Is an exterior perch necessary on a bluebird nesting box?
Since bluebirds have the ability to cling, they are adapted to enter both tree cavities and nest boxes without the aid of a perch. House sparrows do not have the ability to cling, and as such a perch will assist their ability to enter a nesting cavity. It also permits easier access for other predators. It is recommended to eliminate perches not only from bluebird boxes, but also from all nesting boxes for primary and secondary cavity nesters.
