Ok you have me convinced. I had never considered the stomach contents issue, but that makes perfect sense!!
I've always assumed many North American colubrids are pretty flexible in their dietary capabilities, and aren't tied down to a handful of prey species. Snakes are opportunistic feeders, and have to be even more opportunistic as hatchlings to stand a chance at survival. Just as well, north american colubrids, and corn snakes especially, have an extraordinary flexibility in habitat tolerance. This SHOULD translate into at least more-flexible-than-usual dietary tolerance.