Bird song is the most widely studied example of vocal learning outside human language and shares important parallels with it, including the importance of social factors during development. Our understanding of social factors in song learning however remains surprisingly incomplete. Here we examine the possible role of aggressive interactions in determining song "tutor" choice in song sparrows (Melospiza melodia), a songbird in which individuals display song learning strategies ranging from learning primarily from one tutor, to learning a few songs each from a number of tutors. We test two hypotheses: The Competition hypothesis suggests that young birds learn more from tutors with whom they compete especially intensely and predicts that tutees will respond with high aggression to tutor songs. In contrast the Cooperation hypothesis suggests that song learning reflects a cooperative relationship between the tutor and the tutee and predicts low aggressive response to tutors. In a playback experiment we found that birds respond more aggressively to songs of their tutors than they do to songs of strangers and strength of aggressive response correlated positively with how much they had learned from that tutor. These results provide the first field evidence that young songbirds show increased aggression to tutor songs, supporting the hypothesis that young males learn from adult males with whom they most intensely compete during the song-learning phase, and perhaps afterwards.