Migratory birds take breaks to boost their immune systems, shows study

Exercising too much and not getting enough rest is bad for your health. A new study from Lund University in Sweden shows that the same is true for migratory birds. They need to rest not only to renew their energy levels but also in order to boost their immune system.

After a period of physical exertion, vertebrates, including humans, usually need a period of recovery. Apart from the obvious—lowering the heart rate and repairing injured muscles—other, less prominent physiological systems might also need to recover. Intensive physical activity can affect an individual's basic immune defense.

When birds migrate, they regularly stop in one place for a few days to rest and eat. This was previously thought necessary in order to build up new fat reserves that provide fuel for their migration. However, researchers have now shown that birds also build up their immune system during their pit stops. They do so very quickly—a few days' rest is more than enough.

"This is the first time that this has been demonstrated in wild migratory birds. Our study shows that migratory birds' stops serve other purposes, besides just 'refueling.' They also need other physiological systems to recover. You could compare it to pulling off the motorway into a service station. That is not just for the purpose of refueling, you might also need to recover," says Arne Hegemann, biologist at Lund University who conducted the study with colleagues from the Institute for Avian Research in Germany.

Researchers have examined small migratory birds—such as chaffinches, dunnocks and common redstarts—and analyzed how their immune system changes when they take a break during their migration.

"If you see a little bird in your garden or in the park during the autumn and you know that it is heading to southern Europe or Africa, it is fascinating to think about why it is taking a break. If they do not get food or rest, their immune systems cannot recover—which is when they risk becoming ill," says Arne Hegemann.

By collecting and comparing data from different individuals and species, the researchers show that free-flying migratory birds can restore several parameters of immune function during stopovers; stationary periods between flights.

"It is fascinating just how much we are still to learn about avian migration and exciting new things emerge regularly. This provides an important part of the puzzle of how migratory birds cope with the physiological challenges they are faced with on their long journeys," concludes Arne Hegemann.

The research is published in Biology Letters.