Condo living for bluebirds?

I noticed someone flying over toward the purple martin house with nesting materials in their mouth.  I was too far away to see the species, but the long pine needles were evident sticking out on each side of the bird, like whiskers on a cat.  Purple martins appear each year, but fail to move in.
   

I grabbed the field glasses and took a gander.  Well, what do you know, it is a bluebird.

Now, I did have two fledgees of bluebirds out of that condo in the summer of last year.  And the nestbox was only used for two broods in 2011 although in past years it has been used for three.
   

I will have to pay close attention to see if the nest box is used a third time this year.  The baby bluebirds from 2012 brood 2 are a due to fledge this week.  I was saddened that only two of the five eggs hatched, but the two babies look plump and healthy.
   

Now I wonder if perhaps the adult pair that use the nest box try to get an early start on brood 3 by making use of the martin house.  They can build while the young ones are still home in the main house.  They are building in the lower level, unit 1.
   

Of course I'm not sure how the Martins are going to feel about someone moving in.  Maybe they rented it out to the bluebirds, after all, it does have a wonderful water view and I'm sure they could get a tidy sum for it.

p.s., The bird hanging upside down in the photo is a decoy to entice the Purple Martins to move in.  We don't have acrobatic birds...except the plastic ones.