Very local little bird of fresh and brackish marshes and adjacent weedy vegetation in lowlands. Departs Chile in the winter (mainly late March to mid-October) for unknown areas east of the Andes. Usually inconspicuous, hopping low in bushes and weeds, but sings from atop a bush or tall rush stalk. Distinctive in Chilean range: underparts are bright yellow, upperparts plain olive-brown with paler wingbars and soft rusty cap. Similar species occur east of the Andes.