The Evolution of Song Versatility in the Willow Warbler Phylloscopus trochilus: A Case of Evolution by Intersexual Selection Explained by the "Female's Choice of the Best Mate"

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Abstract
Intersexual selection is regarded to be 1 of the mechanisms responsible for the evolution of versatility in bird song. This study examined if intersexual selection could have contributed to the evolution of versatility in willow warbler song. Data on time of arrival, pair-formation and reproductive success was collected for 15 willow warbler males in 1979 and for 15 males in 1980. The degree of song versatility was estimated for 5 1st-time breeding and 4 older male willow warblers by dividing the number of different syllable types by the total number of syllables distinguished. Older males performed more versatile songs than did the males which were breeding for the 1st time. The former also arrived earlier on the breeding ground, became paired earlier and these pairs achieved better reproductive success. Intersexual selection has been involved in the evolution of willow warbler song versatility and the mechanism involved is the choice by the female of the best mate.