I've never had a problem with graybands but I have with mexmex. I'm looking towards 3 potential solutions:
1. Nutrition - a lower fat diet, ie lizards and quail vs. strictly mice.
2. Better nest boxes - Boxes that provide complete darkness might make the snakes feel more secure.
3. Larger cages - Provide greater opportunity for physical excercise.

Another thing to think of is that in captivity there is no natural selection so there's probably a higher percentage of unfit snakes in captivity than in the wild.
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