -2.6 -8.1

(contfnued)

FoodPreference and Availability Exceptfor phalaropesand SnowyPlovers, very littleis knownabout the food preferencesof shorebirdsat Mono Lake or elsewherein the Great Basin (Warnocket al. 1997). Wilson'sPhalaropes depend on both brine shrimpand alkali flies when staging at Mono Lakein fall,and their diet varies by sexand age (Jehl1988). SnowyPlovers rely on alkaliflies and a carabid beetle(Bembidion ephippigerum; Swarth 1983). Red-neckedPhalaropes feed mostlyon alkaliflies (Jehl 1986) and •annot subsiston a diet of brine shrimp alone (Rubegaand Inouye 1994). Mahoney and Jehl (1985) reported that samplesof three stomachsof the AmericanAvocet at Mono Lake containedonly adult alkali flies, althoughRobinson et al. (1997) reportedthat at salineinland wetlands brine shrimp were also important. On the Californiacoast, Ramer et al. (1991) foundWestern Sandpipers feeding

232 MIGRATORY SHOREBIRDS AT MONO LAKE

Table 3 (Continued)

Segment Species South East North West

Fall 1991 23.3 19.9 -5.3 -38 1992 -- 7.8 20 -27 1994 -17.7 -- 54 -34.5 Least/Westernsandpipers Spring 1990 48 -30 38 -57 1991 8.5 44 -21 -31 1992 30 10 -13 -28 1994 26 27 -11 -42 Fall 1991 -14 34 5.4 -25 1992 -4.2 -8.8 20 -7.2 1994 -4.7 29 -11 -14 SpottedSandpiper Fall 1991-94 b -- -2.9 -2.1 6.5 Dunlin c Spring 1990 4.8 7.3 -2.9 -9.2 1992 4.3 3.6 -- -7.3 1994 11 7.1 -6.3 -12 Dowitcherspp. a Spring 1990 -- 3.8 -- -- 1991 -2.6 3.4 -- -- 1992 -- -4.0 7.3 -4.0

aNumbersin boldand Romantype are thosefor whichcounts were significantly greaterand lowerthan expected,respectively, under the null hypothesisthat, within a year, the distributionaround the lake was uniform.Dashes indicate nonsignificantresiduals. Significance was assessedat p < 0.05. bNoevidence was found that the pattern of distributiondiffered by years, hence data for all yearswere pooledand tested for a uniformdistribution around the lake. CNoDunlins were recordedin spring1991. aThenumber of dowitchersrecorded in spring1994 wastoo low to analyze.

predominantlyon ephydridfly larvaeand pupae, suggestingthat at Mono Lake ephydrids,such as alkaliflies, may providean importantfood source. Variationin ShorebirdAbundance and SpeciesRichness Shorebirdnumbers at Mono Lake variedsubstantially from year to year. This may reflectannual differences in habitat availabilityelsewhere in the westernGreat Basin,north-to-south variation in climaticconditions, annual variationin the periodof peak migration,or annualfluctuations in sizesof shorebirdpopulations. Warnock et al. (1998) and Shufordet al. (2002) also foundlarge annualfluctuations in shorebirdtotals at other westernGreat Basin sites. In allyears except 1991, shorebirdtotals at Mono Lakewere greater in fall thanspring. This difference most likely reflects great seasonal fluctuations in

233 MIGRATORY SHOREBIRDS AT MONO LAKE the availabilityof brineshrimp and alkali flies, which attract large numbers of AmericanAvocets and phalaropes to the lake.Although little is known about othershorebirds' food preferences at MonoLake, prey are probablylimited at Mono Lake duringApril but abundantduring most years in August. DuringApril, brineshrimp are immatureand not largeenough for shorebirdsto eat(R. Jellison pers. comm.), and densities of alkalifly larvae, pupae, and adultsare low (Lenz 1984, Herbst 1988). During most years, brine shrimphave grown to a sizeappropriate for shorebirdsby late May or June (Lenz 1984, Jehl 1988). In California,species richness of shorebirdsis oftenslightly higher in fall thanin springbecause of the presencein fallof juvenilesof somespecies that are rare to absentin spring(Page et al. 1979, Shufordet al. 1989). At Mono Lake, we observed six species--the Mountain Plover (Charadrius montanus),Ruddy Turnstone (Arenaria interpres),Sanderling (Calidris alba), Semipalmated(C. pusilia),and Pectoral(C. melanotos)sandpipers, and Red Phalarope(Phalaropus fulicarius)--during fall surveysonly.

Distribution Relative to Habitat Use During our studies,shorebirds were distributedaround Mono Lake nonuniformly,reflecting in part the arrangementof varioushabitats. Most species,except the Killdeerand Spotted Sandpiper,occurred more than expectedon eitherthe south,east, or north shores.A largeproportion of shorebirdsappeared to be attractedto the juxtapositionof alkali mudflats and freshwatersprings found in all but the westsegment. Althoughsubstantial numbers of shorebirdsused dry alkalimudfiats near the edge of Mono Lake, the majority, especiallyCalidris sandpipers, concentratednear rills, especiallythose at Warm and Simon's springs. Another habitat feature that attracted shorebirds was a set of shallow freshwaterpools that formedmost years at the mouthof WilsonCreek and along the south segment.Most shorebirdson the north segmentalso concentratedat freshwatersites. In 1994 and 1995, Mono Lake'slevel rose and inundatedmuch of the shorelinebetween Navy Beachand Simon's Springs.The remainingnarrow beach tended to supportfewer shorebirds, though some shorebirduse here shiftedto narrow lagoonscreated by floodingfrom the risinglake. The westsegment typically had less shorebird habitat than the others,and what was there did not attract large concentrations.The Killdeerand SpottedSandpiper, the only speciesoccurring at a frequencyhigher than expectedin the west segment,use primarilygravel bars and other hard substrates,a predominanthabitat in this area. The SpottedSandpiper, in particular,typically associates with deltasof freshwaterstreams (Gaines 1992) andgravelly or cobblybeaches. The Killdeeris oftenfound on gravel bars(Jackson and Jackson2000), especiallythose adjacent to freshwater (D. Shufordpers. obs.).The creekdeltas, as well as mudfiatsbelow the Old Marina and southernportion of Mono CountyPark, attractedmost of the othershorebird species in the westsegment. Mostof the speciescommonly associated with muddymargins of lakes, such as the SemipalmatedPlover, Western and Least sandpipers,and

234 MIGRATORY SHOREBIRDS AT MONO LAKE

Dunlin (Gaines1992), usedprimarily the south,east, and north segments, rarelythe westsegment. Page et al. (1983) observedSnowy Plovers feeding eitheralong the shoreof MonoLake or at seeps,which is consistent with our observations.Like most migrantshorebirds at the lake, dowitcherswere attractedto muddylake margins (Gaines 1992) on the south,east, and north shores,but they alsouse narrow mudfiats adjacent to marshesand freshwater pools on the west shore.American Avocets, which feed on flies and shrimpobtained by wadingin shallowwater (Robinsonet al. 1997), were oftenobserved in areasaway from sourcesof freshwater, such as the Negit Islandland bridge. Our anecdotalobservations showed that phalaropesconcentrated off- shoreexcept in fall 1995, when large numbersroosted and preenedon alkali fiats at the lake's edge. Wilson'sSnipe used primarily freshwater marshes on the lake's western shore. Freshwater is extremely important to shorebirdsin a hypersalineenviron- mentsuch as Mono Lake.Although shorebirds have developed adaptations to avoidstress, such as excreting excess salt through salt glands and kidneys, avoidingingestion of saltwater,choosing prey low in salt,manipulating or strainingprey to minimizesalt ingestion, and visiting fresh water to drinkand bathe(Rubega and Robinson1997), thesebehaviors consume energy and canreduce fitness. At Mono Lake freshwater is clearly important to Wilson's and Red-neckedphalaropes, Least and Westernsandpipers, and California Gullsfor drinkingand bathing (Mahoney and Jehl 1985; Jehl 1986, 1988; Rubegaand Robinson1997). Mahoney and Jehl (1985) indicatedthat Wilson'sPhalaropes and AmericanAvocets probably require limited fresh waterwhen at Mono Lakebecause they avoid some salt loading by usingthe tongue and bill to squeezelake water from prey items before ingestion. Duringthe lastseveral weeks of their fall visit,Wilson's Phalaropes escalate feedingrates to put on fat for a nonstopflight to SouthAmerica; during this hyperphagicperiod they may inadvertentlyswallow additional lake water and thus requirefresh water as often as twice a day (Mahoneyand Jehl 1985, Jehl 1997). During the 1980s when Mono Lake's salinitywas especiallyhigh, Jehl (1988) observedup to 50,000 Wilson'sPhalaropes bathingand drinking at Mono Lakein the deltasof streamsand springs. Red- neckedPhalaropes also visit fresh water (Winkleret al. 1977).

Future Prospects Concern over ecosystemcollapse from increasingsalinity was largely alleviatedby a water-rightsdecision that mandatedallowing Mono Lake to riseover 20 yearsto a dynamicequilibrium centered around a lake levelof 6391 feet (1943 m; SWRCB 1994a). If water diversionshad continued, hypersalinitywould have greatlyreduced the abundanceof alkaliflies and brine shrimpand increasedthe energeticcost of salt excretionand prey manipulationby shorebirds. Changinghabitat conditions as the lakerises to reachdynamic equilibrium shouldaffect patternsof shorebirduse. Most shorebirdspecies likely will benefitfrom an increasein food.Herbst and Bradley(1993) predictedthat alkalifly abundancewill increasebecause of a reductionin salinityand an

235 MIGRATORY SHOREBIRDS AT MONO LAKE increasein submergedhard substrate used as attachmentsites by fly pupae. Brineshrimp abundance also is expectedto increase(Dana and Lenz 1986). Immediatelyafter the decisionto restrictwater diversionsand raisethe lake levelthese long-term benefits of lesssaline conditions were partiallyoffset whena largeinflux of freshwater in 1995 initiatedan episodeof meromixis or persistentsalinity stratification (Jellison et al. 1998). The absenceof a winterperiod of holomixis(complete mixing) and stronger density stratification in summerreduces internal recycling of nutrientsand algaland zoo- planktonproductivity (Jellison and Melack 1993a). A similarepisode of meromixiswas initiatedby exceptionallyhigh runoffin 1983 (Jellisonand Melack 1993b). As duringthe earlier(1983-1988) episode,however, three years(2000-2002) of modestdeclines in lakelevel have resulted in weaken- ing of meromixisand a returnto averagelevels of productivityeven prior to its breakdown(R. Jellisonpers. comm.).The lake is currentlywell mixed abovea depthof 28 m. Whilemeromixis is expectedto breakdown in 2003, productivityis predictedto remainnear or aboveaverage even if it persists (R. Jellisonpers. comm.). Most types of shorebirdhabitat will likely increasein abundanceand quality,except that mudfiatsmay contractsomewhat. Inundation of pro- tectedcoves by the risinglake will allowfresh water to form a hypopycnal layerover denser, more saline lake water (Stine 1995), benefitingphalaropes by providingadditional sources for drinkingand bathing.Ephemeral brack- ishlagoons are expectedto expand,but someof them will be surroundedby densemarsh vegetation; freshwater pond acreagewill remain about the same (Stine 1995). Wilson'sSnipe will likely lose breedinghabitat as freshwatermarshes and seasonalwet meadowsare inundatedby the rising lake; still, marsheswill remain more extensivethan at prediversionlevels. The totallength of shorelinewill increase(SWRCB 1994b), but not all of it will be availableto shorebirds.During our studythe widthof alkalimudfiats decreasedon the south and west shoresas the lake rose, but on the north and eastshores habitat was still abundant. The extentof mudflatwill vary as the lake reachesand fluctuatesaround the dynamicequilibrium, reducing habitatduring wet periodsover that availableto shorebirdsduring our study. There will alsobe seasonalvariation in the lake level. In April, the lake is generallyrelatively low, and substantialmudfiats should be available(S. Stine pers. comm.). During most years, by Augustsnowpack runoff will have raised the lake level, and mudfiatsmay be less availablethan in April, especiallyalong the westernshore. It is unclearif the suitabilityof alkalifiats and mudfiatswill be altered,as somesprings will migrateupslope of the existinglake level, whereas others will become sublacustrine. Shallow water shouldremain abundant (S. Stine pers. comm.).Although the acreageof alkaliflats will decline, it willstabilize at a levelhigh enough to provideample habitatfor maintainingSnowy Plover populations (SWRCB 1994b:4-99). If shorelineaccess and humandisturbance increase, shorebirds feeding at the lake edgeand roostingphalaropes could be affectednegatively (e.g., Pfister et al. 1992, Burger et al. 1995). Foragingphalaropes may be disturbedif recreationalboating increases.

236 MIGRATORY SHOREBIRDS AT MONO LAKE

Researchand MonitoringNeeds Long-term monitoring is critical to assessingshorebirds' response to changesin lake levelsand the potential effectsof human disturbance. Understandingpopulation changes at the lakewill be possible,though, only within the context of information from broader efforts to assess continental populations,which likewisewill be aided by monitoringat Mana Lake. Knowledgeof shorebirdsat Mana Lake couldbe enhancedby studiesthat betterdocument seasonal use patterns via morefrequent surveys, determine the migratory pathwaysof shorebirdspassing to and from Mana Lake, investigatethe diet of poorlyknown species, and assessthe relationof prey availabilityto shorebirdconcentrations. Research, monitoring, and manage- ment at Mana Lake shouldbe coordinatedwith similarefforts throughout the IntermountainWest (Oring et al. 2000).

ACKNOWLEDGMENTS

The PacificFlyway Project was fundedby the Bay Foundationof Morro Bay, the BradfordFoundation, the Bureauof Reclamation,the CentralValley Habitat Joint Venture, Chevron USA Inc., the Dakin Foundation,Ducks Unlimited, the Katherine W. Dumke and EzekielR. Dumke, Jr. Foundation,Genentech, the Walter and Elise Haas Fund, the J. M. Long Foundation,the Marin, Morro Coast, and Stockton chaptersof NationalAudubon Society, the NationalFish and Wildlife Foundation, the David and LucillePackard Foundation, the San FranciscoFoundation, the Strong Foundation,the True North Foundation,the Dean Witter Foundation,anonymous donors,and members of PRBO. GaryPage provided inspiration and guidance for the PacificFlyway Project, Janet Kjelmyr helped with datasummary, and LynneStenzel offeredsuggestions for data analysis.We wouldespecially like to thank JosephR. Jehl, Jr., MargaretRubega, and associatesfor sharingtheir data from boat countsof phalaropes.We are gratefulto the followingindividuals for participatingin shorebird counts:Sy Baldwin,Reuben Balzer, Lynn Berner,Andy Bradvica,Jane Broer,John Cain, Dave and Janet Carle, Chris Corben, Bob Damschroeder,Claudia Davison, SusanDesbaillets, Jan Dierks,Deanna Dulen, Kathy Duvall, Joel Ellis,Chris Elphick, MelanieFindling, Larry Ford, Steve Glover, Paul and HelenGreen, Tina Hargis,Emily Hart, Patrick Heimgartner,Dave Herbst, Catherine Hickey, Tom and Rosanne Higley, Steve Holland, Eva Howarth, Lisa Hug, Joan Humphrey,Bruce Inman, CaronInouye, Emily Johnson, Bridget Keinel, Dan Keller,Donald Keller, John Keller, DaveMarquart, Mike McClaskey,Bev Mcintosh, Randy McPheron, Geoff McQuilkin, PeterMetropulos, Nancy Metzer, Patrick Mitchell, Jason Montiel, Francis Moore, Ron and LisaNelson, Bryan Obst, Ginelle O'Connor, Jim and DebbyParker, Lina Prairie, MichaelPrather, Bob Richmond,Ed Rodriguez,Steve Schmidt, Patricia Suppa, Curt Sutliffe,Sarah Taylor, Paul Todd, Brian Voll, Tracy Westphal, Judy Wickman, Denise Wight, and David Wimpfheimer.Larry Ford (USDA Forest Service)and Dave Marquart(Mono Basin State Tufa Reserve)provided 4-wheel drive transportation to remoteparts of Mono Lake. SteveBarager, Joel Ellis,Ilene Mandelbaum,and Emily and Dale Johnsonprovided logistical support, Bryan Flaig contributedcomputer expertise,Robert Jellison calculated brine shrimp lengths, Bartshe Miller contributed informationon shorebirdstatus and abundance,Scott Stine calculatedthe lengthof shorebirdsurvey segments,Rusty Scalf kindly drafted Figure 1, and Stillwater Sciencescontributed staff time to manuscriptpreparation. This is contribution940 of PRBO.

237 MIGRATORY SHOREBIRDS AT MONO LAKE

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Accepted 4 April 2002

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