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The importance of the immune system for migrating birds

Biologists at Lund University in Sweden have found out how small migratory birds respond when they become ill on the way to warmer regions in the autumn. The researchers established that the birds handle sickness in different ways depending on their destination and the efficacy of their immune system.
Blood sample is taken from a small bird
Blood sample is taken from a small bird. Photo: Aron Hejdström

Biologists at Lund University have carried out two studies on the effect of sickness on small birds which are migrating to wintering sites in Africa south of the equator and to south-west Europe. The studies were carried out in Falsterbo in Skåne, Southern Sweden.

In one study, the researchers simulated sickness by vaccinating birds. In the other study, the researchers captured birds and examined them to see if they were infected by avian malaria. In order to measure the strength of the immune system, the researchers took blood samples from the birds. Finally, they fitted each bird with a small transmitter weighing 0.3 grams. The birds were then released. Using the transmitters, the researchers were able to follow each individual bird and measure precisely when the sick and healthy birds continued their migration over the Baltic and how long they rested before leaving.

The results show that vaccinated birds seem to believe that they are sick and therefore delay flying on for a few days. The researchers also observed that birds that have a real sickness (chronic malaria infection) left later in the day compared with their healthy conspecifics. The conclusion is that they fly a shorter distance before the next stopover in order to rest and that the entire migration therefore takes longer, which can have unfavourable consequences.

Both studies also show differences that stem from the location of the wintering sites. The sick birds whose destinations are south of the Sahara are in more of a hurry to leave compared with species that spend the winter in south-west Europe. The birds heading for tropical Africa stay an extra day in Falsterbo in order to rest. Those heading for south-west Europe rest for more than twice as long before they leave – almost three days.  

A dunnock with a tiny radio transmitter on its back.
A dunnock with a tiny radiotransmitter on its back. The antenna is visible. Photo: Arne Hegemann

“Regardless of where they are heading, they continue their migration before they are fully recovered. However, the pressure to continue seems to be more intense for those that are heading to southern Africa and have longer distances to fly”, says biologist Arne Hegemann, who led the research.

“If too much time is spent resting, recovering and getting back to normal, there will be consequences for the entire migration. If the birds arrive later at their wintering sites, the conditions may have changed dramatically. There may, for example, have been a considerable reduction in insect numbers as food”, he says.

Antenna at the lighthouse in Falsterbo, Skåne.
One of the antennas of the automated radio-telemetry system at the lighthouse in Falsterbo, Skåne. Photo: Arne Hegemann

The researchers emphasise the importance of a rapid migration. The easiest way to influence that is to minimise the time the birds spend on stopovers. A strong immune system is therefore important. Arne Hegemann wants to continue the studies by monitoring the infected birds all the way to the wintering sites.

“Do they lag behind the whole way or do they catch up when they have completely recovered?

The studies were published in Journal of Animal Ecology and Oecologia.

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