Another Walk: Bluebird Pair at Work

A week ago, I took a walk in the Colvin Run Habitat (post 1, post 2, post 3, and post 4) seeking to photograph the bluebirds nesting in the bird box. A week ago, the bluebirds led me around the Habitat and proved to be elusive. Today's walk was a much different story.
Today, both bluebirds - male and female - were visibly working hard to protect the nest and to tend the content of the box. This first photo shows the male keep watch from one of his perches - on top of the bird box.
It took me a few minutes to understand what the male bluebird was upset about, but from as you can see in the second photo something has caught his attention as he looks over his shoulder.
In the third photo, you can see that he has turned to face what is concerning him. He nearly immediately flew into a a nearby bush.
Over the next ten minutes, the male bluebird would chase this house wren from a bush that is about 15 feet in front of the bird box. This was a constant chase and return. The wren did a lot of singing. The male bluebird would run him each time. Of course, we know from last year that the house wrens can be deadly to occupants of a bird box if the wren is determined to take possession.
Last year, I described the house wren as a not so friendly bird that will evict another bird’s eggs and hatchlings, and then build over the existing nest. Basically, the male builds the nest of twigs on top of an existing nest, then attracts a female that completes the nest lining of feathers, lays and incubates the eggs, then raises the young.