Song characteristics are age dependent in the willow warbler, Phylloscopus trochilus

Abstract

Theory predicts that sexually selected characters should be condition dependent if they are to be used as honest signals of male quality. One consequence of this dependence is that young and older males will differ in the expression of these characters. Many bird song characteristics, such as repertoire size or song length, are considered to have evolved through sexual selection. Consistent with this, several studies have shown that repertoire size increases with age. However, many of these studies used cross-sectional methods, comparing song characteristics in different age classes. This approach has the disadvantage that differential survival related to song can confound the data, emphasizing or hiding individual changes. To avoid this we combined a cross-sectional with a longitudinal analysis, where the same cohort is compared at different ages, to study how song characteristics change with age in the willow warbler. The longitudinal analysis showed that song repertoire size (the set of different song elements) and element rate (number of elements produced divided by song duration) increased between the first and second year. The cross-sectional analysis gave similar results, but there were no changes in element rate. From these results we can infer that the development of several song characteristics is costly in this species. The different results provided by the two methods for element rate suggest a negative correlation between element rate and the probability of survival. The discrepancy between the methods implies that cross-sectional methods can provide erroneous conclusions in the study of changes with age in bird song.