Baby seabirds that have not yet hatched communicate with their siblings in neighbouring eggs by vibrating their shells, scientists have discovered.

A study of yellow-legged gulls revealed one of the most sophisticated known examples of embryonic communication. When exposed to the alarm calls of an adult bird responding to a predator, developing chicks apparently were able to convey the presence of danger to their nestmates by wriggling inside their eggs.

“We were very surprised,” said Jose Noguera, the lead author of the study from the Animal Ecology Group at the University of Vigo, Spain. “We were aware that bird embryos were able to produce egg vibrations, [but they vibrated] even more than we expected.”

Knowing about danger and sharing this information with their nestmates could help the chicks adapt to life on the outside and boost the family’s chances of survival in hostile environments.

“It is already well known that embryos are able to perceive certain cues from outside the egg, but it has not been known until now that they can capture this information from outside and transfer it from one embryo to another,” said Noguera.

The team collected 90 yellow-legged gull eggs from Sálvora Island, a large breeding region off the coast of northern Spain, and sorted them into nests of three.

When the eggs were six days off hatching, two of the three eggs in each nest were temporarily removed from the nest and exposed to either a recording of a predator alarm call or white noise each day until the chicks hatched. The noise was delivered four times a day at random intervals for three minutes at a time. The third egg from each group remained in the nest.

It was found that the embryos responded to the external alarm calls by vibrating more and vocalising less – and that this information appeared to be passed on to the third nestmate, which had not been directly exposed to the sounds.

The third egg was seen to mimic the vibrations. Like the other chicks, it also underwent genetic changes known to delay hatch time and had an increase in the production of stress hormones – responses that are known to occur in embryos in the presence of a potential threat.

“This kind of transfer of information – embryo to embryo – can induce developmental changes that can have potential benefits [to the birds] after hatching,” said Noguera.

A rise in stress hormones makes birds more aware of their surroundings after hatching. When hatched chicks were exposed to alarm sounds, it was found that those who had listened to the noises previously in the egg were quicker to run away and hide.

Scientists are still unsure how the embryos process and understand these vibrations as signals.

Noguera said the phenomenon was likely to occur in other bird species. His team now plans to investigate whether the chicks are able to pick up other clues about their external environment before hatching, such as how many other eggs are in the nest.

Mylene Mariette, a behavioural ecologist at Deakin University in Australia, who was not involved in the research, said “[This study] suggests that this phenomenon is a lot more widespread than currently realised.” She said the results might lead to findings that this type of communication between embryos occurs in other animals, not just birds.

Noguera’s findings are published in the journal Nature Ecology and Evolution.