Nesting Populations of California and Ring-Billed Gulls in California

WESTERN BIR

Volume 31, Number 3, 2000

NESTING POPULATIONS OF CLwO AND RING-BI--F-r GULLS IN CALIFORNIA: RECENT SURVEYS AND HISTORICAL STATUS

W. DAVID SHUFORD, Point Reyes Bird Observatory(PRBO), 4990 Shoreline Highway, StinsonBeach, California94970 THOMAS P. RYAN, San FranciscoBay Bird Observatory(SFBBO), P.O. Box 247, 1290 Hope Street,Alviso, California 95002

ABSTRACT: Statewidesurveys from 1994 to 1997 revealed33,125 to 39,678 breedingpairs of CaliforniaGulls and at least9611 to 12,660 pairsof Ring-billed Gullsin California.Gulls nested at 12 inland sitesand in San FranciscoBay. The Mono Lake colonywas by far the largestof the CaliforniaGull, holding 70% to 80% of the statepopulation, followed by SanFrancisco Bay with 11% to 14%. ButteValley WildlifeArea, Clear Lake NationalWildlife Refuge, and Honey Lake WildlifeArea were the only othersites that heldover 1000 pairsof CaliforniaGulls. In mostyears, Butte Valley, Clear Lake, Big Sage Reservoir,and Honey Lake togetherheld over 98% of the state'sbreeding Ring-billed Gulls; Goose Lake held9% in 1997. Muchof the historicalrecord of gullcolonies consists of estimatestoo roughfor assessmentof populationtrends. Nevertheless, California Gulls, at least,have increased substantially in recentdecades, driven largely by trendsat Mono Lake andSan FranciscoBay (first colonizedin 1980). Irregularoccupancy of some locationsreflects the changing suitabilityof nestingsites with fluctuatingwater levels.In 1994, low water at six sites allowedcoyotes access to nestingcolonies, and resultingpredation appeared to reducenesting success greatly at threesites. Nesting islands secure from predators and humandisturbance are nestinggulls' greatest need.

Conover(1983) compileddata suggestingthat breedingpopulations of Ring-billed(Larus delawarensis)and California(Larus californicus)gulls haveincreased greafiy in the Westin recentdecades. Detailed assessments of populationstatus and trends of these speciesin individualwestern states, however,have been publishedonly for Washington(Conover et al. 1979), Nevada(Yochem et al. 1991), and Utah (Paulet al. 1990). As part of a broad inventoryof nongameaquatic birds at Great Basin lakes,Winkler (1982) estimatednumbers of thesegulls at variouscolonies in northernCalifornia, but hissurveys did not coverall historicalbreeding sites and were conductedlate in the breedingseason during the extremedrought year of 1977. Currenfiy,

Western Birds 31:133-164, 2000 133 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA the CaliforniaGull is listedas a speciesof specialconcern in California (Remsen1978, Calif.Dept. Fish & Game,unpubl., 1992), and the Ring-billed Gull is underconsideration for thisdesignation (L. Comrackpers. comm.). To determinethe currentstatus of breedingCalifornia and Ring-billed gullsin California,from 1994 to 1997 we surveyedall sites where they have nested in recent decades.Here we report the distribution,abundance, habitat associations,and nesting phenology of gulls at these colonies, presentthe historicalrecord, and evaluatedata quality and population trends of thesespecies in California.Finally, we assessthreats to thesecolonies and make recommendationsfor their protection.

STUDY AREA AND METHODS

StatewideSurveys We surveyedgull coloniesstatewide from 1994 to 1997, focusingour efforts on the Modoc Plateauand Great Basin in northeasternCalifornia, wheremost historical colonies have been located. This region,though arid, hasnumerous large wetlands at moderatelyhigh elevations in plateaus,large valleys,or basinsreceiving drainage from nearbymountains. Colonies at Mono Lake in the Great Basin,at San FranciscoBay on the coast,and at the SaltonSea in the ColoradoDesert of southernCalifornia were surveyedas part of other studies,as describedbelow. We supplementedthe statewide surveyswith data from opportunisticsurveys or other studiesfrom 1998 to 2000. Prior to our field surveys,we searchedthe literatureand contacted biologistsand field ornithologistsactive in Californiato determinewhich siteshad supportedgull colonies since the 1970s. Shufordalso scouted for othergull colonies during shorebird surveys by airplane,airboat, and on foot in northeasternCalifornia in late April to earlyMay in 1994 and 1995 and duringstatewide surveys on foot, by kayak,and by planefor variouscolonial waterbirdsfrom mid-May to mid-July, 1997 to 1999 (Shuford 1998, Shufordet al. 1999; PRBO unpubl.data). Our surveysspanned one very dry year and three very wet years. The surveyperiod began at the end of a droughtthat beganin 1986-87 and ended in 1994-95, interruptedonly by above-normalprecipitation in 1992-93. Precipitation(1 July-30 June) in the Great Basinin California averaged31.8, 93.6, 69.8, and 79.7 cm in 1993-94, 1994-95, 1995-96, and 1996-97, respectively;the long-termaverage (n -- 103 years)for this region is 54.0 cm (Western Regional Climate Center; http:// www.wrcc.dri.edu/divisional.html).Water levelsat most coloniesin north- easternCalifornia fluctuated greatly over the surveyperiod. To capturepeak nestingnumbers, we selecteda primarysurvey period of earlyto mid-Mayfor San FranciscoBay and mid- to lateMay for northeastern California,representing the late incubationto very early hatchingperiod of both species(PRBO and SFBBO unpubl.data). Shufordand colleagues countedall gull nestsor breedingadults at variouscolonies in northeastern Californiaprimarily from 11 to 29 May, 1994 to 2000. A few early-season surveys(27 April-3 May)served mainly to confirmthe lackof nestingat a few sitesoccupied irregularly. Shuford also counted nesting gulls at GooseLake in mid-June1999 and 2000; gullnesting at GooseLake in theseyears was

134 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA much later than at other sitesin the region. Dates of nest countsat Mono Lakein allother years, 1983 to 1993, rangedfrom 18 May to 2 June,except in 1983 when they spanned29 May to 16 June (PRBO unpubl.data, J. R. Jehl, Jr., in litt.). Shufordand othersconducted aerial photographic surveys of nestinggulls at the otherwiseinaccessible Sheepy Lake pelicanand cormorantcolony at LowerKlamath National Wildlife Refuge (NWR) on 23 May 1994, 12 May 1997, and 13 May 1999. Gullsmay havenested at this sitein otheryears of oursurveys, but a lackof countsat thissmall colony has littleeffect on statewidetotals. Similarly, we wereunaware until 1995 that in 1994 a few gullsbred at ShastaValley Wildlife Area (WA),Siskiyou County; no gullsbred at thislocation in subsequentyears (M. McVeypers. comm.). From 3 May to 19 June(mostly 8-20 May), 1994 to 2000, Ryan, V. L. Layne, D. Bell, and colleaguessurveyed California Gull coloniesin San FranciscoBay within Santa Clara, Alameda, San Francisco,and Contra Costacounties. Nest counts there from 1980 to 1993 were between4 May and 21 June (mostly4-30 May; SFBBO unpubl.data). Molina (2000) countedCalifornia Gull nestsat a newlyformed colony at ObsidianButte, SaltonSea, ImperialCounty, between 15 May and 6 June, 1997 to 1999. Observersmade most countsby walkingthrough colonies and marking eachnest individually (on the rim or on an adjacentrock or weed)with a dab of spray paint to avoid over- or undercounting.For those colonies,we estimatedthe numberof nestingpairs equaled the numberof nestscounted. At SouthSan FranciscoBay we did not mark nests;the colonies'occupying narrowdikes minimized over- or undercounting.An exceptionwas at the AlamedaNaval Air Station(NAS), where colonysize was estimatedfrom outsidethe colony(L. Collinspers. comm.). At Clear Lake, mostyears we usedthe spray-paintmethod to countone or two gullcolonies not inhabitedby other colonialwaterbirds. Otherwise, observersminimized disturbance to multi-speciescolonies by countingall adultgulls from a smallmotorboat cruising slowing by the coloniesabout 60 m offshore. We estimated the number of nests on these islands as 0.71 of the numberof adultscounted for the Ring-billedand 0.72 for the California, the ratios at Clear Lake in 1994 at colonies where we could count both nests and adults.Though these ratios may varyby siteand year,we appliedthem at Sheepy Lake to countsof adultsobtained from aerial photographsin 1997 and at GooseLake to countsof adultsfrom the groundin 1999 and 2000, when the associationof gullswith nestingCaspian Terns ($terna caspia)precluded nest counts.We made all countsin the morning at the samestage of the nestingcycle. At Lake Shastinain 1994, R. Ekstrom(in litt.) countednesting gulls from shoreby lookingout to the smallnesting islandoff MilkhouseIsland; in otheryears, observers counted nests using the spray-paintmethod. For northeasternCalifornia, we characterizednesting phenology by the firstobservations of chicksand, to a lesserdegree, by the proportionof nests with chicksat the time of our surveys.Survey dates and the extent of coveragewere most suitable for thispurpose in 1994, 1995, and 1997. We estimatedthe approximatedates of first egg layingby backdatingbased on the range of mean incubationperiods from variousstudies reported by Ryder(1993; 25-26 days)for the Ring-billedGull and Winkler(1996; 24-

135 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

27 days) for the California. In San FranciscoBay in 1998 and 1999, biologistsaged eggs by the eggflotation method of Haysand LeCroy(1971) and therebyestimated egg layingdates + 1-2 days.

Historical Data We obtainedother populationestimates from the publishedliterature when possible,including the seasonalreports of IXlorthAmerican Birds (NAB) and its predecessors,and unpublishedsources, including long-term studiesat Mono Lake (PRBO;J. R. Jehl, Jr., in litt.) and San FranciscoBay (SFBBO),the notebookson filewith the regionaleditors of the MiddlePacific CoastRegion of NAB (citedMPCR notebooks),the filesand annual narrative reportsof variousstate and federal wildlife refuges, and biologists' field notes. We reportdates of surveys,methods used, and the numbersof nestsor pairs when thesedata are available.Otherwise, we report numbersof nesting adults,as there is no reliableway to convertraw countsor estimatesof adults to nestingpairs. We alsosearched for historicaldata from the recordsof egg setshoused at the CaliforniaAcademy of Sciences(CAS), Museum of Natural History of Los AngelesCounty (LACM), Museumof VertebrateZoology (MVZ), San BernardinoCounty Museum (SBCM), San DiegoNatural History Museum(SDNHM), Santa BarbaraMuseum of Natural History (SBMNH), and the WesternFoundation of VertebrateZoology (WFVZ). Data Quality Becausethe methodsused to estimatenests, adults,or young varied widely,we categorizedthe accuracyand repeatabilityof each estimateas high (1), moderate(2), or unknown(3). 1: All countstaken near the late incubationperiod when the adult nestingpopulation reaches its peak; individualnests counted in entirecolony, or densityof nestsin a portionof a colonycalculated then appliedto the measuredarea occupiedby the entire colony,or numberof pairs estimatedfrom countsof adultsconverted to nestingpairs from the ratioof adultsto nestsdetermined for a portionof the colony.We considersuch data appropriate for population-trendassessment. 2: Countstaken on knowndate(s) in earlyto mid-nestingseason and based on direct countsor estimatesof adultsor chicks,possibly from a distant vantagepoint. Such data shouldbe viewedcautiously in assessmentof populationtrends. 3: Date(s)and/or methodsof surveysunavailable, or methodsas in category2 but estimatemade late in the nestingseason, or estimatemade visually from fixed-wingaircraft. Such data shouldbe used with greatcaution in interpretingpopulation trends. The qualityof estimates basedon early-to mid-seasonaerial photographs varies with the clarityand sizeof imagesand contrastof adultgulls and nestswith the background.

RESULTS AND DISCUSSION

Current Distribution,Abundance, and Habitat Associations Except for the recentlyestablished colonies of California Gulls in San FranciscoBay (Jones1986) and at the SaltonSea (Molina2000), all known currentbreeding colonies of Californiaand Ring-billed gulls in Californiaare locatedin the northeasternpart of the state in Siskiyou,Modoc, Lassen,

136 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Plumas,and Mono counties(Figure 1). For CaliforniaGulls, the westernmost and southernmostbreeding sites in the species'entire range are at San FranciscoBay andthe SaltonSea, respectively(Winkler 1996, thispaper), whereasRing-billed Gulls reach their southwesternlimit at Honey Lake (AOU 1998). In northeasternCalifornia, gulls nest mainlyon islandsand, rarely, on peninsulasat natural lakes, reservoirs,managed wetlands, and saline or alkalinelakes, primarily between 2700 and 5100 feet (823-1555 m) above sealevel, at 6385 feet (1947 m) at Mono Lake.The substratemay be earth, rock, or, infrequently,broken-down tule mats. Gulls nest in the open or amongrocks, tall weeds,or shrubs(Figures 2b, 3b, 4b, 5). Californiaand Ring-billedgulls often nest together at thesesites and/or with other colonial species,such as pelicans,cormorants, herons, egrets, and terns. Exceptat

Tule Lake

Lower Klamath Lake NWR

Sage Reservoir Lake

- Honey Lake ßLake Almanor

Woodward Reservoir

/ Mono Lake

100 0 100 •

Salton Sea

Figure1. Locationsof Californiaand Ring-billedgull colonies in California.All colonies havebeen active since 1944 exceptfor Riego,Sutter County, and Woodward Reservoir, StanislausCounty.

137 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Mono Lake, most large gull coloniesare at or near extensivefreshwater wetlandsor irrigatedfields (primarily alfalfa). In SanFrancisco Bay, Califor- nia Gullsnest at sea level primarilyon earthenislands and leveesin salt ponds:the small colony at theSalton Sea, on a rockyand sandy island near shore, is 227 feet below sea level.

Figure2. (a)View from landlocked Milkhouse Island of isletin centralportion of Lake Shastina,Siskiyou County, where (b) Ring-billed and California gulls nest amid rocks and shrubs. Photosby W. D. Shuford, 16 May 1995

138 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Figure3. (a) Aerial view of MeissLake, Butte ValleyWildlife Area, SiskiyouCounty, where Ring-billedand Californiagulls attempted to nestduring a prolongeddrought despitethe nestingislands' becoming peninsulas. (b) Ring-billedGulls nesting amid weedygrowth on an islandin MeissLake, with Mt. Shastalooming in the background, five yearslater after water levelshad risen. Photos by W. D. Shuford, (a) 23 May 1994 and (b) 24 June 1999

139 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Figure4. (a)View frombluff along Highway 395 of southeasternportion of Goose Lake, ModocCounty. where small changes in lake levelcan rapidlyexpose or inundatepotential nesting islands. (b) Ring-billed Gull colony on a low-lyinggrassy islandisolated after a post-droughtrise in the levelof GooseLake. Photos by W. D. Shuford, 18-19 May 1997

140 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

.: •.... • '" ..

Figure5. CaliforniaGulls nesting in thegrass in theopen and under shrubs on a small isletin MiddleAlkali •ke, SurpriseValley, Modo• Count, with the Warner Moun- tains in the distance. Photos by W. D. Shuford, 17 May 1994

California Gull. From 1994 to 1997, the numberof pairsof California Gullsnesting in the stateranged from about33,125 to 39,678 (Table1). In theseyears, the Mono Lakecolony constituted 70% to 80% of the statewide total, coloniesfarther north 9% to 16%. and the San FranciscoBay colonies 11% to 14%. Of other sitesoccupied as recentlyas the early 1990s, only Tule Lake NWR and Lake Almanor remainedunoccupied throughout the four yearsof our surveys. Ring-billedGull. From 1994 to 1997, at least9611 to 12,660 pairsof Ring-billedGulls nested annually at five to seven sites in northeastern California(Table 1). Of othersites occupied since the 1970s• onlyTule Lake NWR remainedunoccupied throughout our surveys.Despite their smaller statewidebreeding population, Ring-billed Gulls outnumberedCalifornia Gullswhere they overlappedin Siskiyou,Modoc, and Lassencounties by a ratio of about 1.9 to 3.2:1, a patternWinkler (1982) alsofound•

Phenology In all years, we saw at least a few small chicksduring our nest counts, indicatingwe had timed our countswell with respectto peak nesting numbers.Patterns varied somewhatby species,region, and year. California Gull In northeasternCalifornia, we recordedthe firstCalifor- nia Gullchicks from 16 to 20 May, with no consistentpattern of variationby colonyor year.Backdating, the earliestCalifornia Gull eggs were laid at least

141 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

142 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

143 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA by 19-22 April in 1996 and 1997 butprobably several days earlier based on the largenumbers of early-seasonchicks in thoseyears. On 16 May 1997, 22.2% of 1858 California Gull nestschecked at Honey Lake had small chicks,implying the earliestnesting; no chickswere seen at any other coloniessurveyed from 14 to 19 May 1997. On 17 May 1995, of 47 territorialgulls an islandin Clear Lake had only 7 nestswith eggs,implying the latestnesting. Estimatedmean datesof egg layingin San FranciscoBay in 1998 and 1999 rangedfrom 26 April (range 18 April-2 May, SD 2.67, n -- 121, colonysurveyed 3 May)at Knappto 8 May (range5-14 May, SD 2.39, n -- 100, colonysurveyed 14 May) at Moffett, both in 1998. Ring-billedGull. We sawthe firstRing-billed Gull chicksfrom 16 to 20 May, in all instancesat Honey Lake. Backdatingimplies the earliesteggs were laid about 20-21 April in 1994 and 1997. Nestingwas somewhat advancedat Honey Lake in 1997, when on 16 May 3.1% of 2479 Ring- billedGull nests checked had smallchicks; no chickswere seenat any other coloniessurveyed from 14 to 19 May 1997. In 1999, incubatingadults and a lack of chicksat GooseLake on 20 Juneindicated a very late initiationof nesting.

Historyof BreedingColonies Lake Shastina. It is unclearwhen islandsbecame available or when gulls begannesting at this reservoir,which though created in 1928 took a long time to seal and hold water (DWR 1988, R. Ekstrompers. comm.). Our countsof a mixedcolony of Ring-billedand Californiagulls nesting among rocksand shrubson a smallisland off landlockedMilkhouse Island (Table 1, Figure2) appearto be the only populationestimates for thissite. ShastaValley WA. The onlyknown evidence of gullnesting was in 1994 when low waterat SteamboatLake exposedtwo smalldredge-spoil islands that later in the seasonbecame a peninsula.The attemptsby a few Ring- billedGulls to nest there apparentlywere disruptedby terrestrialpredators (B. Smith pers. comm., Table 1). Butte Valley WA. Historically,Meiss Lake coveredabout 4050 ha, but dikingin the 1940s greatlyreduced the sizeof the wetlands(K. Novickpers. comm.). The islandswhere gullsnest have been availablesince before the dikes;smaller islands created in managedwetlands since the establishment of the refugein 1981 havenot been used.Gulls have nestedat MeissLake sinceat least1970, when a 27 May surveyof onlyone of the lake'sislands revealed1244 nestinggulls (species not distinguished;K. Novick in litt.). Aerialsurveys of MeissLake from 1 to 5 June 1981 and 29 May to 1 June 1984 yieldedestimates of about 1000 and 700 nestinggulls, respectively (B. E. Deuelin litt.).On 7 May 1988, observersestimated 600 to 1000 adult gullson threeof the fourislands with nesting gulls that they checked; they did not estimatethe proportionsof the two species,but Ring-billedswere more numerousthan Californias(K. Novickin litt.). In 1988, a landbridgeformed, leavingthe colonyvulnerable to predation;in 1992, the lake driedup, and gullsdid not nestsuccessfully. The firstaccurate nest countsat MeissLake on 7 May 1993 yielded520 pairsof Californiaand 450 pairsof Ring-billedgulls nesting on fourlow-lying

144 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA islands(K. Novickin litt.). The early-seasoncounts in both 1988 and 1993 may have underestimatedthe nestingpopulation in thoseyears. In 1994, both speciesinitiated nesting on two peninsulasin the lake (Table1, Figure 3a). Signsof coyote(Canis latrans) predation starting in May andthe lake's dryingby earlyJuly made it unlikelythat gullsraised any youngin 1994 (K. Novickpers. comm.). From 1995 to 1999, gullsnested on up to sevensmall islandsin MeissLake, three or four in the centralportion and one to three in the southportion of the lake (K. Novick in litt., Figure 3b). During this period,the nestingRing-billed Gull populationranged from about2525 to 4087 pairswith no trend(Table 1, K. Novickin litt.).By contrast,California Gull numbersincreased steadily from about 327 pairs in 1994 to 2956 in 1999 (Table1, K. Novickin litt.), althoughthis trend is exaggeratedby the artificiallylow countin 1994. Lower Klamath NWR. Before 1900, Lower Klamath Lake consistedof about 22,267 ha of marsh and 12,146 ha of open water (Akins 1970). Currently,Lower Klamath NWR has8907 ha of wetlands;4858 to 6478 ha are seasonallyflooded and 2024 to 3644 ha are permanentlyflooded marshes(USBR 1998). In 1905, Finley (1907) exploredLower Klamath Lake by rowboat, describingfloating tule "islands"stretching for miles separatedby a networkof narrow channels.He and his companion,H. T. Bohlman,found "at least 500 pairs"of Californiaand Ring-billedgulls nestingon the broken-downtules. They ascribedthe nestingsites to Lower KlamathLake, Oregon,but giventhe vastarea of the lakeand the difficulty of travelthey couldhave missedother sites. Since 1952, biologistsat the Klamath Basin National Wildlife Refuges haveestimated numbers of breedinggulls at LowerKlamath NWR, Califor- nia (Table2). From the 1950s to the 1970s, up to 2500 pairseach of the Californiaand Ring-billed gulls nested. The refugeestimated 700 Ring-billed Gull nestsin 1976, but subsequentlyvery few appearto have nestedat Lower Klamath.When thousandsof CaliforniaGulls were nestingthere in the 1980s, M. Robbins(pers. comm.) noted only one or two Ring-billedGull nests,in 1981 and 1983. Sprayingof oil on gull eggs by biologistsin the 1950s, to reducegull predationon waterfowleggs and young(J. Hainlinepers. comm.),did not appearto haveany majoreffect on gullpopulations on the refuge.Probably beginningin the 1950s or 1960s, and increasingfrom the 1970s to early 1980s, refugepersonnel built islands in dikedponds to attractnesting Canada Geese(Branta canaden$is;J. Beckstrandin litt.). Gull coloniesexpanded to occupyislands as they became available and first concentrated in Unit 4 of the refuge(early 1960s), then units 3 and4 (mid-1960s),finally units 4 and6 (late 1970s onward).The gulls'apparent increase from the late 1970s to the mid- 1980s, peakingat 3000-5000 pairs of the Californiain 1985, prompted removalof most of the islandsto reducethe perceivedproblem of gull predationon waterfowlnests and young (J. Hainline, D. Mauser pers. comm.). Drainingof pondsin Unit 4 for maintenancein late April 1986 forcedthousands of gullsto abandonnesting islands by 1 June,and they producedno young(R. Ekstrom,M. Robbinsin litt.). In 1987 and 1988, numbersof gullsdeclined (refuge's annual reports). In 1989, afterremoval of islandsin Unit 4C displacedover 2000 nestinggulls, biologists found only a

145 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Table 2 Numbersof Nests(and Young) of Califor- nia and Ring-billedGulls at Lower KlamathNational WildlifeRefuge, 1952-1991

Year California Ring-billed Unidentified

1952 a 1820 (3600) 2400 (5500) 1953 a 930 (1580) 46 (80) 1954 • ? (3025) ? (1810) 1955 • 1533 (2759) 746 (1720) 1957 • 1220 (1220) 900 (900) 1958 • 800 (1000) 500 (600) 1961 a ? (500) ? (400) 1963 • 900 (2000) 1964 a 2500 400 1965 a 1514 (2000) 1966 • 2000 (2500) 1967 • 2500 (2000) 1968 • 1300 (1500) 800 (1000) 1969a 900 (1300) 600 (850) 1970 • 548 (700) 950 (1200) 1971 • 1550 (620) 700 (350) 1972 • 1670 (508) 685 (322) 1972•,b ? (3000) 1973 a ? (600) ? (580) 1976 • 210 (400) 700 (955) 1977 c present 1980 d 4000 adults 1985 e 3000-5000 -2000 adults 1986•,/ 1000's(0) 1990 • -100 1991 • -100

aDataquality category 3, from refuge'sannual narrative reports. bFrom Am. Birds 26:884. CGullsnested successfully on islandsin a dry lakebed(E. O'Neillfide D. W. Winkler in litt.). aAvisit on 1 Juneproduced a roughestimate of 2000 pairs(data qualitycategory 2; Conover 1983, S. A. Laymonpers. comm.). eBasedon visitsfrom mid-Junethrough mid-July(data quality category2; R. Ekstromin litt. and annualnarrative reports). /FromR. Ekstrom(in litt.). few gullnests on the entirerefuge. Since then the few breedinggulls have nestedin Unit 6A or in SheepyLake (Table1; J. Beckstrandpers. comm., Shufordpers. obs.). Sheepy Lake has been occupied by nestinggulls since at leastthe early1990s (L. A. Moreno-Matiellapers. comm.), but its remoteness may havemasked earlier occupation. From 1994 to 2000, numbersof nestingpairs of CaliforniaGulls ranged from about6 to 269 in Unit 6A of LowerKlamath NWR, wherethey bred on up to 8 smallrocky islets (Table 1). Duringthat period,Ring-billed Gulls

146 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA nestedin Unit 6A only in 2000, but in 1994 and 1997, at least, small numbers nested with even fewer California Gulls on a tule-mat island at SheepyLake (Table1). Tule Lake NIAIR. Before 1900, Tule Lake fluctuatedin sizefrom about 22,267 to 44,534 ha (Akins1970) but todayconsists only of 5263 ha of return-flowpermanent sumps (USBR 1998). The firstreported visits to the colony,in 1931 (Moffitt1942) and on 16 May 1940 (WFVZ),produced no populationestimates. In at leastthe 1950sand 1960s, gulls bred irregularly, mostlyin the uppersump, on flattenedmats of tulessheared off by ice(E. J. O'Neill pers. comm.).Up to about400 pairseach of both speciesnested therein at leastsix years from 1952 to 1964, thoughthe data, from the refuge'sannual report, are of low quality(category 3; see above).Since 1964, gullsare known to havenested at TuleLake only in 1990, whenL. A. Moreno(pers. comm.) found 35 adultand 11 youngRing-billed on a small rockyisland in the lowersump on 19 July.Shuford found no nestingactivity duringour 1994-1997 and 1999 surveys. Conover's(1983) reportof 1980 Californiaand 11,818 Ring-billedgulls breedingat TuleLake in 1976 isin error.Those figures are the sumof "peak population"estimates, not numbersderived from nest counts,for Clear Lakeand LowerKlamath refuges combined; no nestingdata were available for Tule Lake in 1976 (refuge'sannual reports, J. Beckstrandin litt.). Clear Lake NIAIR. Before its outflow was dammed in 1910, Clear Lake consistedof a westlobe of about4048 ha of shallowopen water and an east lobe of about 2024 ha of marshlands(Akins 1970). The dam created a reservoirof open water,which today covers about 9623 ha (USBR 1998). Willett(1919) visitedthe "usualbreeding grounds on islands"in ClearLake on 10 April 1918, prior to nestinitiation, and estimatedabout 2000 gulls present,about 90% Californiaand 10% Ring-billed.On 11 and 12 July 1933, H. M. Worcester(in Lincoln 1933) found California Gulls closeto fiedgingon an islandin Clear Lake from whichthe water had recededthree miles. Estimates for Clear Lake from 1952 to 1989 come from the Klamath Basinrefuges' annual reports, those from 1990 to 1993 fromthe refuges' reportsor L. A. Moreno-Matiella(in litt.),who wasstudying nesting Ameri- canWhite Pelicans(Pelecanus erythrorhynchos)(Table 3). Fromthe 1950s throughat leastthe 1970s up to about850 pairsof CaliforniaGulls nested at ClearLake, andfrom the 1950s throughthe mid-1960sup to about550 pairs of Ring-billedGulls nestedthere, though the estimatesare rough (qualitycategory 3; seeabove). Subsequently, numbers of Ring-billedGulls appearedto increasesharply, as population estimates from 1968 to at least 1972 rangedfrom about 1150 to 2100 pairs.Data from the mid-1970s throughthe early 1990s did not distinguishbetween the two species,but numbersof gullsas a wholeappeared to increase.Most estimates during this periodranged from about2500 to 5000 nestingpairs. Our more precise estimates(category 1) from 1994 to 2000 rangedfrom 1739 to 3922 pairs of Ring-billedGulls and 432 to 1769 pairsof CaliforniaGulls (Table 1). The locationof waterbirdcolonies at Clear Lake variesannually as the availabilityof islandsvaries with changinglake levels (L. A. Moreno-Matiella pers.comm., Shuford pers. obs.). Nesting sites are (1)a largerocky island in

147 Table 3 Numbersof Nests(and Young)of Californiaand Ring-billedGulls at Clear Lake National Wildlife Refuge, 1952-1993

Year California Ring-billed Unidentified

1952 ø 500 (950) 554 (1200) 1953 ø 475 (900) 450 (950) 1954 ø 685 (1850) 565 (1730) 1955 ø 423 (994) 527 (1325) 1956 ø 200 (210) 250 (260) 1957 ø "above 1956 levels.... below 1956 levels" 1961 ø ? (70) ? (30) 1963 a 600 (1750) 1964 ø 500 (600) 432 (500) 1965ø 100 (300) 470 (1500) 1966ø 250 (500) 500 (1000) 1967ø 500 (500) 140 (300) 1968ø 350 (500) 1885 (2300) 1969 a 150 (200) 2000 (2500) 1970 ø 868 (950) 1165 (1800) 1971 ø 878 (500) 2134 (1000) 1972 ø 400 (460) 1690 (1200) 1973 ø ? (520) ? (1080) 1977 b "lower than 1976 .... lower than 1976" 1985 ø ? (4300) 1986o, c near• 8000 1987o, c 3000 1988o, c near]y4500 1989o• 5000 1990d,e 5000 1991e,f 4000+ 1992•,g 2500(2000+) 1993•, h 3400

aDataquality category 3, from refuge'sannual narrative reports. bpeakpopulation and production slightly lower in 1977 thanin 1976 because of human intrusion in colonies. cone to two, of several,mixed gull colonies on islandsnear the ClearLake dam. danaerial survey on 20 Junerevealed 2000 gullnests on an islandnorth of the Clear Lake peninsula(1630 youngon 27 Juneground survey) and 3000 nests on an island near the Clear Lake dam. eDataquality category 2, from L. A. Moreno-Matiella(in lift.) and/or refuge's annualnarrative reports. labour4000 gullnests on anisland at thenorth end of theClear Lake peninsula were later abandonedwhen a landbridgewas exposed,allowing predator access.A countby foot on 30 June found450 adultgulls on Bird Islandand 270 adultson RockyIsland; the formerwas connected to shorein earlyJune and producedno young,the latterheld 265 youngon 24 July. •Surveysby boat estimated2000 gullnests on RockyIsland on 1 May and 500 nestson Bird Islandon 10 June. Surveysby foot found2000 youngon Rocky Islandon 13 July1992. Exposedlandbridges connected both islands to shore, and an electricfence was erectedin late April or early May to protectvarious colonialwaterbirds on Rocky Island. h1000 gullnests on RockyIsland, 1500 on the islandnorth of the peninsula, 400 on Bird Island, and 500 on islandsin the west lobe.

148 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA the eastlobe of the lake that becomesseveral smaller islands at higherlake levels;(2) Bird Island,a sandyisland near the eastshore of the westlobe; (3) a largelow-lying island of soiland small rocks at the northend of the lakein the channelconnecting the two lobes;and (4) varioussmall 1owJlying islands resembling(3) in the northeasterncorner of the lake.In somerecent drought years, when a dropping lake level connectedislands to the mainland, biologistserected electricfences, preventingcoyotes from entering and disruptingcolonies, particularly those of the White Pelican(D. Mauser,L. A. Moreno-Matiellapers. comm.). In 1991, without suchfences, gulls aban- donedabout 4000 nestsafter a landbridgeformed. GooseLake. Sinceat least 1976 gullshave nestedintermittently on low- lyingislands, or a peninsula,near the southeastshore of GooseLake. B. E. Deuel(in litt.)noted both Californiaand Ring-billedgulls nesting there, 1-22 June 1976. D. W. Winkler (in Conover 1983) estimated 1205 California Gullsbreeding at GooseLake in 1977. Conover(1983) mistakenlyreported that S. A. Laymonfound 1200 CaliforniaGulls and <20 Ring-billedGulls at GooseLake in 1977; Laymon(pers. comm.) did not visitthat area in 1977. From 1979 to 1985, B. E. Deuel (in litt.) visitedGoose Lake in mostyears to surveyor bandwaterfowl. On 15 June 1979 he estimated800 California Gullswere nestingon a large island.In 1980, gullswere breedingat Goose Lake but were not counted,and duringaerial surveys1-5 June 1981 an estimated400 gullsof unknownspecies were nestingthere. In 1983, gulls againbred, but risingwaters reduced the sizeof the nestingisland. An aerial surveyon 1 June 1984 detecteda colonyof about400 gullson a peninsula dose to the inundatedformer nestingisland; gulls were againnesting on the peninsulain 1985 but in much smallernumbers than when they nestedon a nearbyisland. Shuford surveyed shorebirds at GooseLake each year in late April from 1991 to 1995 and foundno evidenceof gull nesting;drought prevailed1991-1994. The colonyremained unoccupied in May 1996 (R. Ryno in litt.) but was reo½½upiedin 1997 (Table1, Figure4). A risinglake levelin 1998 and 1999 reducedavailable nesting islands; Shuford's surveys on 20 June 1999 and 19 June 2000 surveysrevealed about 199 and 194 pairsof Ring-billedand 8 and6 pairsof Californiagulls, respectively, nesting at the tip of a long peninsula. Big Sage Reservoir.Big Sage Reservoirwas completedin 1921 (DWR 1988); it now coversabout 2133 ha (DWR 1988). B. E. Deuel (in litt.) first found gulls,mostly Ring-billed, nesting on islandshere on 18 June 1976. During aerialsurveys he foundthis colonyactive in 1980 and occupiedby about600 gulls1-5 June 1981. From shore,on 2 July 1989, S. F. Bailey (pers. comm.) estimated>500 adult Ring-billedGulls with young on an island in the middle of the reservoir;this colony cannot be ½ensused completelyfrom shore.In August1992, the reservoirwas completelydry, forthe firsttime since 1935 (G. Dickpers. comm.), and it seemsunlikely that gullsnested. But gulls had reoccupied the colonyby 1994 (Table1). Shuford visitedthe colony on 21 June 1999 and 20 June 2000 and found it dominatedby Ring-billedGulls in numberssimilar to thosein recentyears but made no formal census.The reducednumber of Ring-billedGulls in 1997 couldreflect a shiftfrom Big Sage Reservoirto nearbyGoose Lake, reo½½upiedthat year for the firsttime in manyyears. In mostyears, the gulls

149 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA nestonly at the eastend of Bird Island,the large islandin the south-central portion of the reservoir,though in 1995 21ø-/owere on a smallunnamed island to the northeast. Middle Alkali Lake. Shuforddiscovered California Gulls nesting on a smallscrub-covered island adjacent to landlockedHansen Island on the east shoreon 30 April 1993, duringan aerialsurvey for shorebirds.On 4 May, by airboat,he foundabout 300 adultson the island,but asmost had not laid determiningthe sizeof the populationwas not possible.On 17 May 1994, Shufordand C. Alexanderfound the island(Figure 5) connectedto the lakeshoreby a peninsulaof saturatedmud, crossedby numerouscoyote trails.They counted71 nestswith eggs, 9 of whichhad recently been preyed on, some destroyednests, and severaldead adult gullsthat had suffered predation.They estimatedthat prior to nestloss that about100 nestshad beeninitiated. The colonywas unoccupied from 1995 to 1997. Eagle Lake. Accountsof the birdsof EagleLake from the late 1880s and early1900s mentionedbreeding by CaliforniaGulls but gave no population estimates(Ray 1915, 1921; Townsendin Grinnell et al. 1930). Ray and otherscollected at leastfive egg setsat EagleLake in 1914, when, as in recent decades,the gullsnested at PelicanPoint (egg data slipsMVZ, SDNHM, WFVZ). On 22 June 1921 Grinnell et al. (1930) found four CaliforniaGull nestson a smallrocky island,and on 13 June 1929 they countedabout 50 CaliforniaGulls along with two nestson an islandwhere White Pelicans nested. Gould (1974) reported that California and Ring-billedgulls were nonbreedingsummer residents at EagleLake in 1970 and 1971. Lederer (1976) counted13 CaliforniaGull nests at EagleLake in 1974, a yearwhen risingwater had isolatedthe nestingisland (Pelican Point, fide Shaw 1998). From 1 to 5 June 1981, duringaerial waterfowl surveys, B. E. Deuel(pers. comm.) estimated150 gullsnesting. In 1990 J. Bogiatto(pers. comm.) estimated1500 to 2000 Californiaand 300 to 400 Ring-billedgulls nesting on PelicanPoint; only the Californiafledged young. By 1991 drought loweredthe lake'slevel and the islandbecame a peninsula;that year and in 1992 gullsinitiated nesting but abandonedtheir effortsmidseason, appar- ently becauseof disruptionby terrestrialpredators. Nesting resumed in 1995 (201 nestsof the California,132 of the Ring-billedcounted) when risingwater madePelican Point an islandagain (Table 1). No gullsnested therein 1996, 1997, or 1999, despiteits remaining an island(Shaw 1998, Shuford pers. obs.), but did in 2000 (no populationestimates made, J. Bogiattopers. comm.). Honey Lake g/A. Ring-billedGulls have nested since at leastabout 1920 at Hartson Reservoir(Moffitt 1942), a natural freshwaterlake separated from the northshore of salineHoney Lake by sanddunes. From incomplete surveys(category 3) on 3 May 1940 and 14 May 1941, Moffitt estimated 150 and 75 Ring-billedGull nests, respectively,and inferred that the population totaled "250 or more pairs." Johnston and Foster (1954) reportedthat when HartsonReservoir was very low in 1950 about750 gull nestswere "countedon a smallduck pond near the refugeheadquarters." AlthoughRing-billed Gulls predominated,they suspectedthat California Gullsalso were breeding. After 1950 the gullsagain used the smallislands in

150 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA the reservoir.Nest counts (category 1) on 17 May 1953 revealed717 pairs of the Ring-billedand 26 of the Californiaon one island. Various visitors from the mid-1950s to the mid-1980s estimated about 1500 to 4000 nestinggulls, the Ring-billedgenerally predominating (Table 4). In the extreme droughtyear of 1977, the numbersnesting were greafiy reduced.On 23 June Winkler (1982, in litt.) found two nestingislands connectedto the shorelineand deadadults and many destroyedeggs strewn about both colonies;on 19 July he found the colonyabandoned. Such landbridgingof naturalislands during dry periods prompted refuge personnel to create additionalislands in Hartson in the mid-1980s (K. Novick pers. comm.).Hartson Reservoir dried up in September1990, and no gullsnested until1993, aftera verywet winter.Then highwater reduced the numberand sizeof the remainingislands, and most of the nestinggulls shifted to small islandsin dikedponds north of Hartson (C. Holmespers. comm.). More accuratesurveys (category 1) from 1993 to 2000 revealed1727 to 2502 pairsof the Ring-billedand 1247 to 1913 of the California(Tables 1 and 2). The vast majority have nestedon 9 to 18 small low-lyingislands primarilynear the north shore of Hartson Reservoirin the Dakin Unit (Figure6b). In someyears, up to 544 pairsof Ring-billedGulls have nested on one to six smallislands in dikedponds of the Dakin Unit north (units6G and 6J) and east (Unit 5A) of Hartson Reservoir(Shuford pers. obs.).In 1994, aboutfive pairs of Ring-billedGulls nested in an isolatedcolony on a smallisland in a dikedpond in Unit 15 of the FlemingUnit, the onlyknown gullnesting at the refugeaway from HartsonReservoir or nearbyponds. In May 1994, islandsin Hartson Reservoirand Unit 6J were separated from the shorelineby only narrow shallowwater or saturatedmud; one formed a peninsula.On 15 May Shufordsaw a coyoteon the islands.B. Tatman (in litt.) found three islandsthat had had 631 nestingparis aban- donedon 7 Juneand 6 Julyand on 13 Julycounted 894 deadchicks on four islandsthat initiallyheld 2398 nestingpairs. Lake Almanor. This reservoirwas created in 1913; today it covers 11,435 ha (DWR 1988). CaliforniaGulls were firstreported nesting here in 1992, when a boat surveyby H. Green (pers.comm.) revealed about 250 adults,many sittingon nestsor courtingor copulating,on a low-lyingisland alongthe eastshore roughly 1-1.5 km southof the causewayat the north end of the reservoir.Distant views of the islandvia spottingscope from the causewayon 5 May 1993 revealedabout 550 adultCalifornia Gulls, some of whichwere copulating,but views were inadequateto revealnests (Shuford pers.obs.). The lake'slevel was rising at the time, and by late May the island was inundated(J. Evanspers. comm.), terminatingany nestingattempt. Similarly,visits from late April to June, in 1995, 1996, 1997, 1999, and 2000 foundthe islandtiny or submergedand yieldedno evidenceof gull nesting(Shuford, H. and P. Green pers. obs.).This islandapparently is availablefor gull nestingunder only a very narrow range of lake levels(H. Green pers. comm.). Sacramentoand San Joaquin Valleys.California Gulls formerly nested at a shiftingstation in the middlestretches of the SacramentoRiver (Dawson 1923, Grinnell and Miller 1944). Egg data slips (MVZ, SBCM, WFVZ) confirmthe collectionin the SutterBasin, Sutter County, of at leastsix egg

151 • ø

152 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Figure 6. (a) PelicanPoint was unoccupiedby nestinggulls when connectedto the shorelinesouth of SpauldingTract. Eagle Lake, LassenCounty, near the end of a prolongeddrought. (b) Aerial view of Hartson Reservoir,Dakin Unit of Honey Lake WA. abuttingthe north shoreof Honey Lake, LassenCounty, in the background;low waterlevels during a prolongeddrought gave coyotes access to the gulls'nesting islets on the north shoreof Hartson and in adjoiningponds. Photos by W. D. Shuford, (a) 23 May 1994 and (b) 24 May 1994

153 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA setsfrom 21 to 24 May 1910 and 46 setson 25 May 1911. On the former dates,C. S. Thompsonfound the colonyabout 7 milesfrom the Sacramento River.On the latter,H. A. Snowestimated about 200 pairsof gullsnesting nearRiego within 2 milesof the SacramentoRiver. Both collectors indicated the colonywas situatedin overflowlands, where snow-meltwaters that initiallysurrounded the nestingisland had recededand left it high and dry at the time of their visits.Extensive construction of water-storageand flood- controlstructures later in the centuryhas virtually eliminated such seasonal habitatin the SacramentoValley. On 14 June 1925, W. B. Sampsoncollected at leastfour sets of California Gull eggs from a small islandin WoodwardReservoir south of Eugene, StanislausCounty, in the northern San Joaquin Valley (WFVZ egg data slips).Grinnell and Miller (1944) apparentlywere unawareof thesedata. Mono Lake. Mono Lake is secondonly to Great Salt Lake in numbersof breedingCalifornia Gulls (Conover 1983, Paulet al. 1990, Winkler1996). The extensivehistorical record on gull nestingat Mono Lake has already been presentedand debatedby Jehl et al. (1984, 1988) and Winkler and Shuford(1988). Regardless,it is clear that numbersof breedinggulls at Mono Lake have increasedfrom the low thousandsearly in the centuryto tens of thousandsin the 1970s to 1990s. Since 1983, the populationhas rangedfrom about17,500 to 32,500 pairswith no cleartrend (Figure7). Storms at Mono Lake precedingthe 1991, 1995, and 1998 breeding seasonsappeared to delay initiation of nesting and prolong egg laying (PRBO unpubl.data), leadingto underestimationof the breedingpopula- tion. Such weather alsolikely reducednesting attempts; in Wyoming many youngerCalifornia Gulls may skip breedingin years of unusuallyharsh weather(Pugesek and Wood 1992, B. Pugesekpers. comm.). Before 1979 the vast majority of nesting gulls occupiedvolcanic Negit Island; they abandonedit that year when a droppinglake level exposeda landbridge, allowingcoyotes to enter the colony(Winkler and Shuford1988). Subse- quently,most gulls have bred on two clustersof smallerislets--the Negit and Paohaislets (Figure 8). The formerare composedprimarily of volcanicrock, the latterof easilyeroded lake-bottom sediments. From 1983 to 2000, 64ø-/o to 91ø-/oof the lake'snesting population has occupied the Negit Islets(Figure 7). Gull numbersincreased on the Paoha Isletswhen their acreageincreased duringa periodof declininglake level in the earlyto mid-1990sand on Negit Islandduring a periodof recolonizationin the mid-1980s to early 1990s. Sincethe late 1970s the loweredlake levelperiodically has enabledcoyotes to reach some of the islandsand causethe gullsto abandontheir nests (Winklerand Shuford 1988, PRBO unpubl.data). Protectionof the gull colonies was one of the factors in a water board's decision to restore Mono Lake by allowingthe lake to risefrom the 1994 levelof 6374 feet (1943 m) abovesea level to 6391 feet (1948 m), a processexpected to take about20 years(SWRCB 1994) but so far hastenedby wet winters. San FranciscoBay. San FranciscoBay supportsthe only coastalcolonies of the CaliforniaGull (Winkler1996), largelyon abandonedlevees, levee roads,and dredge-spoilislands in remoteregions of the saltponds, active and abandoned,at the bay's southend (Figure9). Creation of the ponds beganabout 1860; by the 1930s about 12,145 ha of tidal marshhad been

154 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

--e-- MONO LAKE TOTAL --O.. NEGIT ISLETS .--•-- PAOHA ISLETS --•. NEGIT ISLAND 35000

30000 n, 25000 r• 20000 z

• 15000 z i.u 10000

• 5000 z

I I I I I I I I I 982 1984 1986 1988 1990 1992 1994 1996 1998 2000

YEAR Figure7. Trendsin numbersof nestingpairs of CaliforniaGulls at Mono Lake,Mono County,California, 1983 to 2000. Totalspresented for the entirelake and for three subsections:the NegitIslets, the PaohaIslets, and NegitIsland. convertedinto them. CaliforniaGulls were firstdiscovered breeding in south San FranciscoBay in 1980 on a leveein Pond A6 at the Knapp property near Alviso,Santa Clara County(Jones 1986). The colonygrew exponen- tially as birdsexpanded to an adjacentparallel levee in 1982 and to an adjacentroad in 1986 (Table5, Figure10). Since 1983, CaliforniaGulls havecolonized eight additional sites in San FranciscoBay, establishingthe species'second largest breeding aggregation in the state. Six are in salt pondsin the southbay, two are in the centralbay, one on an islandin a wetlandand on nearbybreakwaters at the AlamedaNAS, Alamedaand San Franciscocounties, the other on BrooksIsland, Contra CostaCounty. The sourceof the bay'spopulation is unknown,but it mustbe immigration,since localchick production alone cannot account for its rapidgrowth. Nevertheless,California Gulls face some threats in south San Francisco Bay. IntroducedRed Foxes (Vulpes vulpes) were first sighted at PondA6 in 1991, subsequentlynear the coloniesat AlvisoPond, Moffett, and Mowry 2. These colonieswere partiallyor completelyabandoned in the following sequence:Pond A6 (1991-1997), Alviso Pond (1991-1995), Mowry 2 (1995), and Moffett (1997). All of thesecolonies are either permanently attachedto the mainlandor becomelandbridged when watersrecede early

155 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Figure8. Aerialviews of CaliforniaGull nesting islands at Mono Lake,Mono County. (a)The NegitIslets in rightforeground flanked to leftby the larger.darker Negit Island, shownhere connectedto the mainlandby a landbridgeat a lake elevationof about 6372 feet: the steep,snow-covered eastern escarpment of the SierraNevada is in the distance.(b) The Paoha Islets(center) off the westernshoreline of Paoha Island,with Negit Islandand the Negit Isletsin the left background. Photos (a) courtesy Mono Lake Committee. circa 1982, and (b) by W. D. Shuford. 24 May 1994

156 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Newark Mowry I

Mowry2

Mountain View

MoffeE

Alviso Pond

2 0

Base map from •e EcoAfins, Version 1.02, I•1ometers May 1997, San FranciscoEstuan/Inslitute.

__ Figure9. Locationof CaliforniaGull colonies in southSan Francisco Bay salt ponds (shaded);see Table 3 fordata on colony sizes. The Alameda NAS and Brooks Island coloniesin the centralbay are not depicted. in the nestingseason. Additionally, the Don EdwardsSan Francisco Bay NWRpurchased the Knapp property from Cargill Salt Division in 1989,and thepond remained an active salt evaporator until 1993. Since then the pond hasnot been actively managed and no waterhas been pumped into it. SaltonSea. Small (1994) reported a failednesting attempt of a singlepair of California Gullsat the southend of the Salton Sea, Imperial County, in July1976, and two pairs laid eggs but failed to fledge young at the north end of the SaltonSea, RiversideCounty, in 1996 (Field Notes 50:996). In 1997, 22 pairsof CaliforniaGulls bred successfully on a smallrocky and sandyislet off Obsidian Butte at the south end of the Salton Sea: this colony increasedto 37 pairsin 1998 and40 pairsin 1999 (Molina2000). Other sites.Gulls have been reported breeding at threeother sites in the

157 •00000•0 II II •00•

••0000

158 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Figure 10. CaliforniaGulls nestingamid weedsand woody debrison abandoned leveesin a salt pond at the Knapp propertyin southSan FranciscoBay near Alviso. Santa Clara County. Photo courtesy $FBBO archives. mid-1

interior of California, but information on these attempts is poorly docu- mented or in error. Dawson's(1923) report of nestingat Lake Tahoe, El DoradoCounty, is unverified(Grinnell and Miller 1944). Cogswell's(1977) report of CaliforniaGulls breeding at Topaz Lake, and perpetuatedfrom that sourceby Small (1994, A. Small pers. comm.), is erroneous(H. L. Cogswellpers. comm.). Finally,Small (1994) reportedthat Ring-billedGulls havebred at ModocNWR, ModocCounty, on the basisof a refugechecklist only (A. Small in litt.), but refuge biologistsknow of no evidencethat the specieshas ever bred there (R. L. Ryno pers. comm.).

PopulationTrends Though Conover (1983) concludedthat gullshave increasedgreatly in Californiaand the rest of the West duringthe past century,the historical recordis limited.We believethat Conover's(1983) comparisonsto the early 1900s are inadvertentlybiased toward an increasebecause no authorearly in the centurymade a carefulattempt to chroniclegull colonies, as Conover did for the recentperiod, and the numberof personslikely to know of or report gull colonieshas increasedgreatly since early in this century. Even knowledgeof the recent record is poor. For example, Conover (1983) reportedonly six gull coloniesfrom the interiorof Californiain the late 1970s to early 1980s, but we now know that 14 coloniesin that area have been active,at leastintermittently, since that time. Furthermore,the

159 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA methodsand datesof surveysare reportedinfrequently, or the estimatesare rough,made incidentally to otheractivities. Exemplifying the latterare two independentestimates of nestinggulls at HoneyLake in 1981 thatdiffer by a factorof 2.5 (Table4). Similarly,Paul et al. (1990) commented"sporadic recordsand unequaleffort make data [on populationsof the CaliforniaGull at Great Salt Lake]difficult to interpret."Also, the year-to-yearfluctuations in gull numbersand nestingsites caused by changingwater levelsdevalue estimatingregional populations for a giventime periodby summingdata collectedfrom various sites in differentyears with wide-ranging environmental conditions.Gilligan et al. (1994) and Conoveret al. (1979) reported fluctuationsat individualcolonies and shiftingamong colonies in Oregonand Washington,Paul et al. (1990) at GreatSalt Lake,the latterduring a period when the overall populationwas stable. Consequently,we believe that apparent trends shouldbe interpretedwith the utmost cautionand only when the usuallylimited data are compelling.Conover (1983) recognized many of the limitationsof the data availablebut still expressedapparent populationtrends in terms of percentageincrease over time, a practicewe consider indefensible. Despite these caveats,we draw some conclusions.The clearestwell- documentedexample of a populationincrease is in San FranciscoBay, where CaliforniaGulls initiated nesting in 1980 and now total over 8000 pairs.At Mono Lake, CaliforniaGulls have increased greatly in thiscentury, but it is uncertainwhether the populationwas recovering from a declinein the 1800s, as at Pyramid Lake, Nevada (Yochemet al. 1991). At Honey Lake, numbersseem to have increased,but even when accuratecounts have beentaken the populationhas fluctuated greatly with climaticconditions. At Clear Lake nestingpopulations appeared to increasefrom the 1950s to the late 1960s, remained relativelystable until increasingagain in the mid- 1980s, then droppedfrom 1998 to 2000. Some of these increasesmay haveresulted from birdsshifting from nearbyTule Lake or Lower Klamath NWRs, particularlyin the 1980s when the Lower Klamath populationof thousandsof gullsdecreased to nearextirpation. While estimates at BigSage Reservoirand Butte Valleysuggest increases, these may be artifactsof a limitedearlier record followed by moreaccurate subsequent counts or, again, shiftingof birdsfrom nearbycolonies. For othersites, the data are too few or colonyoccupancy too intermittentto warrant interpretationof population trends.On the whole, populationsof CaliforniaGulls, at least,have increasedsubstantially in Californiain recent decades,driven largelyby trendsat Mono Lake and San FranciscoBay. The recordbefore 1950 is too scant for trends then to be estimated with confidence. Paul et al. (1990) tracedthe historyof CaliforniaGulls breeding at Great Salt Lake, where the populationremained relativelystable from 1931 throughthe 1980s, then rose dramaticallyfrom 1989 to 1990 (D. Paul pers. comm.). In Nevada Yochem et al. (1991) concludedthat California Gullsincreased from severalthousand birds in the 1870s (followedby a decreasethrough 1920s) to about 15,000 birds in the 1980s. These conclusionswere basedon an inferencethat a descriptionof the Pyramid Lake colonyin 1868 as "immense"and spreadover "severalacres" equaled a populationof 4000-6000 birdsand that thiswas the entirepopulation in

160 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA the region.Their roughestimates of 16,700 breedinggulls in 1985-1986 were followed by more accurateestimates of 10,700 in 1989-1990 (Yochemet al. 1991), suggestinggulls either shifted to otherstates or earlier estimatesexaggerated the sizeof the population.Though these authors also reportedrough estimates of 3000 pairsof CaliforniaGulls at Lahontan Reservoir1983-1985, they omittedAlcorn's (1988) estimatesof only 500 and>400 neststhere in 1986 and 1987, respectively.We are skepticalthat the Nevadapopulation of CaliforniaGulls has increased greatly historically. We remainuncertain whether the benefitto gullsof creationof islandsin reservoirs,agricultural irrigation, and augmentationof the food supplyon the winteringgrounds has compensated for the lossof >90% of California's historic wetlands (Dahl 1990). Further research should concentrate on accuratepopulation estimates via rangewidesurveys and investigating gulls' responsesto fluctuatingenvironmental conditions. ManagementIssues The mainthreat to breedinggulls is the periodicscarcity of isolatednesting islands.This was particularlyevident in 1994 when low water levelsin northeasternCalifornia appeared responsible for a lackof suitablenesting islandsat GooseLake and EagleLake and for the formationof landbridges at ShastaValley, Butte Valley, Clear Lake, MiddleAlkali Lake, Honey Lake, and Mono Lake, allowingcoyotes access to part or all of the colonies. Althoughperiodic droughts are a naturalphenomenon to which gullsare adapted,water diversionsfor human usesmay exacerbatetheir effects. When possible,refuge managers and reservoiroperators should maintain waterlevels that protectnesting islands for gulls.Despite the remotenessof many colonies,human disturbance is a potentialthreat. Seasonalclosure of nestingislands has proved effective at Mono Lake (Shufordpers. obs.), but suchrestrictions or interpretivesigning will likelybe effectiveonly where adequatepersonnel are availablefor enforcementor interpretation.In some instancessuch efforts might be counterproductivein drawingundue atten- tion to nestingcolonies. Protection of gull colonieswill likely be most effectivethrough comprehensive conservation plans that addressthe needs of all colonialwaterbirds that nesttogether on islands.

ACKNOWLEDGMENTS

Manythanks to the followingpeople for helpingon nestcounts, sharing data from variousgull colonies, or providinglogistical support: Chuck Alexander, Jenny Barnett, John Beckstrand,Doug Bell, Ken Blum, Tim Burton, Dave Calleri, Laura Collins, ThomasDarden, Adrian del Nevo, AI DeMartini,Jan Dierks,Ray Ekstrom,Larry Ford, John Fredrickson,Helen and Paul Green, Tim Hansen, Patrick Hendrix, Kathy Hobson,Chuck and BarbaraHolmes, Joseph R. Jehl, Jr., and colleagues,Richard Kaufmann,Valerie Layne, DanielleLefer, Sue Madas, AidanChristine McNeil, Mike McVey,Dave Mauser,Richard Mewaldt, Bartshe Miller, Kathy Molina, the Mono Lake Committee,Pollo Moreno, ShannonNelson, Kit Novick, JenniferParkin, Anthony Ries, MichaelRigney, Ron Ryno, Jeff Scheele,Bob Smith, EmilieStrauss, George Studinski,Bobby and Dick Tatman, Terry Weist, Tricia Wilson, and Peg Woodin. Additional volunteers that assisted with nest counts at Mono Lake and San Francisco Bay overthe yearsare too numerousto list,but their help stillis greafiyappreciated.

161 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Thanksto CaliforniaDepartment of Fishand Game (CDFG), via LyannComrack, Kevin Hunting(Bird and Mammal Conservation), and Paul Kelly (Office of Oil SpillPrevention and Response),for providingairplane time; to our CDFG pilots--LaurenGoehring, Larry Heitz, and Bob Morgan--for impeccableaerial support; and to Harry Carter, GerryMcChesney, and Daryl Whitworth (National Biological Survey) for arrangingand participatingin the aerialsurveys and for providingfilm and cameraequipment. In northeasternCalifornia, CDFG, U.S. Fish and Wildlife Service(USFWS), and U.S. ForestService supported our work by allowingaccess to their landsor supplying personnelto help on surveys.In San FranciscoBay, CargillSalt Divisionand Don EdwardsSan FranciscoBay NWR providedaccess to colonieson theirlands. The followingpeople graciously provided information that allowedus to identify former and current gull colonies,shared unpublisheddata, or confirmedfacts regardingvarious gull colonies:Daniel A. Airola, Doug Bell, Steve Bobzien,Jay Bogiatto,Beverly Clark, Howard L. Cogswell,Bruce E. Deuel, Ray Ekstrom,Helen Green, Jim Hainline, Chuck Holmes, StephenA. Laymon, Dave Mauser,Pollo Moreno, Kit Novick, Ed O'Neill, Mike Robbins,Ron L. Ryno, Dan Shaw, Arnold Small, Bob Smith, Rich Stallcup,Steve Summers,and David W. Winkler. Karen Cebra, Carla Cicero, Krista Fahy, Kimball Garrett, Ned Johnson,Douglas Long, RobertMcKernan, and Philip Unitt provideddata from gull egg sets, or confirmation of a lackthereof, from their respective institutions. Manuel Marin kindly confirmed the validityof CaliforniaGull egg setsfrom StanislausCounty housedat WFVZ. Janet Hanson,Catherine Hickey, and Jennifer Roth helpedin preparationof Figure9; data layersfor the basemap wereprovided courtesy of the EstuarineInstitute. Mike Lynes and Peter LaTourettekindly scannedphotographs into digitalformat for use in figures.John Beckstrandwas especiallyhelpful in compilingdata on gull numbers from the annualnarrative reports of the KlamathBasin National Wildlife Refuges. Fundingof PRBO'sresearch on CaliforniaGulls at Mono Lake hasbeen provided by an anonymousdonor, the AtlanticRichfield Foundation, the ConservationEndow- ment Fund, GoldenGate AudubonSociety, the Los AngelesDepartment of Water and Power via Hubbs-SeaWorld ResearchInstitute, the Mono BasinNational Forest Scenic Area Visitors' Center in partnershipwith the Eastern Sierra Interpretive Center,the Mono Lake Committee,the Mono Lake Foundation,National Audubon Society,Recreational Equipment, Inc., USFWS, and the membershipof PRBO. Funding for researchon California Gulls in San FranciscoBay came from the membershipof SFBBO. Fundingfor surveysof breedingwaterbirds from 1997 to 1999 was providedby CDFG, Klamath Basin NWR Complex, National Fish and WildlifeFoundation, USFWS MigratoryNongame Program (Region 1), the Weeden Foundation,and the membershipof PRBO. Paul Fonteyn,Kevin Hunting, Gary Page, Tom Smith, and Tara Zimmermanprovided support and encouragementfor the latterproject. Earlier versions of the manuscriptbenefited from thoughtfulreviews by StephenN. G. Howell, JosephR. Jehl, Jr., GerryMcChesney, and KathyMolina. This is Contribution 716 of PRBO and 77 of SFBBO.

LITERATURE CITED

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162 NESTING CALIFORNIA AND RING-BILLED GULLS IN CALIFORNIA

Conover,M. R. 1983. Recentchanges in the Ring-billedand Californiagull populations in the western United States. Wilson Bull. 95:362-383. Conover,M. R., Thompson,B.C., Fitzner,R. E., and Miller,D. E. 1979. Increasing populationsof Ring-billedand Californiagulls in Washingtonstate. W. Birds 10:31-36. Dahl, T. E. 1990. Weftand lossesin the Unted States 1780's to 1980's. U.S. Dept. Interior,Fish & WildlifeService, Washington, D.C. Dawson,W. L. 1923. The Birdsof California.South Moulton Co., San Diego. Departmentof Water Resources.1988. Dams within jurisdictionof the state of California. DWR Bull. 17-88. Dept. Water Resources,P.O. Box 942836, Sacramento,CA 94236. Finley,W. L. 1907. Among the gullson KlamathLake. Condor 9:12-16. Gilligan,J., Smith, M., Rogers,D., and Contreras,A., eds. 1994. Birdsof Oregon: Statusand Distribution.Cinclus Publ., McMinnville,OR. Gould,G. I., Jr. 1974. Breedingsuccess of piscivorousbirds at EagleLake, California. M. S. thesis,Humboldt State Univ., Arcata, CA. Grinnell,J., Dixon, J., and Linsdale,J. M. 1930. Vertebratenatural history of a sectionof northernCalifornia through the LassenPeak region. Univ. Calif.Publ. Zool. 35:226-227. Grinnell,J., and Miller, A. H. 1944. The distributionof the birdsof California.Pac. Coast Avifauna 27. Hays,H., andLeCroy, M. 1971. Fieldcriteria for determiningincubation state in eggs of the Common Tern. Wilson Bull. 83:425-429. Jehl, J. R., Jr., Babb,D. E., and Power,D. M. 1984. Historyof the CaliforniaGull colonyat Mono Lake, California.Colonial Waterbirds 7:94-104. Jehl, J. R., Jr., Babb, D. E., and Power, D. M. 1988. Commentary. On the interpretationof historicaldata, with referenceto the CaliforniaGull colonyat Mono Lake, California.Colonial Waterbirds 11:322-327. Johnston,D. W., and Foster,M. E. 1954. Interspecificrelations of breedinggulls at Honey Lake, California.Condor 56:38-42. Jones,P. A. 1986. Aspectsof the reproductivebiology of the CaliforniaGull in Alviso, California.M. A. thesis,San FranciscoState Univ., San Francisco. Lederer,R. J. 1976. The breedingpopulations of piscivorousbirds of EagleLake. Am. Birds 30:771-772. Lincoln,F. C. 1933. White Pelicanslead their youngto water.Condor 35:238-239. Moffitt,J. 1942. A nestingcolony of Ring-billedGulls in California.Condor 44:105- 107. Molina,K. C. 2000. The recentnesting of Californiaand Laughing gulls at the Salton Sea, California.W. Birds31:106-111. Paul, D. S., Jehl, J. R., Jr., and Yochem,P. K. 1990. CaliforniaGull populations nestingat GreatSalt Lake, Utah. Great BasinNat. 50:299-302. Pugesek,B. H., and Wood, P. 1992. Alternate reproductivestrategies in the California Gull. Evol. Ecol. 6:279-295. Ray,M. S. 1915. Nestingof the AmericanOsprey at EagleLake, California. Condor 17:70-74. Ray, M. S. 1921. On the occurrenceof the Buffie-headat Eagle Lake. Condor 23:192-193.

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Remsen,J. V., Jr. 1978. Birdspecies of specialconcern in California:An annotated listof decliningor vulnerablebird species. Nongame Wildl. Invest., Wildl. Mgmt. BranchAdmin. Rept. 78-1. Calif. Dept. Fish Game, 1416 Ninth St., Sacra- mento, CA 95814. Ryder,J.P. 1993. The Ring-billedGull, in The Birdsof North America(A. Poole,P. Stettenheim,and F. Gill, eds.),no. 33. Acad. Nat. Sci., Philadelphia. Shaw, D. W. H. 1998. Changesin populationsize and colonylocation of breeding waterbirdsat Eagle Lake, Californiabetween 1970 and 1997. M. S. thesis, Chico State Univ., Chico, CA. Shuford,W. D. 1998. Surveysof BlackTerns and otherinland-breeding seabirds in northeasternCalifornia in 1997. Report98-03, Birdand Mammal Conservation Program,Calif. Dept. FishGame, 1416 Ninth St., Sacramento,CA 95814. Shuford, W. D., Humphrey, J. M., and Nur, N. 1999. Surveysof terns and cormorantsin California'sCentral Valley in 1998. PointReyes Bird Obs. (Contr. 772), 4990 ShorelineHwy., StinsonBeach, CA 94970. Small,A. 1994. CaliforniaBirds: Their Statusand Distribution.Ibis Publ., Vista, CA. State (of California)Water ResourcesControl Board. 1994. Mono Lake Basinwater right decision1631. StateWater ResourcesControl Board, Division of Water Rights,P.O. Box 2000, Sacramento,CA 95810. U.S. Bureauof Reclamation.1998. 1998 KlamathOperations Plan Environmental Assessment.U.S. Bur.Recl., KlamathBasin Area Office, 6600 WashburnWay, Klamath Falls,OR 97603. Willett,G. 1919. Birdnotes from southeasternOregon and northeasternCalifornia. Condor 21:194-207. Winkler, D. W. 1982. Importanceof Great Basin lakes in northern Californiato nongameaquatic birds, 1977. Wildl. Mgmt. BranchAdmin. Rept. 82-4. Calif. Dept. Fish& Game, 1416 Ninth St., Sacramento,CA 95814. Winkler, D. W. 1996. CaliforniaGull (Larus californicus),in The Birds of North America (A. Poole and E Gill, eds.),no. 259. Acad. Nat. Sci., Philadelphia. Winkler,D. W., and Shuford,W. D. 1988. Changesin the numbersand locationsof CaliforniaGulls nesting at Mono Lake, California,in the period 1863-1986. Colonial Waterbirds 11: 263-274. Yochem,P. K., Jehl, J. R., Jr., Stewart,B. S., Thompson,S., and Neel, L. 1991. Distributionand historyof CaliforniaGull colonies in Nevada.W. Birds22:1-12.

Accepted 5 September 2000

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