Green Heron - Butorides virescens - although you may have to click for the enlarged version to see it. I didn't know what it was until I got home and looked at it full size. All I could tell looking through the telephoto lens was that something shaped rather like an anvil was at a place where no such object should be. I know the features of the pond so well that, even though the bird never moved while I was there, I knew it had to be some sort of animal.