The plasma chemistries may show some changes reflecting hormonal activity. Early in the reproductive cycle, the hen will undergo vitelligenesis. During this process, yolk precursors, primarily proteins and lipid, are synthesized in the liver and transported to the ovarian cortex hematogenously. The regulation of yolk protein synthesis is controlled by gonadotrophin and steroid hormones. Vitellogenin, a phosphoglycolipoprotein, and a low density lipoprotein are both translocated across the oocytes plasma membrane via receptor mediation. Yolk precursors are assembled in the primary oocytes cytoplasm with proteolytic processing by cathepsin D into phosvitin, lipovetellin, triglycerides, cholesterol and phospholipids. Initially, for the first few months of follicle development, an equal amount of protein and lipids are deposited. However, during the final rapid growth phase, 7-14 days prior to ovulation in most species, an increasing amount of lipid is incorporated. It is thought that vitelligenesis ceases approximately 24 hrs prior to ovulation.

During vitelligenesis, the plasma will usually appear lipemic due to the circulating lipoproteins used to plump the yolk. During calcification of the eggs, which occurs in the uterus portion of the oviduct, calcium ions are transported across the mucosa and carbonic anhydrase provides the carbonate ion to form the calcium carbonate shell. Approximately every 15 mins, the uterus withdraws from the blood a weight of calcium equaling the total amount circulating at any one moment. During this time, the plasma calcium level will be greatly elevated, often with values well above 15 mg/dL, reaching levels of 20-30 mg/dL. With some individuals, the phosphorus level will often elevate proportionally, while in others, it may remain within normal limits. Elevated total plasma proteins and total serum solids are increased because of a need for protein for calcium transport, as well as the estrogen-controlled liver synthesis of lipid and proteins produced during yolk formation. Hypercholesterolemia may be seen during reproductive periods. Pre-albumin may be elevated in laying hens, as well. Alkaline phosphatase levels may also increase due to estrogen stimulation.

Radiographs often show the classic calcification of the medullary spaces of the long bones, particularly of the femur and tibia, which occurs approximately ten days before egg formation. In budgerigars, the primary sites of medullary calcification are the humerus and femur. This is called polyostotic hyperostosis.

Hematogenic changes may also occur that are associated with egg-laying. A slight increase in the white blood cell count is typical. The packed cell volume may also increase to the high range of normal.

Not all hens that are reproductively active will go on to oviposit. Some may resorb the eggs, others may lay internally (where the egg enters the coelom instead of the infundibulum), which may or may not cause problems. Others may develop egg-binding or dystocia.

While not directly involved with reproductive activity, the uropygial gland is involved with calcium metabolism and should be evaluated during every physical examination. The gland, which is bilobed and heart-shaped, has a papilla with several wick feathers. The wicks allow the secretion to be picked up on the beak during preening. The secretion is then spread on the feathers. In addition to keeping feathers supple, it also has antibacterial and antifungal properties. The secretion has vitamin D precursors that are spread on the feathers. Once activated by exposure to UVB light, the secretion becomes active vitamin D3, which is necessary for proper calcium utilization. So, if hypovitaminosis A causes squamous metaplasia of the uropygial gland, or if the wick is not present or there are other problems with the gland, or the bird does not receive outdoor natural sunlight (not filtered through glass or plastic) or full-spectrum light indoors, the bird may not have normal blood calcium levels, even though calcium is being supplemented in the diet.

With many healthy young birds showing signs of reproductive activity, we must learn how to ascertain what is normal reproductive activity, and what is a true medical problem. Many physical and biochemical changes are a normal part of avian reproductive physiology. This summary article is intended to help explain what these changes mean for your avian patients.

Contributed by Margaret A. Wissman, DVM, DABVP, Avian Practice