We have 4 black copper marans that are about 6 weeks old. One of them is not growing as fast. He/she is pretty much just as lively as the others, but tends to stay under the heat lamp a lot more than the other 3. The chick sometimes has a runny poop that does not look normal - but not on the poop chart. It has the color of normal poop but runny instead of firm. We feed them organic chick starter and not much else - occasionally organic lettuce and grit. Any ideas?