Hi Kendra Yes you should wait until after the first egg is laid to put up the spooker and then watch to make sure it is accepted. I would put up a trap box near the blues box and if possible I would make the hole 1 1/4" instead of 1 1/2" to discourage the Blues from checking it out. I have another box about 50 feet away and in sight of the box they have chosen. I wait until hosps get interested in it before I put the trap in. I cover the arms with straw and scotch tape and tape a leaf over the orange dot. and that usually does the trick.
Good luck
Cathy

Thanks so much for the information Cathy. I was really worried yesterday because the Bluebirds had built a complete nest, but the HOSP was determined to claim the box. Since I didn't have any replies yet, I decided to just put up the bracket for the spooker (no mylar yet). I don't know if the HOSP thought that was scary in itself, but he hasn't been back since. The Bluebirds and Chickadees don't mind the brackets at all. They use them as perches. I will make a hole reducer with a 1 1/4" hole and put that on a spare box to see if I can entice the sparrow, while keeping the Blues from entering it. That is actually an excellent idea. I will also try covering the arm of the trap and the orange dot. Thanks so much for your reply.

The bluebirds might try to claim the trap box too, if it is in what they think is their territory. But the HOSP will find the empty box the easier box to try for as the path of least resistance. (I must add that I've trapped a bluebird with a 1.25 hole; he somehow squeezed himself in there.) Always monitor traps frequently!
I keep my trap box on a pole that I can easily move. When I see a HOSP make an attempt to take over the bluebird's house, I put the trap box rather nearby so the HOSP can easily find it, then move it at least a few yards every so often; and the HOSP will move with it. Toss some white bread chunks nearby for them to eat. Let them get comfortable with the box. Usually if I set the trap after dark, they will go in the next morning.
BlueBars

BlueBars-
I am responding to your post above about BB's "claiming" a box they feel is in there territory, even if it is not used for nesting. I have 2 boxes in my backyard, one for BB's and one for trapping. I have a BB pair that frequent my back yard but are nesting in a box a few houses away (probably about 100 yards). There have been times that HOSP's will come check out the boxes in my yard and the male BB will chase them off (he will chase them away from the trap box, which I am hoping the HOSP will enter). I am wondering if the BB is being territorial even though he is not using these boxes. I am also wondering if he will keep away another BB pair as well, by defending what he considers his territory.
Thank you,
Alison

Alison,
Yes, I think so. But they can't be at every nestbox all the time. A sneaky HOSP might still have a chance to pop into a nestbox nearby when the bluebirds are not watching, and even nest there. I've had more than one (new) bluebird host tell me that their bluebirds and HOSP nest nearby peacefully; however, it does not last more than one season, as the HOSP multiply to fast, and are more aggressive.
Bluebirds are very territorial, and the size of what they consider "their territory" varies. Bluebirds will often tolerate nearby nesting of non-competitors, such as tree swallows, even in side-by-side nestboxes. Some people have hosted 2 bluebird pairs by blocking their view of one another (one in front yard, one in back, blocked by human house).
I found this at distance for bluebird nestbox spacing at http://www.sialis.org/ :
"Recommended distance between nestboxes: 125-150 yards apart."
They also get very territorial at "their" mealworm feeder, especially during nesting seasion. In winter, they are more willing to share.
BlueBars