Nightjars are cryptically colored, with plumage that primarily is brown; nightjars also have a short bill with a very broad gape, and short tarsi. White-tailed Nightjar is a fairly typical nightjar in appearance, with streaked, grayish brown upperparts, and a rufous buff collar across the nape. The wing coverts are grayish brown, spotted with white and buff. At rest, the male appears to have a thin white line across the forewing. The supercilium and malar are white or buff. The male has a stark white throat, a buffy breast, and largely white underparts. The male has extensive white in the tail, and a bold white patch across the four outermost primaries, both of which are conspicuous in flight. Females are less boldly patterned and are buffier overall, with a light brown wing patch in flight, and brown and black banded rectrices.
White-tailed Nightjar overlaps broadly with Common Pauraque (Nyctidromus albicollis), which also has extensive white in the tail (male) and a white (male) or female (buff) wing band. Pauraque is larger and grayer, however; pauraque also has chestnut auriculars, and lacks the rufous nuchal collar of White-tailed Nightjar. Female White-tailed Nightjar may be confused with females of Todd’s Nightjar (Setopagis heterura) or of Spot-tailed Nightjar (Hydropsalis maculicaudus). Female Todd's Nightjar is browner, with a less distinct collar, and no supercilium, which is browner, with a less distinct collar, and no supercilium, and has a more prominent white throat. Female Spot-tailed Nightjar has a more pronounced supercilium, a darker crown, and no pale band on the wings.
The following description is based on Cleere (1998), and refers to nominate cayennensis; see also Geographic Variation:
Adult male: Forecrown, crown, and nape grayish brown, speckled with brown and streaked with blackish brown; central feathers broadly streaked with blackish brown, and edges with tawny. Broad tawny buff collar across rear of neck. Back, rump, and uppertail coverts grayish brown, and streaked with blackish brown. The outermost rectrices are the longest, giving appearance of slightly forked tail. Tail pattern is highly variable, but generally the outermost pair of rectrices (R5) are white, with one dark brown bar across the inner web, about half way out the feather. The next three pairs (R4-R2) are white, broadly tipped with brown, with the outer webs edged with brown, and with one dark brown bar across the inner web, about half way out the feather. The central pair of rectrices (R1) are grayish brown, and speckled and barred with blackish brown. The scapulars are blackish brown, and broadly edged with buff or buffy white on the outer webs. Wing coverts grayish brown, speckled and heavily spotted with tawny, buff, and pale buff; upper median coverts broadly tipped with white, and occasionally tinged with buffy, forming a narrow white line across the closed wing. The primaries are blackish brown. The four outermost primaries (P10-P7) have a narrow but well defined white band, about halfway out the length of the feather; on P9-P7 there also is a small white spot distal to the white band. There is a small white spot on on the inner web of P6. The three innermost primaries (P3-P1) are tipped with white. The secondaries are dark brown with broad white tips, and with two white spots on the inner web. The tertials are grayish brown, speckled with brown and streaked with blackish brown along the shafts, and with a buff spot at the tip of the outer web. Supercilium and submoustachial stripe white or buffy. Chin and throat white, and often tinged with buffy. Breast buff tinged with cinnamon, and barred with brown and boldly spotted with white. Belly and flanks white, washed with pale buff. Undertail coverts white or very pale buff. Underwing coverts white, barred with brown.
Adult female: Similar to the male, but darker overall, with a buffy throat; brownish breast; and buff underparts, barred iwth brown. Lacks white on the wings; the outermost primaries are brown, heavily spotted with tawny or buff. Central pair of rectrices as in the male; outer rectrices brown with paler tips, barred with dark brown and with buff or tawny.
Juvenile and immature: Undescribed.
Iris: dark brown, brown
Bill: black, dusky brown
Tarsi and toes: grayish, blackish brown, flesh
Bare parts color data from Friedmann and Smith (1950), Haverschmidt (1968), Schmitt and Schmitt (1990), and from specimens in the Cornell University Museum of Vertebrates.
Total length: 20-22.5 cm (Cleere 1998), 21 cm (Stiles and Skutch 1989), 22 cm (Hilty and Brown 1989, Hilty 2003)
Linear measurements (from Wetmore 1968; albicauda):
male (n = 7)
wing length: mean 137.8 mm (range 134.0-142.9 mm)
tail length: mean 110.9 mm (range 106.5-116.5 mm)
tarsus length: mean 17.5 mm (range 15.5-19.3 mm)
female (n = 7)
wing length: mean 135.4 mm (range 129.3-138.5 mm)
tail length: mean 100.7 mm (range 96.7-108.5 mm)
tarsus length: mean 17.1 mm (range 16.0-18.9 mm)
Mass: male, range 33-40 g (n = 16, Haverschmidt and Mees 1994); female, range 32-38 g (n = 17, Haverschmidt and Mees 1994)