X2 on everything said here, I lost my precious hen Daisy last year to egg yolk peritonitis...she had had problems with internal laying before, recovered, and then struggled again....and the second time, we had her put to sleep, we knew it was time, her abdomen was swollen with the infection, and hatchery stock Legs are unfortunately prone to issues, but she was a very special hen to us. If your girl was already immuno-compromised (weakened) to begin with and then caught the secondary cold somehow that your other bird struggled with, there would have been very little you could have done....chickens are small creatures and can take downturns very quickly so don't beat yourself up. I am dealing with some issues with one of my cats right now, her teeth and gums are swollen from periodontal disease and she needs 3/4 of them pulled...I should have taken notice a long time ago, but she is so outwardly healthy and playful (and eating just fine) that I didn't catch it until I was tusseling with her the other day and noticed she is already missing FOUR of her lower incisors. I felt SO SO bad after going to the vet, but animals instinctively hide their pain as a defense mechanism...in any case, hugs to you and your family...it's never easy, keep us posted on a necropsy.