Southern Bristle-Tyrants perches vertically and often remains on a perch for long periods (see Foraging Behavior). It regularly engages in wing flicking behavior, rapidly raising one wing above the back and than snapping wing shut again, in the manner of a Leptopogon or Mionectes flycatcher (Clay et al. 1998). The bristle-tyrant frequently sings while foraging, and generally tolerate the presence of a human observer (Tonetti and Pizo 2016). Despite being a small bodied passerine, Southern Bristle-Tyrant can be detected by its song at a distance up to 90 m (Tonetti and Pizo 2016).
Southern Bristle-Tyrant commonly is found alone or in pairs, apart from or associated with mixed species flocks (Tonetti and Pizo 2016). It presumably is territorial, as it responds aggressively to audio playback throughout most of the year (Tonetti and Pizo 2016). Although the size of the territory or home range have not been quantified, the territory seems to be large (Tonetti, personal observations). At one site, the mean density is estimated as 12.7 individuals per km2 (range of 7.3-20.2 individuals) (Tonetti and Pizo 2016).
Southern Bristle-Tyrant is socially monogamous.
Social and interspecific behavior
Southern Bristle-Tyrant most is commonly is solitary or in pairs, and only rarely is encountered in small family groups, of up to four individuals (Tonetti et al. 2017). Although some authors assert that the species generally does not associate with mixed species flocks (Ridgely and Tudor 1994, Fitzpatrick 2004), Willis and Oniki (2003) and Tonetti and Pizo (2016) reported the opposite, with the bristle-tyrant frequently in association with mixed flocks. Apparently the species is more frequently found in mixed-species flocks in winter (Tonetti personal observation), but this information is based on unsystematic observation. At Cantareira State Park, southeastern Brazil, the bristle-tyrant most commonly occurs in flocks let by Red-crowned Ant-Tanager Habia rubica (Tonetti and Pizo 2016); Willis and Oniki (2003) noted an association with Golden-crowed Warbler Basileuterus culicivorus.
No instances of predation on Southern Bristle-Tyrant have been reported in the literature. A Collared Forest-Falcon Micrastur semitorquatus was observed attacking a mixed species flock that included a bristle-tyrant, which reacted with intense vocalizations, but no act of predation was observed (Tonetti, personal observation).