"Herons aren't white," I said. I tried to observe it more closely using the small binoculars. It had unhelpfully folded itself up so it was just a large white blob.
"It had a long neck," G said.
"But herons are always grey," I said, "though I have seen them perch in trees before. Perhaps it's an egret," I added.
"What's an egret?" G asked.
"I don't know. It's just a name that floated up from the depths of my memory," I said, "but I think they're white and have long necks."
At this point the bird flew out of the tree and flapped off into the distance, but I couldn't follow it with the small binoculars and so couldn't really see any more detail.
So we googled "egret" and discovered that it's a variety of white heron and they're reasonably common and spreading north. So I think it's safe to say that's what it was.