First, you'll want to eliminate the possibility of parasites - both worms, and external parasites like lice and mites. Both can make chickens lose weight.
Examine the bird carefully on her skin, feathers, etc for lice, mites, their eggs, anything that moves. They're almost microscopic so use a flashlight and do it on a light colored sheet so maybe you'll see things on the sheet if you rumple their feathers.
Pay careful attention around the vent and behind the neck.
If you have mites/lice you'll need to use a treatment (not prevention method like DE/cayenne) for the mites on the bird and off. Poultry Dust is easy to apply and reapply in the 14 days or so when the eggs hatch.
Second, if you haven't wormed them yearly, it's recommended.
For adult birds who have never been wormed, particularly those who are thin, I like to start off with Wazine first because it kills roundworms (the most common worm of chickens) and requires retreatment. The reason I like a LESS effective wormer first is just in case the birds have a heavy parasite load about which I don't know.
As a former vet tech, I know very well that animals can have worms without you ever seeing one in their droppings.
I also know that the best way to tell is by having a vet do a fecal egg count (not just a fecal) but not many people can get their vets to do it.
So we worm first with a lighter strength limited spectrum wormer like piperazine 17% (wazine 17 brand for example). Don't bother with puppy/kitten formulations unless you 're great at math.
You worm them once, don't eat their eggs or meat for 2 weeks. It's designed for retreatment. So follow up wither with the wazine or, better yet, a more strong broad spectrum wormer like ivermectin, ,etc.
The reason I personally recommend the weak wormer first is that I've known poultry and other animals that went into shock when being initially wormed with the strong wormer because of the dying parasites inside of them. So I do the traditional light/hard worm method for the first time.
Then I just worm once or twice a year with ivermectin or another very broad spectrum one-time wormer. You can use DE (human grade only) or cayenne as others do in between to help prevent heavy parasite loads.
So if you decide to worm, that's the way I'd recommend it personally.
In the mean time no less than 90% of their food should be their laying pellets because of their calcium needs. YOu can give them a cup full of corn or oats to help build their body condition up. Oats have a little more protein. (Whole oats). But sometimes corn chops are easier to get from the feedstore without buying 50 pounds of them.
Make sure your girls have both a source of grit AND oyster shell or the prepared mixture if you're feeding whole grains. Oyster shell will dissolve while grit will only wear down. So Oyster shell doesn't always do the full job granite grit or similar will.
I'm noticing that a lot of feedstores now carry manno pro brand granite grit as well as their oyster shell. So if that's the oyster you use, look for the grit. Or ask for pigeon grit. Great stuff.
You can boost the absorbtion of the feed by using probiotics to increasee the bacteria in your girls' guts. Bacteria literally are the workers that feed your bird. They line the digestive tract and also help ward off illness.. But most importantly they finish digesting what the gizzard breaks down, secrete vitamins, make enzymes which make food particles into something absorbable, etc etc. They will help your birds use more of their food and resist more 'bugs' in their gut. I like plain yogurt for its simplicity. One teaspoon per adult bird mixed with crumbles usually works. Or you can mix it with babyfood applesauce and crumbles, or boiled egg yolk, etc. Anything to tempt them to eat it.
If you have them you could alternatively use acidophilis capsules/tablets. Crush the tablets or empty the capsules. don't use in water - use on foods.
If they were my birds, I'd rule out external parasites. Then I'd consider worming if you haven't. Then boost their nutrition with a daily LIGHT treat of corn or oats (not scratch) for condition after their worming. Definitely use the probiotics for the day you worm - maybe a couple of days afterwards. Otherwise use weekly for weight gain.
I hope this helps!
