Well, this thread contains end-game spoilers.
I don't know if this was brought up before, but has anyone else noticed how each of Booker, Comstock and Songbird die? One way or another, they all die by drowning. Comstock is drowned by Booker after he smashes his head on the baptismal font; Booker is drowned by Elizabeth in the river; and Songbird is also drowned by Elizabeth when she opens a tear to Rapture.
I personally haven't paid a lot of attention to the discussions on the true identity of Songbird, and, quite frankly, I didn't really care. This, however, struck me as conspicuous that these three characters died in somewhat similar ways, especially Songbird. So, since everyone knows that Booker and Comstock are the same person, do you think Songbird is a variation of either Booker or Comstock?
Then again, when I think about who Ken Levine is influenced by; I can't help but think that this was done on purpose to throw us off. This actually reminds me of the second season of Mad Men (seasons 1 and 2 Mad Men spoilers ahead) when everyone thought that the blonde child Peggy checks on in her mother's house is her love child, when the truth was that she actually gave him/her away.