The Trumpet of the Swan by E. B. White

Louie is very popular. Who wouldn't love a swan who can read, write, and play the trumpet? When Louie goes to camp, he meets a boy named A.G. who doesn't like birds, and since Louie is a bird, that means he doesn't like Louie.

When A.G. pulls a dangerous stunt out on the lake, he realizes that Louie is a hero, after all.