Least Tern Foraging Ecology at Three Major California Breeding Colonies

WESTERN BIRDS

Volume 14, Number 2, 1983

LEAST TERN FORAGING ECOLOGY AT THREE MAJOR CALIFORNIA BREEDING COLONIES

JONATHAN L. ATWOOD, Department of Biology, Universityof California, Los Angeles, California 90024 DENNIS E. MINSKY, Department of Biology, California State University, Long Beach, California 90840

The California Least Tern (Sterna antillarumbrowni) historicallynested commonly on coastalbeaches from Monterey County, California,to Cabo San Lucas, Baja California (Grinnell 1928, Grinnell and Miller 1944). However, substantialpopulation declines were documented in the subspecies'United Statesrange duringthe yearsfollowing World War II, and the populationwas given Federal and State endangeredspecies status in 1969 and 1971. Because most of the Least Tern's decline in California seems to have resultedfrom disturbanceor destructionof nestingareas, recent protective ef- forts have focusedon the breedingcolonies themselves, and researchhas generallyemphasized breeding biology, nesting requirements and population trends(Massey 1974; Masseyand Atwood 1978, 1981; Atwoodet al. 1979). Yet in spiteof earlysuggestions that lossof estuarineforaging habitat may also have contributed to the California Least Tern's decline, little attention has been given to the population'sforaging ecology aside from an indirect analysis,based on fishdropped at nestingsites, of food habits(Atwood MS), a 1-yearstudy of daily and seasonalfluctuations in feedingactivity at a single colony (Collinset al. 1979), and some brief speculationsthat tidal estuaries representthe principalforaging habitat (Wilbur 1974, Massey1977). Recent and dramaticincreases at severalCalifornia Least Tern breeding areas, where formerly extensiveestuarine habitat has been almost entirely destroyed,raised questions of whereterns from thesecolonies were obtaining food, and how importantthe remainingestuarine areas near thesesites were to the colonies'continued growth and success.During 1980-1981 we studied foragingactivities at two of theselarge, growing colonies that are essentially lackingnearby, viableestuarine foraging habitat; in 1982, observationswere made at a third site locatedadjacent to a relativelyundisturbed river mouth. Western Birds 14: 57-72, 1983 57 LEAST TERN FORAGING ECOLOGY

STUDY AREAS AND METHODS Observationsof LeastTern foragingactivity were made during1980 and ].98].at theVenice Beach and Huntington Beach breeding colonies located at DockweilerState Beach, Los AngelesCounty, and HuntingtonState Beach, OrangeCounty, respectively. In ].989_,data were collected in the vicinityof the Santa MargaritaRiver nestingarea, locatedon the U.S. MarineCorps Base, Camp Pendleton,San Diego County. All sitesare historicnesting areaswhich, with protectionduring the springand summermonths, have supportedsubstantial numbers of nestingLeast Terns in recentyears (Table 1.). Not only have these coloniesconsistently been among the largestin Californiasince 1978, VeniceBeach and HuntingtonBeach have alsobeen the mostsignificant in termsof numberof youngproduced; from ].978-1981., approximately4].% of LeastTerns successfully fledged in Californiahave comefrom thesetwo sites(California Department of Fishand Game, unpubl. data). All three studycolonies are situatedrelatively distant (greater than 5 miles)from othermajor Least Tern nestingareas and, during].980-1.989., the smallcolonies at Playa del Rey (0.8 milesESE of Venice Beach), Upper Newport Bay (5.0 milesNE of HuntingtonBeach), and White Beach (3.5 milesN of Santa MargaritaRiver) eitherfailed early in the nestingseason or were not usedat all. Therefore,we have made the assumptionthat mostor all foragingLeast Terns observed in the studyareas were individualsassociated with the breedingcolonies under investigation. All potentialLeast Tern feedingareas were identifiedwithin a 5 mileradius of eachnesting site, and observationstations were selectedthat allowedquick surveysof foragingterns (Figures 1., 9., 3). Habitatwas broadly characterized for eachstation, and included(a) open ocean, (b) floodcontrol channels and channelizedrivers, (c) degradedsaltmarsh channels with little or no tidalflow, (d) freshwaterand sewagetreatment ponds, (e) shelteredmarinas, (f) shallow,brackish lagoons, (g) relativelyundisturbed river channels,and (h) natural, unchannelizedriver mouths.Not all habitatswere representedwithin 5 milesof eachstudy colony. In comparisonwith potentialforaging habitats that historicallyoccurred at Venice Beachand HuntingtonBeach, estuarine and freshwaterareas have been almost entirely eliminated by landfills,chan- nelizationand marinadredging; at the SantaMargarita River, extant estuarine and freshwaterhabitats are reducedand somewhatdegraded compared with historicconditions (Salata 1.981.).

Table 1. Numbers of Least Terns nesting at three southern California colonies, 1978-1982.

Approximatenumber of breedingpairs a 1978 1979 1980 1981 1982 Venice Beach 60-75 80-95 150-165 140-160 150-185 HuntingtonBeach 75-90 80-95 70-90 109-122 85-111 Santa MargaritaRiver 30-40 32-40 47-65 50-105 110-140

aCaliforniaDepartment Fish and Game, unpubl.data.

58 LEAST TERN FORAGING ECOLOGY

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59 LEAST TERN FORAGING ECOLOGY

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Eachstation was visited within a 90-minuteperiod beginning approximately 30 minutesafter sunrise.Although simultaneous observations from each station were beyondthe scopeof the presentstudy, we feel that variationsin in- tensityof foragingactivity during the 90-minute time spanof eachsurvey were probablyso minoras to not affectoverall results (Collins et al. 1979). Further- more, we doubt that occasionalduplicated observations (i.e., the same bird recordedat more than one station)would alter generalpatterns in recorded foragingactivity. During 1980, surveyswere conductedat Venice Beach and Huntington Beach on four dates, spacedso as to representeach major phase of the tern nestingcycle (courtship,incubation, feeding of chicksand post-fledging dispersal).In 1981, sixsurveys were performedat thesetwo sitesat approx- imately2-week intervals. Eleven surveys, including three pairs of consecutive- day observations,were made at approximately2-week intervalsat the Santa MargaritaRiver in 1982; additionalinformation regarding the SantaMargarita River studyis provided in Minsky (1982). Observationswere made with 10 x binocularsfrom each station, and the numberof LeastTerns visibleduring each of five 1-minutescans of the survey area, spacedat 1-minuteintervals, was recorded.The behaviorof each in- dividualwas classified as follows: (a) foraging,including birds actively plung- ing into the water or clearlysearching for prey, (b) transit,including birds engagedin high,direct flight with no evidentsearching behavior, (c) courting, includingbirds in fishflights or aerialglides (Wolk 1974), and (d) bathingor loafing.Only birdsconsidered to be foragingat the time of observationare included in the presentanalysis. Simultaneous with each set of surveysmade from land in 1980, boat transectswere also conductedat approximately 0.5-mile intervalsmoving parallel to the coastlineoffshore to approximately5 miles. Datasupplementing the presentstudy have been collected by Atwood since 1977 at mostsouthern and centralCalifornia Least Tern nestingareas; Minsky hasstudied the specieson Cape Cod, Massachusetts,and in southernCalifor- nia since 1974. Relevant informationbased on this experience,especially concerningforaging behavior during post-fledging dispersal, is included in this report. RESULTS Data collectedduring foraging surveys at Venice Beach,Huntington Beach and the Santa MargaritaRiver are summarizedin Tables2, 3 and 4. At least 75% of allforaging activity occurred in the oceanon 9 of 10 surveysat Venice Beach, on 8 of 10 at HuntingtonBeach, and on 8 of 11 at the SantaMargarita River. Approximately90-95% of suchocean feeding was within 1 mile of shorein waterless than 60 feetin depth.Least Terns were rarely seen foraging in the ocean at distancesfrom shoreof 1-2 miles,and never were encountered farther than 2 milesoffshore. Non-ocean habitatsin the vicinityof the study coloniesreceived limited use by foragingterns; in particular,marina areas, which were well representednear all colonies,were little used. At Venice Beachand HuntingtonBeach, with only one exception,at least60% of all foragingtook place within approximately2 milesof the nestingsites; at the Santa MargaritaRiver, this pattern was observedon only 6 of 11 surveys

62 LEAST TERN FORAGING ECOLOGY

(Table5). Relativelylarge numbers of feedingterns were occasionallyseen at stationslocated more than 2 milesfrom nestingsites at eachof the threestudy areas. LeastTerns appear opportunistic in theirforaging behavior, and havebeen observedon numerousoccasions to shiftto differentfeeding areas in response to localizedconcentrations of suitableprey. Duringexperiments conducted at smallartificial feeding ponds near HuntingtonBeach in 1979, numbersof foragingLea•t Ternsincreased from 2 to 24 individualswithin 10 minutesof the releaseof severalthousand Mosquitofish (Gambusia affinis). Similar vagili- ty in feedingactivity is reflectedin the presentstudy by fluctuationsat par- ticularstations on differentdates (e.g., Table 2, stations7 and 9 on 4 May 1981 and 19 May 1981). Duringthe study,Least Terns at VeniceBeach and HuntingtonBeach fed primarilyon NorthernAnchovy (Engraulismordax), Topsmelt(Atherinops affinis) and Jacksmelt(Atherinopsis californiensis) (At- wood MS); a smallfood sampleobtained at the SantaMargarita River colony in 1982 was comparable.While we have no actual data on day-to-day changesin prey populationlevels at foragingsurvey stations, all of the prin- cipalprey speciesform large,mobile schools and hencemight be expectedto vary in abundanceat a singlelocation on differentdates. Althoughthe movementsof prey speciesmakes specific Least Tern foraging localitiesdifficult to delimit, certain areas did receiveconsistently high levelsof useby feedingterns at Venice Beach (stations5, 6, 7), Huntington Beach(stations 5, 6, 7) and the SantaMargarita River (stations4, 5, 6) (Table 6). In general,foraging activity was especially high in nearshoreocean waters near major river mouths. No clear shiftsin habitatutilization or preferencewere noted near Venice Beach and Huntington Beach during the nesting cycle (Tables 2, 3). However,abundant observations of familygroups, frequently including color- bandedjuveniles identifiable as to natal colony, have indicatedthat many ternsfrom Venice Beach and HuntingtonBeach nesting areas disperse follow- ing the breedingseason to freshwaterand estuarinehabitats located beyond the areas includedin the presentstudy. Use of freshwaterWindmill Lake, locatedapproximately 5 milesinland from the SantaMargarita River colony, increasedduring the period of post-fledgingdispersal (Table 4, station24). Similarly,terns nesting at PurisimaPoint, SantaBarbara County, fed almost entirelyin nearshoreocean areas during periods of courtship,incubation, and feedingof chicksduring 1979; afterjuveniles were capable of sustainedflight, familygroups dispersed from the vicinityof the nestingarea to the SantaYnez River mouth, located5.8 milesS of the colony.Comparable post-breeding aggregationsof LeastTerns at localitiesthat had receivedlittle or no foraging use earlierin the seasonhave been observedannually at numerousfresh- water, estuarineand protectedshallow marine areasin coastalcentral and southernCalifornia, apparently representing a generalpattern in the population's behavior. Thereis no evidencethat failure of previouslyused offshore food resources "forced"dispersal to these newly utilizedforaging localities, as late-nesting pairsat all coloniescontinued to feed successfullyin nearshoreocean areas. Rather, the behaviorof Least Terns at freshwateror estuarineforaging sites duringthe post-fledgingperiod strongly suggests that this shift in habitatutiliza-

63 LEAST TERN FORAGING ECOLOGY tion may be associatedwith young birdsdeveloping their fishingskills. This behaviorhas been best documented at freshwaterHarbor Lake, Los Angeles County, whichis knownto have beenused by foragingLeast Terns since at least 1973 (Bender 1974, Masseyand Atwood 1980). Importantly,Least Ternsseldom appear at HarborLake untilafter the dispersalof fledglingsfrom localbreeding colonies in earlyJuly. Althoughno efforthas been madeto quantifyfeeding rates, older juvenilesat Harbor Lake clearlyforaged more frequentlyand with greatersuccess than did youngerbirds, which spent most of their time on the groundwaiting to be fed by their parent(s).

DISCUSSION The LeastTern is oppor•unisticin itsforaging habits, and effortsto precisely define "essential"feeding habitatsor localitiesmay be met with frustration. Priorto post-fiedgingdispersal from breedingcolonies, most foraging activity occurswithin 2 milesof the nestingsites; within this range, terns will probably feed in almostany body of waterthat supportssuitable prey items. Within a habitattype, certainareas may receiveconsistently higher levels of use,suggesting that somelocalities may be of greaterimportance than others. At Venice Beach, HuntingtonBeach and the Santa MargaritaRiver, most foragingoccurs in relativelyshallow, nearshore ocean watersin the vicinityof majorriver mouths, possibly as a resultof waterdepth, salinityor nutrientsup- plies which might favor concentrationsof suitableprey species.However, thesestations are alsoamong the closestfeeding areas to the breedingsites themselves,and the heavyforaging activity may be relatedmore to proximity to the colonythan to high prey concentrations.Regardless of the cause,we note that even in a superficiallyuniform and widespreadhabitat such as nearshoreocean waters, certain sites may be of primaryimportance in the feeding activitiesof a LeastTern breedingcolony. Prior to the subspecies'decline, at least 82% of known California Least Tern nestingsites (n = 33) were locatedwithin 1 mile of river mouth and/or estuarinehabitats (R. Erickson,unpubl. data). This fact, alongwith the in- creasedprobability of seeingfeeding terns flying over restricted marsh areas as opposedto open ocean, probablyled to the assumptionthat estuariesare the species' required foraging habitat (Massey 1971, Wilbur 1974). Some presentlyexisting colonies, such as thoselocated at BatiquitosLagoon, San DiegoCounty, and AnaheimBay and BolsaChica, OrangeCounty, do feed primarily in estuarinehabitats (Atwood MS). However, other colonies,in- cludingseveral located adjacent to relativelyundisturbed river mouths,appear to forage mostly in nearshoreocean waters. The current absenceof significantfreshwater and estuarineecosystems near bothVenice Beachand HuntingtonBeach coloniesmakes it impossibleto establishhistoric habitat preferences. Similarly, determinationof habitat preferencesis difficult at the Santa Margarita River, where estuarineand freshwaterhabitats are present but substantiallyaltered and reduced.We intendto studyforaging behavior at a nestingcolony situated in undisturbedhabitat in Baja California,where both estuarineand ocean fishingare options,to determineLeast Tern foraging preferencesunder natural conditions.

64 LEAST TERN FORAGING ECOLOGY LEAST TERN FORAGING ECOLOGY

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Table5. Effectof distancefrom nestingarea on LeastTern foragingactivity.

Date Distancefrom Nesting Site (miles)a 0-1 1-2 2-3 3-4 4 - 5b Huntington Beach 05 May 1980 25 60 15 0 -- 06 Jun 1980 60 39 1 0 -- 20 Jun 1980 42 28 30 0 -- 21Jul1980 53 33 14 0 -- 05 May 1981 48 25 21 2 3 18 May 1981 52 16 29 3 0 02 Jun 1981 60 9 27 4 0 17 Jun 1981 45 46 7 0 2 01Jul1981 34 48 15 2 0 13 Jul 1981 64 28 7 0 3 Mean(X--) 48.3 33.2 16.6 1.1 1.3

Venice Beach 06 May 1980 41 25 21 13 -- 04 Jun 1980 71 17 12 0 -- 23 Jun 1980 44 21 31 4 -- 22 Jul1980 59 18 9 16 -- 04 May 1981 11 7 72 0 9 19 May 1981 69 6 24 0 1 01 Jun 1981 54 7 20 2 16 23 Jun 1981 80 20 0 0 0 30 Jun 1981 49 41 5 0 5 14 Jul1981 44 39 9 2 6 Mean (X) 52.2 20.1 20.3 3.7 6.2

Santa Marga•taRlver 03 May 1982 17 20 51 11 3 04 May 1982 12 42 23 16 6 18 May 1982 27 19 39 5 13 01 Jun 1982 36 48 15 0 0 02 Jun 1982 27 64 7 0 0 18 Jun 1982 39 46 15 0 0 29 Jun 1982 55 39 4 0 0 13 Ju11982 56 22 14 5 1 27 Ju11982 26 22 19 7 25 28 Ju11982 42 0 7 15 34 11 Aug 1982 70 12 0 0 19 Mean (X) 37.0 30.4 17.6 5.4 9.2

aValuesindicate percent of foraging activity occurring at stations located given distances from nestingsites. bStationsat distancesof 4-5 milesfrom nesting sites were not established at Venice Beachand HuntingtonBeach until 1981.

68 LEAST TERN FORAGING ECOLOGY

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Increaseduse of freshwaterand estuarinemarsh areas during post-fiedging dispersal,when juveniles are developingtheir foraging skills, suggests to us that suchcalm, protectedwaters may be of major significanceduring this period.Even estuarine and freshwaterlocalities that are distantfrom active nestingsites and that receivelittle or no foraginguse during earlier stages of the breedingcycle may be usedheavily by LeastTerns during post-fledging dispersal;loss or disturbanceof suchareas may reducethe survivorshipof dependent young.

SUMMARY LeastTerns at three largesouthern California breeding colonies foraged primarilyin nearshoreocean waters in thevicinity of majorriver mouths. Most foragingtook placewithin 2 milesof the nestingsites. Substantial alteration and reductionof non-oceanLeast Tern foraginghabitats in the vicinitiesof the studycolonies made determination of historichabitat preferences impossible. However, increaseduse of coastalfreshwater and estuarinehabitats during post-fiedgingdispersal indicates the importanceof suchareas during this period.

ACKNOWLEDGMENTS Linda Belluomini,Charles T. Collins,Jack Fanchef, James HartIcy, Paul R. Kelly, BarbaraW. Massey,Gary Wheelerand AbbyWhite provided valuablefield assistance; we thankthese colleagues and friendsfor theirhelp. All bandingof CaliforniaLeast Terns was conductedunder Endangered SpeciesBanding Permit PRT-8-207-B-C issued to CharlesT. Collins.Access to the SantaMargarita River colony was provided by the NaturalResources Office, U.S. Marine CorpsBase, Camp Pendleton.Financial assistance was providedby EcologicalServices, U.S. Fishand WildlifeServices, Laguna Niguel,California. The finalmanuscript was greatly improved by the com- mentsof BarbaraW. Massey,-Joseph R. Jehl, Jr., JudithL. Hand and Elizabeth N. Flint.

LITERATURE CITED Atwood,J.L., R.A. Erickson,P.R. Kelly& P. Unitt. 1979. CaliforniaLeast Tern cen- susand nesting survey, 1978. NongameWildl. Invest. Prog. Rep., Job V-2.13, CaliforniaDep. Fish and Game, Sacramento. Bender,K. 1974. CaliforniaLeast Tern census and nesting survey, 1973. Spec.Wildi. Invest.Prog. Rep., Job I1-11,California Dep. Fishand Game, Sacramento. Collins,C.T., K.E. Bender& D.D. Rypka.1979. Reporton the feedingand nesting habits of the California Least Tern in the Santa Aria River marsh area, Orange County,California. Final rep., contract no. DACW09-78-008, U.S. Army Corps of Engineers,Los Angeles. Grinnell,J. 1928. A distributionalsummation of the ornithologyof lowerCalifornia. Univ. California Pubi. Zooi. 32:1-300. Grinnell,J. & A.H. Miller.1944. The distributionof the birdsof California.Pac. Coast Avif. 27. Massey,B.W. 1971.A breedingstudy of the California Least Tern. Wiidl. Manage. Br. Admin. Rep. 71-9, CaliforniaDep. Fishand Game, Sacramento.

7O LEAST TERN FORAGING ECOLOGY

Massey,B.W. 1974. Breedingbiology of the CaliforniaLeast Tern. Proc. Linn. Soc. New York 72:1-24. Massey.B.W. 1977. Occurrenceand nestingof the LeastTern and other endangered speciesin Baja California, Mexico. West. Birds8:67-70. Massey,B.W. & J.L. Atwood. 1978. Plumagesof the LeastTern. Bird-Banding49: 360-371. Massey,B.W. & J.L. Atwood. 1980. Applicationof ecologicalinformation to habitat managementfor the CaliforniaLeast Tern, prog. rep. no. 2, U.S. Fishand Wildl. Serv., Laguna Niguel, CA. Massey, B.W. & J.L. Atwood. 1981. Second-wavenesting of the CaliforniaLeast Tern: age compositionand reproductivesuccess. Auk 98:596-605. Minsky, D.E. 1982. CaliforniaLeast Tern foragingstudy of the Santa MargaritaRiver and environs. U.S. Fish and Wildl. Serv., Laguna Niguel, CA. Salata, L.R. 1981. Santa MargaritaRiver estuaryresource values and management recommendations.U.S. Fish and Wildl. Serv., Laguna Niguel, CA. Wilbur, S.R. 1974. The literatureof the CaliforniaLeast Tern. U.S. Bur. SportsFish- eriesand Wildl. Spec. Sci. Rep.-Wildl. No. 175. Wolk, R.G. 1974. Reproductivebehavior of the Least Tern. Proc. Linn. Soc. New York 72:44-62. Accepted 17 September 1982

LeastTerns courtship feeding, May 1981, Venice,California Photoby DanaEchoIs 71