The 1 1/4'' diameter opening is sized for small birds such as wrens, chickadees, titmice, finches and nuthatches. The opening can be enlarged for larger birds as desired. If birds don't choose to nest inside they can pull at the wool for material to feather their own nests (one of our customers HAS reported seeing a bluebird go inside!). Comes partially filled with recycled paper to maintain the shape in shipping that can easily be removed. If exposed to sun and hard weather, the colors and shape should last over a year in hard weather, and forever in a protected place such as a porch or even inside. Clean out by opening the bottom.