Kyla, from my (limited) experiences that's what first happens when a semp gets too much water in soil that doesn't drain well enough and perhaps too much sun. I always see it first in a larger hen. In your pic, you can also see it starting to affect surrounding chicks if you look very closely.
I am trying much more sharply draining soil this year to see what happens. I also only water late after the sun has started to set and temps have cooled. I have noticed fewer losses since.
Anyone with more experience and knowledge, please feel free to correct or contradict me if I'm wrong.
