Symbolism, Imagery, Allegory
There's tons of bird imagery throughout Antigone. For one, there's lots of talk of carrion birds making a buffet of Polyneices. The Chorus also describes Polyneices himself as a bird, a big mean eagle wreaking havoc on Thebes. This description seems to heighten the idea of Polyneices as fearful aggressor against his home town. The Chorus even goes so far as to describe Polyneices the eagle as feasting on their blood. This becomes pretty ironic when the birds are feasting on him.
Another instance of avian imagery is when the Sentry describes Antigone as hovering over Poyneices's body like a mother bird. Here the bird reference seems to strengthen Antigone's symbolism as both a maternal figure as well as representative of the ancient force of nature.
The biggest bit of bird symbolism comes from Teiresias. This is not a surprise, since the prophet is skilled in the magic art of augury or telling the future from the behavior of birds. The seer tells King Creon all about how the birds are fighting each other, which symbolizes the horrible imbalance the King has created in nature. Teiresias goes on to tell Creon that the birds won't talk about the future because they've gorged themselves on Polyneices's blood. (Yuck.) The birds have evidently also pooped all over the altars of Thebes. (A bad day for the altar cleaner.) All this foul bird imagery (pun intended!) seems to symbolize the corruption that Creon has caused by not burying Polyneices.