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Socorro Mockingbird Mimus graysoni
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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.

Taxonomic source(s)
AOU. 1998. Check-list of North American birds. American Ornithologists' Union, Washington, D.C.
Sibley, C. G.; Monroe, B. L. 1990. Distribution and taxonomy of birds of the world. Yale University Press, New Haven, USA.
Stotz, D. F.; Fitzpatrick, J. W.; Parker, T. A.; Moskovits, D. K. 1996. Neotropical birds: ecology and conservation. University of Chicago Press, Chicago.

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

Synonym(s)
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)

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.

Distribution and 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 (Castellanos and Rodríguez-Estrella 1993, Wehtje et al. 1993, Rodríguez-Estrella et al. 1996) and c.350 individuals in 1993-1994, with the highest densities in the sheep-free dwarf forests of Cerro Evermann (Martínez-Gómez and Curry 1996). Of 170 birds ringed in 1994, 56% were subadults (Martínez-Gómez and Curry 1996), suggesting that productivity is high and the population would be capable of increasing if habitat quality improves across the island (J. E. Martínez-Gómez in litt. 2007).

Population justification
Martinez-Gomez & Curry 1996 calculated a total population of 353 (287-419) individuals based upon comprehensive ringing data. Visits by the author to the same sites during 2006 & 2007 reported a similar population. The estimate is best applied to the area of the island where ringing took place. This implies that the total population of the island may be larger (J. E. Martinez-Gomez in litt. 2007). This roughly equates to 190-280 mature individuals.

Trend justification
Largely owing to ongoing control of sheep on the island, the population appears to have stabilised; however productivity may be more severely impacted by the locust swarm in some years.

Ecology
It occurs principally in moist dwarf forest and ravines with a mixture of shrubs and trees at elevations above 600 m (Martínez-Gómez and Curry 1996). 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 (Martínez-Gómez and Curry 1996, Martínez-Gómez et al. 2001). 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 (Martínez-Gómez and Curry 1996, Martínez-Gómez et al. 2001), but is common within fig Ficus cotinifolia patches in the north-west of the island (J. E. Martínez-Gómez in litt. 2007). Fig groves may act as regeneration nuclei for the species, supporting birds when a suitable understorey is present (J. E. Martínez-Gómez in litt. 2007). Nesting may occur from November-July with a peak in March-April (Martínez-Gómez and Curry 1995). Three eggs are laid and the incubation period is no more than 15 days (Martínez-Gómez and Curry 1995). Food includes crab remains, small invertebrates and fruit, particularly those of Ilex socorrensis and Bumelia socorrensis (Castellanos and Rodríguez-Estrella 1993, Martínez-Gómez et al. 2001).

Threats
Sheep had intensively grazed almost one third of the island by 1990 (Castellanos and Rodríguez-Estrella 1993), leaving no suitable nesting or foraging habitat in the south of the island (Martínez-Gómez and Curry 1996). Predation by feral cats was initially thought responsible for the species's decline, but cats were introduced some time after 1972 (Martínez-Gómez et al. 2001), and examinations of cat stomach contents and scats have not provided any substantive evidence (Martínez-Gómez and Curry 1996). However, they are likely to prey upon dispersing individuals that move into areas with little or no understorey (J. E. Martínez-Gómez in litt. 2007). 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 (Castellanos and Rodríguez-Estrella 1993, Martínez-Gómez and Curry 1996). 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 Mockingbird (J. E. Martínez-Gómez in litt. 2007). 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 Actions Underway
The Revillagigedo Islands were declared a Biosphere Reserve in 1994 (Rodríguez-Estrella et al. 1996). 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 (B. Tershy in litt. 1999, B. Tershy and B. Keitt in litt. 1999). Conservation Actions Proposed
Eradicate cats and sheep from Socorro (Martínez-Gómez and Curry 1996, B. Tershy in litt. 1999, B. Tershy and B. Keitt in litt. 1999). Implement a vegetation and soil restoration plan after sheep have been removed (Martínez-Gómez et al. 2001). Establish a captive-breeding population, and a research monitoring station on Socorro (Rodríguez-Estrella et al. 1996). Monitor the population, especially before and after the proposed eradications.

References
Castellanos, A.; Rodríguez-Estrella, R. 1993. Current status of the Socorro Mockingbird. Wilson Bulletin 105: 167-171.

Collar, N. J.; Butchart, S. H. M. 2013. Conservation breeding and avian diversity: chances and challenges. International Zoo Yearbook.

Collar, N. J.; Gonzaga, L. P.; Krabbe, N.; Madroño Nieto, A.; Naranjo, L. G.; Parker, T. A.; Wege, D. C. 1992. Threatened birds of the Americas: the ICBP/IUCN Red Data Book. International Council for Bird Preservation, Cambridge, U.K.

Martínez-Gómez, J. E.; Curry, R. L. 1995. First description of the nest and eggs of the Socorro Mockingbird. Wilson Bulletin 107: 551-555.

Martínez-Gómez, J. E.; Curry, R. L. 1996. The conservation of the Socorro Mockingbird Mimodes graysoni in 1993-1994. Bird Conservation International 6: 271-283.

Martínez-Gómez, J. E.; Flores-Palacios, A.; Curry, R. L. 2001. Habitat requirements of the Socorro Mockingbird Mimodes graysoni. Ibis 143: 456-467.

Martinez-Gomez, J. E. undated. The locust outbreak on Socorro Island: a devastating example of a disrupted avian trophic cascade.

Rodriguez-Estrella, R.; Leon de la Luz, J. L.; Breceda, A.; Castellanos, A.; Cancino, J.; Llinas, J. 1996. Status, density and habitat relationships of the endemic terrestrial birds of Socorro Island, Revillagigedo Islands, Mexico. Biological Conservation 76: 195-202.

Wehtje, W.; Walter, H. S.; Rodriguez E, R.; Llinas, J.; Castellanos V, A. 1993. An annotated checklist of the birds of Isla Socorro. Western Birds 24: 1-16.

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.

Click here for more information about the Alliance for Zero Extinction (AZE)

Explore HBW Alive for further information on this species

Search for photos and videos, and hear sounds of this species from the Internet Bird Collection

Text account compilers
Bird, J., Butchart, S., Capper, D., Isherwood, I., Pople, R., Sharpe, C J, Symes, A., Temple, H.

Contributors
Keitt, B., Martínez-Gómez, J., Tershy, B.

Recommended citation
BirdLife International (2014) Species factsheet: Mimus graysoni. Downloaded from http://www.birdlife.org on 30/07/2014. Recommended citation for factsheets for more than one species: BirdLife International (2014) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 30/07/2014.

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.

To provide new information to update this factsheet or to correct any errors, please email BirdLife

To contribute to discussions on the evaluation of the IUCN Red List status of Globally Threatened Birds, please visit BirdLife's Globally Threatened Bird Forums.

Additional resources for this species

ARKive species - Socorro mockingbird (Mimus graysoni) 0

Key facts
Current IUCN Red List category Critically Endangered
Family Mimidae (Mockingbirds and thrashers)
Species name author (Lawrence, 1871)
Population size 190-280 mature individuals
Population trend Stable
Distribution size (breeding/resident) 15 km2
Country endemic? Yes
Links to further information
- Additional Information on this species