I missed this thread, thanks for the refresh, maeve.
Mockingbirds will imitate to beat the band. I cannot tell you how many times I thought I heard a cat stuck somewhere only to find a mockingbird in a tree. They were uncannily authetic sounding.
I love this last bit:
Males face off in territorial disputes by singing to each other, and Beecher has found that repeating a rival's song back to him counts as a strong move. Learning a variety of the neighborhood songs could mean that a young male has his sassy comeback ready.
So for song sparrows that stay put year-round, choosing songs could be the first step in a world of musical warcraft. For other species, the dynamics differ, but the principle is the same: Song matters.