Silverwing (Silverwing #1) by Kenneth Oppel

Shade is a young Silverwing bat, the runt of his colony, determined to prove himself on the long and dangerous winter migration to Hibernaculum, millions of wingbeats to the south. During a storm, he is swept out over the ocean - away from his family, his friends, and the only life he has ever known. Alone and frightened as winter fast approaches and temperatures plunge, Shade sets out on a remarkable journey to rejoin his colony in the south...