Calcium is primarily protein bound in the blood.
Laying hens require more calcium during egg shell formation. An estrogen-induced hypercalcemia occurs several days before ovulation. Estrogen increases the production of blood-calcium binding proteins. Yolk proteins complexed to calcium are transported to the ovary under the influence of estrogen, resulting in an elevation of blood calcium levels. It begins in hens during the preovulatory period and is most pronounced in the long bones of the skeleton.