I know this would vary from locale to locale, but, presumably, there are some animals that do not require a permit to kill (mice in mouse traps, for example). How does this work? Is there a list of "always OK to kill" species?
Depends where you live. Wild animals, we called local animal control about a opossum we'd caught in a cat trap (trying to catch a local stray cat) wanting to know what they wanted us to do with it. and they let us know about the felony we were about to admit to committing and did we want to shut the fuck up. Apparently we needed a license to trap wild animals. So telling your local animal control after the fact isn't in your best interests, but they could provide guidance before the fact. If you're in Washington State, you leave those (just put on the local endangered list) western gray squirrels the fuck alone.
You just gotta add a sign that says "Warning: trapping wild animals without a license is illegal. This trap is for feral cats only. Any wild animals found in this trap will be shot on sight."
Holy cow. I live in Oregon and there are at least 50 western gray squirrels just around my house. There are tons along the river by our house too. I didn't know they are endangered in Washington. How weird, considering we're neighboring states (although there is a huge river between us).
I'm quite familiar with the eastern gray, and these aren't those, as the tails are much fluffier and also grayer as well as not having any yellow highlights like the eastern. And they're all big boys. Some of them could be eastern but lord either way, there are a lot of squirrels here.