From the intro to the Library of Congress Ref Guide

There are two main populations of monarch butterflies in North America; one in the East and one in the West–new evidence shows that these populations may intermix. Every fall, the monarch begins its migration south to an overwintering site. In the spring the monarch leaves its winter roost and heads northward, about half way back, to lay its eggs on milkweed plants. The offspring of this monarch must continue the journey north on its own. In the fall, this new generation will begin the migration cycle south. To learn more about these amazing creatures, this guide lists selected titles, journal articles and Web sites about monarch butterflies, conservation initiatives, and migration projects.

Source: Library of Congress

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