My 6 month old cockerel has become lethargic and weak. I've been looking at the symptoms for some of the common illnesses but I'm not finding anything that really matches him. Until two days ago he was a super active bird and the head of the flock. Besides being lethargic and weak, he seems to pant with any exertion. He's not really eating, only half-hearted at best. When left alone he will just lay down. Any thoughts?