Approach nest boxes and gently tap on the outside of the box. Many times the bird will fly off as you approach or when the box is disturbed. This is normal and the bird will return after you have checked the nest. Females may also sit quietly on the nest as the door is opened. In this case, close the door and return later.
Bluebirds are very tolerable of humans.
All birds have a poorly developed sense of smell and touching around the box, eggs or birds will not cause the bird to abandon the nest. All of this is part of the enjoyment and learning associated with a bluebird trail. Monitoring nest boxes and observing eggs and birds as they develop is one of the joys of having a bluebird trail. Here, an individual removes an old nest after young birds have fledged. There is one time that nest boxes should not be opened.