I don't know the answer to that. Thermoregulation would keep the body temp at a pretty constant temperature and humidity. Some birds, I am thinking ratites require much lower humidity levels for incubation. Their egg shell's and membrane's are much thicker which reduces moisture loss. I assume it is a pretty complex relationship between each bird and their egg/incubation requirements. Waterfowl require more humidity.
I have hens with eggs tucked up under their wings. I have to be careful when I pull them out of the nest to candle eggs. Now I go a bit slower and try to open the wings up before lifting her off the nest.
Both have pattern gene that makes for the white body when interacting with silver. If my guess is correct, then birds you have pictured are silverquill / redquill with silver S allele over a dark brown / extended black base that my birds lack. The dark brown / extended black base is what make so those birds have the black crescents on wings and breast my bird lacks.
I've seen that but I always thought it was yolk from broken egg causing it.
That could be in some cases but once again I’ve seen it happen otherwise I wouldn’t have said anything.
