The unusually long, oval egg of the Indonesian maleo may look a lot like a potato, but this bird is no slouch.

When the chicks of these anvil-headed animals hatch, they're almost immediately able to fly.

For years, scientists have wondered why bird species have different egg shapes. Some theorized shapes may protect eggs from shattering or allow them to fit snugly in the nest. Aristotle had even (wrongly) asserted that long, pointy eggs were female while rounder eggs were male.

But no comprehensive studies had ever been conducted to test these ideas, which left Mary Stoddard and colleagues skeptical.

“It has not gone unnoticed that birds have evolved to shapes that are quite diverse in form—everything from a spherical owl