In a unique study, researchers at Lund University in Sweden have mapped the origins of migratory birds. They used the results to investigate and discover major differences in the immune systems of sedentary and migratory birds. The researchers conclude that migratory species benefit from leaving tropical areas when it is time to raise their young  as moving away from diseases in the tropics enables them to survive with a less costly immune system.

Analysing the genealogy of over 1300 song bird species, passerines, enabled the researchers to establish that both sedentary birds, which do not leave northern Europe in the winter, and migratory birds, which breed in Europe but spend the winter close to the equator, originate from Africa.

The researchers then went on to study the immune system in three different groups: sedentary birds in tropical Africa, European sedentary birds, and migratory birds. Their findings show that African sedentary birds have a more varied and extensive immune system than that of both European sedentary birds and migratory birds.

Evolution has quite simply ensured that bird species migrating to Europe were equipped with a much less variable immune system, as Europe has far fewer diseases than the tropics. The African sedentary species probably need a more advanced immune system to deal with the richer flora of pathogens to which they are exposed.

To explain the surprising result, the researchers propose the idea that the costs associated with a strong and complex immune system could be much higher than anyone previously thought. Examples of these costs could be autoimmunity and other immune system diseases such as chronic inflammation.

Despite the migratory birds having to deal with pathogens both in Europe and tropical Africa, they have thus lost much of the variation and complexity in their immune system compared to their African ancestors. According to the researchers, this indicates that dealing with pathogens is most difficult for the birds the first time they are exposed to them. For migratory birds, this happens in Europe where they raise their young and there are not as many different pathogens.