NOTES

PREDATION BY ON AT ISLA SOCORRO, RICARDO RODRIGUEZ-ESTRELLA, GUSTAVO ARNAUD, SERGIO ALVAREZ CARDENAS, and ANTONIO RODRIGUEZ, Centro de lnvestigacionesBio16gicas, Div. Biol. Terr., ApartadoPostal 128, La Paz, Baja CaliforniaSur 23000, M•xico

SocorroIsland, the largestand biologicallymost diverse of the fourislands of the RevillagigedoArchipelago (Brattstrom 1990), confrontsserious problems related to humanactivities. Eight endemictaxa of land birdsnow breedon Socorro:Yellow- crownedNight Heron, Nyctanassaviolacea gravirostris; Socorro Red-tailed , Buteo jamaicensissocorronensis; Socorro Ground Dove, Columbina passerina socorroensis; Green Parakeet, Aratinga holochlora brevipes; Socorro , Thryomanes sissonii;Socorro ,Miraodes graysoni; Tropical Warbler,Parula pitiayumi graysoni;and SocorroTowhee, Pipilo erythrophtalmus socorroensis(McLellan 1926, Jehl and Parkes 1982, Brattstrom 1990). Jehl and Parkes (1983) have proposedthat domesticcats (Fells catus), introducedsince 1957, are responsiblefor the extirpationof the endemicSocorro Dove, Zenaida graysoni, and reductionsof other species,especially the . However,no quantitativedata on predationby feralcats on SocorroIsland have been previouslyreported in the literature.We presenthere an analysisof the diet of feral cats on . SocorroIsland, located approximately 450 km southof the tip of the peninsulaof BajaCalifornia, covers about 14,000 hectares(16 x 11 km). Thisisland is a volcanic conewhose highest point is Mount Evermann(1050 m). Climateis arid tropicalwith an averageannual temperature of 24.8 ø C and averageannual precipitation of 327.7 mm. Freshwater is found in the puddlesformed after tropicalstorms and continuouslyin severalcaves. From 16 to 28 Februaryand from 7 to 12 May 1990 we conductedour field researchon SocorroIsland, intensively searching for scats,tracks, and other indirect evidenceof feral cats, speciallyin the southportion of the island.We searchedin Croton scrub(40 m elevation),disturbed grassland, and forest (650 m elevation). Presumably,the scatswe foundwere from differentindividuals, as theywere widely dispersed. We foundand analyzed 31 scats,comparing the remainsof scales,bones, feathers, andhairs with thoseof the collectionof Centrode InvestigacionesBio16gicas. Table 1 showsthe occurrenceof itemsin 31 scatsfound during February and May 1990. Feral cats were feedingmainly on House Mice (Mus musculus),endemic lizards (Urosaurusa u riculatus), some birds,and also crabs,, scorpions, and various vegetablematerials (Table 1). Birdswere presentin 22.6% of the catscats (February and May samplescombined). The SocorroGround Dove and TropicalParula Warbler were the two speciesfound most often in the scats, five and three times, respectively.A recent colonizer,the (Zenaida macroura), was presentin two of the 31 analyzedscats. Although we didnot findremains of another recentcolonizer, the NorthernMockingbird ( polyglottus), in May we found feathersof one in the field, presumablyeaten by a feral . Jehl and Parkes(1982) reportedpredation by catson Townsend'sShearwater (Puffinus auricularis), but we did not find evidenceof this perhapsbecause our searcheswere not madenear the shearwater colonies. Our resultssuggest that predationon birdsby feral cats is not very important. However,even limited may seriouslyaffect a speciesas endangeredas the Socorro Mockingbird.This species'population may be approximately60-70 pairs (Castellanosand Rodriguez-Estrella1990, L. Baptistapers. comm.),and we found evidencethat it is preyedupon by SocorroRed-tailed (a SocorroMockingbird Western Birds 22:141-143, 1991 141 NOTES

Talkie 1 Prey RemainsIdentified in 31 Scatsof Feral Cats on Socorro Island, 1990

February May (12 samples) (19 samples)

Prey N ø % N %

Mammals 11 91.7 17 89.5 Mus musculus 9 75.0 17 89.5 Felis catus 2 16.7 0 0.0 Birds 3 25.0 4 21.1 Columbina 2 16.7 3 15.8 Thryomanes 1 8.3 1 5.3 Parula 2 16.7 1 5.3 Miraodes 1 8.3 0 0.0 Zenaida macroura 0 0.0 2 10.5 Reptiles 4 33.3 10 52.6 Urosaurus auriculatus 4 33.3 10 52.6 9 75.0 14 73.7 Orthoptera 8 66.7 12 63.2 Lepidoptera 2 16.7 0 0.0 Coleoptera 2 16.7 4 21.1 Hymenoptera 1 8.3 1 5.3 Scorpiones 0 0.0 4 21.1 Decapoda 0 0.0 2 10.5 Miscellaneous 9 75.0 17 89.5 Seeds 1 8.3 4 21.1 Vegetablefibers 8 66.7 14 73.7 Unidentified matter 0 0.0 2 10.5

aNumberof scatsin which each speciesor item was identified. wingand feathersshowing predation by a hawk)and cats.The endemicRed-tailed Hawk and Socorro Mockingbirdhave presumablylong been in predatory-prey equilibrium,while cats are of recentintroduction. "Alien predation"(Moors and Atkinson1984) by catsmay be a seriousproblem because it couldlead to declinesin somebird species,as has happenedwith other islandlandbirds and seabirds(Jehl 1972, Taylor 1979, Diamond1982, Moorsand Atkinson1984, Fitzgeraldand Veitch 1985, Veitch 1985, Ebenhard 1988). The populationof feralcats seems to be densestin the southeastportion of the islandapparently because this area is the zone mostperturbed by humanactivities. Here water and domesticgarbage are availablefor cats, and the food supplyin humanrefuse may favor an increaseof the cats'population. The actualnumbers of cats is hard to determine,although their distributionon the islandis increasing (Arnaudet al. unpubl.data). Because feral cats take their prey mainly on the ground, birds foragingon the ground,such as the endemicground dove and wren, are especiallyvulnerable. The SocorroWren population is very dense, but such is not the casefor SocorroGround Dove, which is stillfairly common in the openlowlands of the island,especially in the southeastand eastern portions near the coast(W. Wehtje pers.comm.). This speciesis no longeras tame as reportedby Villa (1960)--"son mansasy se les capturasimplemente con las manos"--butit seemsthat their foragingand reproductive activities are normal.However, whenever we approached

142 NOTES

the ground doves, they immediatelyflew, suggestingthat predation by cats is affectingthe endemicdove's tameness and possiblyits populationsize. At the Centro de InvestigacionesBio16gicas we are studyingthe processof apparentextinction of somebird species,and we want to call the attentionof the scientific community and organizationsconcerned with the conservation of biodiversityabout the needto eradicatethe feralcats from Socorro Island and thereby protectthe endemicavifauna. Otherwise, cats may put in risksome of the landbirds of the island,particularly the SocorroMockingbird. Specialthanks are due to AlfredoOrtega for encouragingour studieson Socorro Island.Philip Unitt, WilliamT. Everett,and an anonymousreviewer made helpful suggestionson a previousdraft. LuisBaptista kindly provided us the informationon the 19 SocorroMockingbirds he recordedin the north-centralpart of the islandin November 1990. For its support on the island we thank the Mexican Navy. Economic support was provided by World Wildlife Fund, the Centro de InvestigacionesBio16gicas, Consejo Nacional de Cienciay Tecnologia,Secretaria de Programaci6ny Preseupuesto,Secretaria de DesarrolloUrbano y Ecologia,and Fundaci6n Ricardo J. Zevada.

LITERATURE CITED

Brattstrom,B. H. 1990. Biogeographyof the IslasRevillagigedo, Mexico. J. Bio- geogr. 17:177-183. Brattstrom,B. H., and Howell, T. R. 1956. The birdsof the , Mexico. Condor 58:107-120. Castellanos,A., and Rodriguez-Estrella,R. 1990. EndangeredSocorro Mockingbird (Nlimodesgraysoni) on SocorroIsland, Mexico. Proceedingsof the 108th StatedMeeting of the AmericanOrnithologists' Union and 60th AnnualMeet- ing of the CooperOrnithological Society, Univ. of Calif., Los Angeles. Diamond, J. M. 1982. Man the exterminator. Nature 298:787-789. Ebenhard,T. 1988. Introducedbirds and mammalsand their ecologicaleffects. Swedish Wildlife Res. 13:1-107. Fitzgerald,B. M., and Veitch, C. R. 1985. The cats of HerekopareIsland, New Zealand;their history, and effectson birdlife.New ZealandJ. Zool. 12:319-330. Jehl,J. R. 1972. On the coldtrail of an extinctpetrel. Pacific Discovery 25:24-29. Jehl,J. R., andParkes, K. C. 1982. The statusof the avifaunaof the Revillagigedo Islands,Mexico. Wilson Bull. 94:1-19. Jehl,J. R., and Parkes,K. C. 1983. "Replacements"of landbirdspecies on Socorro Island,Mexico. Auk 100:551-559. McLellan,M. E. 1926. Expeditionto the RevillagigedoIslands, Mexico, in 1925. VI. The birdsand .Proc. Calif. Acad. Sci., 4th ser., 15:297-322. Moors, P. J., and Atkinson,I. A. E. 1984. Predationon seabirdsby introduced ,and factorsaffecting its severity.ICBP Tech. Publ.2:668-690. Taylor, R. H. 1979. How the MacquarieIsland parakeet became extinct. New Zealand J. Ecol. 2:42-45. Veitch,C. R. 1985. Methodsof eradicatingferal catsfrom offshoreislands in New Zealand. ICBP Tech. Publ. 3:125-141. Villa, B. 1960. Vertebradosterrestres. La Isla Socorro.Monogr. Inst. Geofisica. Univ. Nacional Aut6noma de M•xico 2:203-216. Accepted 15 April 1991

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