I am not sure which are or are not but mushrooms are definately very chemically active and even small doses to a human can be deadly of some varieties.

Are they eating an amount of them or just "tasting" them? If things are bad, they don't normally go back to munching on them.

Not much advice sorry. It's a toughie. And even if they are not good, how do you stop them from eating them? Mushrooms pop up over night with the right weather.