In the opening scene of Fahrenheit 451, why are the books compared to birds?

Bradbury skillfully uses a metaphor when he describes the burning books in the opening scene of the novel, where we see Montag, the main protagonist, finding almost manic glee in the fire he has set. Bradbury describes the books as “flapping pigeon-winged books [dying] on the porch and lawn of the house. While the books went up in sparkling whirls and blew away on a wind turned dark with burning.” To understand why Bradbury compares the burning books to birds, you need to imagine what the scene looks like. Picture the books lying open on the ground. As they burn, the pages are flapping in the wind and heat created by the fire. The books look like birds fluttering in the flames. Their pages are like wings on fire, turning to ash and creating sparks (“fireflies”) that swirl up into the night sky. Like a flock of birds banking and gliding through the sky, the pages are lifting away in the wind and into the darkness. Ironically, the knowledge the books contain is also "going up in smoke" and becoming intangible to the society in Fahrenheit 451 that deems books useless and unnecessary. So, metaphorically, not only do the burning books look like dying, fluttering birds, but the knowledge the books hold is also dying.