Lesser Ground-Cuckoo is a medium sized cuckoo, with relatively long, strong tarsi, a long graduated tail, and a slightly decurved bill. The upperparts are mostly dull brown, with rufous tips to the feathers of the rump and uppertail coverts, and a green sheen to the wing. Lesser Ground-Cuckoo has an ornate pattern on the face, with a black mask on the sides of the head, enclosing a bright yellow orbital ring, broader before the eye, and bright blue bare skin behind the eye. There also is a short white postocular stripe, which can be indistinct. The underparts of Lesser Ground-Cuckoo are cinnamon. Sexes are similar, but the juvenile is duller.
Lesser Ground-Cuckoo is not particularly simlar to any other species in its geographic range. Other ground dwelling cuckoos, such as Striped Cuckoo (Tapera naevia) and Lesser Roadrunner (Geococcyx velox), are heavily streaked and are whiter below (among other differences). Mangrove Cuckoo (Coccyzus minor) also is superficially similar to Lesser Ground-Cuckoo, but Mangrove Cuckoo is arboreal, does not have the ornate face pattern of the ground-cuckoo, and has paler buff underparts.
The following description is based on Ridgeway (1916) and on Stiles and Skutch (1989):
Adult: Crown grayish brown, the feathers with darker shaft streaks, especially on the forecrown. Narrow, and sometimes poorly defined, pale buff or grayish buffy white postocular streak. Narrow lines along upper and lower margins of bare orbital skin and upper portion of auricular area black. Lower portion of auricular area, subauricular area, and malar region cinnamon ochraceous. Nape, back, scapulars, and wing coverts olive brown, the wings and posterior scapulars faintly glossed with bronze greenish. Lower back and rump sooty blackish, feathers of the lower back tipped with buff or pale buffy brown; uppertail coverts olive brown, with browner margins. Rectrices olive brown, glossed with purplish bronze (especially on the central rectrices) and bronze green, the lateral rectrices tipped with dull brownish buff to dull brownish white or buffy white; under surface of rectrices light grayish brown, the lateral rectrices with a well defined dull black subterminal area. Entire underparts cinnamon ochraceous. Sexes similar, although Dickey and van Rossem (1938) describe the female as having paler underparts than the male, and the males as having larger black subterminal spots on the rectrices.
Immature: Similar to adult, but duller. Lateral rectrices lack the blackish subterminal area and the whitish or buffy tips (although according to Dickey and van Rossem 1938, "there is tremendous variation in the amount of black present on the rectrices of both young and old, but this seems to be largely individual and not dependent on age"). Upperparts scaled with grayish buff.
Little information, other than the notes by Dickey and van Rossem (1938). Juvenile rectrices are retained until the subsequent spring (March and April), when they are replaced by rectrices that are indistinguishable from those of the adult. There is a partial prealternate on the body of both adults and first year birds. The period of prebasic molt of adults extends from the end of July to the middle of October (the timing varying among individuals).
Iris: dark brown
Facial skin: loral area and orbital ring yellow, varying from lemon yellow to greenish yellow; narrow area above eye, and larger area behind eye, bright blue; subocular area, dark blue
Bill: maxilla blackish brown, tomia narrowly orange basally and more broadly orange distally; mandible orange
Tarsi and toes: light reddish brown
Bare parts color data from Dickey and van Rossem (1938); see also Tashian (1953).
Total length: 25 cm (Stiles and Skutch 1989), 25.5-28 cm (Howell and Webb 1995)
Linear measurements (from Ridgeway 1916):
male, nominate erythropygius (n = 12)
wing length: mean 98.9 mm (range 90.5-109 mm)
tail length: mean 129.3 mm (range 114.5-140.5 mm)
bill length (exposed culmen): mean 23.7 mm (range 21-26.5 mm)
tarsus length: mean 35.4 mm (range 33-38 mm)
female, nominate erythropygius (n = 13)
wing length: mean 97 mm (range 90.5-101.5 mm)
tail length: mean 126.7 mm (range 117-137 mm)
bill length (exposed culmen): mean 23.5 mm (range 21.5-25 mm)
tarsus length: mean 35.1 mm (range 31.5-38.5 mm)
Mass: male, mean 63.1 g (range 58-70.5 g, n = 5, mexicanus; Payne 2005); female, mean 68.0 g (range 62.9-86.8 g, n =2, mexicanus; Payne 2005); male, mean 61.1 g (range 53.1-66.2 g, n = 10, erythropygus; Payne 2005); female, mean 65.3 g (range 56-76 g, n = 7, erythropygus; Payne 2005)