Birding with Jerry Hall: The Monarch Migration

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Millions of monarch butterflies spend the winter in trees in central Mexico.

October 3, 2009

By Jerry Hall The San Marcos Record Sat Oct 03, 2009, 12:52 PM CDT

October marks the start of monarch butterflies passing through our area on their migration south. It’s a fabulous phenomenon when you consider some 300 million monarchs, each weighing half a gram, will fly up to 3,000 miles from as far away as Canada to reach a 70-square mile forest in central Mexico.

Wow.

There are two major flyways passing through Texas with the central flyway being the larger at about 300 miles wide and the coastal flyway being more narrow, winding along the Gulf coast. A lot depends on wind direction and speed, plus temperatures and rain, but generally, large numbers show up on the central route from early to late October and on the coastal route from mid-October to mid-November.

I’ve personally seen more butterfly swarms just west of San Antonio and in the Uvalde and Concan area than in Hays County. But the pattern varies so it pays to keep an eye out. You can follow the fluttering by visiting www.texasento.net/dplex.htm, an interesting Web site updated weekly during migration.

Not all monarchs make it all the way to Mexico, with some electing to spend the winter along the Gulf coast. And not all migrating butterflies are monarchs as several other species will move around seeking better food sources and fleeing colder climates.

But the monarch is unique in making such a lengthy round trip. Sometimes they fly at high altitudes, taking advantage of thermal air currents and sometimes they are low to the ground, always seeking favorable tailwinds. They will feed along the way and actually arrive at their final destination weighing more than when they started out.

Monarchs require several generations to make their round-trip migration. The southward-bound generation flies to Mexico and spends the winter clinging to trees in a torpid state. Come spring, they’ll perk up and start north, living about eight months total before laying eggs along the flight back north and dying. The next two generations complete the northward journey, each living four to six weeks. Then the last generation starts the cycle all over again when it heads back south.

How does an insect with a brain smaller than a pinhead navigate with unerring accuracy over thousands of miles, when it has never made the journey before? Scientists say these winged beauties have a circadian clock that combines with an internal sun compass to manage precise navigation.

Supposedly, they take their first cue from the angle of the sun and also utilize the earth’s magnetic field as well as landmarks. However they do it, I am in complete awe of their global positioning abilities since I am one who can get lost in a Wal-Mart parking lot.

While most monarchs migrate through Texas, some from west of the Rockies will winter along the California coast and some eastern monarchs will fly through Florida to Cuba.

I hope you see your fair share of these winged beauties. They represent one of the major marvels of the natural world. Enjoy.

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