The single best way to keep your chimney pest-free is to have a good cap on a clean chimney. Bringing lots of other advantages with them, chimney caps are specifically designed to keep pests out of the chimney. Surrounded with wire mesh or grill work of some kind, they act as screens for both pests and debris.
It is only sensible not to cap either a chimney that is not clean or a flue in unknown condition. Often missing entirely from chimneys, caps should be replaced after the chimney has been professionally swept and inspected. Pests and their nests, or water damage, could be too far inside the flue for you to see just peering in from above or below.
The screening features of chimney caps are effective traps for debris as well, so they need to be periodically checked and cleared. That is actually a good thing because caps also serve to keep water out of the flue, so regular inspection is important for other reasons. Multi-functional chimney caps are where the whole system begins when you are looking down, and keeping pests out of it is only part of the picture.
The Pests Within
Pests can also come from indoors, mice that make their way into the home and are attracted to the dark fireplace. Fireplace screens, therefore, also serve not only to keep sparks in and children out, but also to keep the chimney pest free. Decorative and important for safety, too, they are a little like those draft-improving caps that whirl around on the tops of chimneys.
Finally, homeowners who live on the migratory paths of birds are going to want to have chimney caps. A CSIA certified chimney sweep will know what exactly is involved, and can help you make the right choice for your home.