It takes a while for the Purple Martins to learn that mealworms are a good food. Then, the other martins learn from watching the initial birds feed. Ed Donath and a number of others have saved their colonies by feeding mealworms. Protect your martins from house sparrows and starlings. There is no way to have a successful martin colony if sparrows are allowed to nest in the boxes also.
Sparrows are public enemy number one in the bird world.
They are belligerent, malicious birds. One couple has been known to ruin a whole housefull of eggs by pipping or throwing out every egg in the house. They have also been reported to have pecked babies on the head, killing them and moved in, building their nests on top of the dead babies. An effective method of sparrow control is by trapping. By trapping during the non-martin season, most of the sparrows become wary of the houses and are not as much of a problem during the martin season.