How can I keep animals out of my chimney?
Sounds like you need a chimney cap



These come in a variety of sizes and shapes.  They keep out most rain and snow and, most importantly, animals.

You should attach any cap with stainless screws and lead anchors. Sealant is not a big issue, since there is no need for air or water tight joints. While sealant might hold in most conditions, strong winds could rip the cap off if it is not mechanically attached.