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Spring Migration

As winter ends and the days grow longer, the Monarchs become more active, beginning to mate and often moving to locations lower on the mountainsides. They leave their Mexican roosts during the second week of March, flying north and east looking for milkweed plants on which to lay their eggs. These Monarchs have already survived a long southward flight in the fall and winter's cold; they have escaped predatory birds and other hazards along the way, and are the only Monarchs left that can produce a new generation. If they return too early, before the milkweed is up in the spring, they will not be able to lay their eggs and continue the cycle.


Spring Map
(45K)
The migrating females lay eggs on the milkweed plants they find as they fly, recolonizing the southern United States before they die. Soon the first spring caterpillars hatch and metamorphose into orange and black adults. It is these newly emerged Monarchs, the offspring of the butterflies that made the fall journey, that recolonize their parents' original homes. Summer Monarchs live a much briefer life than the overwintering generation; their adult lifespan is only three to five weeks compared with eight or nine months for the overwintering adults. Over the summer there are three or four generations of Monarch butterflies, depending on the length of the growing season. Since each female lays hundreds of eggs, the total number of Monarch butterflies increases throughout the summer. Before the summer ends, there are once again millions of Monarchs all over the U.S. and southern Canada.


We have created a comprehensive map representing both the fall and spring migrations both east and west of the Rocky Mountains. To the extent possible, the map is based on data. Nevertheless, our knowledge of the distribution and abundance of monarchs in some seasons is lacking for certain areas of the country. In these cases, our interpretations are based on what monarchs are doing in surrounding areas. You can read more about our Two-way Monarch Migration Map via our blog. A poster version of this Monarch Migration Map is also available via the Monarch Watch Shop (shop.monarchwatch.org)

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