How many chicks do you have? How old are they? What sort of living quarters are they currently in (size, lighting/heat, indoor/outdoor, etc)? There are several things that can cause this to start happening - some that are easily fixed with an adjustment to how the animals are being kept, which is the reason for the questions above. If you are fairly certain of the culprit, a good first step would be to partition off part of the enclosure to enable you to separate that chick from the rest to see if the feather picking stops (though, once it starts the behavior can spread).
