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. 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
C. s. sharpii. For an overview of finch phylogeny (including canaries) see the entry on
finches. 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
yellow canary, with overlapping range, is also a known confusion species. 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. ==Behaviour==