Ed Yong in Not Exactly Rocket Science:
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 chicks ever survives to become a fledgling, and it’s invariably the larger B-chick.
Evolutionary biologists have been puzzling over this bizarre trend since the 1960s. Why is the A-egg so much smaller than the B-egg? And since it almost always dies, why would crested penguins bother producing it at all? Why not simply concentrate their efforts on a single egg, as the famous emperor and king penguins do?
More here.