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Monarch Migration
Thu Jun 10, 2004
Listen in RealAudio 
With spring, a great northward migration begins, not only of birds, but of Monarch
butterflies. Hi, I’m Bryan Yeaton for The Weather Notebook.
Last fall, a unique generation of Monarchs migrated from the Eastern U.S. to Mexico
where they over-wintered. Summer generations live only a month and reproduce four to
six days after emerging from the chrysalis, but not the Autumn Monarchs.
Reproductive functions in the summer’s last adult cohorts go dormant, and that
generation can survive for as long as eight months, which allows them to travel
thousands of miles south, survive winter, and begin the return journey the following
spring.
Lengthening late-winter days trigger hormones that bring on sexual maturity, and
cause Monarch females to fly hundreds of miles north in search of milkweed plants on
which to lay their eggs.
Her life cycle finally completed, she leaves her offspring to take up the next leg of the
migratory imperative: continue northward and produce a new generation. This relay
continues until the first New England and Canadian Monarchs emerge from their
chrysalides, probably the great-great-great-grandchildren of those who left Mexico five
months earlier.
How do such small creatures travel such great distances? They take advantage of the
weather.
Monarchs are soaring butterflies that use wind and weather to reach high altitudes and
glide northward for hundreds of miles. In springtime, the strong sun heats the surface
to produce rising thermals of warm air. Monarchs hitch a ride on these thermals to
higher altitudes and stronger winds.
Southerly spring winds push the butterflies northward. When headwinds blow, the
Monarchs generally drop down to the ground and forage for nectar among flowers.
Eventually, they find milkweed plants, and the cycle continues.
Thanks to our contributing writer, meteorologist Keith Heidorn. The Weather Notebook
is supported by Subaru of America and the National Science Foundation.
Today's Links
Monarch FAQ’s:
http://www.learner.org/jnorth/search/Monarch.html#Migration
Spring Migration Map:
http://www.monarchwatch.org/tagmig/spmap.htm
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