Interspecific cross-fostering affects song acquisition but not mate choice in pied flycatchers, Ficedula hypoleuca

Laboratory studies suggest that young birds prefer to learn conspecific songs over songs from heterospecifics. However, field studies have shown that many species copy heterospecific song. We cross-fostered pied flycatchers, Ficedula hypoleuca, to nests of blue tits, Cyanistes caeruleus, and great tits, Parus major, in the field. Recent studies have shown that species recognition is genetically determined in pied flycatchers. However, we found that most cross-fostered males included song types from the foster species in their repertoire, with an even stronger effect if reared with heterospecific nestmates. Furthermore, in cross-fostered males frequency range and syllable number per strophe were intermediate between control flycatchers and the foster species. The results contrast with those obtained when blue tits and great tits are cross-fostered in the field; the tits not only adopt songs from the foster species but, in contrast to the flycatchers, they also become sexually imprinted on it. Vocal mimicry may include a cost of reduced species recognition, but this cost may be minimized by also keeping a large proportion of conspecific song in the repertoire. Our finding highlights the importance of early experience for song acquisition, showing that altered social-rearing conditions may cause a mixed choice of song tutors. Depending on different proximate mechanisms, song used in mate choice may more readily respond to environmental change than mate choice itself. Changes in song by learning may thus serve as an essential first step in hybridization and speciation processes.