Greetings, I am Dr. Pat. I have worked with birds for many years. I will do my best to help you. I am sorry for your losses, that is a terrible situation.
At this point, you need help in finding out the causual factors. And it may be quite complicated. The ex-batts are going to be compromised from the start, and may even bring in disease to a regular flock. You need some serious diagnostic work. Post-mortem exams on any deceased will be extremely valuable as well.
The problem you have here is to find out what exactly is the cause. There is nothing specific in what you describe, quite literally 100 different diseases can have the same presentation. Luckily there are labs that work with small producers and an start diagnostics for a very reasonable fee.
There are so many myths in poultry medicine and diseases that I do not have the time to go into every detail, but botXXXXX XXXXXne you need facts to do right by yourself AND your birds.
The signs you describe are of a very sick birds, and not specific to any one disease. At minimum there is severe GI problems, the "sloshing" being fluid in the crop (usually caused by a blockage or irritation further down the GI system). And that means it is not fair to you or her to guess, there are so many possibilities.You are going to need local help on this, and a scientific and solid diagnosis to find safe and effective treatment for the remaining flock.
You can examine any sick bird thoroughly again, including opening the mouth and having a good look in there for mucus, redness, masses or anything else unusual. You can take the temperature gently with a rectal thermometer. Anything above 105F/40C is significant. Palpate the tummy for an egg, fluid, lumps or anything else. Check all the joints for swelling, pain, and mobility.
Move sick birds indoors to a pen, box or carrier with soft towels or hay in the bottom, no perch, and food and water in low bowls that can be reached easily.
Do not try to force food or water. You can offer warm cooked rice, pancakes, cornbread, grapes, melon, greens in addition to normal food.
I know it is expensive, but you may not have many home options, because the first thing you need a vet for is to find out what is going on. Treatment is only as good as the diagnosis. If you call around, you may find a vet to work within your means. We certainly try to do our best in my clinic.
She needs to see an avian/poultry-experienced veterinarian
ASAP for complete examination, diagnosis and appropriate treatment. Checkhttp://www.eaavonline.org/index.php?option=com_content&view=article&id=4&Itemid=10 http://www.parrotpassionsuk.com/Advice/Uk_Avian_Vets.htm http://www.theparrotsocietyuk.org/index.php/Avian_Vets/28 http://www.avianveterinaryservices.co.uk/http://www.birdvet.co.uk/http://chickenvet.co.uk/http://poultrykeeper.com/poultry-vets-uk/poultry-veterinary-practices-services-uk/
The expense for this is going to be a lot less than inefficient, ineffective, dangerous treatments, guesswork, and loss of the flock; not to mention possible implications to human consumption of tainted eggs. Many states/governments have poultry diagnostic labs that charge very reasonable fees to test for common diseases. In the UK, check: http://chickenvet.co.uk/lab/index.aspx
Because you have others and presumably use the eggs, it is important to have a solid diagnosis and treatment safe for egg consumption.
If these were my patient, I would start with complete fecal analysis and direct smear, for multiple parasites; bacterial culture and sensitivity of the feces and choana. Depending on the case I might do a fungal culture. Routine blood work is necessary to rule out other issues. I would very likely order a number of DNA tests for poultry viruses as well. Generally I start them out on antibiotics as indicated by the tests. You may need feed analysis, or even evaluation of emergent plants where they feed, looking for toxic species.
Pet/feed store medications and home remedies are harmful, ineffective, immuno-suppressive, and make them much worse and may interfere with the veterinarian's diagnosis and treatment. Do not use them.
The flock should be on a high quality pelleted diet with extra greens/pasturage. Of late, I have been recommending TOP pet bird pellets (see link below) as they are more appropriate for the pet chicken. Overcrowding, cleanliness, proper water, environmental temperature, humidity, ventilation, photoperiod, and toxic exposures should be addressed. Check this http://www.poultryhub.org/index.php/Welfare_of_poultry_in_periurban_environments
for husbandry advice.
for basic care, set up and maintenance. You need to check for fly and mosquito access, as they can carry certain diseases, and check for external parasites. Mites, lice and fleas (in some areas, ticks) can contribute to over-all health issues, anemia, and disease transmission.
Here are some sites of general interest:backyard poultry backyard poultry statement http://www.freewebs.com/professorchickensdiseases/ http://poultrykeeper.com/poultry-vets-uk/poultry-veterinary-practices-services-uk/ http://www.thepoultrysite.com/about/http://mypetchicken.com/backyard-chickens/chicken-care/guide-toc.aspx http://www.brittonclouse.com/chickenrunrescue/http://www.shopthecoop.com/chickens/links.html