Nunlets (Nonnula) are small puffbirds with a long, fairly heavy black bill, prominent rictal bristles, a slender tail, and plumage that is primarily brown, and, in many species, with little pattern. Fulvous-chinned Nunlet takes its name from the pale rufous of the lores, chin and throat. Otherwise, the upperparts are dull brown, with gray sides to the head. The throat and breast are dull rufescent brown, with a whitish center to the belly, and white undertail coverts.
As a group, nunlets are easily recognized by their small body size, brown plumage, large heavy bill, and "cute" demeanor. Fulvous-chinned Nunlet is broadly sympatric with Rufous-capped Nunlet (Nonnula ruficapilla), which, for a nunlet, has a highly patterned, very distinctive plumage: Rufous-capped easily is distinguishable by the very extensive slate gray of the sides of the head and breast, contrasting with the brighter rufous crown, breast, and belly. Contact also is possible, locally in eastern Peru, between Fulvous-chinned and Rusty-breasted (Nonnula rubecula) nunlets; Rusty-breasted has, as its name implies, a tawnier color to the breast than does Fulvous-chinned, and also differs by having a distinctly white eyering (in contrast to the red orbital ring of Fulvous-chinned).
The following description is based on Hellmayr (1907) and on Rasmussen and Collar (2002):
Adult: Upperparts (including tail and wings) generally dull brown; edgings to secondaries often buffy. Lores rufous brown. Sides of the head, below and behind eye (including auriculars), dark gray. Chin pale rufous, shading to rufescent brown on the throat and breast. Flanks dull fulvous, center of belly whitish. Undertail coverts white.
Juvenile and immature: Undescribed
Bare orbital ring: coral red, reddish purple; narrow
Bill: maxilla black; mandible dark gray, greenish gray
Tarsus and toes: gray, olive gray
Bare parts color data based on specimens in the Museo de Historia Natural de la Universidad Nacional Mayor de San Marcos, Lima, and the Field Museum of Natural History.
Total length: 14–15.5 cm (Rasmussen and Collar 2002, Schulenberg et al. 2010)
Linear measurements (from Hellmayr 1907 and Gyldenstolpe 1945; n= 5, sexes combined):
wing length: mean 63 mm (range 62-64 mm)
tail length: mean 56.1 mm (range 53-59 mm)
bill length: mean 23.3 mm (range 22.5-25 mm)
Mass: male, 15.4 g, 17 g (n = 2; O'Neill 1974); female, 19 g, 19.9 g (n = 2; O'Neill 1974)