Why do hens lay unfertilized eggs?

With Easter just around the corner we will probably all be tucking into chocolate eggs very soon. But have you ever wondered about the non-chocolate variety that we eat… and why hens lay unfertilized eggs? It could be considered a fairly wasteful process; egg production requires protein, fat, nutrients and the calcium to make the shell. That’s a big investment for an egg that may not lead to any offspring. So why so many unfertilized eggs?

A bit of biology

How are the eggs made in the first place? Hens hatch (as chicks) with all their eggs already made, or at least, an unfinished form of the eggs. Once a hen reaches sexual maturity it will lay approximately one egg every 24 hours during the laying season, although this time period may vary slightly for domesticated hens.

Selective breeding

When we talk about hens we are referring to domesticated animals that have been selectively bred to lay more eggs over a longer laying period that their wild relatives. The closest wild bird to compare our domestic hens to would be red jungle fowl. A hen (wild or domestic) will typically lay a number of eggs, called a clutch, over a period of time before stopping and incubating the eggs. If the eggs are taken away once laid (as is the case for the domestic breed) the hen will keep laying in an attempt to build up her clutch of eggs.

Which came first?

Certain domestic breeds have lost this brooding instinct but it is still found in some breeds. As the process of making and laying each egg is a big drain on the hens energy and resources red jungle fowl only lay a couple of clutches a year, with about four to six eggs in each. These wild birds will also only lay when conditions are right… food supply, weather and the likelihood of finding a male to fertilize the eggs.

With enough food, water and daylight hours domestic hens have been selectively bred to keep laying for much longer periods, whether there is a possibility of fertilization or not.

The fertilized bit

In the wild it is likely that roosters would be naturally part of the flock and therefore most eggs produced would be fertilized. In the domestic situation roosters are often not present but the hens are biologically programmed to keep producing the eggs. There is a short window of fertilization, between one egg being laid and the next one beginning the final stages of the cycle. An egg must be fertilized before the yolk membrane (called the vitelline) is added.

Many birds, including hens, have a way to increase the chances of fertilization; they can actually store sperm and use it to fertilize eggs at a later stage. They can also store sperm from more than one male and they can use that store at various stages to fertilize a number of eggs. Resent research suggests that hen cells actually break down special fats, adding droplets to the stored sperm to increase their survival time.

You could also blame the egg shell

Animals that produce soft shelled eggs usually have a requirement for water in their reproductive process; frogs are a good example of this. In fact, for many of these species the eggs are fertilized outside the female body. Animals that lay eggs that are protected in hard shells have the advantage of being less tied to water when it comes to reproduction. The down side of this, of course, is that these shells cannot be fertilized outside of the body.

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