Adults of the nominate subspecies A. f. fuliginosa have a faint grayish-tawny supercilium and a whitish eyering on an otherwise blackish to dark brown face.

Their tail is long and deeply forked with few barbs at the feather ends that give a ragged appearance.

A. f. peruviana has duller upperparts than the nominate, with a darker chin, little or no eyering, and a gray or blue-gray mandible.

A. f. plengei has redder upperparts and tail than the nominate, with a longer and whiter supercilium and a dark gray lower throat with whitish streaks.

[4] The white-chinned thistletail feeds mostly on arthropods but also includes small seeds in its diet.

It feeds in the understory, gleaning prey from foliage and small branches, and sometimes makes acrobatic moves.

One of the white-chinned thistletail's songs is "a high-pitched, weak, slightly accelerating and ascending trill".