Sparrows are natural predators of bluebird eggs and chicks. Studies have show that this oblong slot entrance is effective at stopping sparrows. Also, sparrows don't like a shallow nest box, but bluebirds don't seem to mind. This Coveside Conservation box is only 5" deep but if sparrows are still a problem, a wooden insert is included to reduce the depth even more. The feeder is made of thick pine for insulation, has air vents on the top and a sloped roof to help keep the feeder dry inside. You might try this feeder if you're having problems with sparrows.