I am running multiple broods of chicks in tight quarters with adults over ground that has been getting wet. Those together promote more intense cocci infections that are more likely to get ahead of the chick's developing immune system. When cocci hit particularly hard I have been using medicated water. Outside with hens I sometime see poorer outcomes than birds of similar genetics that have been moved to garage. Two things possibly at play. First is hens stop warming chicks for extended periods (as far as sick chicks are concerned) in when temperature is low while the brooder chicks have a much more consistent environment. Secondly is the brooder is lit 24 / 7 and I have observe chicks in there getting food and water even during the dead of night while the hen-reared chicks shut down for the night. I think the brooder reared chicks, especially when it comes to hydration are in better shape in terms of medication intake plus they may be less inclined to become dehydrated.