Ray Bradbury is a very poetic writer who uses many different images throughout his novel both as symbols of greater meaning, and as tools to create more descriptive images and scenes. In the opening pages of "Fahrenheit 451," Bradbury describes a scene in which Montag is burning books, with great pleasure and enjoyment. To Montag, these books are merely empty objects--they don't possess any meaning or value whatsoever. However, one of the main themes of Bradbury's writing in this novel is that books DO have meaning, they ARE valuable, and the are incredibly necessary for the world to have thinking, happy, fulfilled people. So, when Bradbury compares the books to birds in phrases like this, "the flapping, pigeon-winged books died," it transforms the books from cold objects to beautifully alive birds, whose death is tragic and disturbing. He uses the bird analogy throughout the entire novel; later, he describes one as a "white pigeon...wings fluttering" and the page as a "snowy feather." In describing the books as such, Bradbury is symbolizing how books are indeed alive, they are precious, beautiful things, and how burning them is as much of a crime as burning living creatures would be.
Besides the symbolic underlay of using birds as books in order represent their value, it is simply poetic. He uses a metaphor, comparing them to birds, and it brings to mind images of fluttering wings, of a great fire filled with the beating of wings. It is a much more descriptive way to paint the scene than just stating "and the books burned." It's good writing.
I hope that those thoughts helped a bit; good luck!
Mostly for symbolism, and the effect of the image. This entire book is about how books are burned in their society, and along with it, how any sort of free thinking, independent thought, capability for happiness, and freedom through personal intelligence is completely repressed and imprisoned. People in Montag's society are bound and shackled by apathy, violence, and a twisted kind of mind-control. They are not free; so, when Bradbury compares books to "flapping pigeon-winged" birds that "died on the porch and lawn of the house," he is describing an animal that is free to take flight, soar and take to the skies. It is a symbol for what books can do for our minds. Books help our thoughts to take flight, they help us to be capable of independent thinking, and to not be imprisoned by someone else's viewpoints. So, the books as a bird is a symbol of how books are freedom.
Another reason that a bird is used is simply for the effect of the image. Not only are they birds, but they are birds that "died on the porch and lawn of the house." Bradbury compares them to a real, live creature, which makes their burning even more cruel. Pigeons are harmless, mild, tame birds that live peacefully amongst people, and to go in there and wipe them out, burning them, adds more dramatic impact to the brutality of what Montag is doing. It gives the books a kind of life, and makes it seem more awful when that life is taken.
Literally, the books are dancing around as they burn, as if they might take flight. Symbolically, books have the power to allow your mind to take flight and flee its surrounding and explore and to taste the freedom of flight. This symbolic meaning is in line with the symbol of the phoenix, a mythic bird which burns up and then is reborn and takes flight from its own ashes. Keep that in mind as you continue to read the book, for Montag will be enticed to read the books he is destroying in that opening scene. He will experience the freedom offered by them, but he will have his world - literally - incinerated around him. But will he rise from the ashes?
In the opening scene, the books are refered to as "flapping pigeon-winged books" because the burning pages look as if they are wings of a bird flapping up and down. Montag watches until they burn and "die."
In the book they are described as pigeons, and normally (for example in DC) pigeons are nuisances, pests, a bad thing.
Bradbury's usage of the bird imagery continues throughout his novel. I feel that the "birds" symbolize the freedom one can obtain through gaining knowledge. No matter how society attempts to thwart knowledge, such as via censorship or book burning, man finds a way to learn. He is able to "Fly".