A sick or ill bird often will also stay in the nest box because it can be safe from other birds there. Will not protest when you are near the nest. When walking will often look wrong tail will droop, chicken might walk and then lay down again. An ill bird might lay.
Yes, more like this...
My two year old Red Star Olivia (in my avi) has been acting broody the last couple of days. Yesterday I brought her out of the coop and she drank some water. She had very watery stool. Today I found that she was sitting on an egg with a very soft shell that was cracked open and smooshed. The...