It's an interesting question isn't it? We all know what egg-shaped means but when you look at a selection of birds' eggs, their shape does vary. Some eggs are more spherical than others.
Until now, the variety of shapes was thought to be caused by the bird's habitats. It was believed that it might be beneficial for cliff dwelling birds to lay conical eggs which roll in a tight circle as these would be less likely to roll off of the cliff. Birds which tend to produce large clutches could lay eggs with shapes which make incubation easier.
A group of scientists decided that it was time to take a closer look at eggs!
Mary Caswell Stoddard of Princeton University and her colleagues have analysed the shape of almost 50, 000 eggs from around 1400 different species of birds. The eggs studied were those in museum collections. The team then quantified their shape according to two measurements. Firstly they recorded the eggs' ellipticity, or length relative to width. Then they recorded their asymmetry. In other words, to what extent one end was pointier than the other. Eggs which are both elliptical and symmetrical, like an airship, do not exist in nature.
Eggs and Wings
The researchers compared the shapes of the eggs with data relating to each bird species. Surprisingly, they found no correlation between shape and clutch size, environmental factors or nest characteristics. But they did find one factor which influenced egg shape and that was the hand-wing index, a measure of the shape of the wing.
Birds with a large hand-wing index tend to be better fliers and also tend to have more elliptical or asymmetric eggs. This could be because streamlined bodies are more efficient for flight. A streamlined body will mean a narrow pelvis and so these birds produce longer, narrower eggs. This shape of egg means that the desired volume is retained but the birds can pass the eggs through their narrow pelvis.
The study certainly suggest that there is a correlation between egg size and flight but offers no proof. The research does not explain why more spherical eggs are an advantage for birds who do not engage in high-intensity flight. It is possible that the rounder shape minimises the shell material required to create the required volume.
The researchers also looked at the biophysical mechanisms that create the egg's shape and found that this isn't determined by the shell. If you dissolve an egg shell in vinegar, you find that the egg retains its shape due to its outer membrane. An asymmetric egg shape can be created via the variations in the thickness or elasticity of the membrane, or through the arrangement of molecular constituents such as collagen.
Proving the Theory
The team didn't benefit from sufficient anatomical data to search for a correlation between pelvis width and egg shape and so further research will be required to prove their theory. They also plan to study how the egg develops as it moves through the oviduct to discover how this process influences the shape of the eggs.