David Attenborough has a passion for birds’ eggs. These remarkable structures nurture new life, protecting it from the outside world at the same time as allowing it to breathe. They are strong enough to withstand the full weight of an incubating parent and weak enough to allow a chick to break free. But how is an egg made? Why are they the shape they are? And perhaps most importantly, why lay an egg at all? Piece by piece, from creation to hatching, David reveals the wonder behind these miracles of nature.
In the past, seabird biologists used to have a rather romantic idea about this period spent away from the colony. For a start, they called it the honeymoon period. But that instead missed the point because the female goes away on my own.
So why does she fly hundreds of miles away from home with her mate’s sperm still unused inside her? Because she’s busy building up this yo yoke. This is the birds’ reproductive tract.
Here in the ovary, one of the over is filling with yoke. On the yoke, surface sits a tiny disk that contains all the female genetic material needed to create an embryo. The albatross is now to collect enough food to enable her to amass a yoke so big that it can be transformed into a chick. And only when she’s done, that will the egg be fertilized.
Remarkably, it takes more than one sperm to start a new life. The extra sperm probably releases substances that start the development of the embryo. Minutes after fertilization, the egg starts its 24-hour journey down the oviduct.
To help understand this, we can do a simple experiment. First, it sways without cumin, and the egg white contains the water needed by the growing chick. That is done and imposed within the membrane. It travels on into the uterus, whether it will be given its protective armor or Shell. The Shell is actually quite separate from what it contains.
These are unfertilized quail eggs, and this is vinegar. The vinegar will reverse the process of shell formation by eating away the Shell from the outside. These 1000s of tiny bubbles are carbon dioxide. They’re the result of the acetic acid in the vinegar reacting with Shell’s calcium carbonate. In 24 hours, the Shell has dissolved. And this is the egg as it would have been when it first arrived at the uterus.
A yoke surrounded by a thin layer of albumin, all contained and supported by a loose soft bag. And unexpectedly, it’s this bag, the membrane, not the Shell, that gives the egg its shape. So now, back inside the uterus, the egg is almost complete. Calcium carbonate carried by blood vessels is deposited on the soft egg membrane, which will harden unset, forming the Shell. Then other cells begin to discharge pigment-like paint being skirted from hundreds of tiny paint gums.
As the eggs slowly Rooibos yet more cells spray out spots and streaks is taken just under 24 hours for the egg to be fertilized and enclosed within a hard shell. And now, within the dark uterus in weights like an actor in the wings, ready to make its appearance on life’s stage. Which will emerge first from the bird begin all little end