Autumn brings a great migration of monarch butterflies to Mexico
Autumn’s monarch butterflies will travel along a migratory route covering up to 3,000 miles from breeding grounds in North America to wintering grounds in central Mexico.
Migratory monarchs breeding east of the Rockies have already headed south from Canada and the Midwestern United States. Their journey through Texas will follow a broad route from Wichita Falls through the Hill Country and across the border between Laredo and Langtry
Monarchs from the Northeastern U.S. will follow the Texas coast to Mexico.
The butterflies started their southbound journey when the noonday sun was approximately 48 to 57-degrees above the horizon over their summer breeding grounds. They’re now on a precise path toward Mexico, flying on tissue-thin, 4-inch wingspans while weighing half as much as a dollar bill. They won’t get lost, thanks to a built-in navigation system. If blown off course by sudden winds, they’ll regain their bearings on the route to Mexico. A monarch navigates based on
signals sent to the brain from the antennae and eyes, both registering compass direction by the position of the sun. As the sun moves above the eastern horizon, the monarch orients southward by keeping the sun to its left
Additionally, magnetosensors in the antennae can detect ultraviolet light while functioning as a magnetic compass to orient the monarch’s flight path. It’s a crucial ability when clouds blot out the sun or when the butterfly gets blown off course.
The antennae also function as a biological clock to tell the time of day based on the rising and setting sun.
During the past spring and summer, multiple generations of monarchs bred and fluttered in the air for a lifespan of three to six weeks. Except the last generation, called the Methuselah Generation because they’ll live until next spring. But first, they’ll migrate to Mexico along an ancestral route they’ve never traveled to an ancestral winter home they’ve never been to.
By Thanksgiving, most monarchs will have arrived in the mountainous oyamel fir forest reserve of Michoacán in central Mexico. They’ll then enter a state a diapause or nonbreeding for the winter.
Around late February, wintering monarchs will begin the return trip toward North American breeding grounds. But they won’t live to complete the trip back. They’ll instead breed and lay eggs along the way, leaving new generations of monarchs to continue a journey to ancestral breeding grounds they’ve never before known.