An interjection similar to "get lost" or "quit it"
Entomology: Comes from a story on TardBlog where a retarded boy eats an embryonic bird and later exclaims "eat bird" - it's usage, however, has little to do with the entomology.
Random guy: Hey baby, let's see those tits!
Girl: Eat bird.
Patron: Bartender, could I get bloody mary but with a bit a tanginess.
Bartender: What did you have in mind?
Patron: Could you give it a swirly bird?
Bartender: You are in luck, I haven'twashed my cock for a couple days.
A metaphor for when two people are in a relationship and Person 1 feels like they are a bird wanting to fly but are trapped inside of a cage (aka: Person 2).
It is basically a great way of telling someone you want to break-up due to your animalistic nature of being wild and free and not wanting to be tied down to them.
Anne: "Ricky, I'm feeling like our relationship is based off the Bird and the Cage theory.
Ricky: "Wait, what do you mean?"
Anne: "I mean that I'm a bird and you're a cage; I just want to fly."
Ricky: "Oh... *sobs uncontrollably*