Scientists believe they know why birds' eggs are different shapes. Some eggs are quite round; some are kind of potato-shaped; and others are longer and pointy. Two thousand years ago, the Greek philosopher Aristotle said flatter eggs had female chicks inside them and rounder eggs contained male chicks. But he was wrong. A new study from Princeton University in the USA suggests that the shape of the egg depends on how well and how fast the female bird flies. Professor Mary Stoddard said: "It has not gone unnoticed that birds have evolved to [lay eggs with] shapes that are quite diverse in form - everything from a spherical owl egg to a pointy sandpiper egg."

Professor Stoddard and her colleagues created a mathematical formula to map the shape of different eggs. They looked at almost 50,000 eggs from 1,400 different bird species. The researchers put the shape, length and height of the eggs into a computer database. They also looked at how oval the eggs were. The researchers found that the birds that laid the most-oval-shaped eggs were the best fliers. Professor Stoddard said: "We were shocked to see that one of the best explanations for egg shape variation was flight ability." The researchers found that hummingbirds and sandpipers laid the most-oval-shaped eggs; owls laid the roundest eggs; and sea birds laid the pointiest eggs.