Increased Male Singing in Response to Predator Presence May Represent Reproductive Investment in a Promiscuous Species, the Aquatic Warbler Acrocephalus paludicola

Abstract. 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.