The sooty tyrannulet is an active and restless bird, constantly on the move as it flits from perch to perch, or darts into the air, to the ground or to the water surface to catch insects (2) (4). It may also land on floating vegetation or on rocks or logs in the middle of the water (4). When perched, the tail is regularly pumped up and down, and is often fanned out (2) (3) (4).
The sooty tyrannulet is usually found in pairs (4). Breeding takes places between October and December, with both the male and female helping to build the nest, which is a tightly woven open cup or basket. The nest may be suspended from a branch, twig or root, often under an overhang, and is constructed from roots and grasses, and lined with feathers. Three eggs are usually laid. Nests may occasionally be targeted by the shiny cowbird (Molothrus bonariensis), a brood parasite (2).