WESTERN BIRDS Volume 24, Number 1, 1993 AN ANNOTATED CHECKLIST OF THE BIRDS OF ISLA SOCORRO, MEXICO WALTER WEHTJE and HARTMUT S. WALTER, Departmentof Geography,Uni- versity of California, Los Angeles, California 90024-1524 (present addressof Wehtje:Western Foundation of VertebrateZoology, 439 Calle San Pablo,Camarillo, California 93010) RICARDO RODRIGUEZ ESTRELLA, JORGE LLINAS and ARADIT CASTE- LLANOS VERA, Centro de InvestigacionesBio16gicas de Baja California Sur, Apartado Postal 128, La Paz, Baja CaliforniaSur 23000, Mexico In recentyears, ornithologists have devoted increased attention to insular avifaunasand the problemsthey face (Scott et al. 1986). Many island speciesare criticallyendangered and require human intervention in orderto survive.Island species also provide the opportunityto testdiffering ecologi- cal and biogeographicaltheories (MacArthur and Wilson 1967, Jonesand Diamond 1976). However,many remote islandsare not visitedfrequently enoughor for periodslong enoughto provideinformation on population trendsand the lossand/or gain of breedingspecies. Such has been the case for the lslasRevillagigedo of Mexico(Figure 1). The IslasRevillagigedo are a group of four islandslying off the Pacific coastof Mexico. In ascendingorder of sizethey are Roca Partida,Isla San Benedicto, lsla Clari6n, and Isla Socorro. The last is located at 18 ø 47' N, 110 ø 57' W, about 460 km south of Baja California(Figure 2). In the absenceof a goodtopographic map, we estimatethe sizeof IslaSocorro as 110 km2. The maximumelevation of the island,Cerro Evermann,is 1040 m. The flora of the islandhas been describedby Levin and Moran (1989) and consistsof at least 117 native speciesof plants, 30 of which are endemic.There are six principalplant associations(Le6n de la Luz pers. comm.): mixed scrub, dominated by Dodonaea viscosaand Pteridium caudatum(0-700 m), deciduousscrub, dominated by Croton masonii(0- 250 m), sheep-inducedprairie, mainlywith Mitracarpus hirtus, Aristida spp., and Boerhavia spp. (250-400 m), the shore habitats with Conocarpuserecta, woodlandsdominated by Bumelia socorrensis,Ficus cotonifolia, Ilex socorrensis,Guettarda insularis, and Psidium sp. (350- 850 m), and highland prairie, dominated by Castilleja socorrensis, Gnaphalium attenuatum, Heterotoma cordifolia, and Linaria canadensis(850-950 m). Western Birds 24:1-16. 1993 1 BIRDS OF ISLA SOCORRO The mixedscrub, up to 3 m high,covers much of the islandbelow 700 m elevation.Where it is undamaged,this extremelythick brusheffectively restrictsaccess. The vegetationof the southernhalf of the island,however, has been severelydamaged by feral sheep,introduced during the 1860s. We estimatethe presentsheep population at 2000 individuals.The damage causedby sheepranges from inhibitingregrowth in forestat mid-elevations to the completeremoval of vegetationon someslopes and hilltops.Soil erosionis a major problemin severalareas. In additionto sheep,feral domestic cats and housemice are alsopresent on IslaSocorro. The lattertwo speciesare believedto havearrived on the Miles Los Angeles 0 100 200 300 i i i ,I UNITED STATES Kilometers 0 100 200 300 400 I I I I I -30* I$1Jde Guadalupe MEXICO -25* La Paz Tropic of Can Mazatlan ;abo San Lucas PACIFIC OCEAN Islas% Marias ' San Benedicto Roca Partida. Clari6n- Soc•rro I ISLASREVILLAGIGEDO 115' 1•0ø 105' Figure 1. Location of lsla Socorro in the Pacific Ocean to the south of Baja California. BIRDS OF ISLA SOCORRO islandwhen the Mexican navy establisheda basethere in 1958 (Jehl and Parkes 1982). Miraculously,rats have not invadedIsla Socorro. The navybase is locatedon the southernmosttip of the islandand houses closeto 200 military personneland dependents.There is also a landing strip and radar station, constructedon the eastern side of the island in 1978. The only roadson the islandare one connectingthe airstripto the navybase and anotherleading from this main road to the southernplateau region, terminatingat approximately500 m elevation on the southern slope of Cerro Evermann. Previous authors (Jehl and Parkes 1982, Brattstromand Howell 1956) mentioneda temporarylake calledLaguna Escondidaon the southernside of the island.It hasnever been filled during our visits.There are a numberof deep potholesin the canyonsthat contain limitedquantities of fresh water after the rainy season(July-November). Apart from the navy base and a springlocated in the tidal zone at Caleta Grayson,no known permanentsources of water existon the island. 111ø00'W Cabo Middleton ISLA SOCORRO NW NE QUADRANT QUADRANT • RocaO'Neal Cabo Tosca Caleta 18ø45'N Bahia Cornwallis sw QUADRANT •'•':•....{•iLavaFlow O•,'' Bahia ////•Woodland{•' ' ' •kilometer Braithwaite Cabo Rule SE Contours in intervals of 500 feel QUADRANT Figure2. SocorroIsland. The northeast-southwestline partitions the island into the sheep-freenorthern half and the sheep-degradedsouthern half. BIRDS OF ISLA SOCORRO The Revillagigedosbelong to the federalgovernment of Mexicoand are currently administeredby its navy. Access to Isla Socorro is severely restricted because of the lack of facilities for visitors. The absence of roads limitsthe area that can be easilyvisited. Two trailslead to the peak of the islandfrom the end of the mountainroad. Areas affectedby sheepare also fairlyeasily traversed by humans.The ruggedterrain, lava flows, and nearly impenetrablevegetation of the northernside of the islandmake much of the islanddifficult to survey.Because of these conditions,most earlier reports were based upon studiesin severelyaltered habitat. Time con- straintskept mostresearchers in the pastfrom spendingany lengthof time in undisturbedareas. We have been able to explorea larger portion of the islandfor a greaterlength of time, and over differentseasons during the same year. AlthoughIsla Socorrowas first discoveredby the Spanishin 1533, the firstnatural history expeditions to the islandwere madeby AndrewJackson Graysonin 1865 and 1867. Shipwreckedthere for 10 days(19-28 May 1867), he and his sondiscovered and collectedspecimens of all the island's endemicbirds (Grayson 1872). Ornithologicalassessments of other early expeditionscan be foundin Anthony(1898), McLellan(1926), Brattstrom and Howell (1956), and Jehl and Parkes(1982). Interestin the avifaunaof the IslasRevillagigedo has increasedduring the past decade,and severalarticles dealing with these islandshave recently been published(Jehl and Parkes 1982 and 1983, Jehl 1982, Brattstrom 1990, Howell and Webb 1990, Parkes 1990, Santaella and Sada 1991). The last comprehensiveannotated bird list for the archipelagowas that of Jehl and Parkes(1982). Most visits to Isla Socorro have been ship-based,limiting the time observerscan spendashore. As membersof a joint conservationproject of the University of California, Los Angeles (UCLA), the Centro de InvestigacionesBio16gicas (CIB) in La Paz, and the World WildlifeFund, we visitedthis ruggedisland repeatedly by airplanein April, July, September, November,and December 1988, February,May, and November1990, and August1991, spendinga total of 62 dayson the island.During these visits we improved our understandingof the populationstatus of the resident speciesand other visitors,and observed23 previouslyunrecorded bird species.Here we providean updatedannotated checklist for the avifaunaof Isla Socorro. An asteriskbefore a speciesname specifiesa new record. ANNOTATED SPECIES LIST Pied-billedGrebe (Podilyrnbus podiceps). Brattstrom found a deadspecimen on the beach at Cabo Henslow on 14 November 1971 (Jehl and Parkes 1982). LaysanAlbatross (Diornedea imrnutabilis). Howell and Webb (1990) observedan individualover Cabo Henslow on 16 February 1988. Two birds on 7 May 1990 (Walter)and one on the followingday (Wehtje)were seenflying over the navybase. This speciesis extendingits range, with breedingrecorded on lsla Claribn and lsla Guadalupeand nesting behavior observed on lslaSan Benedicto, 55 km northof lsla Socorro (Howell and Webb 1990). BIRDS OF ISLA SOCORRO Wedge-tailedShearwaters (_Puffinus pacificus) have been observednear Isla Socorro in moderate numbers(10-130) in March, April, May, and November (Anthony1898, Brattstromand Howell 1956, Jehland Parkes 1982, Santaellaand Sada1991). The lackof sightingsin othermonths is probablydue to the absenceof observers.This speciesbreeds on IslaSan Benedicto(Jehl and Parkes1982). Townsend'sShearwater (_Puffinus auricularis). Jehl (1982) is the bestsource for informationon this species,but here we add a few observationsthat confirm and elaborateon hisfindings. Jehl statedthat breedingbirds arrive in Decemberand leave byJune. We spentthe nightof 20 November1990 in a forestedcanyon on the north sideof the islandat 750 m elevationand foundourselves in the midstof a verynoisy shear0vatercolony. This suggeststhat the birdsarrive at leastby mid-November.We capturedand photographedone individualat the mouthof a burrowunder a large boulder.Other birdswere heard insidetheir burrowsduring daylight hours. Most of the burrowswere locatedon steepslopes that were difficultto reach.We alsoheard betweenone and five shearwaterscalling over land on 25 August1991, indicating that this speciesmay visit its breedingcolonies throughout the year. Shearwaters were commonlyseen from shorein numbersof up to a dozenin August. In the eveningof 23 February1990 we foundanother colony, northeast of Cerro Evermannat 500 m elevation,where we estimatedhearing close to 100 birdscalling simultaneously.Because of the impenetrabilityof the vegetation,we were unableto approachit. On 24 FebruaryRodriguez heard a similarnumber