Oh, it's bats. The one I chased (more like opened the doors and encouraged it towards them)out was quite healthy. The thing is, I don't think they are in the flue. I think they are in the back of the wallmounted fireplace, and possibly in the wall. This item is about 40 years old, quite large, and is probably yet another example of the poor skills of a previous owner; there's possibly a small crack or hole around the flue exit.

I'm in Nova Scotia: our bats mostly hibernate. We seem to have a good sized local population where we live. Thanks all for the kind advice!