Egg shells

I have found another broken egg shell near the pond. This and last week's were palest green, similar in size to a hen's egg, and almost certainly mallard. And they were almost certainly predated; today's had very clear peck marks. The normal mallard clutch is about twelve eggs. I hope the female is still incubating the other ten, and we will see duckings on the pond soon.
 
As I pondered the mallard egg, two of the swallows swept around me, drinking on the wing from the pond. They chose the privacy of the water on the other side of island from where I was standing, so I couldn't see the actual drinking. But I googled for images, and found this impressive photo. There are often three swallows on the kitchen garden wall, which bring to mind the traditional Scottish children's song "Three craws sat upon a wa'". But from what I've learnt over these past months, crows tend to go about singly or in pairs, and I would expect the third craw also wisna' there at a'.