This was a conversation on Google Home Improvement groups that was an interesting recommendation for a Chimney Balloon to stop birds from entering a home:
Q: Hi I have never used a fireplace and never needed one but an old house I bought has one in it. I have not moved into it yet and are updating it here and there.
I was working on removing some wood paneling this afternoon and I heard some bird sounds. It seemed to come from the inside? I ignored it and then I heard it again, and from the direction of the fireplace. I walked over to the fireplace and I saw two birds, one was standing at the bottom of the fireplace and one was laying at a corner - dead.
I opened the doors and windows and went to get the live one, he freaked and started to fly around and I lead her to the doors and he flew out. I then took the dead one and disposed of it. Then I heard more bird songs. I stucked my head and looked up the chimney and it's dark. Could the birds be coming from the chimney? Isn't it supposed to be closed? I think there are more birds up there but can't find their way out and they won't come down until they are exhausted or near dead.
What should I do to free them? or should I assume that if they can find their way in they should be able to find their way out? That dead bird obviously did not. - MC
A: MC - It sounds like you have no damper on this fireplace if the birds were just all of a sudden in your hearth. I would go to the roof and inspect the cap. It is likely that there is no screen or a rotted out screen that needs some work. By this rooftop inspection You will also probubly find out if you are dealing with a nest or a couple of curiouse birdies. If the roof is a steep pitch or not easily reachable be sure to hire a chimney sweep to do the inspection.
Considering the birds came right through you may want to consider a chimney balloon to plug the flue to keep from loosing heat through the open fireplace and to block the bird noise.