Blue tit dawn song output, not complexity, predicts female laying and clutch size.

Evidence

Poesel et al. (2001) have studied the song patterns at dawn in 20 blue tit males, demonstrating that its song output, more than song complexity, is a trait connected with sexual selection. In fact, males with a higher song output without drift were found to participate in pairs in which females started laying earlier and laid larger clutches. — Almo Farina, Soundscape Ecology, 1720