You will find a slew of info and photos to identify the HOSP's and information on them. A very useful site dedicated to bluebirds but also contains pertinent other information!
About 6 years ago I began noticing beautiful 'blue' birds entering a nesting box which I had for decoration. I started researching and found them to be Eastern Bluebirds and their numbers were on the decline, in part to non natives taking over nesting boxes or cavity nests by the HOSP.
The TRES also have a problem using nesting boxes in our yard.
After discovering the bluebirds and that website above, I then realized we had been feeding and welcoming the HOSP to our bird feeders. Next-research on how to get the sparrows under control. The first year we captured a couple hundred over the season. That's the adults and juveniles born for that season. To date have caught about 400-500 over 4 years. No you can't be 100% sparrow free but at least it's under better control. WE still have problems with them taking over the nesting boxes but we are at least having success with Bluebirds and Tree Swallows having a brood.
The BlueBirds have two or three broods a year when left alone.
The Tree Swallows have a brood a year on a occasion they have been known to have more than one in a year.
Both have been ousted from nesting boxes at least once or twice during the season.
These are the traps I use-
One is for the nesting boxes-Vanert Universal Trap
The other is a Deluxe Repeating Sparrow Trap
Your traps must be closely monitored at all times. You don't want to trap native birds!