Baker wrote:
Where did you get your box design? They look a bit different than the Kestrel boxes we've built and installed. Ours were designed by a biologist from the PA Game Commission.

It's a multi-purpose kestrel or screech owl box, it was purchased from Massachusetts Audubon Society. Here's their nesting box page with general recommendations and box dimensions for various species:
Nest box specs

For this location there are no squirrels, it's in a stream floodplain with mostly cottonwoods, willow and a few ash trees in an otherwise open marsh and wet meadow. It's heavily patrolled by red-tailed hawks during the day mostly looking for terrestrial rodents and mourning doves etc., squirrels don't have a chance in there. We realize that we probably won't get nesting occupancy the first year. We're planning on putting up more boxes. There are screech owls in the area but the one tree that has good cavities is starting to fall apart so we'd like to give them some options. Screech will move from cavity to cavity on different nights to roost so multiple boxes would be very good for them. Even if we don't get kestrel nesting we'd be happy to provide a screech with a roof for the night every now and again.

In general we won't be doing any evictions of "undesirables", the idea is to increase the cavity nesting and roost options, we'll let the birds sort it out. For bluebird nesting boxes people often do a lot closer management and kick out house sparrows but this is a "let it be" scenario. There will be annual clean out and basic maintenance.

Thanks for the comments and feedback!
-moss