Résumé
Blue tits, as many other species, show much geographic variation in their songs. In blue tits, songs that include a trill (series of rapidly repeated notes) are widespread on the European mainland, but have not been reported in North Africa or the Canary Islands. We studied song structure of two blue tit populations from the French Mediterranean that are exposed to large differences in local selection pressures (food, vegetation, social factors). Songs from populations on the European mainland and the island of Corsica differed much in syntax, tempo, frequency, and the incidence of trills. The observed population differences could potentially be caused by a combination of different factors, including biotic and abiotic habitat characteristics, body size, and cultural drift. We suggest that songs without trill are acoustically adapted for transmission in habitats where the local breeding density of great tits is low. In addition, we suggest that songs with trill may reflect a character shift, to minimise territorial interactions with great tits.