Pale patches could be a sign of anemia. That could be due to many things. That extreme sickness one day, then improvement the next could be due to ingesting a toxin such as botulinum. Any decaying matter can contain the toxin. It may have needed some time to pass through the bird's system. If the bird is eating, drinking on its own and is active, you could help it by using a molasses flush to get any remaining toxins out of the intestinal tract. Here's some info on laxative treatment:
http://msucares.com/poultry/diseases/solutions.html
LAXATIVE SOLUTIONS
The following solutions or mixtures are recommended to flush the digestive system of toxic substances, most notably for treating birds exposed to botulism toxins.
Molasses Solution
Add one pint of molasses to 5 gallons of water
Offer the drinking solution free-choice to the affected birds for about four hours. Treat severely affected birds individually if they cannot drink. Return the birds to regular water after the treatment period.
As a supportive treatment for symptoms resulting from Cryptosporidia infection, often referred to as coronaviral enteritis, use:
One quart molasses in 20 gallons of water
Offer this solution free-choice for a period of up to 7-10 days. It is assumed that the molasses replaces certain minerals lost from diarrhea during the course of the infection.
Epsom Salt Solution
1 lb Epsom Salt per 15 lb feed
-or-
1 lb Epsom Salt per 5 gallons water for 1 day
Give the epson salt feed mixture as the sole feed source for a one day period. This feed can be used only if the birds are eating. If the birds are not eating, use the water solution. If the birds are unable to eat or drink by themselves, use individual treatment with:
1 teaspoon of Epsom Salt in 1 fl oz water
Place the solution in the crop of the affected bird. This same amount of solution will treat 5-8 quail or one chicken.
After the four hour treatment with molasses or the Epsom salt salt treatment, supplement water with vitamin-electrolyte & probiotic soluble powder for at least 3 days. If you don't see improvement during that time, you may want to consider treating for protozoa (Coccidiosis, Histomoniasis) or deworming them if they haven't been in awhile.
