From the almost perfectly spherical eggs of the brown hawk owl to the pointy eggs of the sandpiper, there is a surprising variety in the shape of bird eggs. Yet why birds lay different shaped eggs has remained unanswered, until now. It seems that the shape of an egg is not determined by the environment in which the adult lives, but instead by how it flies.
There have been many drivers suggested to explain the differences in egg shapes seen across the avian world. A popular one posits that the shape is dictated by where the bird nests, using those birds that set up shop on cliffs as the perfect case. Some sea birds have eggs that are incredible pointy, for example, meaning they roll in a tight circle. This has been argued to be an adaptation to prevent them rolling off the cliff.
Yet now it seems that this shape may instead be dictated by how much the species flies. By studying the shapes of over 50,000 eggs from 1,400 different species held in museum collections, the researchers were able to analyze their length, width, and shape. They then plotted how far from perfectly spherical each species' eggs were. While some eggs were found to be either conical or elongated, some both and others neither, no eggs were found to be both short and conical.
It is the membrane inside an egg that alters its shape, not the shell. “Guided by observations that show that the membrane thickness varies from pole to pole, we constructed a mathematical model that considers the egg to be a pressurized elastic shell that grows and showed that we can capture the entire range of egg shapes observed in nature,” explains L. Mahadevan, senior author of the paper published in Science, in a statement.
While this explains how egg shapes vary, it does not yet explain why. When looking at the eggs, the researchers noticed a correlation between egg shape and how much a bird flies. From this, they think that the evolutionary pressure on birds with high flying demands to become more streamlined has narrowed their pelvis and oviduct.
“To maintain sleek and streamlined bodies for flight, birds appear to lay eggs that are more asymmetric or elliptical,” says lead author Mary Caswell Stoddard. “With these egg shapes, birds can maximize egg volume without increasing the egg's width – this is an advantage in narrow oviducts.”
This may explain why both albatrosses and hummingbirds have similar shaped eggs despite being unrelated, as they are both highly specialized fliers, or why penguins’ eggs are different from ostriches even though both are flightless, as penguins effectively “fly” underwater.