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The sound of tawny owls makes young European robins eat less during their southward migration. A new study from Lund University in Sweden shows how the threat from nocturnal predators affects the birds' behavior—and by extension, their survival.

When young robins embark on their first southward migration in the autumn, they make regular stops along the way to rest and replenish their energy reserves. However, each stop entails a risk—predators may be lurking nearby.

In an in the Journal of Animal Ecology, a research team has established that migrating birds are not only aware of threats around them, but also adapt their behavior based on which predator calls they hear.

"For the first time it has been possible to show that the calls of nocturnal predators affect how birds obtain energy during their migration," says Ã…kesson, professor of biology at Lund University.

In the research project, young robins were exposed to calls from two different birds of prey: the tawny owl and sparrowhawk. The results were clear. The call of the diurnal sparrowhawk did not significantly affect the birds' behavior, whereas they reacted strongly to the call of the nocturnal tawny owl. They became more cautious, reduced their night-time activity and ate less. This in turn led to a slower build-up of fat reserves and poorer physical condition.

"There is a clear compromise involved: to dare to eat and build up fuel reserves or steer clear to avoid being eaten," says Ã…kesson.

Eating less also means that the birds have to stay longer at their stopovers in order to sufficiently replenish energy reserves. Longer stays increase the risk that they will arrive late at their wintering grounds—where there is tough competition for the best territories. A late arrival may have consequences for both survival and future reproduction.

"By understanding how migratory birds respond to different threats, we can improve how we plan the design of stopover sites and peri-urban environments. If birds have access to calm and protective surroundings during their stopovers, it increases their chances of surviving the long journey," concludes Ã…kesson.