Pet Bird Magazine, Ezine June 2001 Magazine

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IMPORTANCE OF ADEQUATE CALCIUM

Part II Growing Birds and Laying Hens

by Linda Pesek DVM, Diplomate ABVP (Avian)

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Photo © Linda Middleton

Part II discusses the amounts of calcium required and also the processes and changes in calcium that take place in laying hens.

Calcium is primarily protein bound in the blood. The normal range is 8.5 - 11.5 mg/dl.

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.

Birds are unique in that they develop more medullary bone under the influence of estrogen (and androgens - male hormones). This process is known as "physiologic marrow ossification". It begins in hens during the preovulatory period and is most pronounced in the long bones of the skeleton. During this process, increased amounts of dietary calcium and phosphorous are deposited as medullary bone. This calcium is mobilized from bone for egg shell formation when food intake and intestinal absorption of calcium is insufficient. The excess phosphorous in the bone is excreted by the kidneys.

Growing birds and laying hens have a greater requirement for calcium than adult non breeding birds. Laying psittacine hens should be offered at least 0.3% calcium in a 1:1 or 2:1 ratio with phosphorous to prevent calcium mobilization from the skeleton. Too much calcium in the diet will lead to egg shells that are too thick, leading to decreased hatching and increased chick mortality. In addition, excess calcium will decrease the utilization of many proteins, fats, phosphorous, vitamins, iron, iodine, zinc and manganese.

A deficiency of calcium or vitamin D3 or an improper ration of calcium to phosphorous in the diet results in metabolic bone disease also known as rickets, cage paralysis, osteoporosis, osteomalacia and nutritional hyperparathyroidism. Rickets occurs in immature birds, while the other conditions occur in adults. Soft shelled eggs, cracked or broken shells, and embryonic death due to excessive loss of moisture through the thin shelled egg may occur. Egg binding resulting from uterine inertia may also occur. Chicks and hens often develop folding fractures because their bones are unable to support their weight and lack normal strength. Certain birds - especially African grey parrots - may develop seizures and weakness.

All seed diets - high in phosphorous and low in calcium and D3 - are an imoortant cause of hypocalcemia.

The most important thing you can do to insure the health of your bird is to feed a well-balanced, nutritious diet that meets all the physiologic and metabolic demands of your bird.

Winged Wisdom Note: Dr. Linda Pesek graduated from the University of Pennsylvania School of Veterinary Medicine and is a Diplomate of the ABVP in Avian Practice (a Board Certified Avian Veterinarian). She has a small animal and avian practice in New York. Linda also writes columns for The Long Island Parrot Society and The Big Apple Bird Club and is a frequent lecturer at their meetings. She is the owner of an extensive collection of exotic birds.