Blue Tit song repertoire size, male quality and interspecific competition

A recent hypothesis suggested that bird song repertoire size is a reliable indication of male quality because the number of songs emitted by an indivi dual and the expression of many traits important for fitness are highly dep endent on developmental conditions. To test this hypothesis, we studied the relationship between song repertoire size and a trait known to be strongly affected by developmental conditions: tarsus length of Blue Tits Parus cae ruleus. We studied this relationship in three populations presenting differ ences in density, quantity of food available and relative and absolute dens ities of Great Tits Parus major. Great Tit density may be an important fact or because this species is supposed to be in competition for acoustic space with the Blue Tit. We found a positive correlation between Blue Tit dawn r epertoire size and tarsus length in the three study populations which is co nsistent with the tested hypothesis. However, this correlation was only sig nificant in the population with the lower Great Tit density. We discuss the significance of this variation and propose that interspecific competition may limit the action of sexual selection on song repertoire size.