How to tell what you have
Scratching and chirping usually mean birds or squirrels above the damper. A heavy odor with flies means something died on the smoke shelf. Either way, the flue is partly blocked, which is a carbon monoxide and fire risk.
Why nests are a real hazard
Nesting material is dry, flammable and sits directly in the path of hot flue gases. It also blocks draft, pushing smoke and carbon monoxide back into the house.
The permanent fix is a cap
A stainless steel cap with mesh screening keeps birds, squirrels and raccoons out, keeps rain off the flue, and stops sparks from leaving. It is one of the cheapest components on a chimney and prevents several of the most expensive problems.