Thanks, Marie. Yes, the heron flew off - they're quite timid now because always being chased. Thing is, my fish are real pets, with names, so I feel a responsibility to protect them. And there's a fishing lake not too far away, so the herons should learn to go there...
What I find worrying in the collared dove's choice of nesting place is what happens when there are little fledglings that need to leave the nest? I hope they don't get flattened before they can fly.