Scientists think they know why birds' eggs are different shapes. Some are round, some are potato-shaped, and others are pointy. Two thousand years ago, Aristotle said flatter eggs had female chicks inside and rounder eggs contained male chicks. But he was wrong. A new study says that the shape of the egg depends on how well the female bird flies. A professor said she noticed that birds have evolved to lay eggs that have different shapes. They are all different in form, "everything from a spherical owl egg to a pointy sandpiper egg".

The professor and her team made a formula for the shape of different eggs. They looked at 50,000 eggs from 1,400 different bird species. They put the shape, length and height of the eggs into a computer. They also looked at how oval the eggs were. The researchers found that the birds with the most-oval-shaped eggs were the best fliers. The professor said: "We were shocked to see that one of the best explanations for egg shape [difference] was flight ability." Hummingbirds laid the most-oval-shaped eggs; owls laid the roundest eggs; and sea birds laid the pointiest eggs.