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You might think eggs just kind of... lie there, totally oblivious to the outside world. After all, the building of a baby bird is all happening on the inside.

But science has shown that eggs are actually pretty tuned in to what's happening outside the shell. And in 2019, scientists learned something even more incredible:. Eggs aren't just listening—they can communicate with each other.

Eggs can talk to other eggs! Scientists learned this by studying a colony of yellow-legged gulls on the. Spanish island of Sálvora, where chicks are often eaten by predators like mink.

They'd known from previous research that, in some species, bird-moms communicate with their chicks through the egg wall, and in response, those chicks adapt in a way that prepares them for life's challenges. So in the 2019 study, which was published in Nature Ecology & Evolution, the researchers wanted to see if these gull chicks could be prepped to deal with their predators before they even hatched. To do that, the scientists made artificial nests in the lab and put three eggs in them, about a week before they were due to hatch.

Every day, they removed two of the eggs—always the same two— and put them in a soundproof box where they played recordings of alarm calls from adult birds in the colony. Then, they reunited the two eggs with their nest-mate, which hadn't been exposed to any noise. After a week of this, the scientists returned the eggs to the colony just in time for them to hatch, then brought them back to the lab once the chicks were two days old.

They found that the chicks that were exposed to alarm calls in the egg were quieter than chicks from a control group. They also had higher levels of stress hormones and were faster to hide by crouching any time scientists played the alarm call. Which was pretty strong evidence that even inside the egg, chicks were adapting to deal with threats out in the real world.

But, what was amazing was that the third egg in the nest, which developed in complete peace, behaved just like the other two— even though it had never heard a single alarm itself! Somehow, the siblings were passing on warnings to their nest-mates! Egg to egg!

The researchers think the eggs were communicating through vibrations:. Eggs that heard alarm calls vibrated more, and their siblings picked up those cues. Then, in both cases, there was an increase in a process called DNA methylation, which changes how genes are expressed without changing the genetic code.

The researchers don't know exactly what those changes do to a growing chick, but they seem to shape its response to fear. That could explain how all these eggs instinctively knew to be quiet and hide, even the ones that had never heard warning signals themselves. And birds may not be alone!

Studies from other creatures that lay eggs, like turtles, have also shown that egg-siblings may talk to each other. Like, they may give each other cues to hatch faster if the nest gets flooded, or to time things so that siblings hatch together and have safety in numbers against predators. So, the peaceful eggs you might see in a nest are likely chatting away, gearing up for a tough world—and looking out for each other.

Thanks for watching this episode of SciShow! And if you liked it, I bet you'll enjoy our episode about some amazing bird nests that will change the way you think about birds! You can watch that right after this. [♪ OUTRO].