Birdie is open for interpretation.
Some speculate that the "birdie" is in fact, a plane with bombs/chemical weapons. It is flying over continents, such as Africa and Europe. Leah must stop the plane somehow, being told to kill it and not to trust it by a mysterious person.
'Birdie' is shown at the end of the game as a white bird (possibly a dove) with a yellow beak and black eyes; it is strong enough to carry a 9 year-old child. However, it could be a representation of a war plane carrying lethal cargo (like explosives or harmful chemicals) due to theories.