In addition to these theories, scientists also thought that precocial baby birds—those that are usually mobile and more mature at birth, like ducks—had asymmetrical eggs because the blunt ends have more pores, letting in more oxygen to help their brains develop faster.

Researchers put all these theories to the test by looking at the life histories and the evolutionary relationships of the birds in a new study, published in Science.

They looked at a whole slew of variables, including: adult body mass; diet; number of eggs in a nest; nest type; nest location; environment; and something called hand-wing index (HWI), a proxy for flight capability. A high HWI is linked to better flight performance.

After crunching the numbers, the scientists found the links they’d been looking for: the length of an egg correlates with bird body size. The shape of an egg—how asymmetrical or elliptical it is—relates to flying habits. And the stronger a bird's flight, the more asymmetrical or elliptical its eggs will be.