Before they hatch, developing baby gulls seem to prepare for the worst when they hear the piercing predator-warning calls of nearby adults—and may also alert their less-developed nestmates.

Yellow-legged gull embryos exposed to the warnings of adults and neighboring embryos that had not been exposed to the sounds both displayed a series of behavioral and physiological changes when newly hatched, according to a study published Monday in Nature Ecology & Evolution. By vibrating, the embryos exposed to the call appeared to transfer knowledge of the threat to the others in their nest—the first time such a behavior has resulted in observed changes to clutchmates.

Just as human babies register sounds they hear from the womb, embryos within bird, reptile, amphibian and insect eggs use sensory clues to glean information about their environment. To investigate this process in yellow-legged gulls, study co-authors and behavioral ecologists Jose Noguera and Alberto Velando, both at the University of Vigo in Spain, plucked 90 gull eggs from nests on the rocky shores of Sálvora Island, off the country’s western coast. They then tracked the development of multiple clutches, or groups, of three eggs, each of which was laid days apart from the other two, inside a nearby field station. Four times a day, two eggs from each clutch were placed inside an incubation chamber for one hour. One set of eggs was placed in a chamber that periodically played the shrill warning calls of adult gulls. The other pair of eggs—a control group—sat in the incubation chamber in near silence. 

The embryos that were exposed to warning calls would begin to vibrate, shifting inside their shell in response to the perceived threat, and continued doing so even after they were transferred back to their nest. Noguera and his colleagues say they think this movement alerted embryos inside nearby eggs that something was amiss. In the wild, the translation from an auditory alert to a vibrational one would serve to warn clutchmates whose sense of hearing had not yet developed, the researchers suggest. 

Once the chicks hatched, the team measured their physiological and behavioral responses to the warning calls. Blood samples taken from the chicks revealed a series of changes. The researchers found elevated levels of corticosterone, a stress hormone, and higher levels of DNA methylation, a molecular process that indicates changes in which genes are activated. As the rate of vibration among the eggs increased, so did DNA methylation levels in the hatched chicks, the researchers found. The amount of DNA in the blood cells’ energy-producing mitochondria—an indication of good health—decreased in both chicks that were exposed to warning calls and their nestmates.

The hatchlings’ behavior and physical shape changed, too. Embryos that experienced warning calls not only delayed hatching but were also quieter and quicker to crouch low when they perceived nearby threats after they hatched. Additionally, they were smaller and had a slower-growing lower leg bone called the tarsus. Those chicks’ clutchmates, who had not been exposed to the sounds but had shared the same nest, showed similar changes in behavior and bone structure. Embryos in the clutches that were not introduced to the sounds, however, did not vibrate in their nests nearly as much as those that were, and they showed none of the changes that the latter did.

“Sounds can give a lot of information to embryos, and they seem to be using it to shape their development to their particular conditions,” says behavioral ecologist Mylene Mariette of Deakin University in Australia, who was not involved in the study.

According to Mariette and Lynn Martin, an ecological physiologist at the University of South Florida, who was also not involved in the work, there are still questions about the exact mechanism that causes changes to the birds’ traits, including where and when in development these changes occur.

For example, the DNA methylation samples were collected from the chicks’ blood instead of the brain, where most of their observed changes occur, so it is difficult to say which genes allowed the behavioral ones to happen. Martin adds that identifying levels of DNA methylation from samples collected across different parts of the brain would more specifically reveal which genes unlock the behavioral changes. “The behavioral effect is what's most interesting,” he says. “At the end of the day, evolution sees that. It doesn’t really matter if the behavioral effect is driven by methylation or something else.” Noguera and Velando say they are not yet sure how these changes would impact chicks’ ability to fend off predators later in life.

Next, Noguera hopes to study these embryo-to-embryo interactions in birds whose chicks compete for resources. “If an embryo is able to capture information about its environment, it is probably going to be able to capture information about how many potential siblings, or competitors, will hatch,” he says. He suspects that embryos can use this information “to follow their sibling’s development so they can adapt their development for the future level of competition” they will find after hatching.