Symbolism, Imagery, Allegory
When Alex talks about how hard he's fought for his freedom, he refers to Portland as a cage, keeping everyone inside with rules and walls. He says, "I'd thought that nothing and nobody was free in Portland, but I was wrong. There were always the birds" (14.81). Do you think that's how he views Lena, as a bird in a cage waiting to be freed?
In the bonus features to the paperback version of Delirium, Lauren Oliver states that "birds have always been a symbol of freedom and the possibility of escape." Well, that solves that question, huh? Thanks, Lauren!