Scientists say they cracked the birds’ egg mystery

Scientists have long been trying to answer the question of why are bird eggs so diverse now they think that they’ve finally found an solution to this mystery. 

Scientists have long been wondering about the reasons behind the amazing diversity of bird eggs. A new Princeton study suggest that it could all be tied to the birds’ flight capabilities.

In a study published June 22 in the journal Science, lead author Stoddard and colleagues say that they have found evidence that the birds’ aerodynamic shape has also changed the reproductive system and influenced egg shape. And in this case, it means that the flexible membrane and not the hard shell is what determines the shape, the scientists add.

“In contrast to classic hypotheses, we discovered that flight may influence egg shape,” Stoddard said. “Birds that are good fliers tend to lay asymmetric or elliptical eggs. In addition, we propose that the stretchy egg membrane, not the hard shell, is responsible for generating the diversity of egg shapes we see in nature.”

To find an answer to the bird egg diversity question, Stoddard and her team  analysed the shapes of over 49,000 eggs, belonging to almost 14 per cent of  the existing species of birds. The researchers concluded that an egg that tends toward asymmetry or ellipticity might pass more easily through the narrow oviduct of a bird with an aerodynamically streamlined body, while still having enough volume to hold the nutrients necessary to support an embryo.

The scientists used a software, which they comically called the he “Eggxtractor”, to analyze tens of thousands of digital images of the collection’s eggs and determine their shapes. The data set provided the inspiration for a biophysical model of egg shaping.

“Now that we’ve analyzed an enormous data set of eggs, we can ask the question of whether there is a mechanistic model that makes sense, that can generate all the diversity in shape that we have observed”, Stoddard said. “And the answer is yes. … Just by adjusting the membrane’s properties and the oviduct’s pressure, we can derive the entire world of egg shapes. And we think it’s biologically realistic, what we’ve proposed.”

For example, the limpkin, a crane-like bird with a low hand-wing index, is a relatively weak flier and produces eggs with a low degree of asymmetry. The whooping crane, on the other hand, produces symmetrical eggs, as it is a strong flier.

Claire Spottiswoode of the University of Cambridge, who is familiar with Stoddard’s research but had no role in it, described Stoddard’s work as “a major leap forward in explaining how and why [egg shape] diversity has evolved.

Stoddard’s next step is to conduct an analysis of the membrane and look at museum specimens to see if the dimensions of their pelvises and oviducts can also be correlated with egg shape.

“We have ideas about how the bird body should be modified for flight,” said Stoddard, “but now we can actually look at museum specimens and take measurements of the pelvis, and see if it’s true that birds with small pelvises lay really elliptical or asymmetrical eggs. I’d say, on the whole, that eggs are surprising and interesting for reasons I couldn’t have imagined, getting into these questions years ago. They’re endlessly fascinating.”