I have three Austalorps. Two are pretty big, compared to others in the flock. One is small, littler than my Polish, slightly bigger than my Silkies. All birds are 6 weeks old. This little Australorp is a real lap bird, quite the snuggler. When should I get worried that it is so little? It it "normal" to have a chick be roughly half as big as others of the same type?