Replaces any door in the Martin house and turns it into a Sparrow trap. Uses no bait. a signal flag pops up when a sparrow is in the trap. Trapped Sparrows are captured in a plastic cup with an attached plastic bag, eliminating the need to open the compartment. One reusable bag is supplied. Door is 6 1/2" x 6 1/2" x 4 3/4"