Smithsonian.com

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When you see pictures of dinosaur nests, they usually involve a whole boatload of eggs. But birds tend to only lay a few. If birds came from dinosaurs, then why do they make so many fewer eggs? Possibly because they gave up an ovary in exchange for the ability to fly.

Dinosaurs had two ovaries, giving them the ability to lay a bunch of eggs. Today’s birds, however, only have one working ovary—usually on the left side, apparently—and can only make a few eggs. This new research looked at fossils from China, in which a few early birds were fossilized such that you can see their ovaries. Finding preserved ovaries is pretty rare, and the researchers were actually confused about what they were seeing for a while. They explain, in a Nature press release:

“It took us a while to figure out what these strange circular structures actually represent,” said Dr. Zhou Zhonghe, project leader of the IVPP . The small structures might possibly have been seeds or tiny stones the birds had swallowed to grind food in their digestive system. But on the basis of the size, shape, and position of the rounded structures, the team ruled out the alternative explanations and interpreted them as ovarian follicles.

But once they figured out what they were, the researchers were quite excited. The fact that very early birds seem to have already lost an ovary suggests that the change was key to flying. The fossils suggest that losing the weight of that second ovary could ave been part of the path to flight. Science NOW reports:

Some scientists have assumed that the evolutionary loss of one functional ovary—a weight-saving change that might have proved beneficial to flying birds—took place early in avian evolution. Until the new study, paleontologists hadn’t unearthed any evidence for the notion that early birds, like their modern-day kin, had only one ovary.

In case you’re wondering, the average human ovary weighs about 2-3.5 grams. Probably not enough to allow us to take to the skies, even if we drop it. But we can dream.

More from Smithsonian.com:

How Hummingbirds and Moths Fly
Pterosaurs Were Born to Fly