I've noticed a lot of dead fledglings and raided nests lately. Sometimes, mixed in with all the broken eggshells, I find an unhatched baby bird. And that got me wondering: are grocery store eggs fertilized? How come you never come across one with an embryo or a little chicken inside? If they're not, why does a chicken spend the energy required to produce unfertilized eggs?
When you google "are chicken eggs fertilized?" you get a lot of responses. Guess lots of other people had the same question.
The answer is that chickens will lay eggs even when they've had no contact with a rooster. According to the "Ask a Scientist" feature of the Howard Hughes Medical Institute,
"If an egg has been fertilized, then the embryo inside has already divided several times but remains a group of unspecialized cells [at the time the egg is laid]. When the egg is incubated at about 37 to 38 °C, the embryonic cells differentiate to form a chick, which will hatch after 21 days. If the egg has not been fertilized, then the oocyte [or egg cell] within will never grow or divide, and the egg will never hatch. The eggs you buy at the supermarket are eggs that have never been fertilized.
Domestic chickens lay one egg every 26 to 28 hours (about one egg a day) for a period of 4 to 6 days. In between periods of egg laying, the hen rests. Wild birds may rest for months before laying more eggs, but domestic hens, specially bred for abundant egg production, may rest for as little as 1 day between egg-laying periods."
In commercial egg operations, hens are kept away from roosters, and the eggs are collected as they're laid. Chickens raised to produce eggs only need to mate to replace hens that get too old to lay eggs.
Sometimes you come across a grocery store egg that contains a blood spot. I was told, as a child, that a blood spot indicated a fertilized egg. But that's not so, according to the American Egg Board. Instead,
" Contrary to popular opinion, these tiny [blood] spots do not indicate a fertilized egg. Rather, they are caused by the rupture of a blood vessel on the yolk surface during formation of the egg or by a similar accident in the wall of the oviduct. Less than 1% of all eggs produced have blood spots.
Mass candling methods reveal most eggs with blood spots and those eggs are removed but, even with electronic spotters, it is impossible to catch all of them. As an egg ages, the yolk takes up water from the albumen to dilute the blood spot so, in actuality, a blood spot indicates that the egg is fresh. Both chemically and nutritionally, these eggs are fit to eat. The spot can be removed with the tip of a knife, if you wish."
What about those weird white stringy bits you see when you crack an egg? The American Egg Board says they're called chalazae:
"[Chalazae (singular=chalaza) are] ropey strands of egg white which anchor the yolk in place in the center of the thick white. They are neither imperfections nor beginning embryos.
The more prominent the chalazae, the fresher the egg. Chalazae do not interfere with the cooking or beating of the white and need not be removed, although some cooks like to strain them from stirred custard."
Here's a neat site about the structure of a chicken egg.
And another one about how a hen lays an egg.