This is probably heavily influenced by area... for me most recently it has been hawks. In the past dogs, raccoons, skunks and opossum were all problematic. We had dogs scale the fence to get at birds even. Other listed critters got the birds various ways. As a result when we started keeping again the whole coop area was placed on brick (prevent burrowing) with smaller than chicken wire used in coops (to prevent decapitated birds) surrounded by a steel fence in a yard with another fence around that (keep dogs out)... no digging under and 3 layers of fencing of different types to get through... worked great till the hawks... so now added an additional fencing onto iron fencing to prevent chickens exiting to main garden during day to free range, and mesh covering over top of coop area to prevent day time chicken lunch take out by hawks.... other critters in my area include coyotes, mountain lion (very rare), snakes, cats, bobcat, rats, weasels, owls (though none of these have not been an issue) and yes people... I suspect one bird was stolen. Now locking gates.