Who or what was Songbird?
Solution 1:
Songbird was built at the Fink Factory, you can see the blueprints there. Where they got the technology is the interesting part:
The technology came from another dimension; It is never directly stated but it is quite possible that dimension is the one with the Rapture city from Bioshock 1 & 2. The same technology was used to build the Big Daddies.
This would imply that Songbird is a brainwashed human with machine augmentations.
Solution 2:
The most passed around theory is that songbird is kyle fitzpatrick from bioshock 1 or, songbird was there atleast, as if you listen close at his death scene you hear the songbirds' squawk and cry's. Ign has a clip on youtube that you can hear it pretty well on.
Here is said clip, you have to listen carefully over the talking of fitzpatrick and the piano, hope this helps :)Solution 3:
While Songbird is machine, I don't believe he is human in any aspect except the fact that he protects Elizabeth like the Big Daddies. While this is out there, on an unrelated note, A voxophone with Father Comstock in the Good Time Club says a line that caught my attention:
...Or, drown his flock in waters of the ocean.
This means, in my opinion, that
Comstock knew about Rapture before Songbirds creation.