Like most nightjars, Scissor-tailed Nightjar is cryptically colored, with plumage that primarily is grayish brown (male) or brown (female). Also as is typical of nightjars, this species has a short bill with a very broad gape, and short tarsi. Both sexes of Scissor-tailed Nightjar have a tawny collar across the nape. The outer rectrices of the male are greatly elongated, and are extensively white on the inner webs; the female has a moderately long tail, but the outer rectrices are not elongated. The wings of both sexes of Scissor-tailed Nightjar are uniformly dark, with no white or buff band across the outer primaries.
Scissor-tailed Nightjar broadly overlaps geographically with Ladder-tailed Nightjar (Hydropsalis climacocerca), but these two species generally occupy different habitats, with Ladder-tailed confined to river edge, such as sand bars and river islands. The outer rectrices of male Ladder-tailed are not as elongated as in Scissor-tailed, and the tail of male Ladder-tailed has a distinctive double notch at the tip. Female Ladder-tailed is more similar to female Scissor-tailed, but is slightly smaller and darker. Ladder-tailed Nightjar lacks the rufous nuchal collar of Scissor-tailed, and has a white (male) or buff (female) band across the outer primaries; both sexes of Scissor-tailed Nighjar have uniformly dark primaries.
Long-trained Nightjar (Macropsalis forcipata) overlaps with Scissor-tailed Nightjar in southeastern Brazil and northeastern Argentina. Long-trained Nightjar is larger and darker than Scissor-tailed Nightjar, and the outer rectrices of male Long-trained Nightjar are even longer than those of male Scissor-tailed.
Scissor-tailed Nightjar also is similar to long tailed Uropsalis nightjars, but these occur at higher elevations in more humid habitats, and rarely if ever are sympatric with Scissor-tailed Nightjar. Uropsalis nightjars also are darker and more tawny or rufescent than Scissor-tailed Nightjar.
The following description is based on Cleere (1998), and refers to nominate torquata; see also Geographic Variation:
Adult male: Forecrown, crown, and nape brown, speckled with grayish white; the central feathers are broadly streaked with blackish brown and boldly ocellated or spotted with tawny. Broad tawny collar around hindneck. Back and rump brown, speckled with grayish white. Uppertail coverts brown, boldly spotted with grayish white. The tail has a distinctive shape: the outer rectrices (R5) are greatly elongated; the next pair of rectrices (R4) 120-120 mm shorter; the next two pairs of rectrices (R3-R2) 20-30 shorter than R4; but the central pair of rectrices (R1) 15-25 mm longer than R4. The outermost rectrices (R5) are brown, broadly tipped with whitish, and with white extending along the distal edge of the outer web and diagonally across the distal half of the inner web; the rest of the feather is spotted with buff along the outer web and barred with whitish or buffish white on the inner web. Most of the remaining rectrices (R4-R2) are brown, boldly barred with buffish white and very broadly (ca 40-50 mm) tipped with grayish cinnamon speckled with brown, often tinged whitish. The central pair of rectrices (R1) are grayish brown, speckled and broadly barred with dark brown. Scapulars blackish brown ocellated with tawny, and broadly edged with buff on the outer webs. Lesser wing coverts brown, speckled with tawny. Primary coverts brown, edged or spotted with buff. Remaining wing coverts brown, speckled, spotted, and ocellated with tawny, buff, and grayish white. Primaries brown. The outermost primary (P10) is edged buff on the basal half of the outer webs; the next primaries (P9-P7) are spotted with buff along both webs, except at the tips; and the inner primaries are spotted with buff, especially on the inner webs, and are narrowly tipped with grayish buff. The secondaries are brown, spotted with buff along the inner webs and with buffy or pale buff tips. Tertials are brown, mottled with grayish brown. Lores and auriculars brown, speckled with tawny or buff. Submoustachial stripe white. Chin buffy; throat pale buff or whitish, barred or spotted with brown. Sides of lower throat often dark brown, spotted with tawny. Breast buff or pale buff, narrowly barred with brown. Belly and flanks buff or pale buff, broadly barred with brown. Undertail coverts buffy, usually unmarked. Underwing coverts buff, barred with brown.
Adult female: Similar to male, but tawnier on the wings; undertail coverts barred; and with no white in the tail. The outermost rectrices (R5) are not elongated and are only slightly longer than R4. The rectrices generally (R5-R2) are brown, broadly barred with buffy.
Juvenile and immature: Not described.
Very little information. Wetmore (1926) found birds in molt (body and tail) in Argentina in December. Two specimens collected by Wetmore (1926) "show an interesting molt of the rictal bristles in which the separate bristles are being shed and renewed irregularly. The immature female had developed the posterior bristles first, and then six of the anterior ones had grown in simultaneously, and have the bases still inclosed [sic] in sheaths".
Iris: dark brown, brown
Bill: dull black, dark brown tipped black
Tarsi and toes: brownish drab, brown, pinkish or reddish gray
Bare parts color data from Wetmore (1926), Belton (1984), and Cleere (1998).
Total length: 25-30 cm excluding the outer rectrices of the male; up to 66 cm including the tail streamers (Cleere 1998, Schulenberg et al. 2010)
Linear measurements (from Cleere 1998):
male, torquata (n = ?)
wing length: 168-170 mm
tail length: 270-280 mm
bill length: 14.2-18.2 mm
tarsus length: 17.8-21.1 mm
female, torquata (n = ?)
wing length: 151-165 mm
tail length: 131-141 mm
bill length: 17.0-18.2 mm
tarsus length: 17.8-19.8 mm
male, furcifera (n = ?)
wing length: 169-192 mm
tail length: 260-360 mm
female, furcifera (n = ?)
wing length: 161-180 mm
tail length: 134-169 mm
male, 51-63 g (n = 6, furcifera; Belton 1984)
female, 57-60 g (n = 2, furcifera; Belton 1984)
male, 47.5-52.3 g, (n = ?, furcifera; Cleere 1998)
female, 48.0-57.0 g (n = 2, furcifera; Cleere 1998)