I know there's no absolute-concrete proof to tie this together. But I just noticed as I was listening to the song:
1) It's very natural in it's lyrics, the river/raindrop, erodes us, the clouds, the water your ships roll in. And what also convinces me. "If we could float away fly up to the surface
and just start again" and "Lift off
2) You have "through chaos as it swirls", if you replace "it's us against the world" with "we fly up with the birds" i don't think it fits QUITE as well but it still essentially rhymes and that's probably why they changed the lyrics and thus the title.
3) The song is very peaceful melodically. It definitely makes me think that it could've been named something with "Birds" in it and even at the end, Jonny's guitar riff is gorgeous. But it's just so in a way that I could imagine it emulating birds "lifting off".
4) Chris said "It might be a duet but we don't know who with yet." They probably decided the duet would be within the band, a la with Will. Because he is a fantastic singer after all. :] And he definitely deserves it.
4) To counter this though, Chris also said "I'm trying to make sure there are three songs that start with the same letter. This time it could be U." Which I just realized makes me also really believe "U.F.O." will be a song!!
Maybe the three U's are U.F.O., Us Against the World, and Up with the Birds? Though I'm betting it's probably U.F.O., Us Against the World (PKA Up with the Birds), and another song we don't know/haven't heard.