From late August to early September, millions of adult monarch butterflies of the eastern North American population cease reproducing, become highly gregarious and begin migrating southwards. By mid-October, they migrate through central Texas into Mexico where they follow the Sierra Madre Oriental across the Tropic of Cancer. They then shift direction westwards towards the Transverse Neovolcanic Belt of mountains where they overwinter without breeding. A rapid exodus northwards occurs at the spring equinox, and by early April both sexes reach the Gulf Coast states where the females lay eggs on the resurgent spring milkweed (Asclepias) flora and die. Adults of the new generation continue the migration to the northernmost breeding range, arriving by early June. Two or more short-lived breeding generations are produced over the summer, spread eastwards across the Appalachian Mountains and, by September, the autumn migration is again under way. This paper presents a new hypothesis that the orientation of adult monarchs undergoes a continual clockwise shifting throughout the 3-5 generations, rotating by 360 in the course of the year. This hypothesis is consistent with the timing of arrivals and the relative abundances of the successive generations of monarchs throughout eastern North America, with the directions of movement of their spring, summer and autumn generations, and with the timing of their arrival at the overwintering area in central Mexico.

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