A bird's nest is a hub of communication. There are the squeaks and chirps of hatchlings clamoring for food. And mom telling them to hush when a predator rears its head.
But now scientists say that level of communication begins long before the babies even hatch.
A study published this week in the journal Nature Ecology and Evolution finds bird embryos chat with each other — using vibrations — while still inside the egg.
And, as a result, they know when it's safe to hatch or if they should bide their time in the comfort and relative safety of their shells.
To test that theory, a team of biologists from Spain's University of Vigo looked to birds that hatch in a particularly precarious setting: Sálvora Island, off the country's Galician coast. A popular mating spot for yellow-legged gulls, the island is also home to a population of minks with a taste for baby birds.
As such, knowing when to break out of one's shell is a matter of life and death.
For their experiment, researchers carefully gathered sea bird eggs and organized them into test groups under incubators. One group was regularly subjected to recordings of adult predator alarm calls — essentially a parent's warning that danger was near.
Meanwhile, another batch of eggs remained in a soundproof box oblivious to simulated threats.
When all the eggs were returned to the same incubator and placed in physical contact with each other, scientists made a stunning observation: the eggs that had been exposed to warning calls vibrated more than those that had been undisturbed.
"We were very surprised," lead author Jose Noguera, of the Animal Ecology Group at the University of Vigo, told The Guardian. "We were aware that bird embryos were able to produce egg vibrations, [but they vibrated] even more than we expected."
Those tremors were caused by embryos wriggling nervously in their shells. And, like a Morse-code transmission from behind those thin calcium walls, it found keen ears among the rest of the eggs.
In fact, when the eggs finally hatched, the chicks clearly showed they had already received a kind of heads-up about their environment — even the ones that had only been exposed to the vibrations of their alarmed peers.
The newborns, according to the study, emerged in a state of caution: Compared to a control group, they took longer to shed their shells, remained much quieter and crouched down more frequently.
They also revealed physical signs of pre-induced anxiety, including higher levels of stress hormones and fewer copies of mitochondrial DNA per cell.
The embryonic information highway had equipped all the hatchlings for the uncertain environment they were entering.
"Our results clearly show that bird embryos exchange valuable information, probably regarding risk of predation, with their siblings," researchers noted in the paper.