Are migratory birds really to blame for the spread of infectious diseases?

Not only are birds the primary reservoir of some of the most important emerging infectious diseases today, they are also implicated in the spread of pathogens during migration. However, there is little direct evidence of a free-living bird migrating while actively infected with a pathogen. We can study birds in captivity to gain a better understanding how an active infection with West Nile virus or avian influenza virus affects their migratory behavior as well as how environmental stressors can mediate that response.