Tree Swallows using a Hironbec Pendulum

Hironbec Pendulums were designed by Rene LePage of Quebec to protect tree swallow nests from destruction by house sparrows. They are not traps. They do not harm any birds. They utilize a counterbalance to permit the lightweight tree swallows (20g) to pass through into their box, but tip to the right, blocking the entrance into the box when house sparrows (30g) try to enter.