You can purchase a sparrow spooker for $19.95 on the website Rich listed above. A lot of people make their own. You can google images of different homemade sparrow spookers, which should be easy/inexpensive to make. Since you would have to wait to get one shipped, you may just want to go buy some strips of Mylar & find a way to hang them where bottoms of streamers are all touching your box. If that was a house sparrow egg, I’m afraid your bluebirds may be headed for trouble if you don”t install one. No guarantees, but it”s the most effective tool outside of trapping.
Lastly, make sure your bluebird pair accepts it after installing. Hope it all works out!
Nicole 