A bird that slips out of its cage is traditionally read as a longing for freedom finally finding a way out. It can also stand for something precious that you feel has slipped beyond your control.
When this appeared in your dream, what were you feeling?
My Cosmic Code has a private dream journal. Write a dream down and the app picks out the symbols in it, shows what dream books say about them and, over time, which words recur across your entries. The journal is stored on your device only.