|Adult male S. s. sharpii|
This species is found in open, lightly wooded habitats, such as hillsides with trees or scrub and forest edges. In South Africa it occurs mainly in coastal areas, inhabiting coastal bush, shrubs along streams, gardens, and areas with rank vegetation. It is not truly migratory, but undertakes some seasonal movements.
The brimstone canary is 15–16 cm in length with a heavy bill, which is short, conical and very stout at the base. The bill is light brown with a pinkish or yellowish base. The legs and feet are pinkish-brown. It has yellow-green upper parts with dark green streaking, yellow-green ear coverts and malar stripe, and two yellow wing bars. The underparts are yellow, with a greenish wash on the flanks, and breast. The sexes are similar, but the male is brighter, with a bigger bill, better defined face pattern, brighter yellow wing bars and a greenish rump. Young birds are duller, greyer and less yellow below than the adults.
There are three subspecies.
- S. s. sulphuratus is the nominate race of southwestern and southern Cape Province
- S. s. wilsoni of eastern Cape Province to southern Mozambique is smaller, relatively smaller billed, and paler or more yellowish green than the nominate form. The underparts are entirely yellow.
- S. s. sharpii of northern Mozambique to Kenya is smaller and paler or more yellowish green than the nominate form. Its bill size is intermediate between the other two races.
The degree to which this bird is green or yellow varies considerably across regions. Southern birds are greener than northern birds, even within subspecies, notably with S. s. sharpii.
The brimstone canary can be confused with the yellow-fronted canary, but that species is smaller billed with a much more defined head pattern and a bright yellow rump.
The call notes of the brimstone canary in the south of its range are a trilled, deeply pitched swirriwirrit or chirrup. The song is a jumble of chirps, whistles, warbles and trills, with the same deep pitch as the call. Northern birds have a faster, higher, less jumbled and more tuneful version of the song.
The brimstone canary breeds from August to October. The nest is an open cup built from thin stems and other plant material and lined with plant down. It is placed in a leafy bush or a tree.
The brimstone canary is less gregarious than other canaries. It tends to be found singly or in pairs, or occasionally in small groups, and is a common visitor to gardens. It feeds on fruit, seeds, flowers and shoots. Hard seed cases are cracked with the stout bill.