Huh, the only reference I remember about rat-f**king was in All the President's Men. I have never seen rats getting it on. However, I do remember a part of Beavis and Butthead Do America showing a couple of vultures getting it on and the one was behind the other! As far as know birds are beak to beak, cloaca to cloaca (I hope I spelled that correctly!) Anyway, that was just a cartoon...
OK, I'll look up the rat thing...