Oh yeah the lasw are wack in some places. If you are in the UK its pretty safe to assume collecting most things is okay. Double check species that are rare or unique. Things like cetaceans and otters require a license to own.
If you find something (in the UK at least) and you arent sure of its legality its usually safe to keep it for a few days while you email the wildlife law people (like the DNR equivalent in the US i forget what they were called) and ask them. If they say no you can just throw it back into the woods and they wont get mad. Its just worth asking. The same thing might work in the US but I cant grantee they wont go after you. I don’t have much experience collecting in the US.
Some things worth googling:
“[animal name] parts legal to possess”
“[state/area] roadkill collection legal”
If you’re in the U.S, you can’t have any parts of migratory birds, which is most of the birds, no matter how you got them.