Effect of blue tit song syntax on great tit territorial responsiveness - an experimental test of the character shift hypothesis

A previous study of geographic variation in blue tit (Parus caeruleus) song structure showed that changes in blue tit song syntax (i.e. presence/absen ce of a trill) are correlated with the breeding density of a close competit or, the great tit (P. major), and are not correlated with other environment al factors such as vegetation structure or blue tit breeding density. We te sted the hypothesis that blue tit trilled song represents a character shift that evolved because it reduced territorial interactions with more dominan t great tits. We conducted five sets of playback trials in three study popu lations (mainland southern France, Corsica and Denmark) presenting male gre at tits with blue tit trilled and untrilled songs and great tit songs. We f ound that great tits respond equally strongly to both blue tit untrilled so ngs and Co great tit songs, but show a significantly weaker response to blu e tit trilled songs. These findings are the first experimental evidence tha t interspecific competition may play an important role in macrogeographic v ariation of bird song.