Fig. 1

The annual migration circuit of monarch butterflies (Danaus plexippus) in North America. In the eastern population, the cycle starts in spring with migration of post-hibernation adults to Texas (black arrow), where they breed and die. The progeny then colonise the rest of the eastern North American summer-breeding range over the course of an additional 1–3 generations (dark blue arrows). During the autumn, most monarchs that have emerged in eastern North America migrate back to the central Mexican hibernation site (light blue arrows). However, the situation is complicated by an increasing trend for winter migrants to join the continuous-breeding, non-migratory populations in Florida, Cuba and along the Gulf Coast (dashed light blue arrow). The western population was traditionally believed to winter exclusively in hibernation sites along the Californian coast, involving a two-way migration (dark and light green arrows), but there is also an increasing trend for winter breeding at the coastal sites. However, recent evidence indicates that some western monarchs migrate to the central Mexican wintering ground in autumn (dashed pale green arrow), where they will mix with eastern monarchs. It is not known whether these western monarchs migrate back to the western or eastern summer-breeding range (dashed dark green arrows), but it is clear that population mixing is higher, and migratory connectivity is lower, than was originally believed