Laura Erickson's For the Birds

Embryonic Respiration

(Recording of a Pileated Woodpecker)

Recently a listener asked me the question, how do unhatched chicks breathe inside an egg? After all, an egg shell is hardly an open window, and a bird embryo isn’t hooked up to its mother’s blood oxygen supply like a mammal embryo is, so how can it possibly get enough air?

Baby birds live a dark, cramped existence within their eggs, an existence exactly opposite the brilliant life of freedom and flight that they’ll experience once they mature. An egg is sort of like the perfect efficiency apartment–there is exactly enough room to live, no more, and no less. The embryo forms in a germinal spot between the yolk and the egg white, or albumen. The yolk, or avian cupboard, holds a rich mixture of fats, proteins, and carbohydrates to nourish the embryo. The albumen, which works like an all-encompassing sofa for the ultimate couch potato, is about 88% water and 10% amino acids, with traces of minerals. It cushions the embryo, protects it from drying out, and provides structural support for the yolk to prevent it from flattening. The egg white doesn’t directly feed the chick, but it does transfer water, along with some nutrients, to the yolk as the chick develops. The egg shell protects and keeps the egg from drying out. It’s covered with hundreds of tiny pores which allow oxygen and carbon dioxide to diffuse in and out.

The cells of the new embryo form a chamber called the allantois where waste materials can safely collect. This chamber works pretty much the same way as diapers on babies, only the allantois can’t be changed when it gets dirty. Instead, it gets bigger and bigger as the yolk is absorbed for nutrition and gets smaller and smaller. The allantois is surrounded by a network of blood vessels. As the allantois grows, these thin-walled blood vessels begin to press against the shell, where they can absorb oxygen directly through the porous shell. This is how a developing bird solves both its waste disposal problems and its breathing problems in a single elegant system. As the Church Lady would say, “How convenient!”

A developing chick has simple needs and probably simple pleasures. Imagine being wedged snugly in the ultimate water bed, the only big excitement each day coming when a parent bird rolls you over. But the baby bird, like a developing human, doesn’t just vegetate–it can hear certain outside sounds like the peepings of its brothers and sisters inside their own eggs. And some birds probably learn some elements of their species’s song from within the egg. In spite of this, though, parent birds never seem to need to study how to communicate with their unhatched babies in little bird prenatal classes.

The particular listener whose question inspired this program just happened to go out with me on our first date exactly twenty years ago tomorrow. He and I float through life, working on our nest, raising our nestlings, and saving our best jokes for each other, and this program has been for him.

(Recording of a Pileated Woodpecker)

This is Laura Erickson, and this program has been “For the Birds.”