Unscrambling the Shape of an Egg

Do you think you know the shape of an egg? Think again.

 Chicken and duck eggs, our classic, ideal eggs, don't even look like most eggs. In fact, defining the general shape of bird egg is nearly impossible, according to a study of over 50,000 eggs in a lab at Princeton University.

Not all bird eggs are the same. Chicken eggs are a flattened ovoid. Owl eggs are almost spherical. Humming birds have eggs that are shaped like small 3D rectangles with rounded corners.  Certain shorebirds have conical eggs, pointed at one end and much rounder at the other.

Scientists were egged on by a nagging question: What is the evolutionary advantage of egg shape -- no matter what the shape?

Mary Caswell Stoddard and a team from the Stoddard Lab at Princeton studied specimens and photographs of eggs from over 1,400 bird species. Their goal: to statistically correlate egg shape with as many aspects of bird biology as possible so they could figure out what drives the formation of various shapes.

Egg dimensions were compared to the structure of the nest, wing span, diet, egg number, rate of growth, environment, flight speed, weight, size, placement of the nest and other factors. The researchers fed all this data into a computer for statistical analysis.

Why input data on the placement of nest? One hypothesis states that eggs laid in precarious environments, such as high in trees or on rocky seaside cliffs, will not roll off if they are pointed at one end. If an egg has a cone shape and it starts to roll, it will circle around quickly. A round egg might keep off rolling right down the cliff -- and out of the gene pool.

Perhaps eggs that are very pointed at one end will fit together in the tight space of a next, like figs in a circular carton.  Perhaps the length, width, and largest diameter confer upon an egg greater strength.

All these hypotheses were in the minds of the Stoddard and L. Mahadevan, a biophysicist from Harvard, as they and the team analyzed the mathematical data.

In the end, only one aspect of bird biology was correlated with egg shape: the wings.

Birds with long wingspans relative to the length of their hand bones had eggs that were long, asymmetrical, and quite elliptical.

At first the findings surprised the team, but then things began to make sense.

Birds with wings like this are sleek. They are sleek as an adaptation for fast flight. A long body has long narrow internal organs, including the oviduct through which an egg must pass. As the egg develops, it conforms to the innately narrow oviduct.

The correlation makes sense for poor fliers too. Chickens are not good fliers. Their bodies are anything but sleek. Their eggs, as a result, are not long and spherical but shorter and rounder. A swift, which is aptly named, darts around quickly. Its eggs are narrower than average.