Host immune defence and migration in birds

Migratory birds are exposed to at least two different parasite faunas durin g their annual cycle, while resident birds only experience a single parasit e fauna. Migratory birds should therefore have evolved mechanisms to contro l or reduce the negative impact of infections from a more diverse parasite fauna. In a comparison of pairs of closely related species of birds that di ffer with respect to whether they are migratory or residents, the size of t wo immune defence organs (the bursa of Fabricius and the spleen) was consis tently larger in the migratory species. Since the bursa is only found in ju venile, sexually immature birds, we conclude that immune defence adaptation s to the impact of a more diverse parasite fauna in migrants already exist before the start of the first migration. Interspecific differences in inves tment in immune defence between migratory and resident birds have implicati ons for our understanding of complex host-parasite interactions, the acquis ition of new hosts by parasites, and the susceptibility of migratory birds to environmental perturbations.