I have just posted our new preprint describing an experiment with song sparrows asking whether male song sparrows respond with decreased or increased aggression towards their song tutors (specifically the adult males that they have learned the most from, i.e. their best tutors) compared to strangers. We carried out playbacks of tutor vs. stranger songs inside the territories of song sparrows to test whether tutor-tutee relationships are primarily cooperative (in which case, there should be less aggression to tutors) or competitive (in which case there should be more aggression to tutors). To our surprise we found that tutors elicited a higher response than strangers, and the difference in response strength was correlated with the proportion of the repertoire song sparrows learned from that tutor.
You can find the preprint here.