"Replacements" of Landbird Species on Socorro Island, Mexico

The endemic dove Zenaida graysoni of Socorro Island, an oceanic island in the Pacific south of Baja California, became extirpated between 1958 and 1978, and the endemic mockingbird Mimodes graysoni diminished during the same years and was approaching extinction by 1981. The mainland Mourning Dove Z. macroura became established on Socorro between (probably) 1971 and 1978, and the mainland Northern Mockingbird Mimus polyglottos between 1978 and 1981. Superficially an instance of replacement of island endemics by mainland relatives through competitive exclusion, the case is actually more complicated. We argue that extermination of the endemics was probably caused by predation by feral cats introduced in 1957 or later. There is no evidence of any actual competition between the pairs of species, and their preferred habitats overlap only slightly. We attribute the nearly concurrent establishment of the two invading species, known to have occurred frequently as casual visitors to Socorro and other islands, largely to the provision of fresh water made available as a result of human settlement. Our data show that simple comparison of island species lists compiled at intervals is an inadequate base for theories of island species turnover.