Song stability and neighbour recognition in a migratory songbird, the black redstart

Territorial male songbirds have the ability to discriminate between the songs of their neighbours and those of strangers and for a few species it has been shown that they maintain this ability from one breeding season to the next. To better understand the acoustic basis of this long-term discrimination ability we studied song stability across two breeding seasons in a migratory songbird with high inter-annual return rates and territory stability, the black redstart, Phoenicurus ochruros. Strophe repertoires of 14 males (>= 2 years old) were stable from one breeding season to the next and high strophe sharing occurred for males within the same group of houses or hamlets (81%) in contrast to only limited sharing between different hamlets (15%). However, subtle differences exist between the renditions of the same strophe sung by neighbouring males and these differences equally show an inter-annual stability, providing an acoustic basis for long-term discrimination abilities. Playback tests showed the existence of a strong dear-enemy effect: males reacted less aggressively to the familiar, often shared song of a neighbour than to a stranger unshared song and this pattern was maintained when birds returned from migration one year later. We discuss on one side the possible mechanisms leading to the observed patterns of song sharing and on the other side the significance of stable vocal signatures for neighbour recognition.