Not all bird eggs are hatched equal.
Some eggs, such as that of the common quail, are tiny and pocket-sized. Other eggs fit an entire bowl, such as that of the ostrich. The question, however, is this: why are bird eggs shaped the way they are? Is it only because of the size of the bird that hatched them?
An international team of avian ecologists led by scientist Mary Caswell Stoddard of Princeton University wanted to know the reason for the shape of bird eggs.
Stoddard and her colleagues embarked on a tedious and ambitious mission: compare the eggs of thousands of different bird species with the flight ability of the birds they hatch to determine if a bird's wings is linked to the shape of eggs.
Bird Wings And Shape of Eggs
Researchers investigated photos of 49,175 bird eggs taken from the Museum of Vertebrate Zoology at University of California Berkeley. These bird eggs were collected by naturalists in the 1800s and 1900s from different parts of the world, and they represent a gamut of bird sizes and locations.
To make their research more efficient, Stoddard and her team plotted the shape of eggs in terms of ellipticity or how close it is to an ellipse, as well as in terms of asymmetry. They created a computer program called Eggxtractor to pick out the egg in any image and calculate its width, shape, and length.
"There was an obscure hypothesis that egg shape could be related to flight ability that no one had paid any attention to," explained Stoddard.
Furthermore, Stoddard knows that the shape of an egg is determined by the membrane inside, not the shell. She then worked with scientist L. Mahadevan and student Ee Hou Yong to create a mathematical representation that depended on the properties of the egg membrane and how much pressure it received.
To the researchers' surprise, they did find a correlation between the flight ability of a bird and the shape of its eggs.
For instance, bird species such as murres and sandpipers hatch eggs that are more asymmetrical and more elongated, and it's most likely because lots of flying time requires lightweight bodies, the scientists said.
On the other hand, bird species that do not spend time in the air, such as trogons and tropical pittas, hatch eggs that are more spherical.
Future Plans
A previous bird study found that the shape of the egg was determined by the egg membrane, but Stoddard and her team's findings take it a step further: they unraveled how the shape of the membrane itself is determined in the first place. Stoddard also believes their research does not negate the findings of the previous study.
Meanwhile, Stoddard and her colleagues plan to take a closer look at the egg membranes and body structures of different birds to see whether they support the model in this study.
Details of the research are published in the journal Science.
