Wildlife utilize chimneys as they would a hollow tree. Animal such as raccoons commonly nest in chimneys year-round and rear their young in them as well. Often the decision to live in a chimney of a home compared to the attic of home is based on learned behavior passed from female to young. If a female gives birth to young in a chimney they are likely to seek out a chimney of their own to occupy when adults.
Other animals such as birds and squirrels can become trapped in chimneys due to their inability to the climb terra-cotta flue liners or fly vertically up and out of the chimney in case of birds. However, the chimney swift, a federally protected bird, does have the ability to nest and fly vertically up and out of an uncapped chimney.
The solution to wildlife in chimneys in the installation of stainless steel chimney caps after removal of the offending wildlife, or as a preventative measure to stop such entry. As an added bonus, chimney caps limit water intrusion which is much more damaging than wildlife ever could be.
Stainless steel chimney caps should be obtained from companies such as HY-C, Olympia or Copperfield as they offer true wildlife protection. Unfortunately, the caps offered at big box stores are not secure against wildlife entry or high wind due to lack of mounting tabs. Caps of aluminum are often gnawed into and black galvanized caps rust and cause staining to the chimney.