An international team of researchers led by Princeton University and Harvard University may have finally cracked the mystery behind the different sizes and shapes of eggs.

Their discovery, described in a paper published in the journal Science, showed that the bird's flight ability may serve as one of the best predictors of egg shapes.

"We discovered that flight may influence egg shape," said Mary Caswell Stoddard, Assistant Professor of Ecology and Evolutionary Biology at Princeton University and lead author of the study, in a press release. "To maintain sleek and streamlined bodies for flight, birds appear to lay eggs that are more asymmetric or elliptical. With these egg shapes, birds can maximize egg volume without increasing the egg's width -- this is an advantage in narrow oviducts."

To investigate different factors influencing the shape of bird eggs, the researchers first developed computer software capable of scanning and cross-checking nearly 50,000 eggs with 1,400 different bird species. Dubbed as Eggxtractor, the computer software classified the eggs based on their ellipticity and asymmetry. Eggs that are elongated and round at both ends are considered elliptical, while eggs that are pointier at one end are classified as asymmetric.

Previously, biologists have theorized that the shapes of bird eggs were influenced by their nesting place. Birds that have their nests in cliffs or high places were believed to have asymmetric eggs so that the eggs would roll in circle and not fall off. On the other hand, elliptical eggs were better at packing together and providing better incubation.

Interestingly, the new study found a strong correlation between the shape of the eggs and to the so-called hand-wing index of the birds. The bird's hand-wing index is a measure of how pointed and elongated the bird's wings are. This is also being used by biologists to determine the bird's flight ability. Birds that were known to fly frequently or over a long distance often have higher hand-wing index.

The researchers observed that birds that fly frequently or over a long distance have asymmetric eggs, while birds that only flies in their small territory or doesn't migrate at all have elliptical eggs. The researchers noted that frequent or long distance flights require sleeker and more streamlined bodies. Due to this, the abdominal cavities of migratory birds have become smaller, compressing the internal organs of the birds, as well as affecting the egg-shaping process inside the bird.

The researchers noted that more studies are needed to determine if the correlation between the hand-wing index and egg shape is really true or just coincidence.