Several Nests Built, Territorial Activity With Swallows

There are seven pairs of boxes in the South end of the park, over on the other side of the bridge. The first pair can be seen soon after the bridge in an open area. Following trails to the West end (near Coyote Point), two pairs of boxes have Bluebird nest building activity. It looks like the Bluebirds have put grass in both boxes but one box is a complete nest. With no eggs. The pairs are defending both boxes, but will nest only in one. Tree Swallows are flying around the same area, some occasionally landing at a box. One even stuck it's head in. This pair immediately came to the box:

The male took turns sitting on the box or watching from a tree. They are vigilant but since they have no eggs in the box, there is time to feed and fly around.

The second pair has chosen boxes near the middle of Ben Verley Trail, in the same prairie. It was a bit comical with the Bluebirds both defending a box against the swallows in the area. The male is on top, the female on the bottom.

The male moved from one box to the other and the female flew off. Then the male defended against the tree swallow (flying bird): 

(CLICK THE PICTURE TO GET A BIGGER VIEW)

The tree swallows were chasing each other and catching food. They know these are the next boxes, but have not built nests or claimed boxes yet. I presume once the female Bluebird lays eggs the swallows will claim the box next door.

There is still lots of time and most of the nest boxes are unclaimed. My guess is the Bluebirds choose the box based more on the location than the condition.