Research on the function of acoustic signals has focused on high-amplitude, long-range song (LRS) and largely ignored low-amplitude songs produced by many species during close-proximity, conspecific interactions. Low-amplitude songs can be structurally identical to LRS (soft LRS) or they can be widely divergent, sharing few spectral and temporal attributes with LRS (short-range song (SRS)). SRS is often more complex than LRS and is frequently sung by males during courtship. To assess function, we performed two playback experiments on males of a socially monogamous songbird. We compared responses of males whose mates were fertile or non-fertile to differences in song structure (SRS v. LRS and soft LRS), amplitude (SRS and soft LRS v. LRS), and tempo (slow v. fast SRS). Males responded more strongly to SRS than to LRS or soft LRS, indicating that song structure had a greater effect on response than song amplitude. SRS tempo did not detectably affect male response. Importantly, males responded more strongly to SRS when their mates were fertile, presumably because hearing SRS can indicate that a male's mate is being courted by an intruding male and a strong response can deter extra-pair competitors. We conclude that low-amplitude songs can function in both inter- and intra-sexual communication and should receive greater attention in future studies of mate choice and male-male competition.