Figure 1.

Two cases under which migration may evolve from a resident ancestor. (a) In the shared-breeding case, migrants initially share a breeding site with residents and migrate to a new site during the non-breeding period. (b) In the shared-non-breeding case, migrants initially share a non-breeding site with residents and migrate to a different site during the breeding period. Under either case, if a migratory allele invades a resident population, it may remain partially migratory (both residents and migrants) or may evolve to be completely migratory (loss of residents). Grey region, resident site; black ring, migrant site.