Baby birds that have not yet hatched from their shells can communicate and warn each other in the presence of danger.

A new study revealed that developing baby yellow-legged gulls vibrate their shells to alert their neighbors if a predator is around. This, according to experts, is one of the most sophisticated known examples of embryonic communication.

The new study was published in the journal Nature Ecology and Evolution.

How Baby Birds Communicate

They made the discovery after they had studied 90 yellow-legged gull eggs from a large breeding region in Sálvora Island in northern Spain. The eggs were placed into nests of three.

When the eggs are six days away from hatching, two of the three eggs were temporarily removed from each nest and exposed to either a recording of a predator alarm call or white noise. The third egg remained in the nest.

The researchers played the recorded predator alarm call or white noise four times a day for three minutes at a time until the chicks hatched.

The study revealed that the eggs exposed to the noise by vibrating more and vocalizing less. Moreover, this behavior was also passed on to the third bird that remained in the nest and was not exposed to the sounds.

Baby Birds Warn Each Other Of Danger

Like its nestmates, the third egg underwent genetic changes that delayed hatching. The researchers also observed an increased production of stress hormones in the embryos, which is known to occur in the face of a threat.

These responses affect baby birds even after they hatched. Chicks that were previously exposed to predator alarm calls were quicker to run and hide.

"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,"Jose Noguera, the lead author of the study, explained to The Guardian.

The researchers admitted that they still are unsure how embryos understand the vibrations. However, Noguera is positive that a similar phenomenon occurs in other bird species.

He and his team hope to continue the study. Their next step is to investigate whether the unhatched birds can pick up other external information from inside the egg.