Any evidence of wild possum learning ability?

I have “vermin” killing terriers and possums/ raccoons coming into my backyard. My dogs generally go out on a schedule (10pm last call then 630am and remaining daylight). To prevent altercations, I’ve put bells back door and ring them loudly hoping to give animals time to get out of yard or up a tree. Doesn’t always work. Had an incident at 6am yesterday Anybody have a better solution?