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Susana Cortés-Manzaneque, Sin-Yeon Kim, Jose C. Noguera, Francisco Ruiz-Raya, and Alberto Velando: Read the article
This study provides strong evidence that prenatal signals of predation risk prepare chicks to cope with predators during their postnatal development, based on findings from a field experiment on the yellow-legged gull
I magine you’re in an egg, you haven’t even hatched yet, but you’re already learning how dangerous the world outside may be. Before a chick even cracks its shell, it’s already listening and learning. Inside the egg, the chick can hear the outside world: the waves, wind, and the calls of its parents and surrounding nesting ground. If those calls include urgent alarm cries, it might mean danger is near. New research shows that these before-hatch warnings can shape how a chick behaves and grows long after it hatches, especially if the warnings turn out to be wrong.
Researchers set-up an experiment on Sálvora Island, Parque Nacional das Illas Atlánticas de Galicia, Spain, where some unhatched yellow-legged gull chicks heard recordings of alarm calls signaling predators were nearby, while others heard only normal colony sounds. After hatching, chicks either experienced “predators” (a stuffed American mink, a real threat to gulls, attached to a remote-controlled toy off-road car) or something harmless (a stuffed European rabbit attached instead). This created a match-mismatch situation where the chicks’ pre-hatch “expectations” —set by the calls they heard in the egg—either matched their post-hatch reality or didn’t.
The results were striking. Chicks that heard alarm calls before hatching spent more time frozen in a “don’t move” defense called tonic immobility. They also crouched faster when they thought danger was near, though only if they did not meet predators later. Chicks that did face predators after hatching begged for food less often, likely to stay hidden from danger. Early on, the chicks all grew at similar rates, and their stress hormone levels didn’t differ much. But weeks later, when the chicks left the nest, those whose early-life predictions didn’t match reality—whether they prepared for danger that never came or faced danger without warning—were smaller in skeletal body size and showed more damage to their DNA, a sign of physical stress. Their bodies also produced more antioxidants, probably trying to repair the damage. On the other hand, chicks that heard alarm calls before hatching ended up heavier in body mass at fledging, no matter what happened afterward.
Why does this matter? It confirms that chicks aren’t passive passengers before hatching. They are already adjusting their bodies and behavior to fit the world they expect. But if that forecast is wrong, it can cost them in growth and health. More research is necessary to fully understand the lifelong consequences of a developmental mismatch and the mechanisms that govern development. As the environment becomes more unpredictable with climate change and human disturbance, such mismatches between early cues and reality could become more common, and more harmful. Yellow-legged gull chicks are born with a kind of weather report for survival. When it is accurate, they are ready. When it is not, they may pay the price. Just like us, their early expectations can shape their future, for better or worse.
Kate Blackwell is a Ph.D. candidate in Ecology and Evolution at Stony Brook University, where she investigates how marine populations are delineated by studying their genetic variation, gene flow, and linking physical traits with ecological roles and environmental adaptation. Her research focuses on identifying the nesting locations of Antarctic petrels using satellite imagery and understanding their connectivity using genetics and morphology. Passionate about intersection of science, policy, and public engagement, Kate collaborates internationally with scientists and policymakers on conservation priorities for the Arctic and Antarctic. During her downtime, she loves traveling with friends and reading a good fantasy or science fiction book.
