Catherine, Called Birdy

``You can run, but you can't hide'' is the rather belated conclusion reached by Catherine, called ``Birdy'' for her caged pets, in this fictive diary of a medieval young woman's coming-of-age and struggle for self-determination. Escaping regularly into a fantasy life of daring escapades and righteous battles, Birdy manages to postpone the inevitable sale of herself as a wife to a very unwelcome suitor. Just as she resigns herself to her fate with the comforting knowledge that ``I am who I am wherever I am,'' word comes that she will not have to marry the oaf after all. Birdy's journal, begun as an assignment, first wells up in the reluctant and aggressive prose of hated homework, and then eases into the lighthearted flow of descriptive adventures and true confessions; the narrative device reveals Birdy's passage from rebellious child to responsible adult. Despite the too-convenient ending, this first novel introduces an admirable heroine and pungently evokes a largely unfamiliar setting. Ages 12-up. (Apr.)