So I've been doing some research on Egyptian culture and symbols just in case I came across anything relative to Lost. I was reading about common Egyptian tattoos, and it turns out that the phoenix is incorporated into Egyptian culture.
I never really thought of it before, as I always presumed the phoenix was just a mythical creature made up in ancient storytelling, but it is quite significant in Egyptian history. What if, the Hurley Bird is a phoenix? It would make sense with the Egyptian theme coming into play massively in the last season, and would give the Producers another reason to say they knew how it was going to end all along (because the Hurley Bird was seen in Season 1). I know phoenix's aren't usually green but this could just be one of the producer's unique interpretations of the history (remember, there have been criticisms over the fact that the statue isn't exactly like Tawaret).
Anyway, I think this would make sense because otherwise it would just be a random giant bird flying around the Island with no explanation!
(For those who don't know what the Hurley Bird is, or looks like, just write it in the Lostpedia Search.)