Young birds lose strength when they hear predators at night

Each autumn, young European robins set off on their first migration south. The journey is perilous. To survive, they must rest often, build fat reserves, and time their movements carefully. But unseen threats lurk in these resting grounds. Sometimes, it is not the flash of talons but the echo of a predator’s call that changes their fate.

A recent study from Lund University shows how such sounds can shift survival strategies. Robins respond differently to the calls of predators depending on whether they hunt by day or by night. This subtle adjustment carries far-reaching consequences for their condition, speed, and even future reproduction.

A landscape of fear

Stopovers are vital. Birds need about a week of feeding to fuel even one day of flight. Yet each pause increases their vulnerability.

The “landscape of fear” describes how prey shape their behavior not only in response to attacks but also to the mere possibility of predation. For robins, this means that a single predator’s voice can dictate how boldly they eat or how cautiously they move.

“For the first time it has been possible to show that the calls of nocturnal predators affect how birds obtain energy during their migration,” said Susanne Åkesson, professor of biology at Lund University.

Predators influence young birds

In the experiments, researchers exposed juvenile robins to recordings of two predators: the daytime sparrowhawk and the nocturnal tawny owl.

The contrast was striking. Sparrowhawk calls barely shifted robin behavior. Tawny owl calls, however, triggered marked caution. Robins reduced food intake, delayed activity, and ultimately carried less fuel for flight.

“There is a clear compromise involved: to dare to eat and build up fuel reserves or steer clear to avoid being eaten,” explained Åkesson.

The robins that heard owls ended the experiment leaner, with shorter predicted flight ranges. They would likely need more stopovers to compensate, increasing their chances of arriving late to wintering grounds where competition for territories is fierce.

Flight timing disrupted

The study uncovered another surprising detail. Control birds gradually began leaving earlier in the evening as their condition improved.

Robins exposed to owls did not show this adjustment. Instead, their departure remained delayed, pushing their travel window later into the night. Over time, this built into significant migration slowdowns.

These delays reveal how fear influences not only energy intake but also the very timing of long flights. Even without direct attacks, the presence of a predator’s voice alone can ripple through the entire migration strategy.

Temporary stress or lasting fear

The reduced foraging was strongest during the first days of exposure but gradually lessened. By the second week, birds exposed to owls had nearly matched the control group in feeding.

This suggests the robins were not locked into chronic fear but rather experienced a sharp, temporary response.

Still, even short-term hesitation has consequences. A few days of reduced feeding can make the difference between strong arrival and exhaustion. In migration, small setbacks accumulate into large survival costs.

Experienced birds respond differently

The findings also suggest that fear responses may vary depending on experience. These were young, first-year migrants, unfamiliar with the dangers of their route and naïve to the complex threats they would encounter.

Older birds, in contrast, may behave differently, drawing on past encounters to guide their decisions and adjusting their strategies more flexibly. With memory and experience, they might balance caution and foraging in ways that minimize the costs of hesitation while still avoiding unnecessary risks.

This raises new questions about how experience, physiology, memory, and environmental signals combine to shape migration outcomes, and whether seasoned birds can better compensate for predator pressure than inexperienced juveniles.

Understanding these differences could reveal how fear is learned and how survival strategies evolve across varying landscapes and seasons.

Protecting birds from predators

The results carry practical weight. Stopover sites are not just refueling stations; they are arenas where survival balances on fear and opportunity. Calm, sheltered habitats give birds a chance to feed without hesitation, arriving stronger at their destinations.

“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,” concluded Åkesson.

Migration, then, is more than distance and endurance. It is a negotiation with unseen predators, where even a distant owl’s call can tip the scales of survival.

The study is published in the Journal of Animal Ecology.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates. 

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–