An Analysis of the full song of five free-living urban European Blackbirds (Turdus merula); a network approach.

Is the song repertoire of male blackbirds safe-guarded against loss of variability during the breeding season? In order to answer this question the repertoires of five males were analysed from the viewpoint of network theory. In four of the five males a strong coherence of the repertoire was found to exist in anastomoses between strophe types, same elements being reached from different beginnings. The succession of strophe types in song sessions betrayed a mixture of relatively predictable cycles and chaotic connections. The former should facilitate repeated retrieval of a considerable part of the repertoire, while the chaotic component keeps the whole repertoire readily accessible from any point in a singing session. The same mixture may be considered optimal for binding the attention of conspecific listeners. It is concluded that the existing structure of blackbird song repertoires is favourable for the conservation of its richness, but it is as yet uncertain whether this is due to evolutionary (i.c. sexual) adaptation.