Males of the promiscuous Aquatic Warbler are regarded as emancipated from any parental duties, and in most of the broods nestlings are sired by two or more males. During a long-term study on the reproductive biology of the species on fen mires on the river Biebrza in north-eastern Poland, we frequently heard males singing more intensely or uttering warning calls close to nests. Here we test whether this behaviour is reproducible and therefore constitutes a mate investment or paternal investment. During the incubation periods, hides were erected ca. 30 m from nests. During 30-min periods male song bouts were counted while a test person was either concealed inside or placed outside the tent, hence well visible as a potential predator. The production of song showed a sevenfold increase during the presence of a clearly visible test person. This observation indicates that male song in Aquatic Warbler serves as a warning signal and hence can be regarded as a type of mate or paternal investment.