Chimney Swifts are making their fall migration to South America. They fly by day and roost in chimneys at night. I have received calls for
“hundreds of bats are entering my home from the fireplace”. In every instance the “bats” turned out to be chimney swifts. In every instance the flue damper had been left open. I would close the damper or somehow block the flue to prevent further entry, then spend an hour or two catching any birds inside the home and releasing them outside. If the customer demanded it, the next day a chimney cap would be installed; preferably installation of the cap would be delayed until after migration was over, problem solved.
To verify that chimney swifts are still using the chimney, conduct a sunset to dark inspection.
Here are a couple of YouTube videos.
Chimney Swift Migration Roost- Huntsville, Alabama
https://www.youtube.com/watch?v=tQ06zyx9Z_Q Chimney Swifts on Birmingham Ala. Southside-
https://www.youtube.com/watch?v=HflLHULGgmo