In Freak the Mighty by Rodman Philbrick, what do the boys do after the bird is rescued?

The short answer is that they hang out and become friends.

This happens in Chapter 3, right as Max sees Kevin leaping up, trying unsuccessfully to reach an object that has become stuck in a tree. It's a toy of Kevin's: a little birdlike object that really flies. Kevin clearly is upset; he wants his beloved toy back. But he's too short to reach it. Even his long crutch doesn't help.

Max, though, is very...

The short answer is that they hang out and become friends.

This happens in Chapter 3, right as Max sees Kevin leaping up, trying unsuccessfully to reach an object that has become stuck in a tree. It's a toy of Kevin's: a little birdlike object that really flies. Kevin clearly is upset; he wants his beloved toy back. But he's too short to reach it. Even his long crutch doesn't help.

Max, though, is very tall. After he helps Kevin by rescuing the toy from the tree, the two new friends talk about the bird toy (which is called an ornithopter) and Kevin impresses Max with his large vocabulary. Almost an hour goes by, and they have to stop playing with the ornithopter because its elastic part breaks. Kevin assures Max that it can be fixed later, and then Max wheels Kevin over to Max's house in the wagon to show him his room. That's when the chapter closes, as the friendship has begun.

This scene is important because it shows us how well Max and Kevin get along. They are instantly friends, even though they are so different--perhaps because they are so different. Max, with his enormous height, can get the toy down from the tree with no problem, and Kevin, with his enormous vocabulary, can dazzle and entertain Max.