Yellow-margined Flatbill

The yellow-margined flatbill is 13 to 13.5 cm (5.1 to 5.3 in) long and weighs 12 to 17.5 g (0.42 to 0.62 oz). The sexes have the same plumage. Adults of the nominate subspecies T. a. assimilis have a mostly olive-tinged gray head with a thin white broken eye-ring. Their back, rump, and uppertail coverts are olive-green. Their wings are dusky with yellow edges on the greater coverts and remiges that appear as a pale speculum and a faint wing bar on the closed wing. Their tail is dusky. Their throat is pale gray, their breast and flanks pale olive, and their belly pale yellow. Voice: The yellow-margined flatbill's vocalizations vary geographically. In the Guianas it makes a "series of typically 2‒5 drawn-out extremely nasal, almost screaming notes". In Ecuador it sings "a leisurely series of 3 whistled notes, each slightly higher-pitched and shriller, e.g. weeeuw...weeeu...weee?