“St. Lucia Thrasher”

Frontiers of Taxonomy: Two Species of White-breasted Thrasher.

The Martinique and St. Lucia forms of White-breasted Thrasher are very similar in appearance, occupy similar niches on adjacent islands, and have always been classified as a single species.  They share approximately the same shape, color pattern, and habitat.  Nevertheless, there are consistent differences between them, and a growing body of evidence suggests that they are best regarded as separate species.

The “St. Lucia Thrasher” is about 20% larger overall and has more cleanly contrasting bicolored chocolate-brown-and-white plumage, whereas the “Martinique Thrasher” is paler and grayer above and less immaculate below.  Its crown, back, wing coverts have whitish fringes that give it a subtly frosted appearance, and the border between brown and white on its sides tends to show more of a ragged, spotty, gradual color transition—however, both forms could be described this way, so the difference is a matter of degree.  In strong light “St. Lucia” appears warm-brown above, whereas “Martinique” appears mostly cold, grayish-brown with a warmer tone on the wings.

Behaviorally, “Martinique Thrasher” appears to be more purely terrestrial than “St. Lucia.”  Though they are not fully documented, vocalizations of the two forms also appear to differ.  Finally, Miller et al. (2019) have stated that their as-yet unpublished genetic studies indicate that the reproductive isolation of the two forms is older than the divergence of other lineages among the Lesser Antillean mimids.