I have 8 week old Wyandotte chicks, 1 little rooster became 'sleepy' quite soon and died after a few days. For weeks all went well. Now my 3 hens become 'sleepy' suddenly as well and 1 died last week. The 2 other hens are also 'sleepy' and I'm afraid I'll loose them too; the last 2 nights they also didn't find strenght anymore to walk upstairs with the others to the night coop. The last one, also a rooster, is perfectly fine; running around with the adults who are strong and healthy as well. There is nothing to be seen on the young ones, they look perfect, eat and drink, have clean butt's, the weather is not hot but they are just getting weak, drop their wings and sleep more and more untill they die. What could this be?