Security: Everybody needs it. We all provide some of our own on different levels. Some of us have had security broken on some level, which causes more drastic measures for protection, and the bottom line is, we have property to protect. It is no different for Bluebirds. They have duties to perform for nature and life, so protecting their home is of extreme importance.
Male Eastern Bluebird on sentry duty protecting his home nearby.
Guarding territory is a Bluebird trait during breeding season. NOTE: At the house opening, a "real" conservation lady placed some wet wipes into the hole to keep a sparrow out. We felt bad for the bluebird wanting to use this box, and eventually pulled the wipes out ourselves, when the sparrow was gone.
I found 2 separate sites specific of this behavior.
Female Bluebird on sentry duty.