Initial deadline for comments: 31 January 2012.
Bahama Warbler Dendroica flavescens has been split from Yellow-throated Warbler D. dominica following an assessment of its taxonomic status, including analyses of the morphology, vocalisations and genetics of D. (dominica) flavescens and continental D. dominica (McKay et al. 2010) and a subsequent decision by the American Ornithologists’ Union (AOU 2011). D. flavescens is endemic to Grand Bahama and Abaco (Great Abaco and Little Abaco) in the Bahamas, where it appears to be restricted to mature Caribbean Pine Pinus caribaea forest (McKay et al. 2010).
There are estimated to be 900-1,000 km2 of pine forest remaining on Grand Bahama and Abaco combined, although the remaining area of mature pine forest is apparently not known. D. flavescens has been estimated to occur at densities of 3.5-17.1 individuals/km2, suggesting a total population of 3,150-17,100 individuals, although more recent density estimates suggest a population of only 3,150-3,500 individuals (McKay et al. 2010 and references therein, W. K. Hayes in litt. 2011). Surveys in the pine forests of Grand Bahama found the species to be patchily distributed and existing at low densities (Lloyd and Slater unpubl., Lloyd et al. 2008). Its population is thought to have declined since the onset of major logging of the pine forests in the mid-20th century (W. K. Hayes in litt. 2011). Some areas of pine forest, such as those located west of Freeport, have been fragmented by roads and residential development, while their structure and composition have been altered by changing fire regimes, human activity and hurricanes (Lloyd and Slater unpubl.). Caribbean Pine forests are at high risk of further development and renewed logging (McKay et al. 2010, White 2011). This situation is probably exacerbated by the presence of freshwater recharge and supply areas in the pine forests of Grand Bahama coupled with the conversion of the Bahamas to a reverse osmosis water supply, suggesting that these areas may be prone to further development (W. K. Hayes in litt. 2011).
The species’s Extent of Occurrence (EOO) is estimated at c.2,300 km2 (see range map). It is important to note that the EOO is a measure of the spread of extinction risk and not the extent of suitable habitat (ESH), and is thus far less precise compared to the ESH, although areas of sea are excluded because they are completely unsuitable. This estimate for the EOO meets the range size threshold for Endangered under criterion B1 of the IUCN Red List, and there may be continuing declines in the area, extent and/or quality of habitat, thus potentially fulfilling one of the subcriteria (B1b(iii)). However, there is no indication that the species’s population is severely fragmented or restricted to fewer than 11 locations (fewer than 6 to qualify as Endangered) (subcriterion B1a).
In terms of the IUCN criteria, a ‘location’ is defined according to the most important threat, and is regarded as a geographically or ecologically distinct area in which a threatening event can rapidly affect all individuals of the species in question. The major threat facing the species is the logging of pine forest for timber or development, a threat that would normally be expected to affect discrete portions of habitat during each event, thus the species should probably be regarded as occurring at more than 10 locations. For the purpose of Red List assessments, a species’s population is regarded as ‘severely fragmented’ when over 50% of suitable habitat exists in patches that are too small to support viable populations and separated by distances several times greater than the average long-term dispersal distance of the species. There is currently insufficient information on the level of fragmentation in the species’s habitat and population, thus further information is requested.
With a population that probably includes fewer than 10,000 mature individuals, which may be inferred to be undergoing a continuing, but unquantified decline, the species might qualify as Vulnerable under criterion C2; however, information is required on the species’s sub-population structure. For the purposes of Red List assessments, sub-populations are defined as geographically or otherwise distinct groups between which there is little or no demographic or genetic exchange, i.e. typically one successful migrant per year or less. Information is required on the level of movement between the populations on Grand Bahama and Abaco. If they are considered sub-populations according to the IUCN definition, then the proportion of the total population present in each sub-population should be estimated.
There are an estimated 600 km2 of pine forest on Grand Bahama, although the effective area of suitable habitat may be 10-15% lower owing to the effects of recent storms (W. K. Hayes in litt. 2011). With the total area of pine forest being put at 900-1,000 km2, the implication is that Grand Bahama holds 51-67% of the total population. Following surveys in April 2007, Lloyd et al. (2008) estimated a population of 2,116 individuals (95% CI: 1,239-3,614) on Grand Bahama, based on a density of 3.6 birds/km2. Even at this density, it seems unlikely that either of the two island populations number fewer than 1,000 individuals, although clarification is needed on this, and it should be noted that all population extrapolations so far do not take into account the remaining area of mature pine forest, which has apparently not been estimated. If the level of movement means that the species has one sub-population, it would probably qualify as Vulnerable under criterion C2a(ii).
Based on current information the species would probably qualify as Near Threatened on the basis that it appears to nearly meet the thresholds for Vulnerable under criteria B1a+b(iii), and probably also criterion C2a(i). However, further information is requested, particularly on the level of habitat fragmentation and the likely sub-population structure. Comments would be welcomed on the estimation of the total population size and the likely number of mature individuals. Information is also requested on the severity of threats faced by the species, with emphasis on whether the extent and/or quality of habitat is in decline and whether a continuing population decline can be inferred in the present or projected to occur in the future.
AOU (2011) Fifty-second supplement to the American Ornithologists’ Union Check-list of North American Birds. Auk 128: 600-613.
Lloyd, J. D., Slater, G. L. and Metcalfe, A. E. (2008) Taxonomy and population size of the Bahama Nuthatch. Report for the National Geographic Society Committee for Research and Exploration, Grant No. 8146-06. Available at www.ecoinst.org/files/Taxonomy_and_population_size_of_the_Bahama_nuthatch.pdf.
McKay, B. D., Reynolds, M. B. J., Hayes, W. K. and Lee, D. S. (2010) Evidence for the species status of the Bahama Yellow-throated Warbler (Dendroica “dominica” flavescens). Auk 127: 932-939.
White, T. (2011) The False Kirtland’s: A cautionary tale. Birding, November 2011: 34-39.
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