In the first paragraph of Fahrenheit 451, Bradbury describes the books as "flapping pigeon-winged books." Bradbury creates this metaphor for a couple of reasons. Firstly, it reinforces the idea that books are disliked and forbidden in this society. A pigeon, for example, is a very common bird, generally regarded as a nuisance in big cities across the world. So, by comparing a book to pigeon, Bradbury is highlighting the idea that this society views books as troublesome pests which must be removed.
Secondly, by comparing the books to birds, Bradbury is foreshadowing Montag's rebellion later in the story. The flapping, for instance, is symbolic of Montag's escape from society's oppression and censorship. Just as a bird can fly away, Montag flees the city and joins a group of outlaw professors who are determined to reintroduce books to society as soon as they get the chance.