One of the first creepy images in a series of creepy images in this book is that of a pair of squished baby birds in the driveway of Libby's house. Libby describes them as if she's on all fours leaning in really close to their carcasses: "On the pavement near my car are the smashed skeletons of two baby birds, their flattened beaks and wings making them look reptilian. They've been there for a year. I can't resist looking at them each time I get in my car. We need a good flood, wash them away" (1.11).
We wonder if those two baby birds ever remind Libby of her two dead sisters, who never got a chance to grow up. Libby later describes the noises she heard during the massacre, including a "screeching sound like a bird banging into the walls at the end of the hallway" (3.154), so perhaps at one time, she did. If so, this whole book, and Libby's quest to find the real killer, is like a flood to wash the bird skeletons away.
Of course, Libby never actually mentions the birds again, so maybe they're nothing more than road kill.