At this point calcium, magnesium and various phosphates are withdrawn from the bones, to lay down the shell around the egg. Following this process, the egg is passed to the outside, the cycle being repeated on alternate days. This gland is one of the glands which form the endocrine system or the ductless glands as they are commonly known. The thyroid glands in budgerigars produce a hormone called thyroxine. They cannot produce this hormone if the bird is deficient of iodine and should this happen they enlarge and eventually the bird dies.
Iodine deficiency is a metabolic illness and used to be very common in pet budgerigars at one time.
It was not unusual to go into an aviary and hear an odd clicking sound from a bird as it was breathing. This was almost certainly a sign that they thyroid glands were enlarged and they were pressing on either side of the wind pipe. Excessive thyroxine incidentally causes excessive moulting, a high state of nervousness and the incomplete growth of feathers. The condition, when applied to mammals, is known as goitre. This in turn affects the ratio regulation between calcium and phosphorous which is delicately balanced.