It’s a scary world out there.
That’s why yellow-legged gulls (Larus michahellis) have figured out a clever way to gauge their surroundings while still encased within their speckled eggs. Before they hatch, these birds can tune in to the alarm calls of their parents, who emit a series of signature squawks when they sense the presence of ravenous, chick-chomping mink.
These chicks then take Mom and Dad’s warnings to heart—and after hatching, they behave in ways that may help them avoid getting eaten.
But even before busting out of their shells, these little gulls do something else unusual.
After hearing their parents’ calls of distress, embryos prepping for potential predators will toss and turn in their eggs. The resulting vibrations, it seems, are then picked up by their unhatched nestmates, which, when they emerge, look and act similarly to their siblings—even if they never got the original memo.
It’s a bizarre game of avian telephone, and researchers still aren’t completely sure how the underlying process works. But this strikingly sophisticated display of communication, reported in a study published today in the journal Nature Ecology & Evolution, reinforces the notion that eggs are more than passive embryonic vessels. The findings also suggests that, by picking up subtle signals from siblings, chicks-to-be can prep for hardship without actually having to experience it firsthand.
“Any information an animal gleans from its environment...will help it prepare for the world it’s about to be born into,” says Rebecca Calisi Rodríguez, a bird behavioral biologist at the University of California, Davis who was not involved in the study. “But the mind-blowing thing here is that there’s communication between siblings...in a way that changes how all of them develop.”
This isn’t the first time researchers have eavesdropped on animals chattering at their unborn offspring. In 2016, Deakin University researchers Mylene Mariette and Katherine Buchanan discovered that rising temperatures can prompt adult zebra finches to belt out ballads that broadcast the warm conditions to their eggs, which in turn hatch chicks better adapted to the heat. Others have shown that siblings, too, can convey prenatal messages to their nestmates—and several birds, reptiles, and insects are known to coordinate hatching times through cues passed from egg to egg.
But neither of these processes is well understood, and they’ve never been linked together like this in the same species, says study author Jose Noguera, a behavioral ecologist at the University of Vigo in Spain.
So Noguera and his advisor, Alberto Velando, decided to see if eggs could act as yolky intermediary in ferrying crucial intel—like the presence of a predator—from parent to offspring to sibling.
Such a familial relay might seem redundant, since all the eggs in a single nest should be exposed to the same cries. But some birds, like the yellow-legged gull, lay their eggs several days apart, which means each embryo in a given nest is at a different point along its developmental trajectory.
Because hearing crystallizes relatively late in development, older siblings are better equipped to receive their parents’ vocal warnings. Touch, on the other hand, is an early-emerging sense—which could make inter-egg vibrations a way to get a hard-to-hear message across to the junior members of a clutch, Noguera says.
To test this theory, Noguera and Velando gathered 90 second-laid yellow-legged gull eggs—the natural middle children of clutches—and divvied them up into 30 trios. During each day of the eggs’ last week of development, the researchers briefly exposed two of the eggs in each clutch to either recordings of adult alarm calls or static from a speaker, then returned them to incubate with an undisturbed foster sibling.
Clutches whose eggs weren’t privy to the prenatal panic yielded uniformly carefree chicks. But embryos that had regularly encountered alarm calls took longer to emerge from their eggs—and though they eventually hatched, these chicks never fully came out of their figurative shells. Their blood brimmed with stress hormones, and they were quieter and quicker to hide when they heard more of the adults’ cautionary caws. There were even changes in how the chicks broadly expressed different sets of genes.
And the hatchlings didn’t just internalize these subliminal messages: They passed them on to their uninformed siblings, too. After listening to squawks of distress, eggs shook and shimmied, knocking up against their nestmates—which then made moves of their own.
These secondhand signals, Noguera says, were enough to seal the fates of the unexposed chicks—even in the absence of direct auditory input. From blood to behavior, these eggs and the hatchlings they bore were identical to their stressed-out siblings by every metric measured.
“It’s not just about what happens to you during development,” Noguera says. “It matters what happens to your siblings, too.”
Because the researchers didn’t follow the chicks into adulthood, it’s difficult to say just how adaptive these behavioral and physiological changes were, says Mariette, who wrote a commentary to accompany the study, which she wasn’t involved in. (The changes also probably come with costs, she notes: For instance, chicks exposed to alarm calls—both directly and by proxy—grew slower and were smaller than their counterparts.)
However, the sophisticated demonstration of clutchmate coordination still sets this paper apart, Mariette adds. Unhatched chicks aren’t just aware of the world around them; they’re able to convey information about it, too. “Embryos are really not as isolated as we tend to think of them,” she says.
Fussy bird parents might deliberately chirp at their eggs to convey information. But Noguera cautions that the study’s findings don’t necessarily mean that unhatched siblings are engaging in any kind of intentional conversation among themselves. Rather, there’s a good chance these eggs are just sponging up whatever intel they can get. And though he hasn’t yet shown it for this species, Noguera thinks there are probably other cues, such as smells, that might play a role in the prenatal avian milieu.
That might not be all that surprising, given how sensitive other embryos (including those of humans) are to their environments. But when it comes to the nuclear family, knowledge may flow more freely than once thought. The routes it takes probably aren’t always direct—and maybe that’s the point.
“We’re subconsciously gathering information from each other all the time,” Calisi Rodríguez says. “There’s that saying: ‘By failing to prepare, you are preparing to fail.’ By having these particular capabilities, these organisms are preparing to succeed.”