Nightjars are nocturnal birds with plumage that is cryptically patterned, and is dominated by shades of brown. They have long wings, short tarsi, very wide but short bills, and prominent rictal bristles. Spot-tailed Nightjar is a small species of nightjar, with generally buffy plumage and a relatively short tail. Both sexes of Spot-tailed Nightjar have a narrow rufous collar across the nape, and the primaries are entirely dark (lack a white band across the primaries, as is present in many other species of nightjar). All but the central pair of rectrices of the male are prominently tipped with white; the tips of the rectrices of the female are gray.
Spot-tailed Nightjar is similar to several other sympatric species, although most other species of nightjars primarily occur in more densely wooded habitats, and many species of nightjar also have a white or buff band across the primaries. Common Pauraque (Nyctidromus albicollis) is much larger and longer tailed, with a prominent white or buff band across the primaries, rufous sides to the face, and, in the male, long white streaks along the outer rectrices. Female Spot-tailed Nightjar might be confused with females of White-tailed Nightjar (Hydropsalis cayennensis) and Little Nightjar (Setopagis parvulus), but differs from both having a blacker crown, a more prominent supercilium that contrasts with the duskier sides of the head, and more conspicuous spotting on the breast (Hilty 2003).
The following description is based on Cleere (1998):
Adult male: Forecrown and crown blackish brown, spotted with buff or tawny. The nape is blackish brown, barred with tawny and buff. There is a buff, tawny or cinnamon rufous band across the back of the neck. Lores and auriculars tawny or rufous, densely speckled with dark brown. The supercilium is broad and buff. The back is brown, regularly but thinly barred with tawny, becoming paler on the rump and the uppertail coverts. The scapulars are blackish brown on the inner webs, and mostly buffy or cinnamon buff on the outer webs. The wing coverts are brown, heavily speckled with pale buff, with large buff or cinnamon spots on the tips of the outer webs of the coverts. Primary coverts brown, spotted with tawny. The three outermost primaries are brown, with large tawny spots along the outer two thirds of each feather; the remaining primaries are brown, regularly spotted with tawny. The secondaries are brown, broadly tipped with pale tawny and regularly spotted with tawny, these spots becoming paler and buffier on the inner secondaries. The tertials are brown, mottled and spotted with buff and grayish brown. The outermost four pairs of rectrices are dark brown, spotted or barred with tawny along the outer webs, with two or three widely spaced white spots along the inner webs, these spots becoming larger towards the central rectrices. These outer rectrices also are broadly tipped with white (the white tips 10-15 mm wide), the distal half of these white tips washed with cinnamon or pale buff, and speckled with brown. The central pair of rectrices are grayish brown, speckled and thinly barred with brown, and slightly longer than the outer rectrices. There is a thin buff submoustachial stripe, bordered with a broad, triangular black lateral throat stripe. The chin, throat and breast are cinnamon buff or buff, the feathers with brown tips. There is a distinct cinnamon rufous band, with regular thin brown barring, around the lower breast and upper belly. The belly and flanks are buff, indistinctly barred with brown. The undertail coverts are buff. The underwing coverts are buff, barred with brown.
Adult female: Similar to the male, but the tail lacks white markings; the outer four pairs of rectrices are dark brown, indistinctly barred with pale tawny or buff and tipped buffish brown and speckled with brown.
Immature and juvenile: "Similar to the adult (female?) but all primaries and secondaries narrowly tipped pale buffish".
Downy young: "Blackish above with sparse, very pale brownish-buff markings; Below, it is light dusky marked with buffy-brown, except on the belly which is plain, pale buffy-brown" (Zimmerman 1957).
Undescribed (Cleere 1998).
Bill: blackish, brownish dusky
Tarsi and toes: grayish, purplish gray
Bare parts color data from Zimmerman (1957) and Cleere (1998).
Total length: 20.3 cm (Hilty 2003), 20.5-21.5 cm (Howell and Webb 1995)
Linear measurements (from Cleere 1998):
male (n not specified, but a minimum of 5)
wing length, range 127-146 mm
tail length, range 87-114 mm
bill length, range 10.6-16.5 mm
tarsus length, range 14.8-17.2 mm
female (n not specified, but a minimum of 5)
wing length, range 122-137 mm
tail length, range 88-105 mm
bill length, range 10.6-13.1 mm
tarsus length, range 14.0-15.5 mm
Mass: male, range 28.3-35.2 g (n = ?; Cleere 1998); female, range 26.0-39.0 g (n = ?; Cleere 1998)