Abstract
The phenotype-linked fertility hypothesis states that functional male fertility covaries with male phenotype and that females engage in extra-pair copulations with males with more attractive phenotypes than their partner to obtain direct fertility benefits. We tested this idea in the sedge warbler,Acrocephalus schoenobaenus, a species in which females prefer males with larger song repertoires, and in which extra-pair copulations occur. In this study paired males had larger song repertoires than unpaired males. However, we found no significant relationships between song features and any measure of sperm numbers or sperm quality. We also examined the hypothesis that directional asymmetry in testes mass covaries with the expression of secondary sexual traits, such as song features, but again found no significant effects. However, older males tended to have larger repertoires and testes and a greater degree of directional asymmetry in testes size than young males.
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