This is an example of the powerful imagery for which Bradbury is famous. Here we have a simile, a comparison using "like" or "as". The books, which have been set on fire, are being compared to flying birds. As they burn, they move about, looking as though they are leaping and dancing. It is a very dark, rather disturbing comparison, as they look like birds on fire. The papers fly about in the flames, resembling "red and yellow feathers." The fact that the books are being compared to living creatures is important. Bradbury is emphasizing the peculiar nature of books. Each story has a life of its own, and each reader brings a different perspective to that life. When those lives burn, they resemble, in Montag's eyes, burning birds that attempt to fly to freedom.