What is the theme in Catherine, Called Birdy by Karen Cushman?

Catherine's independence or freedom is one of the most important themes of the book. Catherine often comes up with many plans to be free and do exactly what she likes - travel overseas, spend her time reading and creating books in a monastery - however, at the end of the day, she is trapped and must submit to her father and society's wills. This is strongly symbolized by the birds she keeps in cages in her room. Even though they are in beautiful cages (shaped as castles), they - like she - are trapped, only to be set free at the end of the book, before her wedding to Shaggy Beard. Catherine's encounters with numerous characters also underscore this desire for independence from social hierarchies and rules.