Why Are Poultry Eggs Egg-Shaped?

Poultry eggs are not exactly oval. Rather, they are asymmetric — round at one end and more pointy at the opposite end. This unique shape is “ovoid,” which fittingly enough means “like an egg,” or “egg-shaped.” Poultry eggs are egg-shaped for many reasons, beginning inside the hen where an egg is made.

A hens’ anatomy shapes her eggs.

All eggs start out as round yolks. The eggs of some bird species remain roundish from start to finish. Others develop into asymmetric ovoids. The shape of a hen’s shell gland, or uterus, controls the final shape of an egg.

Just before entering the shell gland, the egg-to-be spends about one hour in the isthmus, where it acquires two thin shell membranes. The membrane-enclosed egg then enters the shell gland, or uterus, where albumen plumps it up until it presses against membranes of the uterus to achieve its approximate final shape. Then layers of calcium carbonate crystals cover the egg with a hard shell.

The thickness of the shell membranes and the distribution of the calcium carbonate crystals control the precise curvature of the egg. The shell basically solidifies the shape. Carefully peel away the shell, leaving only the softer membranes, and the egg will still be the same shape.

Once you know what each specific hen’s eggs look like, you can tell in the future who laid what. Unlike industrial hens bred for uniformity, our backyard chickens are individuals in every way, including the shape of the eggs they lay.

The shape of an egg conserves calcium.

Although a round egg has less surface area, thereby conserving calcium, an ovoid egg maximizes volume while keeping the surface area minimal. Further, the tapered end helps minimize the amount of calcium needed for the shell.

Since a good layer can’t absorb dietary calcium fast enough to build sturdy shells, she uses calcium from her bones. Any reduction in the amount of calcium needed for the shell helps preserve the hen’s skeletal structure.

Egg-shaped poultry eggs are easier to lay.

Another reason why poultry eggs are egg-shaped is that they’re easier to lay. The hen’s oviduct is basically a long tube, toward the end of which is her vagina. As the egg leaves the uterus and enters the vagina, it rotates so the blunt end goes first.

The pointy end, having more surface area than the blunt end, gives the hen’s muscles more surface to push against. An asymmetric egg is therefore easier to push from the vagina through the cloaca and into the nest.

An egg’s asymmetry gives it structural strength.

Yet another reason why poultry eggs are egg-shaped is so they hit the nest on the blunt end, which is much less likely to crack than the pointy end. Since a hen tends to stand up when the egg comes out, it has even farther to drop, making it essential that it not break.

Why doesn’t the egg break? Because the shape of the curve at the blunt end distributes pressure evenly across the entire surface. It’s the same reason sports arenas and igloos are built as domes.

Ovoids have a built-in anti-roll feature.

As a hen enters and exits a nest box, she may jostle eggs that are in the nest. If the eggs were spherical they might roll out of the nest and break.

But asymmetric eggs don’t roll in one direction. Instead they roll in a circular pattern around the tapered end. They thus remain safely in the nest.

Asymmetric eggs fit together in the nest.

Unlike spherical eggs, which don’t fit well together in the nest, pointed eggs nestle together without leaving gaps between them. This arrangement allows more eggs to fit into the nest.

It also minimizes the total exposed surface area, allowing a setting hen to more efficiently cover her eggs and transfer heat more evenly. When she briefly leaves the nest during incubation, eggs snugly tucked together keep each other warm.

Poultry eggs are the right shape for hatching.

An asymmetric ovoid has greater volume, relative to its girth, than a spherical egg. The wider end provides room for the nutrient-rich yolk and has more shell pores to allow oxygen to enter and carbon dioxide to exit as the embryo develops.

While the curved surfaces of the shell distribute pressure evenly for strength, the middle of the shell must be weak enough to allow an emerging chick to peck all around and break out.

It’s a matter of compression versus tension. Compression pushes or squeezes on the outside, pushing inward. Tension pulls or stretches from the inside outward.

Thanks to its asymmetric shape, an eggshell is strong in compression but weak in tension. By pecking on the inside of the eggshell, a chick uses tension to break out of the shell.

For these reasons, egg-shaped is the perfect structure for poultry eggs, starting with an egg’s creation and ending with the egg’s entire purpose — to produce a new chick.