Birds leave diseases behind on migration

People are often required to get vaccinations against deadly diseases when they make trips to tropical areas, but birds don’t have that sort of luxury, even though these world travelers face similar risks.

“They’re moving between different regions of the globe twice a year, between sub-Saharan Africa and Europe,” said Emily O’Connor, a research scientist at Lund University in Sweden, referring to European-African migratory birds. “If we make that kind of trip, we need vaccinations. They manage without any help.”

In a study published in Nature Ecology and Evolution, O’Connor and her colleagues looked at how birds cope with diseases when they move into different areas. They studied the immune systems of resident birds in tropical Africa, resident birds in Europe and migratory birds traveling between Africa and Europe. They found that African resident birds have more diverse immune genes than European resident or migratory birds.

The team first looked at where over 1,300 European and African passerine birds — both sedentary and migratory — originated from by determining their ancestral history. They found the birds originated in Africa and colonized in Europe either permanently or seasonally by migrating to breed there.

Then, they looked at the birds’ immune systems by sequencing immune genes, and their findings surprised them. They expected to find higher immunity genes in migrating birds such as the tree pipit (Anthus trivialis), barn swallows (Hirundo rustica) and yellow wagtails (Motacilla flava), but they found the opposite. “There was a reduction in immune diversity,” she said. “This was both in migratory birds and ones that colonized Europe from Africa. This suggested that these birds were actually escaping pathogens.”

This means the migratory species benefit from leaving tropical areas where pathogen prevalence is high when it’s time to raise their young. Moving away from diseases may allow them to survive with less cost to their immune systems, she said.

O’Connor said the study can help provide information on how species change in the presence of new diseases. “It’s important to understand how adaptable they are in response to these things and how those changes come about,” she said. “The fundamentals of how immune systems are shaped by the environment we live in is important to understand.”