Keep the critters out of the rain barrel

The openings into a rain barrel system need to be covered to keep out animals.  Some "hardware cloth" coarse screening will keep out most rodents, and many people also use mosquito screen to keep out smaller insects and debris.  Some people use mosquito control tablets to kill the larvae.  Some people put fish in the barrel to eat the insects, although if you have to winterize then the fish is something else to deal with.

However, a little hardware cloth won't solve all critter problems.  (I couldn't find the original source of the bear, merely references to Colorado)