Cooped the chickens last night. Fed & watered them this morning & all was well. Upon returning home, (9 hrs. later), both of our Silky roosters were dead & one Americana hen. We let the rest of the flock out & they were fine. Not even an hour later 3 more Americana hens returned to the coop & dropped dead! They are not at all "old", they hatched in April. What the heck gives?