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Day 14: The Skeleton

How are Bones Different in Birds?

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The cavities of avian bones are often lined with extensions of the bird's air sacs. Bones containing air sacs are said to be pneumatized. When a bird takes off for flight, the exaggerated movement of the wings creates an air current which fills the air sacs within its bones and makes the bird light enough to fly. The air current created is referred to as "flight wind". Pneumatization of the bones does not take place until after the bird hatches.

During periods when a hen is laying many eggs, she stores extra calcium from her diet as bony masses within the cavities of her long bones. The hen's body draws calcium from these masses to make egg shells. If the hen cannot get enough calcium in her diet to form the bony masses, calcium will be drawn directly from her bones. This will weaken her bones and may still produce a weak shell. The chick embryo relies on its shell, not only for protection, but also as a source of calcium once its bones begin to ossify.

In humans the last five vertebrae of the spinal column fuse to form the sacrum and the joints between the ilia and the sacrum, the sacroiliac joints, are movable. The sacrum makes the pelvis strong and stable by transferring the weight of the body through the sacroiliac joints to the pelvis and legs. In birds fusion occurs in the embryo between the last thoracic vertebrae, the five lumbar vertebrae, the two sacral vertebrae, and the first five caudal vertebrae. The resulting synsacrum is fused to the ilia. and pelvic girdle and cannot move. This structure is a flight adaptation in birds to reduce the shock of landing.

Text by Janet Sinn-Hanlon

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