Tachornis are small swifts with a distinctly forked tail and long narrow wings. Antillean Palm-Swift is mostly black above, with a white band on the rump, and is mostly white below, with a narrow black breast band and black undertail coverts. The tail is shorter and less deeply forked than in other species of Tachornis. The sexes are similar; immatures have a similar pattern, but the plumage is duller and paler than in the adult.
Antillean Palm-Swift is very distinctive within its range. White-collared Swift (Streptoprocne zonaris) is much larger and is mostly black, with a broad white collar across the breast and nape. Black Swift (Cypseloides niger) and Chimney Swift (Chaetura pelagica) also are larger than the palm-swift, and are entirely dark.
The following desciption is based on Ridgway (1911), and refers to nominate phoenicobia; see also Geographic Variation:
Adult: Crown and nape dark sooty brown. Back, scapulars, a narrow strip down the center of the rump, the uppertail coverts, and rectrices black or sooty black; sides of rump white, all but obscuring the dark median stripe. Wings sooty blackish; inner webs of primaries paler, and tips of the secondaries also narrowly margined with paler. Anterior portion of lores, suborbital region, auriculars, and sides of neck grayish brown. Chin, throat, and breast dull white. Narrow breast and and flanks dark sooty brown. Lower breast and belly dull white. Undertail coverts dark sooty brown.
Immature: Similar to adult, but white of underparts duller, especially on the belly, and brown of flanks and undertail coverts paler.
Iris: dark brown
Bill: dull black
Tarsi and toes: dusky
Bare parts color data from Ridgway (1911).
Total length: 9-10 cm (Chantler 2000), 10-11 cm (Raffaele et al. 1998), 11 cm (Garrido and Kirkconnell 2000)
Linear measurements (from Ridgway 1911):
male, phoenicobia (n = 3)
wing length: mean 101.7 mm (range 100.5-102.5 mm)
tail length: mean 39.3 mm (range 39-40 mm)
bill length (exposed culmen): mean 4.3 mm (range 4-4.5 mm)
tarsus length: mean 7.2 mm (range 7-7.5 mm)
female, phoenicobia (n = 5)
wing length: mean 100.7 mm (range 97-102 mm)
tail length: mean 41.5 mm (range 38.5-44.5 mm)
bill length (exposed culmen): mean 4.4 mm (range 4-4.5 mm)
tarsus length: 7 mm
Mass: mean 10.2 g ± 0.7 g (range 9-11 g, n = 12, sex not determined; Arendt et al. 2004)