New York, June 26 (IANS): In what could perhaps crack the long-term mystery behind the astonishing variety of bird egg shapes, an international team of researchers has found that the egg shape is related to flight ability, with good fliers tending to lay pointy or elliptical eggs. Avian eggs have fascinated humans for millennia because they come in different shapes -- elliptical in hummingbirds, spherical in owls, pointy ovoids in shorebirds and almost everything in between. But we still lack the answer to this simple question -- why did different egg shapes evolve, and how? The new study published in the journal Science suggests that egg shape is related to flight ability, and that the egg membrane may play a critical role in determining shape. "In contrast to classic hypotheses, we discovered that flight may influence egg shape. Birds that are good fliers tend to lay asymmetric or elliptical eggs," said the study's lead author Mary Caswell Stoddard of Princeton University in New Jersey, US. "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," Caswell said. To unravel the mystery of egg shape, the researchers used a multi-step, multidisciplinary process, applying tools from computer science, comparative biology, mathematics and biophysics. First, the team used photographs to analyse the shapes of nearly 50,000 eggs representing 1,400 species. 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. Using computer code, the researchers quantified each egg's asymmetry and ellipticity. By combining the power of high-throughput digital image analysis with the wealth of data in the museum egg collection, the team was able to map the world of egg shapes. The team then developed a biophysical model to explain how processes in the bird's oviduct might generate different egg shapes. The team also used an evolutionary framework to test hypotheses about egg shape. Using a recently constructed phylogeny, or family tree, of birds, the researchers 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 suggest that as birds' bodies became adapted for powered flight, this resulted in morphological changes like reduced body size and a reduced abdominal cavity. The discovery that morphological constraints associated with flight may contribute to egg shape challenges the conventional wisdom that egg shape is largely influenced by clutch size or nest location.