how often do indian runner ducks lay

I guess my ducks have started to lay. I saw two ravens this morning by the pond looking like they were having a conversation with my ducks. He was stepping gingerly towards them and they were swimming closer and closer to him. He stepped in the water at the edge and took off with an egg in his beak. I went out there and there was another shell that had been eaten as well. Do they just lay them in the mud ? How often do they lay? I have to find a way of collecting them before the ravens or chickens or dogs or? finds them. I think I am fighting a losing battle.