Repertoire size, sexual selection, and offspring viability in the great reed warbler: changing patterns in space and time

Abstract

Only a few studies have focussed on the consistency of sexual selection patterns in space and time. One such case is the great reed warbler (Acrocephalus arundinaceus), for which studies in Germany in 1981–1982 and Sweden in 1987–1991 strongly suggested that the size of a male's song repertoire was the target of mate choice and sexual selection. Studying the same German population once again in 1994–2000, we investigated the consistency of these patterns over time as well as between populations. Our reanalysis of the data from 1981–1982 shows that male repertoire size was positively correlated with male pairing success (harem size) and with clutch size (adjusted for seasonal effects), whereas no such correlations were found during 1994–2000 in the same population. We suggest that the earlier correlations were probably caused indirectly by covariation with territory quality, and that a decline in population size has changed the role of territory quality. In the Swedish population, an earlier study found a striking correlation between the size of a male's repertoire and the viability of its offspring, suggesting good-gene effects. In contrast, we found no such correlation in the German population, neither in 1981–1982 nor in 1994–2000. We conclude that repertoire size does not seem to be a very reliable indicator of variation in male quality. Interestingly, the analysis of data from 1994–2000 showed that male pairing success was strongly correlated with measures of strophe length and immediate versatility, traits that have been found to reflect male longevity. Future studies will have to show whether these performance-related traits are more powerful indicators of male quality than is repertoire size.