im not sure, the experts would have more than this.
but how is it that it will be blocked off when they are out?
i know birds dont like to fly up generally but if they flew in by accident, fear could drive them upwards and they could get out and fly away.
or can that not happen? i really dont know how a fireplace works haha. sorry im a bit useless
I'm not the best expert, but my birds lived with a wood-burning fireplace for a couple of years, so here's the best I can say : first, make sure the fireplace is properly cleaned by professionals. Once a year, before starting the first fire. Birds are sensitive to smoke, but with an open flue that has been cleaned only the smoky smell, and not the smoke, will come in. But make sure you have a mesh screen and a solid screen. I would not have them out when there is a fire burning, but if there is a good screen it might be a judgement call. Whenever the fire is not burning I would make sure the solid screen is in front of the fireplace so the birds could not get in at all if they get curious. We never had drafts come in that way, but it might be a thing to think about. Oh, and get a real stand for fireplace tools (poker, etc.) that a bird can't knock over by accident. I think that's all. My birds loved being in the living room with the fireplace! Only because it was warm though. They couldn't see the fire from their cage, it might have scared them.
I had the same concern when I got our budgies. We also have a natural fireplace. We have fires 1-2 times a week during the winter and our budgies do just fine (They are about 20 feet away from the fireplace).