Birds that migrate are infected by more nematode species than birds that do not migrate. Why is that? Well, it could be because migrating birds visit many more habits than non-migrating birds, and they therefore encounter many more nematode species. It might also be that migrating birds are more susceptible to nematodes due to the stress associated with migrating. Or, both explanations might be important! Check out the recent paper that described this pattern, and/or check out the summary of the paper on the Oikos blog!
Koprivnikar, J., and T. Leung. Flying with diverse passengers: greater richness of parasitic nematodes in migratory birds. Oikos.