Designed to be the most functional and bird-friendly bluebird house on the market today, our Ultimate Bluebird House is enjoyed by customers and birds nationwide! Designed to allow nest-viewing, provide protection from predators and blowflies, and for easy cleaning, this house is the premier destination for bluebirds everywhere. 1.5" entrance hole. Built with recycled poly-lumber that is guaranteed to never crack, split, or fade. Mount on a post or a pole with a flange and watch the bluebirds come.