Avian Science Vol. 2 No. 1: (2002) ISSN 1424-8743 1 The analysis of geographic variation in bird song has been used to examine mechanisms of song develop- ment (Marler & Pickert 1984), the process of cultural evolution (Lynch & Baker 1993) and the different func- tions of song forms (Rothstein & Fleischer 1987). Geo- graphic variation can be expressed in many song fea- tures. Average repertoire size, for example, can vary geographically among populations of the same species (Kroodsma & Canady 1985). In blue tits Parus cae- ruleus , repertoire size (the number of different song types per individual) differs noticeably among popula- tions. Bijnens and Dhondt (1984) found an average re- pertoire size of 5.25 song types in their Belgian popu- lation whereas French and Corsican blue tits possessed on average three and four different song types respec- tively (Doutrelant et al. 2000a). In contrast, the mean repertoire size of a blue tit population in La Gomera, one of the Canary Islands, was 19 different song types (Schottler 1995). The syntax of blue tit song also shows great variation from northern Europe to the Mediterranean region, North Africa and the Canary Islands. In northern Europe, blue tits frequently sing trills (Fig. 1), a series of rapidly repeated notes in the last subphrase of a strophe (Becker 1980, Bijnens & Dhondt 1984, Dout- relant & Lambrechts 2001), whereas no trills have been reported in North Africa and the Canary Islands (Be- cker 1980, Schottler 1995). As for other life history Differences in song repertoire size and composition between two populations of blue tit Parus caeruleus Leen Gorissen 1 , Ellen Janssens, Rianne Pinxten and Marcel Eens We recorded songs of male blue tits Parus caeruleus during the fertile and egg-laying period in a Belgian and Spanish population and compared several song characteristics in order to investigate the extent of macrogeographic variation in song features such as syntax, repertoire size and the proportion of trills in the repertoire. Repertoire size differed consider- ably between populations: it was twice as large in southern Spain (11.6 song types) as in Belgium (5.4 song types). Song syntax also differed significantly between the two popula- tions. The most remarkable difference, however, was found in the proportion of trills. Trilled song types were rare in southern Spain (18 %) whereas more than half of the song types in the repertoire of Belgian blue tits included a trill (53 %). A recent hypothesis explains varia- tion in blue tit song as a result of interspecific competition, suggesting that blue tit repertoire size decreases with increasing density of great tits Parus major and that the proportion of trills increases with increasing great tit density. This hypothesis does not appear to explain our findings. Although both populations differed strongly in both song parameters, the rela- tive great tit density appeared to be very similar. Furthermore, we provide the first evidence of a striking negative relationship between repertoire size and the proportion of trills in the repertoire. Thus, birds with a large repertoire tend to have only a small proportion of trills, whereas birds with a small repertoire usually have a large proportion of trills. Key words: Blue tit song, geographic variation, Parus caeruleus, Parus major , song reper- toire, repertoire size, trills. Department of Biology, University of Antwerp, U.I.A., Universiteitsplein 1, B–2610 Wilrijk, Bel- gium; 1 e-mail: lego@uia.ua.ac.be 1-029.qxd 05.03.02 11:42 Seite 1