I wish I could help, but there are a lot of things it could be! When you have chicks outside they are a lot more susceptible to a variety of illnesses. I'm not a vet, but it could be gapeworm because of the pheasants. I think you would be better off waiting for somebody who knows what they're talking about to answer (I never let my chicks out till they've feathered, so I don't know too many outdoor sicknesses). I'm sorry! Your best bet is to take them inside. Watch their behavior closely. And DEFINITELY separate the sick chicks from the healthy ones, because it might be contagious. Even so, prepare for the worst. Generally when chicks that young get a sever respiratory problem, they don't recover. They don't have a good immune system. Do you have a vet you could consult?