Why mated dusky warblers sing so much: Territory guarding and male quality announcement

The fertility announcement hypothesis states that males sing most intensively during the period of female fertility in order to assure their paternity, as females would prefer to sexually mate with males singing at a high rate. The dusky warbler (Phylloscopus fuscatus) is one of the species in which singing intensity clearly peaks when females are fertile. As a test of the fertility announcement hypothesis, we study why males of this species sing so much after pairing. Dusky warblers have two distinct kinds of song, an individually specific, stereotype song type (S-song) and a highly variable song type (V-song). S-song is used as an individually recognisable signature to mark the claimed territory area. Playback experiments indicate that males can even memorise their neighbours from the previous breeding season. S-song is used while patrolling over the territory area, presumably to ward off intruding males. Males do not guard fertile females, but intensity of S-song peaks when a male's partner is fertile. In contrast, V-song appears to advertise male quality, functioning to attract females for copulation. This song type is used at the highest rate when any female, the own or the neighbouring, is fertile. Our Study clearly supports the idea that song can function as a paternity guard. However, analysis of extra-pair paternity (shown elsewhere) revealed that females choose copulation partners on the basis of the quality of their V-song, but not on the quantity of song. This means that the fertility announcement hypothesis does not apply to the dusky warbler, as the presumed mechanism - selection on son, rate - is not given. We discuss what selective pressures may favour and maintain such ways of paternity assurance.