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Nesting Populations of California and Ring-Billed Gulls in California

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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 and San 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
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