I've gotten in to purple martins and had most first successful season last year, hoping for a good one this year. I always have my eyes open for these things, they can wreck a pm colony.
I expect you already know about starling-resistant entry holes
https://www.purplemartin.org/purple-martins/competitors/64/non-native/The half-moon crescents block starlings most all of the time. Sparrows can be trapped out using a house opening or gourd with a entrance shimmed down too small to let a Martin in. Starlings are the same size as martins but they can be trapped with nest box traps set against a wall, martins usually won’t enter cavities without 360 deg clear vision all around.
