wrote

have one of those aluminum caps. The house apparently went decades
with out one. On my street I say only 50% of the houses have chimney
caps.

Get one. Its not just for animals. It's also rain. Over time, rain coming
in damages the chimney just like eventually you get small mortar repair
needs on the outside. It can't be reached on the inside though very easily
so is $expensive$ to fix. For 35$, you won't have that problem.

There are 2 types. The 'tension' type that just holds ther with a sort of
'spring' which i do not advise, and the bolted sort (small bolts are used to
hold it firmly). Stainless Steel is best (mine is aluminium and we replace
it every 3 years or so).

As to putting one on, any handyman capable of getting on the roof, can do
this one. No special 'roofer' or 'chimney person' required.