Dutch scientists have suggested an explanation for why Dutch barnacle geese have a less active immune system once they’ve migrated to Spitsbergen, Norway than when they winter in the Netherlands. “The birds on Spitsbergen appear to invest much less energy in their immune systems, particularly general resistance to disease. Researchers suspect that this might be because there are far fewer pathogens [like bird flu] in the North than here in the Netherlands.”
With their immune systems taxed less, the geese have more energy available to reproduce and change feathers, which the Dutch birds don’t. This means that the geese appear able to adjust their immune systems according to the risk of catching a disease.