As you thaw the skins make sure they dry completely. After drying you shouldn't have a problem with keeping them.

As for the leagal aspect. Call your local department of environmental conservation or equivellent. Tell them your situation: You found some old bird skins with feathers and are unsure if any of the skins are of endangered species. They will tell you if they want to idendify them or if you are safe and or what you should do if you are not safe. After the call do what they say. If you can keep the feathers make sure they never leave your home so there is no confussion by other officers that may see them. But good find, good luck with them. Oh, MAKE SURE THEY NEVER CROSS STATE LINES if you can keep an endangered feather in your state. That only complicates matters if you are ever caught.