This is a general fanciers term which covers a number of diseases, but there is one major cause in my opinion. Always the birds lose condition over a long period. They never appear to eat properly. They peck about in the seed dishes and whenever you examine the dishes they always appear empty and full of ground dust. When you perform a post mortem the flesh appears to have a 'dried out' appearance because the individual is dehydrated.
Some birds contract a gizzard ailment.
A normal gizzard is a nice pink colour and is composed of well toned muscle both externally and internally. There should be grit inside as well as seed. An abnormal gizzard is flabby. Inside, while there is sometimes some food, there is a complete absence of grit. The horny lining can be peeled away from the outer gizzard muscles very easily and there are often signs of ulceration.