User blog comment:D.Ryan/As Sonbird is to Big Daddy as Elizabeth is to Little Sister/@comment-2250991-20110714215826

It's pretty obvious to what this answer is; Their similarities are just in their relationship. Even Ken Levine mentions this several times. Here's one excerpt from IGN's BioShock Infinite and Beyond article:

''People have started to notice this, that relationship [between Elizabeth and Songbird] has some thematic connections to the Big Daddy and Little Sister.

Elizabeth's not a child, that's a huge difference. Elizabeth is a woman. A Little Sister has no capacity to understand what she wants in life, but Elizabeth knows what she wants. She wants to control her own destiny. She was locked in this tower when she was the age of a Little Sister, five years old or so, and had no idea why she was locked in there, no idea she has these powers, completely cut off from human contact except for this creature, the Songbird. All she did was look out the window and say, "I want to be out there. I want to control my own destiny." And then she gets out there and she realizes she can't control her powers. She doesn't know she has these powers, then she learns she has these powers and on top of that she can't control them. What an adult wants is to control their own life, control their own destiny. That's the primary difference between Elizabeth and a Little Sister, Elizabeth's an adult.''

As for Songbird, he's mostly like a Big Daddy, only he's 30 feet tall, has been heavily conditioned, looks like a bird, and can fly.