drhome is correct. You need to get that bird out of there!! His recommendations wouldn't work in my house, though, as we have a high efficiency furnace and water heater. They vent out the basement wall, no flue. And we have an zero-clearance fireplace, not a masonary one. Therefore, no clean-out for it.
If you have a similar situation, call a licensed chimney sweep. The sweep has probably doen this before and can replace the cap. Besides, the flue probably needs cleaning anyway.