Justification of Red List Category
This species is listed as Near Threatened because it has an extremely small range on one island, but it remains very common. Ongoing efforts to eradicate invasive species within the range are expected to show a positive effect on the population trend; should these efforts however stop or fail, the species could quickly be driven to CR or EX in the near future. Likewise, any evidence of a decline in the population would qualify it for a higher threat category.
Partners in Flight (2019) estimated the population to number fewer than 50,000 mature individuals, thus it is placed in the band 20,000-49,999 mature individuals here.
The population trend is difficult to determine because of uncertainty over the impacts of habitat modification on population sizes. The population may now be increasing in southern Socorro following sheep and cat eradication efforts (J. E. Martínez-Gómez in litt. 2016); however, further evidence is required to confirm this increasing trend.
This species inhabits arid lowland scrub and semi-deciduous forest on Isla Socorro (c.150 km2) in the Revillagigedo Islands, Mexico (Howell and Webb 1995). It was described as the second most common landbird on the island in 1991 (Santaella and Sada 1991) and remains very common at all elevations (J. E. Martínez-Gómez in litt. 1998).
This species inhabits woodland and brushy hillsides (del Hoyo et al. 2005). It is more common near forest patches, individual trees, dead trunks and occasionally bushes and is now observed near human settlements (J. E. Martínez-Gómez in litt. 2016). Its diet is unknown, but it has been observed foraging low in vegetation, on the ground, and on tree trunks. Families with fledged young have been observed in April, suggesting early breeding. One nest has been found in the knothole of a decayed tree (del Hoyo et al. 2005).
The proliferation of sheep on the island since the 19th century led to deforestation of large parts of the southern lowlands, where the species is no longer common (J. E. Martínez-Gómez in litt. 1998). The effect of intensive grazing was a considerable threat to its already very small range and removed over 30% of the island's suitable habitat (Ortíz-Alcaraz et al. 2017). However now that sheep have been eradicated from Socorro (Ortíz-Alcaraz et al. 2016), surveys will be required to assess whether the population is able to recover and expand. Feral cats also pose a threat however eradication efforts are underway and were predicted to be completed in early 2017 (J. E. Martínez-Gómez in litt. 2016; Ortíz-Alcaraz et al. 2017). There is no direct evidence that it is preyed upon by Socorro Mockingbird Mimodes graysoni (J. E. Martínez-Gómez in litt. 1998 contra Parkes 1990).
Conservation Actions Underway
A programme to eradicate feral sheep from Socorro was conducted from 2009 to 2012 (Ortíz-Alcaraz et al. 2016). Cat eradication efforts are ongoing (J.E. Martínez-Gómez in litt. 2016). 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).
Text account compilers
Ashpole, J, Capper, D., Harding, M., Isherwood, I., Keitt, B., Martínez-Gómez, J.E., O'Brien, A., Sharpe, C.J., Taylor, J. & Tershy, B.
BirdLife International (2021) Species factsheet: Troglodytes sissonii. Downloaded from http://www.birdlife.org on 15/01/2021. Recommended citation for factsheets for more than one species: BirdLife International (2021) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 15/01/2021.