As autumn slides into winter every year, many birds in Europe and Asia pack up and fly south to bask in the tropical African sunshine. When spring rolls around, they return to the temperate Palearctic zone to mate and raise their offspring. Researchers wanted to know why these long-distance fliers do not get travelers’ flu.

“When we go abroad on holiday, we need all sorts of vaccinations,” says Emily O’Connor, an ecologist at Lund University in Sweden. “But birds don’t have the option of pharmaceutical protection. It puzzled us: How is it they can cope so well with something so difficult for us to cope with?”

To find out, O’Connor and her colleagues classified more than 1,300 songbird species as migratory, sedentary African or sedentary Palearctic—an example of the last is the meadow pipit (shown). They then trapped wild birds from a representative subset of 32 species, taking blood samples for genetic analysis. The researchers were looking for genes that encode a class of immune system proteins called MHC-I, which are involved in recognizing pathogens. The greater the number of such genes, the more kinds of invaders an animal’s immune system can detect, O’Connor says.

By this measure, sedentary African birds had the most robust immune systems. Because most Palearctic birds first evolved in the tropics and later spread northward, the researchers suspect these species developed less MHC-I diversity. The results were published in May in Nature Ecology & Evolution.

“Migratory birds, because of the lifestyles they have, have to deal with two separate sets of pathogens,” O’Connor says. “I was expecting them to have the highest gene diversity of all the groups, so I was really surprised to find it was really similar to [that of] the European birds.”

Young birds are most susceptible to pathogens just after hatching, and the stress of reproduction makes their parents more likely to get sick then, too. For both reasons, O’Connor suspects that evolution may have pushed migratory species to favor genes associated with resistance to pathogens common in the north, where they are born, at the expense of those that protect against tropical germs.

Alternatively, migratory species may have invested in other forms of immunity that are not pathogen-specific, says University of Exeter evolutionary biologist Camille Bonneaud, who was not involved in the study. “We now need to further explore whether migratory species invest less in fighting pathogens,” Bonneaud says, and “more in other types of immune processes.”