Bird song is thought to have a function in both inter- and intra-sexual contexts with song complexity serving as an honest signal of male quality. Theory predicts that males use repertoire sizes to estimate rivals’ fighting ability. Here we tested whether element repertoire size plays a role in an intra-sexual context in the Common Blackbird Turdus merula, a songbird with a large repertoire. In a territory intrusion playback experiment, we broadcast Common Blackbird song with different element repertoire sizes to territorial males. The test birds did not respond differently to the different repertoire sizes. This was also the case when we considered the test birds’ own repertoire sizes in the analysis. Thus, we found no evidence for the hypothesis that males use repertoire size for estimating rivals’ fighting force. However, this does not exclude the possibility that repertoire size plays a role in different intra-sexual contexts in Common Blackbirds. In the discussion, we address alternative scenarios.