A new study published in Biology Letters has found that migratory birds need to rest not only to renew energy levels, but to boost their immune systems too.
Following periods of physical exertion, vertebrates, including humans, typically need a period of recovery. This is for fairly obvious reasons, such as lowering the heart rate and repairing injured muscles, but also to restore basic immune defence levels.
Common Redstart was one of the migratory species examined in the study (Derek Lees).
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 from Lund University in Sweden have now shown that birds also build up their immune system during their pit stops. They do so very quickly – a few days of rest is more than enough.
Arne Hegemann, biologist at Lund University who conducted the study with colleagues from the Institute for Avian Research in Germany, said: "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 'refuelling'.
"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 refuelling, you might also need to recover."
Researchers examined small migratory birds, such as Chaffinches, Dunnocks and Common Redstarts, and analysed how their immune system changes when they take a break during their migration. By collecting and comparing data from different individuals and species, the study showed that free-flying migratory birds can restore several parameters of immune function during stopovers (stationary periods between flights).
"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," added Hegemann.
"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."
Eikenaar, C, Ostolani, A, Hessler, S, Ye, E Y, & Hegemann, A. 2023. Recovery of constitutive immune function after migratory endurance flight in free-living birds. Biology Letters. DOI: 10.1098/rsbl.2022.0518