The full song of the Grey Thrush Turdus cardis consists of a ‘whistle part’ and a ‘trill part’ though a song bout usually consists of full songs and whistle-only songs (previous study). To clarify the importance of trills, two field experiments were conducted, a playback experiment with thirteen males and a female removal experiment with three males. The males continued singing the same number of songs before, during, and after playback. However, the males changed their song structure. They decreased the number of whistle-only songs and increased the number of trill-only songs during playback. Trill-only songs were thought to be delivered with aggression toward other males. Observations suggested that full songs were also delivered with aggression in the direction of other males. In the removal experiment, all three males increased song production dramatically when their mates were removed. The proportion of full songs was higher than before and after removal in all three males. Males were observed uttering trill-only songs while approaching their caged mates just before release. The current field experiments demonstrate that songs including trills have important roles in both intra-sexual aggression and pair formation.