Hmmm, are all of their poos brown and runny or are they having green firm ones too. I've found that my chicks have a few different types of poo. Sometimes it's more yellow, sometimes it's green, green with white caps, or loose brown poos. I called our feed store about it, and they said chick poo can range b/n yellowish to brown to green. This rainbow is all normal. Keep an eye on them to be on the safe side. Have you been giing them sugar water or electrolyte mixed water. That changes their poo too.
Sonja