Hire a chimney sweep.

I suggest you have a chimney sweep clean and inspect the chimney. The first time they cleaned the chimney of the masonry fireplace in my last home, the chimney sweep pulled a very dead, very well smoked, and very dehydrated duck off the smoke shelf. I wouldn't have believed it if I hadn't seen it. He said it wasn't the first time he removed waterfowl from a chimney. We removed the rotary damper and replaced it with a spring-loaded damper and screen at the top of the chimney. The spring-loaded damper worked well. The only drawback was that a woodpecker opted to peck at the shiny metal chimney hood seasonally-- and early in the morning. It was an unwelcome wake-up call.