September 26 – Bird Is The Word

Today’s factismal: Birds are born with belly buttons but they lose them as they grow older.

At first blush, it seems like an odd question: “Do birds have belly buttons?” but it turns out that, as is usually the case in science, it is the odd questions that are the most revealing and interesting.

A boy bonds with his chick over their shared love of belly buttons (Image courtesy National Library of Medicine)

A boy bonds with his chick over their shared love of belly buttons
(Image courtesy National Library of Medicine)

Before talking about birds and their vanishing belly buttons, it would help to review why mammals have belly buttons. Most mammals have a few things in common: they are warm-blooded, have hair, the females nurse their young with milk, and give live birth to young. (The only exceptions to that last are the duck-billed platypus and the echidnas.) That last is the important part because it means that the mammals need some way to feed the fetus while it grows inside the mother; for most mammals, that way is the placenta which links to the growing baby critter with an umbilical cord. When they are born, that umbilical cord dries up and falls away, leaving a scar that we call a “belly button”.

Though birds don’t give live birth, they do have to feed the developing chick as it grows in the egg. And the way that they do that is with a large sack of fat, protein, and minerals that we call a yolk and biologists call a vitellus (Latin for “yolk”). The yolk attaches to the chick with a cord similar to the umbilical cord. And, like the umbilical cord, the yolk sack withers away and drops off as the chick gets ready to hatch.

This leaves a tiny little scar – a belly button of sorts. But after the bird hatches and gets older, the scar fades and is covered with feathers; the belly button has disappeared! If you think the disappearing belly button is neat, then wait until you find out what else birds do! And the best way to do that is to head over to Celebrate Urban Birds, where they have tons of facts about birds, an opportunity for groups to earn mini-grants to support urban bird watching activities, and even a place for you to enter information about the birds that you’ve seen!

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