Egg-shaped, according to the Oxford English Dictionary, means “having the oval shape of an egg”. Not all birds’ eggs, however, are as oval as a chicken’s. They are “spherical in owls, elliptical in hummingbirds, conical in shorebirds, and a range of forms in between,” noted a team of scientists who investigated the how and why of egg shapes. They looked at prevailing ideas, such as conical eggs for cliff-nesting birds to prevent them rolling off. Or robust, spherical eggs for birds that lay them in tightly packed nests. The conclusion they came up with, however, was far removed from any of these.
“We discovered that, on a broad scale across birds, egg shape is related to flight ability,” said Mary C Stoddard, an evolutionary biologist and behavioural ecologist at Princeton. Birds that fly better have evolved their bodies in a way that their eggs will be less symmetrical and more elliptical than those of lesser fliers.
“We think that birds, to maintain sleek bodies for flight, have evolved elliptical and asymmetric eggs to increase egg volume without increasing egg width — this is advantageous for narrow, streamlined bodies,” Stoddard told The Indian Express.
The study has been published in Science.
What gives the egg its shape, the researchers propose, is not the shell but its membrane. The yolk and albumen are packed into a sphere, around which the membrane is deposited, before the egg moves into the shell gland where the shell is deposited. The scientists constructed a mathematical model for predicting egg shape based on variations in forces acting on the membrane, and in the membrane properties.
They plotted eggs of 1,400 bird species on a graph for ellipticity against asymmetry. Eggs of red junglefowl, for example, are low on both asymmetry and ellipticity; those of common murre, a stronger flier, are high on both counts.
To quantify flight ability, the team used a standard called Hand-Wing Index, the ratio between two key distances along the wing. They found that a higher Hand-Wing Index, or better flight ability, is often associated with increased asymmetry or ellipticity, and less often with decreased asymmetry or ellipticity.
“We do not suggest that a female’s flight behaviour during the egg formation period directly affects egg formation,” the researchers clarified, “nor do we suggest that egg shape so strongly influences the flight abilities of female birds during their egg-laying period that selection has produced an aerodynamic egg.”
Also, Stoddard said, “there are exceptions — kiwis are flightless, for example, but they lay elliptical eggs.”
Not all flightless birds lay round eggs either. “We predict that birds that have lost the ability to fly might have round eggs. This is true in ostriches — but not in penguins.”