Formicivora antwrens are small antbirds with slender bills and long graduated tails. The sexes in Formicivora are highly dimorphic. Males of most (but not all!) species have a long white stripe extending from the sides of the head and neck down to the flanks, or at least have white flank patches; males always are black or blackish below, but the upperparts of most species are brown. Female Formicivora usually have brown or gray upperparts and underparts that are white or buff. Male Restinga Antwren is mostly black or blackish, with white tips to the wing coverts and rectrices, and gray flanks. Female Restinga Antwren is very different, with a brown crown and back, white supercilium above a black "mask" on the sides of the head, and creamy white or pale buff underparts.
Species of Formicivora are closely similar, mainly the females. Restinga Antwren originally was described by Gonzaga and Pacheco (1990) as a subspecies of Serra Antwren (Formicivora serrana), and these two species are very similar. Serra Antwren occurs in Rio de Janeiro, Espírito Santo and Minas Gerais, and it is not sympatric with Restinga Antwren; Restinga Antwren is restricted to Rio de Janeiro state, where it occurs only in restinga, a sandy plain coastal vegetation. Females of the two species are virtually identical, without diagnosable differences. Male Serra Antwren has brown upperparts, however, and is dark only below; has a prominent white supercilium; and larger white spots on the undersurface of the rectrices. In contrast, male Restinga Antwren is totally dark (upper- and underparts), has small white tips on the rectrices, and only rarely has a white supercilium (a feature that apparently is not related to age). Vocalizations of these two species also are very similar, and Restinga Antwren will respond to playback of vocalizations of Serra Antwren vocalization. Due to these similarities in both morphology and vocalizations, the taxonimic status of Restinga Antwren is controversial. As noted above, however, Restinga and Serra antwrens are geographically isolated.
Restinga Antwren is sexually dimorphic:
Adult male: Males are totally dark (black) with white feathers in the flanks, white tips to the wing coverts, and small white spots at the tips of the undersurface of the rectrices. Some individuals also have a white supercilium or a brownish back.. Some males adults have brownish back.
Adult female: Females are brownish above, with darker wings that have white tips to the wing coverts, and a white supercilium. The tail is similar to that of the male, with small white tips to the undersurfaces of the rectrices. The underparts are pale, creamy white or pale buff.
Nestlings: When hatched, totally naked with a yellow bill (Chaves et al. 2013).
Molt occurs in March, April and May.
Iris: dark brown
Bill: completely black in all adults; yellow in the nestlings (Chaves et al. 2013).
Tarsi and toes: black
Data from Chaves and Alves (2013):
mass - mean: 13.9 g mass – mean: 13.0 g
total length - mean: 122.7 mm total length – mean: 119.0 mm
wing length – mean: 58.2 mm wing length – mean: 56.0 mm
tail length – mean: 55.0 mm tail length – mean: 52.3 mm
exposed culmen – mean: 14.7 mm exposed culmen – mean: 14.7 mm
nostril-tip – mean: 9.7 mm nostril-tip – mean: 9.5 mm
head to beak tip – mean: 33.8 mm head to beak tip – mean: 33.4 mm