If you plan on breeding these chickens then I would most definitely cull the chick. If it isn't getting any better within the next day or so then I would cull the chick. Personally I cull most chicks that are failing to thrive such as this one. I have birds that I breed and I want to use the healthiest birds as possible. If I were to save a chick that didn't thrive well while it was younger and put it back in the breeder pen while an adult. Then my chicks that came from that one wouldn't be the best( small and unhealthy looking ) and I would just be causing my self un due stress because I wanted to save one chick. So my answer is cull your chick so it doesn't have to suffer Some one else may come on here and argue that it can and should be saved but this is all a matter of personal opinion.