second chicken gone, both with same symptoms. What can it be?

[belwelch]

I have now lost two chickens with the same symptoms. They have a high pitched chirp and severe weight loss. By the time they start chirping it is to late to save them. I am not able to catch them easily so I don't try because I am afraid I will over stress them. A friend gave me these chickens and I lost one with in days of getting them. I thought it was from stress. It has been a couple of months and now I have lost another one. They show no signs or symptoms until it is too late to do anything for them. Any suggestions?

 

[belwelch]

I failed to mention the ages of the chickens. They were both around 15 weeks old. Mixed breed. Both pullets.