The symbol of a Bluebird as the bringer of happiness is found in many cultures and may date back thousands of years. In Chinese mythology the Bluebird appears in a circle, often with three legs, as a symbol of the sun.
The Navajo identify the Bluebird as a spirit in animal form, associated with the rising sun. In Russian fairy tales, the Bluebird is a symbol of hope. The Bluebird, inhabitant of the pays bleu, the fabulous blue country of our dreams, stands for happiness.
Hiding your emotions, brings out the mask of yourself, just hiding your vulnerability from the outside World, afraid of being judged. Strength is simply the mask you put on to hide your emotions.
The death of the Bluebird would represent the loss of all innocence, of all emotions and as a result, one would become a mere shell of a person.