Bahama Warbler (Setophaga flavescens): revise global status?

BirdLife species factsheet for Bahama Warbler

Bahama Warbler (Setophaga flavenscens) is known from Abaco (Great Abaco and Little Abaco) and Grand Bahama islands in the northern Bahamas. It occurs exclusively in mature stands of Caribbean Pine Pinus caribaea (McKay et al. 2010), where it is patchily distributed and occurs at low densities (Lloyd and Slater 2011). The population is estimated to number fewer than 10,000 mature individuals and to be in decline due to the large-scale logging of pine forests since the mid-20th century (W. K. Hayes in litt. 2011).

The species’s restriction to Caribbean Pine forests makes it highly dependent on the fate of these forests. Overall, pine forests are among the most threatened ecosystems in the Caribbean. They are rapidly cleared for infrastructural developments, mainly for roads as well as for tourism and residential developments. Large areas of pine forest have been fragmented and their structure and composition have been altered by changing fire regimes, human activity, hurricanes and subsequent saltwater intrusion (Lloyd and Slater 2011, W. K. Hayes in litt. 2011). Caribbean Pine forests are at high risk of further development and renewed logging (McKay et al. 2010, White 2011). It has been suggested that around half of the existing suitable habitat on Grand Bahama Island may be at risk, which may also hold true for Abaco Island (J. Lloyd in litt. 2011).

Bahama Warbler is currently listed as Near Threatened on the IUCN Red List, approaching the threshold for listing as threatened under Criteria B1ab(ii,iii,v); C2a(i). However, using new information regarding the population size and the availability of suitable habitat, this species appears to warrant a change in Red List status. Therefore, we present here our reassessment against all criteria for the species.

Criterion A – The population is suspected to be in decline due to loss and fragmentation of its habitat. There is evidence that the species declined by around 30% between 1969 and 2007 on Grand Bahama (J. Lloyd in litt. 2011). The current rate of decline has not been quantified, but is assumed to be <10% over three generations (BirdLife International 2018). This number is based on estimates of forest loss within the Bahama Warbler’s range between 2000 and 2012, which has been small (2.4% over three generations; Tracewski et al. 2016). However, the population decline might accelerate in the future if development activities increase and logging were to resume, which would be detrimental to the species. However currently, Bahama Warbler may be listed as Least Concern under Criterion A.

Criterion B – Using a Minimum Convex Polygon, the Extent of Occurrence (EOO) has been calculated as 13,800 km2. This meets threshold for listing as Vulnerable under criterion B1 (EOO < 20,000 km2). Tracewski et al. (2016) estimated the remaining tree area within the species’s range, to be c.360 km2 in 2012. This number is supported by Global Forest Watch (2014), which report c.300 km2 of Caribbean Pine forest (canopy density > 50%) on Abaco and c.100 km2 on Grand Bahama for the year 2010. We can tentatively assume that for a forest-dependent species like Bahama Warbler, the area of forested habitat equals the maximum Area of Occupancy (AOO); as such the the maximum AOO meets the threshold for Endangered under criterion B2 (AOO < 500 km2). However, in order to be listed under this criterion, at least two of three further conditions have to be met.

Even though its habitat shows some degree of fragmentation on both Abaco and Grand Bahama (Global Forest Watch 2014), the species should not be considered severely fragmented per IUCN definition (see IUCN Standards and Petitions Subcommittee 2017). The species is patchily distributed and suspected to occur at 11-100 locations* (BirdLife International 2018). Assuming that the true number of locations* is closer to the lower band of the estimate, it approaches the threshold for listing as threatened (≤ 10 locations*). Bahama Warbler therefore almost meets condition (a). AOO, area, extent and/or quality of habitat as well as the number of mature individuals are declining, and hence the species meets condition (b) under subconditions (ii,iii,v). However, Bahama Warbler is not known to undergo extreme fluctuations and so does not meet condition (c).

Therefore, even though Bahama Warbler occurs in a very small range and consequently in restricted EOO and AOO, it only triggers condition (b), and hence it cannot be listed as threatened under criterion B. However, it additionally approaches the threshold for condition (a). As such, Bahama Warbler may be listed as Near Threatened under Criterion B1ab(ii,iii,v)+2ab(ii,iii,v).

Criterion C – The remaining pine forest on Abaco and Grand Bahama islands has been estimated between c.360 km2 (Tracewski et al. 2016) and c.400 km2 (Global Forest Watch 2014). The most recent information of population density for the Bahama Warbler, obtained during a survey in 2007, reports 3.6 individuals/km2 on Grand Bahama Island (Lloyd and Slater 2011). This value, however, does not account for non-territorial males and assumes a 1:1 sex ratio (J. Lloyd in litt. 2011). Nevertheless, in the absence of further information, we can tentatively assume that the population density on both islands is 3.6 individuals/km2, and because of the design of the survey they are likely to represent mature individuals. This gives an estimated population size of 1,296-1,440 mature individuals in total, which is rounded here to 1,250-1,450 mature individuals. Therefore, the population size meets the threshold for Endangered under Criterion C. However, in order to be listed under this criterion, further conditions have to be met.

The population decline of Bahama Warbler has neither been observed, estimated nor projected, so Criterion C1 cannot be applied. Instead, the rate of decline has been inferred from the rate of forest loss per Tracewski et al. (2016). This allows the species to be assessed against Criterion C2, where it has to meet one of three conditions. In order to be listed as Endangered under condition a(i), no subpopulation can hold more than 250 mature individuals. We can assume that Bahama Warbler forms two subpopulations, one on Abaco and one on Grand Bahama. Using the area of Caribbean Pine forest on the islands per Global Forest Watch (2014) and the population density value as a starting point, we can derive a maximum subpopulation size of roughly in the range of c.935-1,090 mature individuals for Abaco and a maximum within the range of c.310-360 mature individuals for Grand Bahama. The subpopulations are consequently too large to justify a listing as Endangered, even though the overall population size might suggest so. However, as both subpopulations potentially contain less than 1,000 mature individuals, precautionarily they meet the threshold for Vulnerable under condition a(i). None of the subpopulations contains more than 90% of all mature individuals, and the population size does not fluctuate extremely; Bahama Warbler hence does not meet conditions a(ii) or (b). In summary, as the population of Bahama Warbler consists of < 10,000 mature individuals, while no subpopulation holds > 1,000 mature individuals, the species may be listed as Vulnerable under Criterion C2a(i).

Criterion D – The population size of this species is estimated at 1,250-1,450 mature individuals. This does not meet the threshold for Vulnerable (< 1,000 mature individuals), but approaches it. Therefore, the species may be listed as Near Threatened under Criterion D1. AOO and number of locations are too large to warrant listing under Criterion D2.

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.

Therefore, it is proposed that Bahama Warbler (Setophaga flavescens) be listed as Vulnerable under Criterion C2a(i). We welcome any comments on this proposed listing.

Please note that this topic is not designed to be a general discussion about the ecology of the species, rather a discussion of its Red List status. Therefore, please make sure your comments are relevant to the discussion outlined in the topic.

*The term ‘location’ refers to a distinct area in which a single threatening event can rapidly affect all individuals of the taxon present, with the size of the location depending on the area covered by the threatening event. 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).

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.

References

BirdLife International. 2018. Species factsheet: Setophaga flavescens. http://www.birdlife.org (Accessed 10/12/2018).

Global Forest Watch. 2014. World Resources Institute. www.globalforestwatch.org (Accessed 10/12/2018).

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. www.iucnredlist.org/technical-documents/categories-and-criteria

IUCN Standards and Petitions Subcommittee. 2017. Guidelines for Using the IUCN Red List Categories and Criteria. Version 13. Prepared by the Standards and Petitions Subcommittee. http://www.iucnredlist.org/documents/RedListGuidelines.pdf

Lloyd, J. D.; Slater, G. L. 2011. Abundance and distribution of breeding birds in the pine forests of Grand Bahama, Bahamas. Journal of Caribbean Ornithology 24: 1-9.

McKay, B. D.; Reynolds, M. B. J.; Hayes, W. K.; Lee, D. S. 2010. Evidence for the species status of the Bahama Yellow-throated Warbler (Dendroica “dominica” flavescens). The Auk 127: 932-939.

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.

White, T. 2011. The False Kirtland’s: A cautionary tale. Birding 2011 (11): 34-39.



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1 Response to Bahama Warbler (Setophaga flavescens): revise global status?

  1. Red List Team (BirdLife International) says:

    Preliminary proposal
    Based on available information, our preliminary proposal for the 2019 Red List would be to adopt the proposed classifications outlined in the initial forum discussion.
    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.

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