Subspecies

The Thick-billed Vireo has a disjunct range, with the main populations occuring on The Bahamas, the Turks and Caicos, and The Cayman Islands, and two small populations off the coasts of Cuba and Haiti. The nominate form (Vireo crassirostris crassirostris) occurs in The Bahamas and the Turks and Caicos. There are five additional subspecies recognized by Clements et al. (2012). V. c. stalagmium is restricted to the Turks and Caicos; V. c. cubensis occurs on Cayo Paredon Grande in the Sabana-Camaguey archipelago off northern Cuba; V. c. alleni occurs only in the Cayman Islands; V. c. tortugae only occurs on Tortue Island off north-west Haiti; and V. c. approximans is restricted to Isla Providencia and Isla Santa Catalina (east of Nicaragua).

V. c. stalagmium is smaller than V. c. crassirostris in wing length, tail length and bill depth; differences are most marked in males (Buden 1985). Subspecies approximans is similar to nominate crassirostris, but the "tarsus is longer, tail more rounded, and coloration paler throughout, the bill light brown instead of dusky" (Ridgway 1904: 192). Additionally, approximans do not sing, whereas males of the rest of the crassirostris subspecies do sing.

The Bahamas individuals were previously split into two subspecies: V. c. crassirostris (western Bahamas islands), and V. c. flavescens (eastern Bahamas islands) (Ridgway 1904). This taxonomic split is no longer recognized.

Gene flow is highest among the northern Bahamas islands (Andros, Abaco, New Providence, and San Salvador), and low in the Turks and Caicos (Walker 1998). The Cayman Island populations had a divergence of 1% from the most common haplotype in the Bahamas (Walker 1998). Genetic analysis of the Turks and Caicos population shows low variation, indicating a founder effect or genetic bottleneck (Walker 1998).

Color

Individuals collected from the three Cayman Islands all had yellow underparts, and no differences were found among those three islands (Bangs 1916).

Within The Bahamas, there is some color variation from north-west to south-east. Individuals from Abaco were slightly grayer than those from New Providence, though the grayest individuals collected by Ridgway (1904) were from Andros and Inagua. However, there is variation even among individuals on the same island.

Description of type specimen of V. c. alleni (from Richmond 1917): "the under surface (except lower abdomen and crissum) tinged with maize yellow, becoming naples yellow or straw yellow on chest, sides and flanks; auricular region and sides of neck pale isabella color; under wing-coverts white, with a buffy tinge, more or less mingled with dusky on the under primary coverts. In Vireo c. crassirostris the under surface is tinged with sulphur yellow, the auricular region and sides of neck are light yellowish olive, and the under wing-coverts have a tinge of primrose yellow. The new form is slightly darker on the mantle, and the size is a little inferior to that of the Bahaman bird."

As described by Buden (1985), "specimens from Ile de Tortue are darker (more brown or buff) on the venter than are those from elsewhere in the range, though one or two (of nearly 200 examined) from the Bahamas approach this condition"; and "specimens from the Cayman Islands tend to have darker bills than do those from elsewhere in the range of the species. Cayman birds usually have a dark brown culmen that appears almost black in many individuals with dark pigment especially prominent on the proximal half of the bill; the tomial region is paler, more gray than brown or black. In Bahaman specimens, the culmen is medium brown with tan and/or reddish tones, especially toward the tip; the coloration of the tomial region is similar to that of the Cayman birds. The contrast in coloration between the mandibular tomial region and the darker parts of the gonys and rami is greater in Cayman brids than in Bahaman examples".

From Buden (1985): "Except for the measurement tail length in females, the specimens from Grand Cayman average slightly smaller in bill, wing, and tail measurements than do those from Little Cayman and/or Cayman Brac."

From Buden (1985): "The whitest birds are from the northwesternmost islands generally, whereas individuals with the greatest amount of yellow are from the central and southern Bahamas. Individuals that are relatively more yellow on the venter also are relatively more yellow-green (less olive, brownish-green, or grayish-green) on the dorsum. Distribution of these “color-phases” however, is not consistent geographically; many samples, especially among those from the more central part of the archipelago, show much variation between (in some cases including) the two extremes in coloration. Also, “yellowish” individuals are well-represented in samples from the Bimini and Berry islands in the northwestern Bahamas where white-ventered individuals predominate. Todd and Worthington (1911) cite other examples of color extremes found outside of the areas they usually occur".

Analysis of DNA sequence data from the mitochondrial control region indicates that White-eyed Vireos (Vireo griseus) have a 2.3% sequence divergence from Thick-billed Vireos, representing a divergence time of 110,000 years before present. A similar analysis based on cytochrome b suggests a sequence divergence of 1.1%, and a divergence time of 250,000-500,000 years before present (Walker 1998). Flat-billed Vireos (V. nanus) had a 11.5% divergence from White-eyed Vireos, and Mangrove Vireos (V. pallens) had a 5% divergence (Walker 1998).

Five groups of Vireo were proposed for the 12 West Indian vireos by Bond (1934), and he placed the Thick-billed Vireo and White-eyed Vireo into a single group.

The taxonomy of the (Old) Providencia Vireo has historically been unclear. It is alternately considered a subspecies (Vireo crassirostris approximans, Ridgway 1884) (Clements et al 2012) or separate full species(Vireo approximans) (IOC World Bird Names). The distribution of the Providencia Vireo is restricted to Isla Providencia and Isla Santa Catalina (Caribbean Sea) and is distinguishable from the Thick-billed Vireo vocally, as Providencia Vireos do not sing; morphologically the two species are indistinguishable, except perhaps by the bill (more compressed in the Thick-billed Vireo) (Bond 1950). The nominate Thick-billed Vireo form does not occur on these islands, and so these species/subspecies have nonoverlapping ranges.

Historically, the smaller Vireo ochraceus has also been considered conspecific (Chapman 1891). This is now recognized as a subspecies of the Mangrove Vireo (V. pallens ochraceus), which occurs in the mangroves of the Pacific coast of Guatemala and El Salvador (Clements et al 2012).