This afternoon June 9, 2016 I took a few photos of purple martins descending on my driveway to eat and collect egg/oyster shells for their babies. Every season I place egg/oyster shells on my driveway to provide a source of calcium/grit for the martins and their young. Female martins need calcium for egg development and the babies need it for bone growth. All martins may need some grit to help in the digestion of food. Martins have a gizzard which is muscular and may contain grit that helps to grind food down for digestion.
Martins in my colony definitely prefer egg shells but I can’t possibly save enough of them for my super large colony. So I also use pullet size oyster shells I get from a local feed store. Oyster shells have high calcium content and probably are a good source of grit, too. The martins will eat the oyster shells and feed them to their babies.
Here are two photos show a good size flock of martins “attacking” egg/oyster shells I had just placed in my driveway:
Steve