Why are the birds so angry?

If you've ever played "Angry Birds," you might have found yourself wondering — why am I sling-shotting cartoon birds at grinning green pigs? Why are these birds so angry? What have the pigs done to deserve this destruction? "The Angry Birds Movie" (Sony Pictures, PG) is here to fill in that back story. The film proves to be more than just a gimmick, and doesn't skimp on any of the quirky wackiness that you might expect from a film about blob-shaped, flightless birds battling pigs.

KATIE WALSH, Tribune News Service