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The Caribbean pine (Pinus caribea) forests of Grand Bahama
support a rich assemblage of birds, including several endemic
species and subspecies. One of the most enigmatic is the
Brown-headed Nuthatch (Sitta pusilla), which, unlike any of the
other avian inhabitants of the pine forests, is found only on
Grand Bahama and is absent from the pine forests on the
neighboring islands of Andros and Abaco. The form that
inhabits Grand Bahama was originally described as a subspecies
(S. p. insularis) by pioneering ornithologist James Bond (Bond
1931, Proc. Acad. Nat. Sci. Philadelphia 83: 389), on the basis of
morphological measurements obtained from two individuals.
Subsequent studies have both challenged this distinction and
sought to amplify it: Smith and Smith (1994, Bahamas Journal
of Science 1:22-26) suggested that additional specimens
revealed that Bond’s analysis overestimated the degree of
difference between insularis and the nominate subspecies (S. p.
pusilla) from the United States, whereas Hayes et al. (2004,
Bahamas Journal of Science 12:21-28) presented new
morphological data and argued that insularis should be
considered a distinct species.
support a rich assemblage of birds, including several endemic
species and subspecies. One of the most enigmatic is the
Brown-headed Nuthatch (Sitta pusilla), which, unlike any of the
other avian inhabitants of the pine forests, is found only on
Grand Bahama and is absent from the pine forests on the
neighboring islands of Andros and Abaco. The form that
inhabits Grand Bahama was originally described as a subspecies
(S. p. insularis) by pioneering ornithologist James Bond (Bond
1931, Proc. Acad. Nat. Sci. Philadelphia 83: 389), on the basis of
morphological measurements obtained from two individuals.
Subsequent studies have both challenged this distinction and
sought to amplify it: Smith and Smith (1994, Bahamas Journal
of Science 1:22-26) suggested that additional specimens
revealed that Bond’s analysis overestimated the degree of
difference between insularis and the nominate subspecies (S. p.
pusilla) from the United States, whereas Hayes et al. (2004,
Bahamas Journal of Science 12:21-28) presented new
morphological data and argued that insularis should be
considered a distinct species.
Our research, up-to-date results of which can be viewed in full in our report to the National Geographic Society, in an
upcoming paper in The Journal of Caribbean Ornithology, and in the abstract of our presentation at the
2010 Joint Meeting of the American Ornithologists' Union, Cooper Ornithological Society, and the Society
of Canadian Ornithologists, aims at resolving the taxonomy of Brown-headed Nuthatches on Grand
Bahama and estimating the distribution and size of the remaining population. Using two genetic markers
from mitochondrial DNA (mtDNA), we estimated that the average level of sequence divergence between
individuals collected on Grand Bahama and in the United States was 1.37%. This level of divergence,
although somewhat less than is typically seen between sister species, suggests that insularis and the
nominate subspecies diverged from a common ancestor approximately 685,000 years ago. Given the
geologic history of the northern Bahamas, including at least two periods in the last 100,000 years during
which the pine islands were completely inundated by rising sea level, insularis almost certainly colonized
Grand Bahama after it had begun diverging from the common ancestor that it shared with S. p. pusilla.
This novel finding challenges conventional wisdom about the evolution of Grand Bahama’s pineland
avifauna.
Furthermore, our results show clearly that Brown-headed Nuthatches on Grand Bahama are a genetically
distinct evolutionary lineage and, at the very least, constitute a unique subspecies. As with previous
studies, we also found that Brown-headed Nuthatches have become exceedingly rare and localized on
Grand Bahama. We detected only two individuals during line-transect surveys of the entire island,
although we found 21 additional individuals (14 adults and 7 juveniles) using ad hoc survey methods that
involved broadcasting recorded vocalizations of Brown-headed Nuthatches. Because we had to rely on an
ad hoc approach, we were unable to generate rigorous estimates of population size. However, our results
were comparable to the findings of previous surveys that estimated less than 2,000 individuals remained.
All of the individuals detected were found in Lucaya Estates (the same was true of the surveys reported by
Hayes et al. 2004), a subdivision in the center of the island that has yet to be built-out. Because it
appears to support the only remaining population of insularis, this area is vital to the persistence of this
unique piece of the Bahamas natural heritage. Residential development of Lucaya Estates will almost
certainly lead to the extinction of insularis. Thus, we recommend the protection of these pine forests as
the first and most important step in the development of a more comprehensive strategy for the
conservation of Brown-headed Nuthatches on Grand Bahama.
upcoming paper in The Journal of Caribbean Ornithology, and in the abstract of our presentation at the
2010 Joint Meeting of the American Ornithologists' Union, Cooper Ornithological Society, and the Society
of Canadian Ornithologists, aims at resolving the taxonomy of Brown-headed Nuthatches on Grand
Bahama and estimating the distribution and size of the remaining population. Using two genetic markers
from mitochondrial DNA (mtDNA), we estimated that the average level of sequence divergence between
individuals collected on Grand Bahama and in the United States was 1.37%. This level of divergence,
although somewhat less than is typically seen between sister species, suggests that insularis and the
nominate subspecies diverged from a common ancestor approximately 685,000 years ago. Given the
geologic history of the northern Bahamas, including at least two periods in the last 100,000 years during
which the pine islands were completely inundated by rising sea level, insularis almost certainly colonized
Grand Bahama after it had begun diverging from the common ancestor that it shared with S. p. pusilla.
This novel finding challenges conventional wisdom about the evolution of Grand Bahama’s pineland
avifauna.
Furthermore, our results show clearly that Brown-headed Nuthatches on Grand Bahama are a genetically
distinct evolutionary lineage and, at the very least, constitute a unique subspecies. As with previous
studies, we also found that Brown-headed Nuthatches have become exceedingly rare and localized on
Grand Bahama. We detected only two individuals during line-transect surveys of the entire island,
although we found 21 additional individuals (14 adults and 7 juveniles) using ad hoc survey methods that
involved broadcasting recorded vocalizations of Brown-headed Nuthatches. Because we had to rely on an
ad hoc approach, we were unable to generate rigorous estimates of population size. However, our results
were comparable to the findings of previous surveys that estimated less than 2,000 individuals remained.
All of the individuals detected were found in Lucaya Estates (the same was true of the surveys reported by
Hayes et al. 2004), a subdivision in the center of the island that has yet to be built-out. Because it
appears to support the only remaining population of insularis, this area is vital to the persistence of this
unique piece of the Bahamas natural heritage. Residential development of Lucaya Estates will almost
certainly lead to the extinction of insularis. Thus, we recommend the protection of these pine forests as
the first and most important step in the development of a more comprehensive strategy for the
conservation of Brown-headed Nuthatches on Grand Bahama.
| “A genetically distinct evolutionary lineage”: Grand Bahama is home to a unique subspecies of Brown-headed Nuthatch (Sitta pusilla insularis), which new data show may have split from its more common American cousin 685,000 years ago. This individual was captured in 2007, along with its mate, at a nest on the island. |
As the taxonomy of the Brown-headed Nuthatch was argued, its numbers on Grand Bahama appeared to
plummet. Common as recently as the late 1960s (Emlen 1977, Ornithological Monographs 24:1-129),
surveys in 1993 and 2004 yielded only a handful of individuals in a single tract of forest (Smith and Smith
1994, Hayes et al. 2004). The best available evidence therefore suggested that Brown-headed Nuthatches
on Grand Bahama were in a perilous situation and at risk of extinction, but precise estimates of population
size were lacking. At the same time, due to taxonomic uncertainties, it remained unclear what the loss of
nuthatches from Grand Bahama would represent. Were they simply a peripheral population of a common
North American species or, as others argued, a distinct species, new to science?