|Purple martin pair.|
Photo courtesy of Dori on Wikimedia Commons
From experience, you may have to evict the starlings and house sparrows more than once before the martins move in. It is amazing how quickly house sparrows can stuff a space full of grass for their nests.
Once pairs are setting up house, I don't bother the martin house any more. True, some starlings and sparrows will move back in, but the martins seem to do a decent job of protecting their apartments, once they claim them.
We've had enough rain that it should be a pretty good year for these noisy little birds.