Egg shapes reveal flight abilities— pointed for strong fliers like common murres, round for earthbound heavies like cassowaries. Peregrine falcon Common murre Scrub jay Pheasant-tailed jacana Cassowary Tinamou Bird Drawings not to scale Egg shapes reveal flight abilities—pointed for strong fliers like common murres, round for earthbound heavies like cassowaries. Peregrine falcon Common murre Scrub jay Pheasant-tailed jacana Cassowary Tinamou Photo: Frans Lanting

ILLUSTRATIONS: DAISY CHUNG, NGM STAFF

Source: René Corado, Western Foundation of Vertebrate Zoology

This story appears in the January 2018 issue of National Geographic magazine. Bird drawings not to scale

Birds lay eggs of surprising variety, and scientists have long wondered why. Could it be that certain shapes protect eggs from shattering? Or perhaps allow them to fit snugly in a nest? Or was Aristotle correct when he asserted that long, pointy eggs contained females while rounder eggs held males? (He wasn’t.)

To crack the mystery about egg shapes, Princeton University evolutionary biologist Mary Caswell Stoddard and her colleagues examined almost 50,000 eggs from more than 1,400 species. They classified the eggs based on their asymmetry and ellipticity, discovering that the more pointy or oval shaped an egg, the more likely it came from a strong flier. “We were shocked to see that one of the best explanations for egg-shape variation was flight ability,” says Stoddard.

Common murres, for instance, have particularly pointy and elliptical eggs (see above)—and are expert divers. Perhaps a streamlined bird needs a streamlined egg; stout flightless birds like ostriches and emus hatch from eggs that are nearly round.