Archived 2019 topic: Bicol Ground-warbler (Robsonius sorsogonensis): request for information.

BirdLife species factsheet for Bicol Ground-warbler:

Bicol Ground-warbler (Robsonius sorsogonensis) occurs in central and southern Luzon and Catanduanes Island, Philippines, although it is known from more than 10 locations now (P. Hosner in litt. 2012). It appears to inhabit broadleaf evergreen forest, also occurring in secondary growth, at forest edges and around limestone rocky areas (see Collar et al. 2018). The predominant threat to the species is habitat loss due to agricultural expansion and logging.

The species has been considered Near Threatened under Criterion B1ab(ii,iii,v) (see BirdLife International 2018a). However, this is no longer tenable because this was based on an Extent of Occurrence (EOO) value calculated as the ‘area of mapped range’. This is no longer appropriate and the EOO should be calculated using a Minimum Convex Polygon (see IUCN 2001, 2012, Joppa et al. 2016), as EOO is a measure of the spatial spread of areas occupied by a species, not the actual area it occupies. The resulting EOO value now exceeds the thresholds required to maintain the species’s current listing, and as such it potentially cannot retain its current Red List status. Therefore, we have fully reviewed the species here against all criteria.

The initial topic on this analysis can be found here.

Criterion A – The population trend for this species has not been directly estimated. However, deforestation data from between 2000 and 2012 (Tracewski et al. 2016), suggests that the area of suitable habitat for the species on average is declining by c.3.2% over three generations (16.5 years). Therefore, while it may be possible to consider the species to be in decline, the rate of decline is likely to be slow and would not approach the threshold for Vulnerable under this criterion. Thus, Bicol Ground-warbler is assessed as Least Concern under Criterion A.

Criterion B – The newly calculated Extent of Occurrence, using a Minimum Convex Polygon, is 47,000 km2. Tracewski et al. (2016) estimated the maximum Area of Occupancy (calculated as the remaining tree area within the species’s range) to be 5,228 km2. Both of these values are large enough that they do not approach the threshold for Vulnerable under this criterion. Thus, Bicol Ground-warbler is assessed as Least Concern under Criterion B.

Criterion C – Given the current information, it is very difficult to assess Bicol Ground-warbler against this criterion. At the moment, the population size for the species has not been quantified. It has been described as uncommon, but given that it has very secretive habits, it could be more common than suggested (Collar et al. 2018). It is plausible that the species could approach or even meet the threshold for Vulnerable under this criterion (e.g. the population size for the closedly related Cordillera Ground-warbler Robsonius rabori is tentatively placed in the range 2,500-9,999 mature individuals [BirdLife International 2018b]), but the lack of information makes it very difficult to outright give a population size estimate.

Even if it meets this threshold, it would still need to trigger other conditions to warrant listing under Criterion C. Without a clear direct estimate of the rate of decline, Criterion C1 cannot be used. The species is not reported to undergo extreme fluctuations, so Criterion C2b cannot be used either. The species also does not appear to occur in just one subpopulation given its current mapped range (see here), and so Criterion C2a(ii) would not be triggered. That leaves Criterion C2a(i), which requires no subpopulation to contain >1,000 mature individuals. Again, given the lack of any quantified population estimates, or density estimates, it is not possible to ascertain whether this is the case. Further information is therefore sought to better assess the species against this criterion.

Criterion D – The species’s range is too large to warrant listing under this criterion and, despite the high level of uncertainty regarding the population size, it is suspected not to approach the threshold for Vulnerable under this criterion. Thus, Bicol Ground-warbler is assessed as Least Concern under Criterion D.

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

Therefore, while the species cannot retain its current listing, further information regarding population and subpopulation size estimates are required to allow for a clearer assessment of its status. Given the tentative population size estimate that is currently used for Cordillera Ground-warbler (R. rabori), though,it is likely that the Red List category for Bicol Ground-warbler should fall in the range of Least Concern to Vulnerable. We therefore request any comments regarding this, butplease note 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 relevant to the species’ Red List status and the information requested.

An information booklet on the Red List Categories and Criteria can be downloaded here and the Red List Criteria Summary Sheet can be downloaded here. Detailed guidance on IUCN Red List terms and definitions and the application of the Red List Categories and Criteria can be downloaded here.


BirdLife International. 2018a. Species factsheet: Robsonius sorsogonensis. Downloaded from on 27/09/2018.

BirdLife International. 2018b. Species factsheet: Robsonius rabori. Downloaded from on 27/09/2018.

Collar, N.; Robson, C.; Sharpe, C. J. 2018. Bicol Ground-warbler (Robsonius sorsogonensis). 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 on 27 September 2018).

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

IUCN. 2012. IUCN Red List Categories and Criteria: Version 3.1. Second edition. IUCN Species Survival Commission. IUCN, Gland, Switzerland and Cambridge, U.K. Available at:

Joppa, L. N.; Butchart, S. H. M.; Hoffmann, M.; Bachman, S. P.; Akçakaya, H. R.; Moat, J. F.; Böhm, M.; Holland, R. A.; Newton, A.; Polidoro, B.; Hughes, A. 2016. Impact of alternative metrics on estimates of extent of occurrence for extinction risk assessment. Conservation Biology 30: 362-370. Tracewski, Ł.; Butchart, S. H. M.; Di Marco, M.; Ficetola, G. F.; Rondinini, C.; Symes, A.; Wheatley, H.; Beresford, A. E.; Buchanan, G. M. 2016. Toward quantification of the impact of 21st-century deforestation on the extinction risk of terrestrial vertebrates. Conservation Biology 30: 1070-1079.

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1 Response to Archived 2019 topic: Bicol Ground-warbler (Robsonius sorsogonensis): request for information.

  1. Rob Martin (BirdLife International) says:

    Preliminary proposal

    Based on available information, our preliminary proposal for the 2019 Red List would be to list Bicol Ground-Warbler as Near Threatened as the population is suspected to approach the thresholds for listing under Criterion C2a(i).

    There is now a period for further comments until the final deadline in mid-July, after which the recommended categorisations will be put forward to IUCN.

    Please note that we will then only post final recommended categorisations on forum discussions where these differ from those in the initial proposal.

    The final 2019 Red List categories will be published on the BirdLife and IUCN websites in December, following further checking of information relevant to the assessments by both BirdLife and IUCN.

Comments are closed.