Scientists may have solved the mystery of why bird eggs come in an astonishing variety of shapes, like ellipses in hummingbirds, spheres in owls and pointy ovoids in shorebirds.
According to a new study published Thursday in the U.S. journal Science, it's related to birds' flight ability and the egg membrane may play a critical role in determining shape.
"In contrast to classic hypotheses, we discovered that flight may influence egg shape," lead author Mary Caswell Stoddard of Princeton University said in a statement.
"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."
Many different theories exist as to why the shape of bird eggs varies so much across species.
One theory was that cliff-nesting birds lay more cone-shaped eggs, which roll in a tight circle and are less likely to tumble off the cliff.
Another theory suggested that different egg shapes exist to maximize incubation efficiency in a clutch.
To unravel the mystery, an international team of researchers analyzed the shapes of nearly 50,000 eggs representing 1,400 species in 35 orders, including two extinct orders.
The eggs, from the online database of The Museum of Vertebrate Zoology at Berkeley, came from across the globe and were largely collected by naturalists in the late 19th and early 20th centuries.
With simple observations, the team found that an egg's shape is not determined by the shell: dissolve away the calcified shell and the shell-less, membrane-encased egg still retains its shape.
"By adjusting two basic properties -- changes in the thickness of the egg membrane as a function of location, and a pressure jump across the membrane -- we show that our model can produce a wide variety of egg shapes, encompassing the entire range of observations," Senior author L. Mahadevan of Harvard University said.
"This mechanistic approach to shape has a long history in biology, and our work suggests how tinkering with just two functional forms could allow evolution to move through the two-dimensional morphospace of egg shapes."
Using a recently constructed family tree, of birds, the researchers then compared egg shapes across different bird lineages. In this analysis, they included details about nest type and location, clutch size, diet and flight ability.
The analysis revealed that birds tend to lay eggs that are more asymmetric and more elliptical if they are better fliers.
The researchers suggested that as birds' bodies became adapted for powered flight, this resulted in morphological changes like reduced body size and a reduced abdominal cavity.
"Variation across species in the size and shape of their eggs is not simply random," said co-author Joseph Tobias of Imperial College London, "but is instead related to differences in ecology, particularly the extent to which each species is designed for strong and streamlined flight."