Birds often carry ticks infected with Borrelia burgdorferi sensu lato 1 — the spirochaete that causes Lyme disease — but their role as possible hosts and amplifiers for this illness has long been discounted. We find, however, that migratory birds are able to carry Lyme disease as a latent infection for several months, and that this infection can be reactivated and passed on to ticks as a result of migratory restlessness. Our results indicate that migratory birds may be efficient long-distance carriers of this pathogen.