This is part of a consultation on the Red List implications of extensive changes to BirdLife’s taxonomy for passerines
Lynx Edicions and BirdLife International will soon publish the second volume of the HBW-BirdLife Illustrated Checklist of the Birds of the World, building off the Handbook of the Birds of the World series, and BirdLife’s annually updated taxonomic checklist.
The new Checklist will be based on the application of criteria for recognising species limits described by Tobias et al. (2010). Full details of the specific scores and the basis of these for each new taxonomic revision will be provided in the Checklist.
Following publication, an open and transparent mechanism will be established to allow people to comment on the taxonomic revisions or suggest new ones, and provide new information of relevance in order to inform regular updates. We are also actively seeking input via a discussion topic here regarding some potential taxonomic revisions that currently lack sufficient information.
The new Checklist will form the taxonomic basis of BirdLife’s assessments of the status of the world’s birds for the IUCN Red List. The taxonomic changes that will appear in volume 2 of the checklist (for passerines) will begin to be incorporated into the 2016 Red List update, with the remainder to be incorporated into subsequent Red List updates.
Preliminary Red List assessments have been carried out for the newly split or lumped taxa. We are now requesting comments and feedback on these preliminary assessments.
Rusty-faced Babbler Robsonius rabori has been redefined and R. thompsoni recognised as a new species, following the application of criteria set out by Tobias et al. (2010).
Prior to this taxonomic change, R. rabori (BirdLife species factsheet) was listed as Vulnerable under criterion B1ab(ii,iii,v), on the basis that it had a small range in which it was found at a limited number of locations and its habitat was declining in both area and quality as a result of deforestation. However it was noted that it may be present at more localities than previously thought. The pre-split species was characterised as being very secretive, and although considered uncommon it was latterly considered to be more common than suspected following more intensive searches.
R. rabori was previously assumed to occur throughout the northern cordillera of Luzon, but the holotype actually came from the central cordillera, and those in the Sierra Madre mountains have subsequently been described as belonging to a separate species, R. thompsoni (Hosner et al. 2013). Consequently, R. rabori Cordillera Ground-warbler (as defined following the change) occurs in the lowlands of the northern Cordillera Central on Luzon, while R. thompsoni Sierra Madre Ground-warbler occurs in the northern Sierra Madre, and may also occur in the southern Cordillera Central (Hosner et al. 2013).
All of the Robsonius are considered to have similar habitat requirements, namely lowland and lower montane forest including secondary, edge and logged second-growth (Hosner et al. 2013). Forest loss has been extensive within the altitudinal range of the species and while absolute rates have declined recently loss and degradation is continuing.
R. rabori is now the most restricted of the three species of Robsonius, with a newly calculated Extent of Occurrence (EOO) of 8,500km2. To date there have only been records from three sites however it is suspected to occur throughout a larger area of forest that is very poorly studied (Hosner et al. 2013). Consequently it is likely that with habitat loss as the principle threat to the species then it can reasonably be expected to occur at more than 10 locations*. However it is suspected that the population is suffering an ongoing decline due to the continuing conversion of forest within its range. The population size and density has not been estimated for any of these species, but R. rabori is conservatively suspected to number fewer than 10,000 mature individuals, based on known records, low population density estimates for species of similar body size and assuming only a proportion of its range is occupied.
It is suggested that Cordillera Ground-warbler Robsonius rabori is listed as Vulnerable under criterion C2a(ii), on the basis that it is estimated to have a single, small population size that is suspected to be in continuing decline.
Should there be an assessment that indicates that the population is larger than this estimate, then the species may warrant listing as Near Threatened or Least Concern.
The EOO of R. thompsoni is 33,000km2, and the species has been observed at numerous sites now, hence is not considered to approach the thresholds for listing as Vulnerable under criterion B. Equally, making similar assumptions as for R. rabori the population is considered to exceed 20,000 mature individuals, hence while the population is suspected to be undergoing a slow, ongoing decline it is not considered to approach the thresholds for listing under criterion C. Information on the potential rate of decline that the population may be suffering within its range is sought. Should it be in the order of 20-30% within a 16.5 year period (three generations) then the species may be warrant listing as Near Threatened under criterion A2c + A3c + A4c. Rates of forest loss (Hansen et al. 2013) indicate that actual loss would indicate that this level of decline is unlikely to be currently occurring, especially given the stated tolerance of logged and secondary growth. Consequently it is proposed that Sierra Madre Ground-warbler R. thompsoni be listed as Least Concern, unless there is evidence of moderately rapid to rapid population decline.
Comments are invited on these proposed categories and further information would be welcomed.
*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).
Hosner, P.A., Boggess, N.C., Alviola, P., Sanchez-Gonzalez, L.A., Oliveros, C.H., Urriza, R. and Moyle, R.G. 2013. Phylogeography of the Robsonius ground-warblers (Passeriformes: Locustellidae) reveals an undescribed species from northeastern Luzon, Philippines. The Condor 115: 630-639.
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.
Tobias, J. A., Seddon, N., Spottiswoode, C. N., Pilgrim, J. D., Fishpool, L. D. C. and Collar, N. J. 2010. Quantitative criteria for species delimitation. Ibis 152: 724–746.