Entrance hole is 1 1/2 in. diameter. You will appreciate this beautiful Bluebird house. Made of 3/4" Western Red Cedar, both sides open to reveal a plexiglass partition to allow for safe observation of the nest. An aluminum screen covering attached above the bottom floor discourages larvae from attacking nesting birds. A copper insert predator guard protects the entrance hole and a generous sized roof offers more protection from the elements. Brass hinges and zinc chromate screws. Made in the USA. Approved by the North American Bluebird Society.