New evidence! Here's a quote from The Boston Phoenix's interview with Ed Helms:
"The chicken is the great McGuffin of "The Hangover." It's never explained. But, you know the only reason it's there is because . . . I mean there is no rational explanation for it. Although Todd will tell you, Todd Philips, the director, when I called him on the chicken I was like `What's the deal with the chicken?' He says that we stole the chicken to feed the tiger."
Helms goes on to theorize that the chicken is a "symbol of chaos," with possible "Bunuel-esque" overtones.
The chicken's brooding, smoldering presence in the hotel suite made what might have been an ordinary bawdy comedy into something transcendent. A less capable actor, a cowardly actor, might have tried too hard, might have been unable to resist hitting the audience over its collective head with a hammer. But that beautiful, cocksure chicken trusted the material, and it trusted its chicken instincts. It trusted us, too — trusted us to get the joke without having it explained to us, without diagrams and maps. There was no talk of motivation, no origin myth, no script-by-numbers ribbon tying, everything grounded in context and meticulous backstory. Life isn't like that. Life is chaos, the thrill of the unknown, the beauty of mystery and what comes next. Sometimes we don't need a reason or a signpost. Sometimes, life just is.
And so it was with that magnificent chicken, with its perfectly timed squawks and walk-throughs.
That chicken was present. That chicken was elemental, man. That chicken belonged."