This is a simplified DIY version of the classic Hironbec Pendulum, a device that is designed to gatekeep birdhouses based on the weight of birds attempting to enter. This allows you to protect some vulnerable species, e.g. Tree Swallows from larger and more predatory birds e.g. European House Sparrows.

I highly Suggest watching the video if you're unsure of how it works or what you would need to utilize it. Thanks.