Deterring House Sparrows and European Starlings. As a nest box monitor, your goal is to provide a safe environment for local cavity-nesting species to breed. For the reasons outlined above, we strongly encourage that you do not allow House Sparrows or European Starlings to breed in your nest boxes.We recommend that you take measures to prevent them from breeding in your boxes. Unfortunately, a completely sparrow-proof nest box does not exist.or the reasons outlined above, we strongly encourage that you do not allow House Sparrows or European Starlings to breed in your nest boxes. We recommend that you take measures to prevent them from breeding in your boxes. Unfortunately, a completely sparrow-proof nest box does not exist.