TropicalChickies
Crowing
Agree. If I had access to such resources as vets and necropsies, I'd scrimp and save to use them.It will cost money to get a correct diagnosis, whatever route you choose. You can check with your state lab to find out if a necropsy cost less than the live test for Marek's disease. If so, you can wait until you lose another bird, then submit the body then. Identifying the invisible, unknown monsters (avian diseases) afflicting a flock is in my book, priceless.
@Chopper923 identifying the disease present in your flock is worthwhile for other reasons not yet mentioned:
You have a fairly large number of birds for a backyard flock. If the disease identified is treatable, you have a much better chance at saving more of them.
Some avian respiratory diseases are a virus, like IBV (infectious bronchitis virus). They simply have to run their course. But just like a person with a virus, fortifying the immune system with nutrients, clean water, fresh air, sunshine, and rest will help. And keeping them parasite-free.
Other diseases like MG (mycoplasma) are bacterial. This is essentially "chicken pneumonia" and can be helped with antibiotics.
Some diseases impart immunity but the birds are carriers for life. (However some studies show that the viral load drops over time). So depending on that factor, you may wish to "close" your flock (not bring in any new birds) for a period of time.
I waited more than two years before introducing new birds. All of my original flock have passed on, sadly, although a few made it to four years old and had good vigorous lives after recovering from their original disease.
Now I have roosters and hens who love to go broody, so they can hatch their own offspring here on our farm. Which reduces the chances of "importing" disease considerably. It does get better!
So you see, knowing what your birds have would really help in many ways.