The same retard who though a bird was interesting enough to loudly notify a large crowd about.
Awesome post, but in all honesty, if one sees something completely ridiculous (IE a flying man), one's mind tends to automatically come up with the most logical explanations possible. From a(n extremely long) distance, those three things could look fairly similar. I mean, if I saw Bigfoot from a distance, I'd probably just assume it was a bear. That's just how our minds work. :)
Maybe if Superman flew with his arms out to the side he would look a lot like a plan from a distance, and if he flapped them he would look like a bird. That's definitely what I would do if I were Superman, just to freak people out.
I always imagined the airforce looking on their radars. They pick up something small. Could it be a bird? No, way too fast for that. Perhaps a plane then? No, too small for that. Oh wait! That's superman! Because as a child it didn't make sense to me that people would mistake Superman for a bird. Maybe a plane. But never a bird. Maybe if you were facing a wall when someone cried out 'There's something in the sky!' But I was a weird kid, with a weird imagination. :)