Although there is no definitive answer, the common consensus among spiritual leaders is that a dead bird symbolizes a new beginning. More specifically, it symbolizes the end of something and the start of something else.
The "something" that is ending in ones life can be a number of things, from a relationship that is coming to an end to a bad habit that is ready to be broken. Similar to the death tarot card, a dead bird is a symbol of a fresh start from an end and not simply an end. Overall, a dead bird symbolizes a new beginning.