Sulphur-bellied Tyrannulet

The sulphur-bellied tyrannulet is 11 to 12 cm (4.3 to 4.7 in) long and weighs about 11 g (0.39 oz). The sexes have the same plumage. Adults have a medium gray crown. They have a thin white supercilium and a blackish stripe through the eye on an otherwise grayish white face. Their upperparts are dark olive. Their wings are dark dusky with wide buff edges on the flight feathers. Their wing coverts have wide buffy tips that show as two bars on the closed wing. Their tail is dusky olive with buffy-tinged edges on the feathers. Their upper throat is white, their lower throat and breast deep yellow with a darker olive wash, and their belly and undertail coverts unmarked bright yellow. Both sexes have a brown iris, a thin, pointed, black bill with a pale base to the mandible, and medium gray legs and feet.

The sulphur-bellied tyrannulet is found primarily on the east slope of the Andes from central Colombia through Ecuador and into Peru as far as Huánuco Department, though it is patchily distributed along the chain. There are also a few scattered records in western Venezuela's Táchira state and on the west side of the Andes in northern Ecuador. It inhabits humid montane forest in the subtropical and temperate zones. It favors secondary forest, forest edges, patchy cloudforest, and clearings with scattered trees rather than unbroken mature forest.

The sulphur-bellied tyrannulet feeds mostly on insects and also includes small fruits in its diet. It usually forages singly or in pairs and often joins mixed-species feeding flocks. It forages mostly at the outer edges of trees, though it also forages lower at forest edges. It takes most of its food by gleaning from leaves and twigs while perched and also makes short upward flights to briefly hover.