CR
Socorro Mockingbird Mimus graysoni



Justification

Justification of Red List Category
Past intensive sheep-grazing and ongoing persistent locust swarms have dramatically reduced and degraded the available habitat for this species. Combined with cat predation, which effectively removes mockingbirds from areas with little or no understorey, declines in its extremely small range are considered likely. Sheep eradication appears to have led to the population trend stabilising, but despite this and ongoing cat eradication, the species has not yet expanded its range. Therefore, it is listed as Critically Endangered.

Population justification
Martínez-Gómez and Curry (1996) calculated a total population of 353 (287-419) individuals based upon comprehensive ringing data. Visits 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 locust swarms in some years. The species has yet to expand its range following sheep eradication and ongoing feral cat eradication efforts (J. E. Martínez-Gómez in litt. 2016). Juveniles have moved into disturbed areas in the south-east of Socorro Island, but failed to successfully establish breeding territories. Adult males have occasionally established in these areas, but without adult females (J. E. Martínez-Gómez in litt. 2016).

Distribution and population

This species 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). However, despite sheep eradication on Socorro (Ortíz-Alcaraz et al. 2016) and ongoing feral cat eradication efforts, the species has so far failed to expand its range (J.E. Martínez-Gómez in litt. 2016).

Ecology

It occurs principally in moist dwarf forest and ravines with a mixture of shrubs and trees at 300-950 m asl (Martínez-Gómez and Curry 1996). Vegetation in these areas is dominated by the trees Ilex socorrensis, Guettarda insularis and Oreopanax xalapensis, as well as the understorey species Triumfetta socorrensis and Eupatorium pacificum (Martínez-Gómez and Curry 1996, Martínez-Gómez et al. 2001). It is absent from areas of Croton masonii scrub 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). Feral sheep however, were completely eradicated by 2012, allowing for a rapid recovery of the local vegetation cover (Ortíz-Alcaraz et al. 2017a). Predation by feral cats was initially thought responsible for the species's decline, but cats were only 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, Ortíz-Alcaraz et al. 2017b). However, cats are likely to prey upon dispersing individuals that move into areas with little or no understorey (J. E. Martínez-Gómez in litt. 2007). Cat eradication efforts were intended to be completed by early 2017 (Ortíz-Alcaraz et al. 2017a), but this work has the potential to indirectly threaten the species via construction of tracks through dense understorey vegetation (J.E. Martínez-Gómez in litt. 2016). 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 urban developments on Socorro, including the 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

Conservation Actions Underway
The Revillagigedo Islands were declared a Biosphere Reserve in 1994 (Rodríguez-Estrella et al. 1996). There is an ongoing control programme for invasive species in the region, with the Mexican navy effectively having reduced the sheep population to c. 300 heads (B. Tershy and B. Keitt in litt. 1999). Additionally, a programme to eradicate feral sheep from Socorro was conducted from 2009 to 2012 (Ortíz-Alcaraz et al. 2016), and cat eradication efforts are ongoing (J. E. Martínez-Gómez in litt. 2016, Ortíz-Alcaraz et al. 2017a). The latter has seen some positive trends for overall bird abundance on Socorro Island (Ortíz-Alcaraz et al. 2017a). The species is included on the watch list of the State of North America's Birds as a species of high conservation concern (NABCI 2016).

Conservation Actions Proposed
Continue the eradication of cats and sheep from Socorro (Martínez-Gómez and Curry 1996, B. Tershy and B. Keitt in litt. 1999). Ensure that infrastructure development for the cat eradication efforts do not negatively impact the species (J. E. Martínez-Gómez in litt. 2016). Implement a vegetation and soil restoration plan after sheep have been completely removed (Martínez-Gómez et al. 2001). Establish a research monitoring station on Socorro (Rodríguez-Estrella et al. 1996). Monitor the population, especially before and after eradication efforts. Consider establishing a captive breeding programme (Collar and Butchart 2013).

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.

Acknowledgements

Text account compilers
Everest, J.

Contributors
Ashpole, J, Bird, J., Butchart, S., Capper, D., Isherwood, I., Keitt, B., Martínez-Gómez, J.E., Pople, R., Sharpe, C.J., Symes, A., Temple, H., Tershy, B. & Westrip, J.R.S.


Recommended citation
BirdLife International (2023) Species factsheet: Mimus graysoni. Downloaded from http://www.birdlife.org on 03/02/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 03/02/2023.