Justification Intensive sheep-grazing and a persistent locust swarm are reducing and degrading habitat for this species. Combined with cat predation, which effectively removes mockingbirds from areas with little or no understorey, declines in its very small population and extremely small range are considered likely. This combination of factors qualifies the species as Critically Endangered.

Family/Sub-family Mimidae

Species name author (Lawrence, 1871)

Taxonomic source(s) AOU checklist (1998 + supplements), Sibley and Monroe (1990, 1993), Stotz et al. (1996)

Synonyms Mimodes graysoni BirdLife International (2004), Mimodes graysoni BirdLife International (2000), Mimodes graysoni Collar et al. (1994), Mimodes graysoni Collar and Andrew (1988), Mimodes graysoni Stotz et al. (1996), Mimodes graysoni Sibley and Monroe (1990, 1993)

Taxonomic note Formerly treated in the monotypic genus Mimodes; transferred to the genus Mimus following AOU (2005).

Identification 25 cm. Largish, mostly plain brown passerine. Brown upperparts, darker wings with two narrow, white wing-bars and darker tail. Brown head with dusky lores and short, pale supercilium. Whitish underparts, streaked brown on flanks. Blackish bill and legs. Similar spp. Northern Mockingbird M. polyglottos is greyer with white wing-patches and outer rectrices. Voice Grating warble song. Loud whichoo call. M. polyglottos and M. graysoni mimic each other.

Population estimate

Population trend

Range estimate (breeding/resident)

Country endemic?

290 - 420

stable

15 km²

Yes

Range & population Mimus graysoni is endemic to Socorro in the Revillagigedo Islands, Mexico. It was the most abundant and widespread landbird in 1925, and was still abundant in 1958. By 1978, it had declined dramatically and was feared on the verge of extinction. Subsequent surveys have estimated the population at 50-200 pairs in 1988-1990^1,5,8 and c.350 individuals in 1993-1994, with the highest densities in the sheep-free dwarf forests of Cerro Evermann³. Of 170 birds ringed in 1994, 56% were subadults³, suggesting that productivity is high and the population would be capable of increasing if habitat quality improves across the island⁹.

Ecology: It occurs principally in moist dwarf forest and ravines with a mixture of shrubs and trees at elevations above 600 m ³. Vegetation in these areas is dominated by the trees Ilex socorrensis, Guettarda insularis and Oreopanax xalapensis and the understorey species Triumfetta socorrensis and Eupatorium pacificum^3,4. It is very rare at low and mid-elevations (0-500 m), and absent from areas of Croton masonii scrub near sea-level and sheep-damaged habitat in the south-east half of the island^3,4, but is common within fig Ficus cotinifolia patches in the north-west of the island⁹. Fig groves may act as regeneration nuclei for the species, supporting birds when a suitable understorey is present⁹. Nesting may occur from November-July with a peak in March-April². Three eggs are laid and the incubation period is no more than 15 days². Food includes crab remains, small invertebrates and fruit, particularly those of Ilex socorrensis and Bumelia socorrensis^1,4.

Threats Sheep had intensively grazed almost one third of the island by 1990¹, leaving no suitable nesting or foraging habitat in the south of the island³. Predation by feral cats was initially thought responsible for the species's decline, but cats were introduced some time after 1972⁴, and examinations of cat stomach contents and scats have not provided any substantive evidence³. However, they are likely to prey upon dispersing individuals that move into areas with little or no understorey⁹. Competition with the immigrant Mimus polyglottos is probably not a factor because Mimus graysoni is much larger, has different habitat preferences and is not outcompeted in undisturbed habitats^1,3. Since 1994, c.30 ha of forest have been lost owing to a now permanent locust Schistocerca piceifrons swarm on the island which irrupts twice yearly. Its effects are thought to be more severe owing to the degradation of native vegetation by introduced grazing mammals, and the suppression of native bird populations (which typically exert top-down control of insect populations on the island) by introduced cats. Locusts cut leaves, flowers and fruit from trees and thus represent a serious threat to fruit eaters such as Socorro Mockingbird⁹. Potential developments on Socorro including enlargements to the airstrip and the possibility of a new federal prison could destroy breeding habitat and increase the risk of accidental introduction of other invasive species.

Conservation measures underway The Revillagigedo Islands were declared a Biosphere Reserve in 1994⁵. There is an ongoing control programme in the region (the Mexican navy has effectively reduced the sheep population to c. 300 heads), and there are plans to eradicate cats and sheep from Socorro^6,7.

Conservation measures proposed Eradicate cats and sheep from Socorro^3,6,7. Implement a vegetation and soil restoration plan after sheep have been removed⁴. Establish a research monitoring station on Socorro⁵. Monitor the population, especially before and after the proposed eradications.

References Collar et al. (1992). 1. Castellanos and Rodríguez-Estrella (1993). 2. Martínez-Gómez and Curry (1995). 3. Martínez-Gómez and Curry (1996). 4. Martínez-Gómez et al. (2001). 5. Rodríguez-Estrella et al. (1996). 6. B. Tershy and B. Keitt in litt. (1999). 7. B. Tershy in litt. (1999). 8. Wehtje et al. (1993). 9. J. E. Martínez-Gómez in litt. (2007). 10. Martínez-Gómez (undated).

Further web sources of information

Alliance for Zero Extinction (AZE) species/site profile. This species has been identified as an AZE trigger due to its IUCN Red List status and limited range.

Text account compilers Jeremy Bird (BirdLife International), Stuart Butchart (BirdLife International), David Capper (BirdLife International), Isabel Isherwood (BirdLife International), Rob Pople (BirdLife International - European Division), Rob Pople (BirdLife International), Andy Symes (BirdLife International), Helen Temple (BirdLife International)

Contributors Brad Keitt (Island Conservation), Juan Esteban Martínez-Gómez (Island Endemics Foundation/Endémicos Insulares), Bernie Tershy (Island Conservation)

IUCN Red List evaluators Jeremy Bird (BirdLife International), Stuart Butchart (BirdLife International), Andy Symes (BirdLife International)

Recommended citation BirdLife International (2009) Species factsheet: Mimus graysoni. Downloaded from http://www.birdlife.org on 9/2/2010

This information is based upon, and updates, the information published in BirdLife International (2000) Threatened birds of the world. Barcelona and Cambridge, UK: Lynx Edicions and BirdLife International, BirdLife International (2004) Threatened birds of the world 2004 CD-ROM and BirdLife International (2008) Threatened birds of the world 2008 CD-ROM. These sources provide the information for species accounts for the birds on the IUCN Red List.

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