Socorro, politically part of Mexico, is the largest and most diverse of four volcanic oceanic islands that comprise the Revillagigedo Islands, each rising independently from the ocean floor (see also Secondary Area s008). The islands are located c.450 km south-south-west of Cabo San Lucas in Baja California, and 640 km west from the mainland state of Colima (Brattstrom 1990, Rodríguez-Estrella et al. 1996).

Socorro consists of one high central volcanic peak, Cerro Evermann (1,130 m), and has a mainly undulating landscape with small hills, but there are also steep slopes, dry canyons and flatter areas at mid-elevation. The natural vegetation of the west, south and east sides of the island below 600 m consists predominantly of a thick mass of the shrub Croton masoni and the cactus Opuntia engelmanni. The north side of the island and all the higher elevations have semi-deciduous forest, including trees such as Prunus capuli, Ficus continifolia, Guettarda insularis, Ilex socorrensis and Bumelia socorrensis, and some bromeliads and orchids (Brattstrom and Howell 1956).

The nine native terrestrial landbirds are all endemic to Socorro, one (Mimodes) to genus level, three to species level, and five to subspecies. Of the four restricted-range species, there are some differences in habitat and altitudinal requirements. Aratinga brevipes is found in the forests mostly above 550 m (Rodríguez-EstrellaThryomanes sissonii appears to prefer shrubby habitat at lower elevations. Zenaida graysoni was found all over the island when first discovered in 1867, but principally in the forests with a good understorey.

Interestingly, Mimodes graysoni was found by different ornithologists to be commoner in either the shrub or the forest habitats, suggesting that seasonal movements might take place. However, recent surveys have found it principally in forest and shrubby ravines at elevations above 600 m, appearing to prefer a relatively unbroken, shrubby ground cover and moderately dense trees for nesting (Castellanos and Rodríguez-Estrella 1993, Martínez-Gómez and Curry 1996).

Socorros flora and fauna have declined drastically owing to the introduction of sheep and cats. Sheep proliferated after their release in 1869 (Brattstrom 1990), and their overgrazing has caused accelerated erosion and has produced extensive savannas that were probably not present before their arrival. Trees are now absent over large parts of the southern end of the island (Martínez-Gómez and Curry 1996, Rodríguez-Estrella et al. 1996). Cats probably arrived on Socorro in the late 1950s after the establishment of a military base (Veitch 1989). Their abundance and distribution are poorly understood but evidence of their presence has been found from areas at all altitudes on the island (Martínez-Gómez and Curry 1996).

Habitat destruction by sheep is thought to be the primary reason for the decline of Mimodes graysoni, which is currently the most threatened bird species on the island, with a provisional estimate of c.350 individuals (Martínez-Gómez and Curry 1996). Recent surveys found that the grassy areas on the south side of the island no longer support the species, and that forests which had overgrazed understoreys held no birds or far fewer than did areas less affected by sheep. However, the widespread Northern Mockingbird Mimus polyglottos, which became established on the island in 1978, favours these areas (Rodríguez-Estrella et al. 1996).

The arrival of the cats almost certainly caused the extinction of the tame and ground-nesting Zenaida graysoni, the last sighting being apparently in 1972 when several birds were shot (Velasco-Murgía 1982). However, c.200 individuals are kept in captivity (Collar et al. 1992), hence its status Extinct in the Wild.

Aratinga brevipes is also classified as threatened: a recent study indicated a population of only 400-500 birds although numbers appear to be stable (Rodríguez-Estrella et al. 1992).

Socorro is also the stronghold of the threatened seabird, Townsends ShearwaterPuffinus auricularis (classified as Vulnerable), endemic as a breeder to the Revillagigedo Islands. Its population was estimated to be 1,000 pairs in 1981, with feral cats now known to be causing substantial losses (Collar et al. 1992, R. L. Curry verbally 1994).

Socorro needs an immediate programme to eradicate sheep and cats (Santaella and Sada 1991, Castellanos and Rodríguez-Estrella 1993, Martínez-Gómez and Curry 1996, Rodríguez-Estrella et al. 1996) in order to avert the eventual extinctions of at Mimodes graysoni and Puffinus auricularis, and to allow for the restocking of Zenaida graysoni. Encouragingly, the Revillagigedo Islands and surrounding waters were declared a Biosphere Reserve in 1994, indicating that the Mexican government acknowledges the great importance of these islands to biodiversity conservation.

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Socorro. Downloaded from https://datazone.birdlife.org/eba/factsheet/10 on 23/11/2024.