In the past 6 weeks I had two pullets die at approximately 20 weeks. Same symptoms, as they first became listless and lethargic and then their wings began to droop and they kept their head tucked into their body. Next they would basically lie on their side and shortly thereafter they died. This process occurred over a few days. Today I noticed one of the pullets demonstrating similar characteristics. I have limited experience with chickens and zero experience with determining illnesses. Any advice would be greatly appreciated as I don't want my entire flock to suffer the same fate. Thanks.