I feed Martins egg shells once they start laying eggs. When you see them taking green leaves into the gourds. I know Ed supports oyster shells but our Martins just wouldn't eat them no matter how small they were ground up, so egg shells it is.
I did learn quickly the eggshells have to be white, Martins aren't interested in brown egg shells. I have all of our neighbors save egg shells in freezer bags and stored in the freezer in the off season. I have noticed that once the babies start developingg feathers the adults will feed them egg shells to strengthen their bones.
The Martins will land beside me when I'm putting them out in the mornings. That's how important the egg shells are to their process.
Welcome to the internet's gathering place for Purple Martin enthusiasts