Rocks at back door !

I’m wondering if anyone could shed some light on why magpies or crows are dropping rocks from our roof please? It’s happened everyday for a week now and at the same time each morning! I’m not sure if the rocks are dropped by accident, but we hear them tumbling from the roof and find them lying by the back door and also now around the front on the decking. I would love to know why they do this ! Many thanks in advance.