Adaptations for flight may explain egg-shape variety among birds

Bird eggs populate a diverse continuum of shape and size. Photo by Harvard Museum of Comparative Zoology

June 23 (UPI) — Bird eggs encompass a wide variety of shapes and sizes. Some of that variety, new research suggests, can be explained by adaptions for flight.

Researchers developed a model combining mathematics and biological data to better understand how and why egg shapes have such tremendous diversity.

“Our study took a unified approach to understanding egg shape by asking three questions: how to quantify egg shape and provide a basis for comparison of shapes across species, what are the biophysical mechanisms that determine egg shape, and what are the implications of egg shape in an evolutionary and ecological setting,” L. Mahadevan, an evolutionary biologist at Harvard University, said in a news release. “We showed that egg shapes vary smoothly across species, that it is determined by the membrane properties rather than the shell, and finally that there is a strong correlation linking birds that have eggs that are elliptical and asymmetric with a strong flight ability, the last a real surprise.”

Scientists used their model to analyze the shapes of 50,000 eggs, representing 14 percent of all living bird species across 35 orders. The physical analysis revealed a continuum of egg shape from perfectly spherical to severely oblong, or conical-shaped. Eggs of different species overlapped across the entirety of the spectrum.

The evidence also showed — as previous studies have — that the physical properties of membranes, not shell material or structure, dictate the egg’s shape. The analysis, detailed in the journal Science, also revealed a strong correlation between egg shape and species capable of flight.

“It’s clear from our study that variation in the size and shape of bird eggs is not simply random but is instead related to differences in ecology, including the amount of calcium in the diet, and particularly the extent to which each species is designed for powerful flight,” said study co-author Joseph Tobias, a researcher at Imperial College in England.

The link between flight adaptations and egg shape explains why hummingbirds and albatross, two very different species in size and appearance, lay similarly shaped eggs.

“To maintain sleek and streamlined bodies for flight, birds appear to lay eggs that are more asymmetric or elliptical,” Harvard ecologist Mary Caswell Stoddard said. “With these egg shapes, birds can maximize egg volume without increasing the egg’s width — this is an advantage in narrow oviducts.”


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