At only 15-19 cm in total length, Least Nighthawk is one of the smallest species of caprimulgid. Like many nighthawks and nightjars, the plumage is cryptically patterned. There is some sexual dimorphism, but both sexes have brown upperparts that are spotted gray, rufous and buff. There is a narrow white throat patch, or small white patches on either side of the throat. The underparts are whitish or buff, barred with brown. Both sexes have a white band across the four outermost primaries, which is conspicuous in flight. The male also has broad white tips the secondaries and inner primaries; the trailing edge to the remiges in the female is narrower and buffier. The six subspecies vary mostly in size, color, and barring of the underparts.
Least Nighthawk is most similar to Lesser Nighthawk (Chordeiles acutipennis), and these two species are broadly sympatric. Least Nighthawk is smaller than Lesser Nighthawk, has white trailing edge to the wing (secondaries and inner primaries), and the white band across the outer primaries is closer to the tip of the wing in Least Nighthawk than it is in Lesser Nighthawk.
Least Nighthawk also occurs sympatrically with both Bahian Nighthawk (Nyctiprogne vielliardi), which is restricted to northeastern Brazil, and with the widespread Band-tailed Nighthawk (Nyctiprogne leucopyga). All three species are similar in shape and color pattern, although Least Nighthawk is the smallest of the three. Band-tailed Nighthawk is a much darker, earthy brown mottled with rufous and buff; lacks the white bar on the primaries; and has a distinctive white band that crosses the midpoint of the tail. Bahian Nighthawk is dark chestnut overall, with no white on the wing or on the throat.
The following description is based on Gould (1861) and Cleere (1998), and refers to nominate pusillus; see also Geographic Variation:
Adult male: Forecrown brown, spotted with tawny. Crown and nape brown, spotted with grayish white and pale tawny. Back, rump, and uppertail coverts brown, spotted with grayish white, tawny, buff, and pale buff. Scapulars brown, spotted with grayish white, buff, and tawny. Wing coverts brown, heavily spotted with buff and pale buff. Primaries brown, the four outermost primaries with a large white spot at about the midpoint of the primary, collectively forming a white band across the outer wing. Secondaries brown. The secondaries, and the three innermost primaries, are broadly tipped white or buffish white. Tertials brown, mottled with grayish white and spotted with buff and pale tawny at feather tips. Rectrices brown, faintly and indistinctly barred with brownish white; outer four pairs of rectrices tipped white (but often only on the inner web), central pair brown indistinctly barred grayish brown. Lores and auriculars brown, spotted with pale buff. Chin and upper throat brown, spotted with pale buff. Small white patch on lower throat, or a white patch on either side of lower throat. Breast brown, heavily spotted with buff and pale buff, becoming brown, barred dirty buff, on lower breast. Belly and flanks whitish, barred with brown. Undertail coverts whitish, lightly barred with brown. Underwing coverts grayish white, tinged with cinnamon, and barred brown.
Adult female: Similar to adult male, but the pale tips to the primaries are smaller, and the tips to the secondaries are duller and buffier. The white tips to the rectriced also usually are lacking.
Immature: Similar to adult.
Nestling: Covered in rufus buff down.
There is no information available on molts of Least Nighthawk; presumably this species follows the complex basic molt strategy.
Iris: dark brown
Bill: blackish, gape lacking rictal bristles (Gould 1861)
Tarsi and toes: brownish; tarsi unfeathered (Gould 1861)
Bare parts colors from Cleere (1998).
Total length: 25 cm (Hilty and Brown 1986), 15-19 cm (Cleere 1998), 16 cm (Hilty 2003)
Linear measurements from Cleere (1998):
male (n = at least 5)
wing length (flat): range 130-141 mm
tail length: 72-79 mm
bill length (from base): 10-15 mm
tarsus length: ca 8.5-13.5 mm
female (n = at least 5)
wing length (flat): 132-139 mm
tail length: 75-79 mm
bill length (from base): 10-14 mm
tarsus length: ca 8.5-13.5 mm
Mass: female, mean 30.6 g (range 27.8-33.4 g, n = 2; Dunning 2008)