The Rufous Hornero (Furnarius rufus: Family Furnariidae), common in southern Brazil, is a territorial, sexually monomorphic bird in which pairs sing duets. This family of birds is known for its lack of sexual dimorphism and for the simplicity of its song types. Here I report that each sex consistently sings a distinct song during the duet: the song starts with both sexes singing identically, followed by a divergence of the female, who slows her singing rate. Also, since both birds pump their wings while singing, at the same frequency as their singing, it possible to see which bird sings at which rate. Sexually dimorphic songs have been observed in other bird species, and suggested in other species in the Family Furnariidae, but this is the first report in which the nature of the song difference is clearly documented. I suggest that many, or all, territorial and sexually monomorphic species of furnariids (and other families, especially suboscines) show similar, consistent sexual differences in song types.