NOTES

SNOWY PLOVER DIETS IN 1995 AT A COASTAL SOUTHERN CALIFORNIA BREEDING SITE

MARK A. TUCKER and ABBY N. POWELL, United States GeologicalSurvey, BiologicalResources Division, California Science Center, Department of Biology, San Diego StateUniversity, San Diego, California92182-4614 (currentaddress for Powell:USGS• Biol. Res. Div., Northern PrairieWildlife Research Center. Depart- ment of BiologicalSciences, University of Arkansas,Fayettevilleq Arkansas 72701)

The SnowyPlover (Charadrius alexandrinus) feeds by peckingat the groundfor invertebratesand catching insects in the air whileon the run. Alongthe PacificCoast, Snowy Plovers feed on both marine and terrestrialinvertebrates. but little more specificinformation is available(Reeder 1951, Page et al. 1995). Variousmethods are availablefor studyingavian diets, including stomach content, fecal analysisqand direct observation(Rosenberg and Cooper 1990). Stomach- contentanalysis requires sacrifice of a large numberof birdsor the use of stomach pumps(Ramer et al. 1991, Martin and Hockey 1993). Poulinand Lefebvre(1995) reportedtartar emetic to be safeand more effectivethan fecalanalysis for investigat- ingavian diets, yet 70 (2.9%) of the birdsthey sampled died. They foundsome families were more sensitiveto the chemicalthan othersand did not testit on plovers.Use of thesetechniques on a threatenedbird like the SnowyPlover would not be acceptable or appropriate(Ralph et al. 1985). Directobservation of the preyof smallshorebirds is difficultbecause their prey are so small(Baker 1977• Rundle1982). Fecalanalysis has been used successfully in diet studiesof othershorebirds (Swarth 1983, Nicholls1989, Le V. Dit Durreland Kelley1990, Shafferand LaPorte1994). Althoughthere is some bias associatedwith fecal analysisdue to the differential digestionand passage rates of preyitems, there is generally good agreement between fecaland stomach-contentanalyses (Rosenberg and Cooper 1990). Fecalsamples are relativelyeasy to collect,and prey fragmentsare easilyextracted because birds lack digestiveenzymes capable of breakingdown chitinous exoskeletons (Swarth 1983). Soft-bodiedorganisms such as polychaeteworms, however, break down readilyand are not well representedin fecalsamples (Rundle 1982, Shafferand LaPorte 1994). The objectivesof thisstudy were to describethe diet of the SnowyPlover during the 1995 breedingseason at a singlecoastal southern California breeding site and to identifyavailable invertebrate prey in the samearea. We choseto usefecal samples as the least invasivemethod to investigatediets to minimize the impacts on this threatenedspecies. The SantaMargarita River mouth, in Camp PendletonMarine Corps Base (33 ø 13' 57" N, 117 ø24' 37"W), SanDiego County, provides a varietyof habitatsfor foraging and breedingSnowy Plovers.As part of ongoingresearch on Snowy Ploversin southernCalifornia, Powell et al. (1995) estimatedthat the populationbreeding at this sitein 1995 was92 malesand 69 females.Sandy beach and saltflats were the major habitatsused for nesting. We collectedfecal samples opportunistically between April andJuly of 1995 during routinenest monitoringand banding.We observedadult and young ploversfirst through binocularsor spottingtelescopes, then searchedthe ground where we observeda bird defecating.We found that binocularsworked better than spotting scopes,because of betterdepth of fielddefinition, especially when we workedalone. We easilydistinguished fresh feces from old, dried droppings.We did not collect samplesif therewas any doubt whether they had beendeposited by a SnowyPlover. Activelyforaging plovers defecate about every eight minutes (Swarth 1983). Snowy

44 WesternBirds 30:44-48, 1999 NOTES

Ploverscan foragefar from their nests.so we couldnot assumethat fecal samples collectedin a particularhabitat containedfragments of prey that were actually consumedthere. Collectedfeces were labeledwith the date, location,and identityof the individualplover, then preservedin 70% ethanol. We identifiedfecal contents through an OlympusSZ10 researchstereo dissecting microscope.and used our reference collections to helpidentify exoskeleton fragments (Table1). We attemptedto identifyfragments to familyor orderby directcomparison of partsto our referencecollection, combined with keysto invertebrateidentification (Rickettset al. 1985. Borror et al. 1989). In addition to fecal samples, we sampled invertebratesto build a reference collectionand obtain a qualitativeinventory of prey items. The habitats at the samplingsites, all locatedsouth of the Santa Margarita River mouth, were unpro- tectedsandy beach, protected sandy beach, salt flat, and mudflat.The unprotected sandy beach site• which was exposed to wave action, included upper (above the tideline)and lower (intertidal)beach. The mudflatconsisted of a 5-cm sun-bakedclay layer coveredwith benthicalgae, and underneaththe claylayer there was coarsewet sand.Salt flatswere sampledduring both •'normal"dry conditionsand after very high spring tides had inundatedlarge sectionsof this area. The Santa Margarita River changedcourse in 1993• leavinga smalltidal lagoon in its old path, andthe protected sandybeach site, which was not exposedto wave action,was locatedin this area. We did not sample much in sand dunes to avoid disturbingthe Least Terns (Sterna antillarum) nestingthere. We usedsticky traps and sweepnets to collectflies and other flyinginsects and stickytraps to capturecrawling insects. Subsurface invertebrates were collectedwith corestaken to 5 cm (the distancelikely to be accessibleto a probingplover) and then siftedthrough a 1-mm sieve.We occasionallycaptured by hand. Pit traps were not usedsince they couldtrap ploverchicks. Invertebrates were labeledwith locationand date and preservedin 70% ethanol. The prey items identifiedin 32 fecal samplesfrom adult ploversincluded at least nine familiesin six ordersof insects(Figure 1). Beetles(Coleoptera) occurred in 23 (72%) of the samples,and rove beetles(Staphylinidae) were the mostfrequent prey. Half of the fecal samplescollected at the unprotectedsandy beach site contained remnantsof rove beetles only. Flies (Diptera) occurredin 14 (44%) of the fecal samples,and a singlefecal samplefrom the unprotectedsandy beach site contained over 35 fly heads. Long-leggedflies (Dolichopodidae)and shoreflies(Ephydridae) were the primarydipteran families found in the feces(Figure 1). Insectlarvae were

Table 1 FragmentsFound in Snowy Plover FecalSamples Used for Inver- tebrate Identification

Order Family Partsused for identification

Coleoptera Staphylinidae elytra.wings, carapace, head, mandible, antennae, legs Carabidae carapace,head, mandible Cicindelidae carapace,elytra Diptera Ephydridae wings,head. abdominal segments Dolichopodidae wings,head. legs, abdominal segments Anthomyiidae wings,head Hymenoptera Braconidae wings Hemiptera Saldidae wings.body, head Decapoda Hippidae carapace NeogastropodaNassaridae shell

45 NOTES

Figure 1. Frequencyof variousfamilies of invertebratesin fecal samplesof adult Snowy Plovers.Numbers to the right of barsindicate number of samples. found in 16 (25%) of the fecal samples,especially in salt flat and protectedsandy beach habitats.Ninety-seven percent of all fecescollected from adultscontained at least one of these three insect families:the Staphylinidae,Dolichopodidae, or Ephydridae. The fecesof threefledglings and one two-week-old chick were collectedfrom sandy beach sites.All four samplesincluded rove beetles.Long-legged flies were in two samples,Braconidae in one sample,and insectlarvae in one sample.The fecesof the two-week-oldchick contained only rove beetles. We identified22 familiesof invertebratesin samplesfrom all habitats.The insect familieswere the Ephydridae,Dolichopodidae, Anthomyidae, Coelopidae, Asilidae (Diptera);Staphylinidae, Carabidae, Cicindelidae, Curculionidae, Tenebrionidae (Co- leoptera);Braconidae, Dryinidae, Formicidae(Hymenoptera), and Saldidae(Hemi- ptera). Non-insectfamilies were the Hippidae (Decapoda),Conidae (Gastropoda), Halacaridae(Acarina), and five of polychaetes(Annelida): the Spionidae,Glyceridae, Arabellidae,Capitellidae, and Opheliidae. Dolichopodidae and Ephydridae were present in all habitatssampled, and Staphylinidaeoccurred in all but one of the habitats. Unidentifiedinsect larvae were collectedat bothsandy beach sites. The greatestvariety

46 NOTES of potentialprey itemsavailable to foragingplovers occurred at the unprotectedsandy beachsite, mainly because of the presenceof fivefamilies of polychaeteworms. Snowy Ploversappeared to concentratetheir foragingeffort on a few terrestrial insectfamilies (mainly flies and beetles)at the Santa Margarita River mouth. The families most abundant in our sampleswere widely distributedamong habitats. However, polychaeteworms are digestedtoo completelyto be identifiedin fecal samplesby our technique,so the extent to which worms contributedto the diet of Snowy Ploversin our studywas unknowmthey may be important.Reeder (1951) foundpolychaete worms in the stomachsof three SnowyPlovers collected during the breedingseason along the southernCalifornia coast.We observedSnowy Plovers feedingon worms on severaloccasions. Polychaete worms couldbe importantprey for Snowy Ploversforaging in intertidalareas. Hofmann and Hoerschelmann(1969; cited in Cramp and Simmons 1983) reported that the stomach of one Kentish Plover (Charadrius a. alexandrinus) contained 124 beetle heads. Grover and Knopf (1982) reported rove beetlesas abundantwhere SnowyPlovers forage in Oklahoma.We found rove beetlesin the majorityof fecal samplesof WesternSnowy Ploversat the Santa MargaritaRiver mouth.However, because the percentoccurrence of an organismin fecalsamples is basedon its presenceor absence,the actualnutritional or volumetriccontribution of rove beetles may not exceed that of other abundant prey, long-leggedflies or shoreflies.Flies formed large, denseswarms, and ploverswere frequentlyobserved foragingwithin these swarms.Our resultswere consistentwith the findingsof diet studiesof the Snowy Plover elsewhere.Swarth (1983) found beetlesand shoreflies constitutingthe majorprey of SnowyPlovers at Mono Lake, California;Grover and Knopf (1982) found them as prey on the Great Salt Plainsof Oklahoma. Kentish Plovers in Europe also feed on terrestrialinvertebrates, mainly flies and beetles (Cramp and Simmons1983). SnowyPlover chicks are highlyprecocial and leave the nestwithin a dayof hatching to foragein the samehabitats as their parents.Although the numberof fecalsamples fromjuveniles in our studywas small, fledglings fed on the sameorganisms, with the exceptionof dipterans,as adults.Plover chicksmay be unableto catchfast-moving insectssuch as dipteransand hymenopterans(Shaffer and LaPorte 1994). This project was fundedby the U.S. National BiologicalService, San Diego Field Station.We thankChristine Collier and BonniePeterson for their help in the field,Joy Zedlerand Pacific Estuarine Research Laboratory for useof laboratoryfacilities, Kathy Boyer and JenniferLewis for their assistancewith invertebrateidentification, and Jill Terp, Kathy Williams, Mark Stern, Philip Unitt, and Tricia Campbell for valuable commentsand suggestionsfor improvingthis report.

LITERATURE CITED

Baker, M. C. 1977. Shorebirdfood habits in the eastern Canadian Arctic. Condor 79:56-62. Borror, D. J., Triplehorn, C. A., and Johnson,N. F. 1989. An Introductionto the Studyof Insects,6th ed. Sanders,Orlando, FL. Cramp, S., and Simmons,K. E. L. 1983. The Birdsof the WesternPalearctic, vol. 3. Oxford Univ. Press,Oxford, England. Grover,P. B., and Knopf, F. L. 1982. Habitat requirementsand breedingsuccess of charadriiformbirds nesting at Salt PlainsNational Wildlife Refuge, Oklahoma. J. Field Ornithol. 52'.139-148. Le V. Dit Durrel,S. E. A., and Kelly,C. P. 1990. Dietsof DunlinCalidris alpina and Grey Plover Pluvialis squatarola on the Wash as determinedby droppings analysis.Bird Study 37:44-47.

47 NOTES

Martin, P. A., and Hockey, P. A. R. 1993. The effectivenessof stomach-flushingin assessingwader diets. Wader StudyGroup Bull. 67:79-80. Nicholls,J. L. 1989. Distributionand other ecologicalaspects of Piping Plovers winteringalong the AtlanticGulf Coasts. M.S. Thesis,Auburn Univ.• Auburn, AL. Page,G. W., Warriner•J. S., Warriner,J. C., andPaton, P. W. C. 1995. SnowyPlover (Charadrius alexandrinus), in The Birdsof North America (A. Poole and F. Gill, eds.), no. 154. Acad. Nat. Sci. Philadelphia. Poulin, B., and Lefebvre,G. 1995. Additionalinformation on the useof tartar emetic in determiningthe diet of tropicalbirds. Condor 97:897-902. Powell,A. N., Collier,Co L., andPeterson, B. 1995. Statusof WesternSnowy Plovers (Charadriusalexandrinus nivosus) in San Diego County, 1995. Report to U.S. Fish and WildlifeService, Portland, OR, and Calif. Dept. Fish & Game, Sacra- mento, 1416 Ninth St., Sacramento,CA 95814. Ralph,C. P., Nagata,S. E., and Ralph,C. J. 1985. Analysisof droppingsto describe diets of small birds. J. Field Ornithol. 56:165-174. Ramer•B. A., Page,G. W., andYoklavich, M. M. 1991. Seasonalabundance, habitat useand diet of shorebirdsin ElkhornSlough, California. W. Birds22:157-174. Reeder,W. G. 1951. Stomachanalysis of a groupof shorebirds.Condor 53:43-45. Ricketts,E. F., Calvin,J., Hedgpeth,J. W., andPhillips• D. W. 1985. BetweenPacific Tides, 5th ed. Stanford Univ. Press,Stanford, CA. Rosenberg,K. V., andCooper, R. J. 1990. Approachesto aviandiet analysis. Studies Avian Biol. 13:80-90. Rundle,W .D. 1982. A casefor esophagealanalysis in shorebirdfood studies. J. Field Ornithol. 53: 249-257. Shaffer,F., and LaPorte, P. 1994. Diet of Piping Ploverson the MagdalenIslands, Quebec. Wilson Bull. 106:531-536. Swarth,C. W. 1983. Foragingecology of SnowyPlovers and the distributionof their arthropodprey at Mono Lake, California. M.S. Thesis•Cal. State Univ., Hayward.

Accepted 18 June 1998

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