Nanogirl Michelle Dickinson: The strange link between flying and eggs

Kiwis lay huge eggs in proportion to their body size. The eggs can weigh up to a quarter of the mother's body weight. Picture / 123RF

Asked to describe an egg, most of us would probably think of a typical chicken egg - an ellipse-shaped shell with a pointy top and fatter, blunt bottom.

The reasons why eggs are "egg-shaped" have long been sought by scientists, and new research out this week gives some exciting new insights.

Eggs come in all shapes, colours and sizes, varying dramatically between species.
According to the Guinness book of records, the largest egg from a living bird was laid by an ostrich in Sweden and weighed more than half a kilogram. Interestingly, although ostriches lay the largest eggs of all the birds, their eggs are only 2 per cent of their adult body weight, which actually makes ostrich eggs the smallest when considered in proportion to their mother.

The iconic kiwi, in contrast, lays huge eggs in proportion to its body size. Kiwi eggs can weigh up to a quarter of the mother's body weight and are about 10 times larger than the eggs of a similar-sized chicken. It's not just size that varies either with eggs - we see all sorts of different colours and shapes as well. For example, the morepork lays white eggs that are more spherical in shape whereas the dunnock's eggs are bright turquoise and pear-shaped!

It was once proposed that birds' eggs with pointy ends were laid by birds which nested on cliffs, the idea being that their shape would result in the egg spinning in a circle when knocked, rather than rolling off the edge. Another theory suggested that birds with low-calcium diets laid round eggs to minimise them the amount of shell material needed, while a third theory proposed that the shape of an egg was designed to ensure equal incubation while in the nest.

None of these theories have ever been fully tested, and this week many of them were rendered obsolete by new research published in the journal Science. Using photographs from almost 50,000 eggs from 1,400 bird species - two of which are now extinct - the researchers were able to analyse exactly how pointy and elongated each egg was, mapping the eggs' shapes using a custom piece of software. By plotting the pole-to-pole asymmetry and ellipticity of the eggs, they compared the egg shapes against factors including what the birds ate, where the birds made their nests and how big the birds were.

After a full analysis, they determined that the shape of an egg correlated only to one factor - how good a bird is at flying.

Bird eggs are created in a stretchy tube called the oviduct. There, the egg white and yolk are encapsulated in an elastic double membrane. The shape of this membrane determines the final shape of the egg, and the eggshell that forms around the membrane solidifies this shape.

The new results conclude that birds which spend much of their time flying tend to lay more elongated, more asymmetrical eggs than birds that fly less. Their theory is that birds that fly more tend to have more streamlined bodies to help them fly better. This streamlining results in more tightly packed internal organs which reduces the width of the oviduct, resulting in the formation of narrower, pointier eggs that can maintain the same volume without having to be any wider.

This new theory could explain why the eggs of the flightless kiwi are so large and reminds us that science continues to ponder the big questions many of us never thought to ask. Although most of us would take the shape of an egg for granted, this research provides a leap forward in our understanding of a biological phenomenon.