Twice a year in southern Sweden, the skies fill with migrating birds. Streams of them funnel through the country’s southern tip, traveling to and from wintering grounds in Africa and nesting grounds in Europe.
It’s a tough and strenuous journey in the best of times. So ornithologist Emily O’Connor wondered: How do these compulsive wanderers deal with infections?
“One of the big biological questions regarding migration is how these birds cope with diseases in two entirely different geographical regions,” said Dr. O’Connor, a researcher at Lund University in Sweden.
“If I was to travel from my home in Europe to Africa for a holiday, then I would need all sorts of vaccinations to protect me from diseases. But migratory birds move regularly between Europe and Africa with no help from medicine.”
The evolutionary origins of bird migration are a longstanding puzzle for ornithologists, and the role of disease in influencing the behavior has never been entirely clear. A recent study by Dr. O’Connor and her colleagues examined the genealogy and immune responses of about 1,300 species of songbirds from both continents.
The surprising result: Migratory birds have weaker immune systems than tropical species that stay put.
Dr. O’Connor began by looking at the evolutionary history and relationships between European and African songbirds, including data on their current ranges and relationships.
Most songbirds in Europe, including those that breed there, descended from African ancestors, the researchers found. Because areas closer to the Equator tend to have a greater incidence of disease, both European migrants and permanent residents had in effect spread from a deeply infectious environment to a relatively benign one.
For European birds, this lends support to the idea of “pathogen escape,” which suggests that species tend to move away from more disease-heavy environments. But migratory species still spend a lot of time in Africa and therefore around African pathogens.
The obvious implication, Dr. O’Connor said, is that they have particularly tough immune systems.
To check, the team examined the pathogen-recognition genes in 32 sedentary and migratory species. These genes govern cellular proteins that help the immune system recognize foreign molecules.
The greater the diversity of these genes, the theory goes, the greater the exposure to infectious diseases during evolution.
To their surprise, the team found that African species that stayed put had significantly tougher immune systems than their European or migratory cousins. That suggested something different: that migration might also be part of an evolutionary trade-off to get away from diseases.
According to prior research, Dr. O’Connor said, birds are most vulnerable to infection after hatching. Birds like the African plain-backed pipit combat this with a powerful immune response.
But maintaining a complex immune response comes with significant costs. Animals that have them are more likely to suffer autoimmune disorders, chronic inflammation and other related ailments.
European redstarts and other birds that shifted their nesting grounds to Europe no longer bring up chicks in a disease-rich environment, and thus don’t need to bear the costs of a powerful immune response, whether they visit Africa or not.
“Ours is the first study, to my knowledge, to show that migration and changes in the immune system are linked on a broad evolutionary level,” Dr. O’Connor said.
“I am not saying that escaping pathogens was necessarily one of the factors driving the evolution of migration, just that the two processes are linked.”
The paper makes a compelling case, said Joel Slade, an evolutionary ecologist at Michigan State University, but the precise relationship between migration and pathogen escape still needs untangling.
Traveling between continents is taxing, and birds that haven’t managed to stock up their energy reserves often die along the route. The strain of such journeys can leave birds more susceptible to disease, and the metabolic cost of fighting off infections can leave birds unable to complete their journey or successfully breed at the end of it.
In addition, he said, temperate regions have plenty of diseases of their own.
“Equatorial diseases can travel to more temperate zones, and it is a concern that climate change is shifting the range of these diseases,” Dr. Slade said. “What’s also notable is avian malaria seems to be a generalist, by which one type can infect multiple species, and these species can be found in what this paper considers ‘pathogen-light’ zones.”
While ornithologist Frank La Sorte, of Cornell University, also finds the study convincing, he’s cautious about applying the findings to other regions. Migration is a local phenomenon that has evolved, disappeared and re-evolved in different areas, he said.
For example, songbirds that migrate between North America and South America seem to have done the opposite of African species, with northern ancestors colonizing the equatorial south. That means the evolution of their immune systems may have been quite different.
Still, Dr. La Sorte said, the study strongly supports the idea that diseases are an important selection pressure in shaping bird migration strategies — at least, between the African and European flyways.
It’s also handy at highlighting the many remaining mysteries in how bird migration originated in the first place. “Each region might have a unique combination of factors that promotes the development of migratory behavior in bird populations,” he said.
For Dr. O’Connor, the research also offered an opportunity to consider how living in different environments can shape the evolution of immune responses.
All vertebrates — including humans — share similar immune systems, she pointed out. In a world where diseases and species are shifting their ranges because of climate change, among other factors, it’s more important to than ever to understand how animals evolve to deal with illness.
“There are lots of factors that we believe play a role in the evolution of migration, and probably the traditional answers of food and competition are the main players. But I do think we’ve potentially overlooked the role that pathogen avoidance may have played.”
“After all, disease is a strong selective force,” she said.
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