With their elaborate yellow eyebrows, crested penguins are both unmistakable and slightly clownish.

But to see the strangest part of these birds, such as the macaroni and rockhopper penguins, you shouldn’t look to their comical plumes. Instead, you need to watch what happens when they lay their eggs.

They produce two in any given breeding season. The first—let’s call it the A-egg—is always smaller than the second, or B-egg. It’s smaller by between 18 and 57 percent, a greater difference than in any other bird. Because it’s smaller, the A-egg is almost always doomed. The mother penguin might kick it out of her nest. She might refuse to incubate it. On the off-chance that both eggs hatch, only one of the two