1/4 inch welded wire.... 1 foot wide.... buried 1-2 inches underground.... from right under main above ground fence to 1 foot away.... no diggers will be able to get in.. raccoons, fox, coyotes, wolves, badgers, dogs, cats,
Above ground, as long as you have 2x2 welded wire almost nothing will get in except rare weasels like mink (in you are close to water), martens(if you live in Northern Canada).
Even bears won't waste time getting in. They do not care about chickens, they only would want the food so keep it in the coop and not in the run.
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