Effects of social rearing conditions on song structure and repertoire size: experimental evidence from the field

Through a cross-fostering experiment, we studied song learning of blue tit, Cyanistes caeruleus, and great tit, Parus major, males reared by heterospecific parents. This was done in the wild, and so potential song tutors, territorial neighbours, potential mates and other social factors were all natural and not affected by the treatment. Normally, the song repertoires of the two species are completely discrete. However, the cross-fostered great tit males altered their song in several aspects, including repertoire size and composition, and temporal and frequency parameters, thus becoming intermediate between the normal songs of the two species. For the cross-fostered blue tit song, only repertoire composition was affected. However, an analysis of repertoire size in both species proved this to be larger in cross-fostered than in control males. This increase in repertoire size shows that repertoire size is influenced by social conditions in these species and is not strictly constrained by memory capacity. Furthermore, several of the cross-fostered great tit males included trilled songs in their repertoires. In these two species, trilled songs have been regarded as unique to the blue tit. In conclusion, our results show the importance of early social experiences for song learning in these wild tits.