Chapter 1 sets the stage for Doug's adventure in his new town of Marysville, though at first it doesn't seem like his move there will be an enjoyable or beneficial one, especially since he has an unpleasant "red in the face" father and a vicious brother (brothers or sisters who hurt their other brothers or sisters is now called "sibling abuse" and is not mistakenly labeled "natural" as it once was). All elements in Chapter 1 are therefore important to characterization and plot while the central idea is introduced well into the chapter. The central idea is then confirmed with a symbolic representation that also serves as an element of foreshadowing. What is the central idea?
Immediately after Doug moves to Marysville, he comes across the path of the girl riding the pink bicycle and looking at him in the same way that the sleepy lounging dogs on the front porches looked at him: they looked at him as if they knew he did not belong there. The girl with the pink bicycle and the pink-wrapped chain lock go into the marble library and Doug, in an odd mixture of resistance, desire and doggedness, goes in after her.
While he is hoping, as he watches her go in, that she might trip on the top marble step so he might decide not to help or maybe to help her, he is glad when she does not see him trip on the top step. Inside the library, at the top of another flight of marble steps, Doug finds "a big open room with not much" but a painting and a "square table with a glass case on top." In the case is a "huge, huge book" with its pages open to one displaying only one picture: "Of a bird." This picture of the bird presents the central idea that is expanded upon and confirmed later by the girl's bottle of Coke.
Symbolically, Doug is the bird. The central idea is how alone Doug is. The expansion and confirmation of the idea is that Doug finds he isn't that alone after all (foreshadowing the ending): There is the girl and she shares her Coke and her understanding of life with Doug, so Doug isn't so alone after all.
This bird was falling and there wasn't a single thing the world that cared at all.
It was the most terrifying picture I had ever seen. ...
... Dang, he was so alone. He was so scared.
She put the bottle down and wiped her lips. "That's how you drink a really cold Coke," she said. "Now you."
So what would you do? I lifted the Coke to my lips, tipped the bottle up, and gulped, and gulped, and gulped.