Nightjars are cryptically patterned nocturnal birds, with plumage that usually is dominated by shades of grays and browns. Nightjars also have short bills with a very broad gape, and very short tarsi. Cayenne Nightjar is a small, dark nightjar. The plumage of the female is unknown. The upperparts of the male generally are grayish brown, with broad blackish brown streaks on the crown, and an indistinct tawny collar on the hindneck. The wing coverts are grayish brown, but are heavily spotted with buff. The underparts are buff, but are heavily barred with brown. There is a small white spot on the outermost primaries, and there are broad white tips of the inner webs of the outer rectrices.
In view of its apparent rarity, great care must be taken in the identification of Cayenne Nightjar. Perhaps the two most similar species that occur in French Guiana are Blackish Nightjar (Nyctipolus nigrescens) and Common Pauraque (Nyctidromus albicollis). Indeed, Blackish Nightjar also was collected at the type locality of Cayenne Nightjar. Blackish Nightjar is slightly smaller than Cayenne Nightjar, and its plumage is significantly darker and overall appears more spotted. The lores and auriculars of Cayenne are chestnut, speckled with dark brown; these regions on Blackish Nightjar are darker, generally blackish brown with some buff and cinnamon speckling. Also, the crown of Cayenne Nightjar is grayish brown, broadly streaked with black; Blackish Nightjar lacks streaking on the crown. It is not clear how apparent the crown pattern may be under field conditions, but this distinction easily is noted with the bird in the hand. Generally, the upperparts of Blackish are mottled blackish brown; Cayenne Nightjar is a paler grayish brown. The wing coverts of Blackish Nightjar have small tawny, buff, and cinnamon spots; the spots on the wing coverts of Cayenne Nightjar, which are buff, are larger. Cayenne Nightjar has a narrow, indistinct tawny collar on the hind neck; Blackish Nightjar lacks a tawny collar. Males of both species have only a limited amount of white on the outer primaries, but on Cayenne this white band extends to the outermost primary; on Blackish Nightjar the white is limited to P9-P7. The white tips to the outer rectrices of the male are broader in Cayenne Nightjar than in Blackish Nightjar.
Common Pauraque is much larger than Cayenne Nightjar, with rounder, broader wings and a longer tail. Male Common Pauraque has a much broader, more extensive white band across the primaries, and the three outer pairs of rectrices are mostly white. Female pauraque has less white in the tail, but still has a broad buff band across the primaries.
The following description is based on Cleere (1998); see also Todd (1920):
Adult male: Forecrown, crown and nape grayish brown, spotted with cinnamon and broadly streaked with blackish brown. Very narrow and indistinct tawny collar on hindneck, this collar barred with brown. Back grayish brown spotted with cinnamon. Scapulars blackish brown, outer webs broadly edge with buff. Rump and uppertail coverts grayish brown spotted with buff. Rectrices dark brown, indistinctly barred with tawny. Broad white tips on the inner webs of the outermost pair of rectrices (R5), and on both webs of R4-R3; central pair of rectrices mottled grayish brown, and barred with blackish brown. Lesser wing coverts dark brown, speckled and spotted with chestnut; remaining wing coverts brown, heavily spotted with buff and cinnamon. Remiges brown. Small white spot, about midway along the length of the feather, on the inner web of the outermost primary (P10); on both webs of P9-P7; and a very small white mark on both webs of P6. Inner primaries, and the secondaries, barred and spotted with tawny along the feather edges. Lores and auriculars chestnut, speckled with dark brown. Chin, throat, and upper breast buff, tinged with chestnut, and barred with brown. Belly, flanks, and undertail coverts buff, barred with brown.
Adult female: Not known with certainty. An unidentified nightjar captured by hand, but later released, may have been a female or immature male Cayenne Nightjar (see Distribution). The outermost two primaries (P10-P9) of this individual had a yellowish brown band on the outer web, becoming yellowish white on the inner web (Ingels 2001). P8-P7 had a whitish patch on both webs. This individual also had whitish tips to some of the outer rectrices (R4-R3) (Ingels 2001).
Juvenile and immature: Undescribed.
Iris: undescribed; probably dark brown
Bill: undescribed; probably black or blackish
Tarsi and toes: undescribed; probably dark brown
Total length: 22.5 cm (Cleere 1998), 23 cm (Cleere 2010)
wing length: 139 mm (Todd 1920); 144 mm (Cleere 1998)
tail length: 112 mm (Todd 1920), 115 mm (Cleere 1998)
bill length (exposed culmen): 8 mm (Todd 1920), 13.1 mm (Cleere 1998)
tarsus length: 17.5 mm (Todd 1920). 18.4 mm (Cleere 1998)
All mensural data are from the holotype (and only known specimen), a male.
Mass: no data.