The adult breeding male has a blackish forehead with a pale grey median stripe which often extends back as a narrow indistinct pale median crown-stripe. The rest of the crown is quite bright rufous, often with some fine black streaking, and with narrow black edges on the lateral crown. The Swamp Sparrow has a broad grey supercilium and slightly buffier-grey lores and ear-coverts. The eye ring is pale greyish-white. The eye stripe (from behind the eye) and narrow moustachial stripe (reaching to the base of the bill) is blackish-brown, framing the ear-coverts. The submoustachial stripe is whitish, and there is a narrow malar stripe that is blackish. The nape and neck-sides are greyish, with darker fine streaks. The mantle and scapulars are dull rufous-brown, heavily streaked black and also finely streaked with pale straw/buff. The rump and uppertail-coverts are more olive-brown, the uppertail-coverts have broad, well-defined black central streaks. The lesser coverts are chestnut. The greater and median coverts are blackish with broad chestnut feather edges. The greater coverts also have a narrow buff tip. Alula and primary coverts are blackish-brown with the alula having a narrow white edge. Flight feathers are blackish with narrow grey edges to primaries, narrow rufous edges to outer secondaries and broader rufous edges to inner secondaries. The tertials are blackish with rufous edges, becoming buffy-white round the tip. The tail is rufous-brown with pale buff feather edges. The throat is whitish. The breast is grey with a few fine dark streaks, occasionally merging to form an obscure central spot. The belly is greyish-white and the flanks and undertail-coverts are buff, with the flanks having obscure darker streaks. The iris is dark reddish-brown. The bill is dusky-grey with mid-flesh lower mandible. The legs are flesh.
The adult breeding female is very similar to the male and not always distinguishable, but tends to have less extensive rufous crown which is more heavily streaked with blackish. The differences are most noticeable in mated pairs.
The non-breeding adult is similar to the breeding adult but the head is rather duller, the crown is noticeable less rufous and more heavily streaked black, often with a narrow pale grey median stripe. The ear-coverts also tend to be buffier. The sexes are generally indistinguishable, or monomorphic.
The first year nonbreeding is similar to the non-breeding adult but noticeably less grey and rufous on head. The crown has very little or no rufous and the narrow median stripe may be buffier. The supercilium and nape are brwonish- or buffy-grey, not pure grey.
The juvenile is much buffier overall with black streaking on the crown, nape, neck-sides, breat and flanks as well as mantle and scapuolars. The streaking on the crown is usually quite heavy but can be noticeably finer than that on the upperparts. The bill is flesh at first rapidly becoming the adult's. Inside of the mouth is yellow to yellowish-white.(Byers 1995)
Average mass: 14.9 g.
Average basal metabolic rate: 0.211 W.
