MONARCH FALL MIGRATION
Monarch butterflies cannot survive a long, cold winter. Instead, they spend the winter in special roosting spots with certain characteristics that enable their survival. Monarchs east of the Rocky Mountains fly south to forests in the mountains of Mexico, while monarchs west of the Rockies migrate to the northern California coast. The monarch’s migration is influenced by seasonal changes, especially day length and change in the angle of the sun as fall approaches.
In all the world, no butterflies migrate like the monarchs of North America. They travel up to 3,000 miles, and are the only butterflies to make such a long, two-way migration every year. They fly to the same winter roosts, often to the exact same trees. However, individuals only make the round-trip once. It is their children’s grandchildren that return south the following fall.
HOW IT HAPPENS
When late summer and early fall monarchs emerge from their chrysalises, they are biologically different from monarchs that emerged in the summer. Instead of mating and laying eggs, they prepare to fly south. By feeding on nectar, the butterflies store fat in their abdomens in order to fuel the long flight to Mexico.
A mystery to be solved is how monarchs find the over-wintering sites each year. Somehow they know their way, even though they have never flown this route before. No one knows exactly how their navigational system works, but recent research suggests that monarchs can track the position of the sun to guide them. They also may use the earth’s magnetic field.
WHERE DO THEY SPEND THE WINTER?
Monarchs east of the Rockies migrate to the mountains of central Mexico to roost on the oyamel fir trees. In 1975 scientists tracked down the wintering sites: 11 different areas high in the mountains. Climate conditions there are perfect for the butterflies: just under freezing and protected from snow and winds by the trees. The fog and clouds in the region provide moisture.
The butterflies cluster together, covering whole tree trunks and branches in multiple layers, so that the trees resemble deciduous trees in fall colors.
SPRING MIGRATION
As winter ends, the monarchs become active. During the second week of March they begin leaving their roosts and mating. After mating, the males die, but the females fly north and east, looking for milkweed plants to lay their eggs on. A female can lay as many as 400 eggs and since she lays only one egg per plant, she must find a lot of plants. As soon as her egg-laying is completed, the female dies. The eggs she has laid will develop into butterflies in a month’s time and the emerging butterflies will continue flying north to mate and lay eggs on milkweed. A rule of thumb is that the monarchs usually appear in an area as soon as the milkweed is seen above ground. For Nebraska this is usually late April or May.
Over the summer there can be three or four generations of monarchs, depending on the length of the growing season. With each female laying hundreds of eggs, the total number of butterflies increases throughout the summer.
These summer butterflies can live for two to six weeks, while the migrants have lived 8 or 9 months. By summer’s end, there are again millions of monarchs throughout the U.S. and Canada.
See sheet entitled “Following the Migration of Monarch Butterflies”