With Purple Martin season in full swing and with nesting starting, the Starlings are trying to lay their eggs in there for the Martins to raise or the English House Sparrows are trying to steal a cavity and will peck holes in Martin eggs to do so.
Don't worry about the legality of shooting them because both Starlings and English House Sparrows are listed as Non-Native Invasive species the same as feral hogs so it's open season all year long.

I originally built both of these about 10-12 years ago and both had small leaks when aired up this season.
They are powered by 12 gram CO2 cartridges and get 30+ good consistent shots per powerlet.

The top one is a .22 cal heavily modified using a true 45 grip frame (regular 1911 grips will fit) from a paintball marker with hand bent stock.
It's tuned to push the .22 caliber pellet a little faster and does better in the wind.
It has flowed valve body, turned down valve stem, heavier hammer spring with adjuster, polished hammer, trigger job, etc.
Crystal clear 6x original Bug Buster scope with AO focus down to 3 yards!

The bottom one is .177 and was originally built as an in-town squirrel shooter and is cut down to be handy out the back door.
It's running stock valve with minor smoothing/matching of air port to barrel.
Polished hammer, trigger job, not-so-good hand me down scope for now.

I also have a Williams match peep/open sight that fits on back in place of the scopes on either one.


This one will hit the work bench next.