I have 6 1/2 week old chicks. Every once in awhile, I will find one or two black runny/gooey poops on their roost or in their outdoor run. This has been happening on and off for the past few weeks. They have been on medicated feed since day one; I started transitioning to grower feed this week. They all seem very healthy and normal in their behavior. I do give them treats - yogurt, cucumbers, sunflower seeds, etc. Any ideas? Should I be concerned?