The genus Phylloscartes includes a diverse suite of small tyrant flycatchers, all of which have a relatively long tail and a slender bill; almost all species also have a more or less conspicuous black or dusky mark framing the auriculars. As currently constituted, Phylloscartes includes two groups of behaviorally distinct groups of small tyrant flycatchers: "true" Phylloscartes, which are more active foragers, with a horizontal (warbler-like) posture, and which frequently cock the tail; and species that formerly were classified in the genus Pogonotriccus), which have more upright posture, do not cock the tail, and are less active when foraging. Southern Bristle-Tyrant belongs to the Pogonotriccus group. The upper parts generally are bright olive, and the underparts are bright yellow, although the upper throat is white. The most distinctive feature of this species is the variegated pattern on the head: there is a large supralocal patch, a dark gray crown bordered below by a pale gray supercilium, and a broad black mark framing the rear edge of the auriculars. As is typical in Phylloscartes, the sexes of Southern Bristle-Tyrant are similar.
Southern Bristle-Tyrant is similar to several other species Phylloscartes that occur in the Andes, in the tepuis, or in the coastal mountains of Venezuela, such as Variegated Bristle-Tyrant Phylloscartes poecilotis, Chapman's Bristle-Tyrant Phylloscartes chapmani, Marble-faced Bristle-Tyrant Phylloscartes ophthalmicus, Venezuelan Bristle-Tyrant Phylloscartes venezuelanus, Antioquia Bristle-Tyrant Phylloscartes lanyoni, and Spectacled Bristle-Tyrant Phylloscartes orbitalis, but of course all of these species are allopatric. The only sympatric tyrant flycatcher with a similar facial pattern to Southern Bristle-Tyrant is Eared Pygmy-Tyrant Myiornis auricularis, which is much smaller with a very short tail, and which has very different vocalizations.
The following description is based on Fitzpatrick (2004):
Adult: Sexes similar. Crown and nape gray; center of crown washed with olive. Lores blackish. Broad white broad supraloral streak, becoming narrower and slightly darker (pale gray) behind the eye, but extending to above or just past the rear edge of the auriculars; interrupted just behind the eye by a black vertical streak, which is continuous with a broad black border to the rear edge of the auriculars. The auriculars and a vertical crescent behind the auriculars are yellow. The upperparts generally are bright olive; the wings are dusky, the wing coverts and remiges broadly margined with yellowish olive. The underparts are bright yellow, with a light olive wash on the breast.
Juvenile: Undescribed, but apparently similar to adult (Tonetti et al. 2016); or with duller, more uniform plumage (Clay et al. 1998).
Molts of Southern Bristle-Tyrant are undescribed.
Iris: dark brown or dull red
Bill: maxilla black; mandible pinkish white
Tarsi and toes: pale gray
Bare parts color data from Belton (1985) and Fitzpatrick (2004).
Total length: 11 cm (Ridgely and Tudor 1994), 11.4 cm (Belton 1985).
Mass: 7.5 g (n = 1, ,male; Belton 1985)