Nick Park does the inventive and very funny “Wallace & Gromit” short claymation films for the BBC. This is his first of hopefully many long-form features for Dreamworks Pictures. It is essentially a send-up of classic war prison break films like “The Great Escape” and “Stalag 17.” The claymation is amazing and the characters are somehow all very real. “Chicken Run” does not patronize its viewers or let them off the hook. By not backing away from the life and death consquences of the chickens’ situation, Park creates real drama. It also helps make the lighter moments funnier. And the wild chase scenes are truly exciting and over the top.