A starling-deterrent wood duck nest box
In many parts of the United States, the starling ( Sturnus vulgaris ) has I become a serious competitor for nest boxes erected for wood ducks ( Aix sponsa ). Research at the Patuxent Wildlife Research Center and at Eastern Neck National Wildlife Refuge near Rock Hall, Maryland, demonstrated that horizontal nest structures with semicircular entrance holes 11 inches in diameter were acceptable to nesting wood ducks but discouraged nesting by starlings. Starlings seemed to prefer boxes in open impoundments to those in wooded impoundments, whereas wood ducks seemed to show no preference.
Maryland↗