Ricardo Rodriguez-Estrella’s scientific contributions

Socorro Island, the largest and biologically most diverse of the four islands of the Revillagigedo Archipelago (Brattstrom 1990), confronts serious problems related to human activities. Eight endemic taxa of land birds now breed on Socorro: Yellow-crowned Night Heron, Nyctanassa violacea gravirostris; Socorro Red-tailed Hawk, Buteo jamaicensis socorronensis; Socorro Ground Dove, Columbina passerina socorroensis; Green Parakeet, Aratinga holochlora brevipes; Socorro Wren, Thryomanes sissonii; Socorro Mockingbird, Miraodes graysoni; Tropical Parula Warbler, Parula pitiayumi graysoni; and Socorro Towhee, Pipilo erythrophtalmus socorroensis (McLellan 1926, Jehl and Parkes 1982, Brattstrom 1990). Jehl and Parkes (1983) have proposed that domestic cats (Fells catus), introduced since 1957, are responsible for the extirpation of the endemic Socorro Dove, Zenaida graysoni, and reductions of other species, especially the Socorro Mockingbird. However, no quantitative data on predation by feral cats on Socorro Island have been previously reported in the literature. We present here an analysis of the diet of feral cats on Socorro Island. Socorro Island, located approximately 450 km south of the tip of the peninsula of Baja California, covers about 14,000 hectares (16 x 11 km). This island is a volcanic cone whose highest point is Mount Evermann (1050 m). Climate is arid tropical with an average annual temperature of 24.8 ø C and average annual precipitation of 327.7 mm. Fresh water is found in the puddles formed after tropical storms and continuously in several caves. From 16 to 28 February and from 7 to 12 May 1990 we conducted our field research on Socorro Island, intensively searching for scats, tracks, and other indirect evidence of feral cats, specially in the south portion of the island. We searched in Croton scrub (40 m elevation), disturbed grassland, and forest (650 m elevation). Presumably, the scats we found were from different individuals, as they were widely dispersed. We found and analyzed 31 scats, comparing the remains of scales, bones, feathers, and hairs with those of the collection of Centro de Investigaciones Bio16gicas. Table 1 shows the occurrence of items in 31 scats found during February and May 1990. Feral cats were feeding mainly on House Mice (Mus musculus), endemic lizards (Urosaurus a u riculatus), some birds, and also crabs, insects, scorpions, and various vegetable materials (Table 1). Birds were present in 22.6% of the cat scats (February and May samples combined). The Socorro Ground Dove and Tropical Parula Warbler were the two bird species found most often in the scats, five and three times, respectively. A recent colonizer, the Mourning Dove (Zenaida macroura), was present in two of the 31 analyzed scats. Although we did not find remains of another recent colonizer, the Northern Mockingbird (Mimus polyglottus), in May we found feathers of one in the field, presumably eaten by a feral cat. Jehl and Parkes (1982) reported predation by cats on Townsend's Shearwater (Puffinus auricularis), but we did not find evidence of this perhaps because our searches were not made near the shearwater colonies. Our results suggest that predation on birds by feral cats is not very important. However, even limited predation may seriously affect a species as endangered as the Socorro Mockingbird. This species' population may be approximately 60-70 pairs (Castellanos and Rodriguez-Estrella 1990, L. Baptista pers. comm.), and we found evidence that it is preyed upon by Socorro Red-tailed Hawks (a Socorro Mockingbird