I have 4 Orpingtons and 3 Barred Rocks; the Barred Rocks are about 5 months old, and the Orpingtons are about a year and a half old. The Orpingtons have never put on their full weight; they are only about 5 1/2 pounds right now, which is drastically underweight for the breed. They have prominent keels, and two of them have slightly paler combs (the other two are vibrant). I have wormed them with piperazine, once about 7 months ago, and once a month ago, but I haven't seen any weight gain. I know that piperazine only affects roundworms, but I don't have access to anything else, because I live in Canada and piperazine is the only antihelminthic available (I even tried ordering broad-spectrum dewormers from the US, and they wouldn't ship them to me). They are seemingly active birds, I can't see any external parasites, and they continue to lay even though it is December. I feed them 16% layer pellets, they have access to oyster shell, and I throw them a handful of scratch once a day. They have the entire backyard fenced off for their run. There are no overt health problems, but I am really concerned about their weight. Any advice?