This in turn affects the ratio regulation between calcium and phosphorous which is delicately balanced. This is one of the reasons why we feed cuttlefish bone. Calcium deficient birds sometimes faint or throw fits from which they may not recover. What happens is there is a lowering of the calcium within the blood stream, the bird faints and while it is in that condition everything slows down. Sometimes the calcium level returns to normal, the bird wakes up and returns to the perch.
Milk incidentally is a good source of calcium.
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.