Sao Tome Thrush (Turdus olivaceofuscus): revise global status?

Sao Tome Thrush (Turdus olivaceofuscus) is endemic to the island of São Tomé, São Tomé and Príncipe. Favouring the lowlands, it inhabits predominantly forest habitats, mainly primary and secondary forest, but will also occur in shaded plantations, orchards, dry woodland in savanna, cloud forest and non-forest habitats like gardens (Atkinson et al. 1991, Christy and Clarke 1998, Dallimer et al. 2010, de Oliveira Soares 2017, Collar 2018). It does, however, appear to be more abundant in forest compared to degraded habitat like plantations (de Lima et al. 2014). Thus, given suspected ongoing habitat loss and degradation, as well as predation by introduced species and persecution by humans the species is currently considered to be undergoing a continuing decline (del Hoyo et al. 2005, R. F. de Lima in litt. 2010). With its restricted range and small population size, the species is therefore currently listed as Near Threatened (see BirdLife International 2018).

Conversely though, it has been suggested that the species may in fact be stable (R. F. de Lima in litt. 2018) and, as it is relatively frequent throughout the island, it may warrant downlisting (R. F. de Lima in litt. 2018). Therefore, in light of this, the species has been reassessed here against all criteria.

 

Criterion A – Even if the species is considered to be declining (see above) it is very unlikely that the species would approach the threshold for Vulnerable under this criterion (population size reduction of 30% over 3 generations or 10 years).

 

Criterion B – While the species has a restricted range (Extent of Occurrence = 930km2) and at least the area/quality of habitat may be declining, it is not thought to undergo extreme fluctuations and it is not severely fragmented. Additionally, as the main threat to the species is habitat loss/degradation, the number of locations* where the species is found is likely to be >>10. Therefore, the species likely does not warrant listing under this criterion.

 

Criterion C – The population size has not been directly estimated, but it is frequently observed (found at 1,111 out of 3,056 point count observations [de Oliveira Soares 2017]), and it is thought to number more than 2,500 individuals (R. F. de Lima in litt. 2010). Thus it is currently placed in the range 2,500-9,999 mature individuals. This is sufficiently low to meet the threshold population size estimate for listing as Vulnerable, but if the species were thought to be stable as per R. F. de Lima (in litt. 2018) then the species would not meet the other required conditions for listing under this criterion.

However, given that it is thought that there are ongoing threats to the species’s habitat, and it is less abundant in more degraded habitats compared to forest (de Lima et al. 2014) it may be more appropriate to suspect that the species is undergoing a continuing decline. The uncertainty over this though means that, at the moment it would likely not warrant listing at a higher level of threat than Near Threatened under criterion C2a(ii). We therefore request further information regarding the population trend.

 

Criterion D – The population size and range of the species are too large for the species to warrant listing under this criterion.

 

Criterion E – To the best of our knowledge no quantitative analysis of extinction risk has been conducted for this species. Therefore, it cannot be assessed against this criterion.

 

Overall, therefore, Sao Tome Thrush could be downlisted to Least Concern. However, if the threat from habitat loss and or degradation is suspected to potentially be causing a slow population decline it may be more appropriate to retain the species as Near Threatened under criterion C2a(ii), given the uncertainty. Comment and further information are welcome, but please note though that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of the species’ Red List status. Therefore, please make sure your comments are about the proposed listing.

 

 

*Note that the term ‘location’ defines a geographically or ecologically distinct area in which a single threatening event can rapidly affect all individuals of the taxon present. The size of the location depends on the area covered by the threatening event and may include part of one or many subpopulations. Where a taxon is affected by more than one threatening event, location should be defined by considering the most serious plausible threat (IUCN 2001, 2012).

 

References

Atkinson, P.; Peet, N.; Alexander, J. 1991. The status and conservation of the endemic bird species of Sao Tomé and Príncipe, West Africa. Bird Conservation International 1: 255-282.

BirdLife International. 2018. Species factsheet: Turdus olivaceofuscus. Downloaded from http://www.birdlife.org on 01/03/2018.

Christy, P.; Clarke, W. V. 1998. Guide des Oiseaux de Sao Tome et Principe. ECOFAC, Sao Tome.

Collar, N. 2018. Sao Tome Thrush (Turdus olivaceofuscus). In: del Hoyo, J., Elliott, A., Sargatal, J., Christie, D.A. & de Juana, E. (eds.). Handbook of the Birds of the World Alive. Lynx Edicions, Barcelona. (retrieved from https://www.hbw.com/node/58286 on 28 February 2018).

Dallimer, M.; Melo, M.; Collar, N. J.; Jones, P. J. 2010. The Príncipe Thrush Turdus xanthorhynchus: a newly split, ‘Critically Endangered’, forest flagship species. Bird Conservation International 20(4): 375-381.

del Hoyo, J.; Elliott, A.; Christie, D. 2005. Handbook of the Birds of the World, vol. 10: Cuckoo-shrikes to Thrushes. Barcelona, Spain: Lynx Edicions.

De Lima, R. F.; Viegas, L.; Solé, N.; Soares, E.; Dallimer, M.; Atkinson, P. W.; Barlow, J. 2014. Can management improve the value of shade plantations for the endemic species of São Tomé Island? Biotropica 46: 238-247.

De Oliveira Soares, F. M. C. 2017. Modelling the distribution of São Tomé bird species: Ecological determinants and conservation prioritization. Masters Thesis, Universidade de Lisboa.

IUCN. 2001. IUCN Red List Categories and Criteria: Version 3.1. Gland, Switzerland and Cambridge, UK: IUCN Species Survival Commission.

IUCN. 2012. Guidelines for Application of IUCN Red List Criteria at Regional and National Levels: Version 4.0. Gland, Switzerland and Cambridge, UK: IUCN.

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