Guadalupe Island is the westernmost territory of Mexico, located c.280 km west of northern Baja California Norte state and 250 km south of San Diego (USA). It is an oceanic volcanic island with a rugged topography, which is highest in its northern and north-central parts. This northern end rises to a sharp ridge that drops off abruptly on the west side, but less so to the east, and widens to a plateau in the centre, with land from there gradually descending to the south. Close to the islands southern tip are two islets known as Zapato (Inner) and Toro (Outer) Islet.

The islands climate is one of dry, hot summers with cold winters. Abundant shrubs and forest once covered most of the island. These forests were especially prevalent at higher altitudes in the north and included cypress Cupressus guadalupensis, pines Pinus radiata var. binata, oaks Quercus tomentella and fan palms Erythea edulis, with more open and scrubby habitats at lower altitudes (Howell and Cade 1953).

Both the two restricted-range species are often treated as subspecies of more widespread taxa: Polyborus lutosus may be a race of Crested Caracara P. plancus, and Junco insularis has been considered a subspecies of Dark-eyed Junco J. hyemalas, but differences in voice, morphology and plumage indicate that the juncos specific status, at least, is justified (Howell and Cade 1953, Mirsky 1976, Howell and Webb 1995a).

Today little is left of the once-abundant vegetation. Virtually all of the native shrubs have gone, so that only vertical cliffs inaccessible to feral goats now hold the last patches. The forests are also greatly reduced in size and, because the understorey is absent in those remaining, there is no regeneration. The total extent of remaining forest includes a small area of oaks and pines on the northern ridge of the island, and a cypress grove (c.1 km in length in 1988) on the north-central plateau.

This lack of vegetation and predation by introduced cats has had a catastrophic effect on Guadalupes birds, although Polyborus lutosus became extinct about 1900 as a result of heavy persecution, mainly by shooting, often when birds gathered at water-holes (del Hoyo et al. 1994). Of the islands six endemic landbird subspecies, three have been lost: Northern Flicker Colaptes auratus rufipileus (although there were sightings possibly relating to this subspecies in 1988: Howell and Webb 1992a), Bewicks Wren Thryomanes bewickii brevicauda and Rufous-sided Towhee Pipilo erythrophthalmus consobrinus. Of all the endemic subspecies, only the local Rock Wren Salpinctes obsoletus guadeloupensis survives in good numbers (Jehl and Everett 1985).

Junco insularis has been found during the twentieth century mostly in the remaining forest patches in the northern half of the island but, in 1988, a group of four or five were found in stands of tobacco Nicotiana glauca shrubs on the beach, suggesting some level of adaptability (Collar et al. 1992). There have been various population estimates put forward for the species, and altogether these indicate a steep decline and fewer than 100 individuals in the late 1980s (hence the species threat classification as Critical).

The island also held the only known breeding colonies of the seabird, Guadalupe Storm-petrel Oceanodroma macrodactyla, which was discovered in 1885 and has not been sighted since 1912 (Collar et al. 1992). The little information collected on the species in this period suggests that its major nesting area was along the steep north-east ridge, where it made burrows in the soft soil under the pines and oaks (Jehl and Everett 1985). The main cause of the storm-petrels demise was presumably heavy predation by cats, and the reduced availability of nesting areas through goats causing forest loss (Jehl 1972). There are very faint hopes that the species could still persist as, although searches for it have been carried out, a thorough survey of the entire island during the breeding season has not been made since 1906.

Removal of goats from the island is needed to help the vegetation regenerate and provide habitat for the birds, especially the junco (Collar et al. 1992). Nearly 35,000 goats were removed in 1970 and 1971, but current numbers are conservatively estimated at 10,000 individuals and unless they are completely eradicated the junco is doomed (P. Sweet in litt. 1996). This EBA is embraced entirely within the Isla Guadalupe Special Biosphere Reserve.

Recommended citation
BirdLife International (2024) Endemic Bird Area factsheet: Guadalupe Island. Downloaded from https://datazone.birdlife.org/eba/factsheet/2 on 23/11/2024.