This is bird note. The tiniest hummingbird and the 250 pound ostrich have at least one thing in common. Like all birds, they lay eggs, precision engineered and wrapped in an exquisite shell made of calcium.

That key ingredient, calcium, is needed on demand and in larger quantities than the female typically has in her bloodstream.

So birds have a trick or two up their sleeves or in their legs. Just as their bodies form the first cells of an

egg, the birds start to assemble a special stash of mostly calcium in the hollow space

in their leg bones.

It's called medullary bone.

As the egg develops, that stored calcium is rapidly dissolved

and sent from inside the leg to the shell gland then formed into the new shell

ready for laying in the nest. Just how every bird species supplies or stores calcium

for egg laying isn't fully known yet. Some birds appear to rely on other

more immediate sources of calcium, like consuming calcium-rich grit from the ground,

and some Arctic nesting shorebirds

have evolved an especially clever trick.

Just in time for egg laying,

they ingest the teeth and bones from lemming skeletons to stock up on calcium.

For Bird Note, I'm Michael Stein.