Résumé
In birds, trill syntax (a trill is a long series of rapidly repeated identical notes) degrades more rapidly by dense vegetation than do syntax with wider spaced notes. Thus, selection might lead to avoidance of trill in dense vegetation. Blue tits (Parus caeruleus) show much geographical variation in the proportion of songs with and without trill and occupy two main types of vegetation while singing, i.e. open broad-leaved deciduous woodland before leaf development and closed evergreen woodland. Blue tit songs with trill are more common in open habitats. We hypothesised that blue tit songs with trill are sung less frequently in dense vegetation because they reduce the efficiency of communication in these types of vegetation. We degraded blue tit songs with and without trill in dense vegetation. To test if degraded songs with trill are less efficient in communication than degraded songs without trill, we presented territorial blue tits to these degraded and undegraded songs and quantified blue tits' territorial responses. Tests were carried out in two blue tit study populations situated in deciduous woodlands where individuals produce both songs with trill and without trill. Undegraded songs provoked stronger responses than degraded songs. Degraded songs with and without trill induced similar responses. These results suggest that, in dense vegetation, songs with trill are as efficient in communication in adults than songs without trill, perhaps because songs without trill degrade in a different way than do songs with trill.