My grandparents still maintain Purple Martin houses and have for at least 25 years. In Southern Missouri you have to mitigate all the mosquito bites in some way, just so happens, they chose to encourage Purple Martins to next near their house. The cats used to get close and the birds would dive bomb them promptly. I never saw the cats catch one, but of the dozens of Martins around I suspect they got one from time to time.
The best Purple Martin habitat almost always contains a large open field. I've seen small colonies (25 birds plus their young) on as small as a half acre tract. My Grandfather's colony consisting of well over 50 adults has about 75 acres of open terrain. The one thing I almost immediately noticed is the Purple Martin's tendency to fight for the gourd houses more intensely than the tradition multi unit houses.
I hear that a newspaper recently did an article on Grandpa's Martins. Can't wait to see it.