What scene do Mutto, Beans and Henry enact at the park? How does Palmer react to their display? Why does he react this way?

The three boys pretend Henry is a pigeon. Beans and Mutto pretend to shoot him, and then one screams "wringer! wringer!" and they pretend to wring Henry's neck. They are laughing and having fun. Palmer remembers the day he saw the pigeons shot and killed at the field and the wringing of one pigeon's neck in front of him. Palmer runs away from the scene, upset.