Don't think I've ever seen a stranger duet (coming from a girl who sang with a dog, before). Albeit, I think it's really cute, though.

When an Australian magpie looks for a worthy singer to harmonize with, he ends up at the window of an apartment, singing to another bird, commonly native to australia, the much smaller Cockatiel. However, this cockatiel is captive bred, and so they have to be separated by a window. That doesn't seem to stop their fun, though.