Calcium in the Avian Patient

Key Points

  • Calcium used for egg production comes from the diet and mobilization of medullary bone.
  • Calcium deficiency may arise from inadequate vitamin D, excessive dietary phosphorus, or inadequate dietary calcium (i.e. all-seed diet or muscle/organ meat diet). Chronic egg laying is also an important drain on calcium stores.
  • Clinical signs of the hypocalcemic syndrome of African grey parrots may include weakness, ataxia, and seizure activity. Clinical signs are responsive to calcium administration, and exposure to ultraviolet B spectrum may also help long-term.
  • Good dietary sources of calcium for pet birds include dark, leafy greens and cooked egg with the shell.

Introduction

The most widespread mineral in the body, calcium is required for normal metabolism and bone mineralization.

 

Physiology

Calcium homeostasis is under the control of calcitonin, which is produced by the ultimobranchial gland, vitamin D, and parathyroid hormone. In laying hens, most dietary calcium is used for egg production. Rising estrogen levels promote increased intake of calcium supplements like cuttlefish bone and calcium-rich foods, however the quantity of calcium ingested daily is insufficient for the massive deposition of calcium required for eggshell calcification.

Therefore eggshell calcification also requires mobilization of calcium from bone. Hens of many species deposit medullary bone in the marrow cavity of long bones (Fig 1). This process, called osteomyelosclerosis, allows medullary bone to provide calcium for eggshell formation when calcium levels are low. In chickens, medullary bone is deposited in the morning and utilized at night.

Osteomyelosclerosis/bone marrow ossification

Figure 1. Osteomyelosclerosis or bone marrow ossification of the long bones occurs secondary to rising estrogen levels. This process allows medullary bone to serve as a source of calcium for eggshell formaton. Click image to enlarge. 

Mobilization of medullary bone and ingestion of calcium-rich foods creates a physiologic hypercalcemia in the hen. Normal serum calcium levels range from 8-10 mg/dL in non-laying hens, while healthy layers have calcium levels ranging from 15-30 mg/dL.

Calcium requirements
Life Stage% CalciumSpecies
Growth*0.90-1.00precocial*
Maintenance<0.1 to 0.2
Egg production0.35cockatiel
0.85Budgerigar
3.30chicken
* If balanced with 0.6% phosphorus; requirements for altricial chicks like the parrot are unknown.

For most species, 0.1% calcium appears adequate for a maintenance diet. With the exception of laying hens, the calcium: phosphorus ratio should be 2:1, although ratios ranging from 1.4:1 to 4:1 may be well tolerated in chickens with adequate vitamin D.

 

Calcium toxicity

In avian species that have been studied, harmful calcium levels are actually only slightly higher than required levels. Excess dietary calcium leads to minimal absorption of calcium (and other minerals) and elevated serum calcium. Prolonged hypercalcemia may lead to nephrosis and soft tissue mineralization.

 

Calcium deficiency

Calcium deficiency may be caused by insufficient dietary calcium, excess dietary phosphorus, or inadequate vitamin D. In granivores, all-seed diets are associated with excess phosphorus and deficient calcium, while diets of muscle or organ meat without bone are calcium deficient for carnivores. Chronic egg laying is another important cause of depleted calcium stores in the parrot.

Profound calcium deficiency leads to decreased bone mineralization and abnormalities of the long bones and vertebral column, particularly in growing birds. Skeletal abnormalities may include lameness, rickets, dyschondroplasia, lameness, and enlarged, painful joints. Skeletal abnormalities may be particularly common in young grey parrots (Psittacus erithacus).

An osteoporosis-like condition may eventually develop in adult birds with calcium deficiency. Bones may become weak and porous until the weight of the bird or pull of muscles creates bone deformities or fractures.

 

Hypocalcemic syndrome of African grey parrots

The hypocalcemic syndrome of grey parrots is frequently diagnosed in young birds, particularly individuals on an all-seed diet. The cause is unknown, but may be related to an inability to effectively mobilize calcium from bone. Signs of hypocalcemia in these patients, such as weakness, ataxia, falling off the perch, and seizure activity, respond to calcium administration. Gradual conversion to a health diet is important, and exposure to ultraviolet B radiation (285 to 315 nm) may also promote normal calcium levels in these birds.

 

Sources of dietary calcium

In the wild, many birds supplement their food intake with mollusk shells, eggshells, calcium-rich grit, and bone fragments to maintain adequate dietary calcium levels.

The availability of calcium in foodstuffs can be extremely variable (download Table 1, Calcium content of selected foods). Invertebrates and grains are generally poor sources of calcium, while some vegetables such as dark, leafy greens are a good source of calcium. Unfortunately the digestibility of calcium in plants is often limited by the degree to which it is bound to oxalate or phytate.

 

Calcium content of select foods

Calcium content of selected foods.

Figure 1. Calcium content of selected foods. Click on image to enlarge.

 

 

References