I recently had a broody Welsummer hatch out a brood of 2 chicks and had a shipment already coming in. I put them in the brooder together 18 total but plenty of room. She took to the new babies no problem. its been about 4 days and on the fifth day I go to look in the brooder in the morning and 2 of the older chicks (I got a mixed shipment of 1 to 3 week old pullets) was flattened out and dead. No other injuries I could see and they were fine last night? I got these chicks the same local breeder I got them from last year. Any suggestions? I removed the Mom but she is mad and doing her best to try and get back.