The Cloud Roads [Books of the Raksura 01] - Martha Wells

Moon has spent his life hiding what he is - a shape-shifter able to transform himself into a winged creature of flight. An orphan with only vague memories of his own kind, Moon tries to fit in among the tribes of his river valley, with mixed success. Just as Moon is once again cast out by his adopted tribe, he discovers a shape-shifter like himself… someone who seems to know exactly what he is, who promises that Moon will be welcomed into his community.