Scientists crack the mystery of eggs shapes

Researchers might have just cracked an old egg mystery: “Why are birds eggs shaped the way they are, and why they differ so much from one bird species to another?” By developing a specialized ‘egg-recognition’ software, a team of scientists at Princeton University seemed to have found a correlation between the shape of a bird species’ egg and their ability to fly.

The shape of an egg correlates with a bird species’ ability to fly, researchers found (credit: Frans Lanting/MINT Images/Science Source).

The shape of an egg correlates with a bird species’ ability to fly, researchers found (credit: Frans Lanting/MINT Images/Science Source).

Bird eggs all do one thing – they provide the nourishing and protected environment for a chick to develop. Yet if one looks at the different eggs existing out there, the variety is somewhat staggering. Owls, for instance, lays almost perfectly round eggs; sandpipers’ eggs, on the other hand, are shaped like a teardrop, and hummingbirds’ have eggs that remind of jelly beans, which leads to think that maybe there is more to the shape of an egg than just chance.

Driven by the desire to solve the mystery, leading author of the study biologist Mary Stoddard and her colleagues developed a computer program – Eggxtractor – which is able to analyze the image of an egg and measure its length, width and shape. They used the software to determine how each of the 50,000 eggs they had access to differed from a perfect sphere, meaning how each egg is elongated or pointy. They then made a family tree of 1000 bird species and ran their findings about the shape of each egg against different factors such as bird species’ nest type, nest location, number of chicks in a clutch, etc. – which had been previously proposed as possible explanations for why eggs are shaped the way they are.

Amongst all these factors, the one that stood out the most was the ability of a bird species to fly – which the software correlated to having a more elongated and asymmetrical egg. “There was an obscure hypothesis that egg shape could be related to flight ability that no one had paid any attention to,” Stoddard says. Good flyers like sandpipers and murres tend to lay eggs that are more elongated and asymmetrical. The reason – the researchers speculate – is that rounder eggs require a wider pelvis. So birds which spend most of their time airborne as well as having evolved more streamlined bodies and lighter small skeletons, have also potentially evolved streamlined egg to fit through the pelvis.

The research is interesting as “eggs aren’t just a favorite breakfast food,” Stoddard explains. “They kick-started a revolution,” she adds, that made it possible for developing young to survive on land, and thus allowed our land vertebrate ancestors to leave the seas about 360 million years ago.

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Carlo Bradac

Carlo Bradac

Dr Carlo Bradac is a Research Fellow at the University of Technology, Sydney (UTS). He studied physics and engineering at the Polytechnic of Milan (Italy) where he achieved his Bachelor of Science (2004) and Master of Science (2006) in Engineering for Physics and Mathematics. During his employment experience, he worked as Application Engineer and Process Automation & Control Engineer. In 2012 he completed his PhD in Physics at Macquarie University, Sydney (Australia). He worked as a Postdoctoral Research Fellow at Sydney University and Macquarie University, before moving to UTS upon receiving the Chancellor Postdoctoral Research and DECRA Fellowships.

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