Migrating monarchs

VANDENBERG AIR FORCE BASE, Calif. -- Although eucalyptus trees are not native, some groves provide the only available protection to the monarchs during critical wintering months, including groves on Vandenberg. Because VAFB has many groves of eucalyptus trees available for the monarchs, there are generally smaller numbers of monarchs at each site, than other sites along the coastline. (Photo by Robert Bumpass)

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Migrating monarchs

Published Dec. 17, 2008
By Liz Bell

VANDENBERG AIR FORCE BASE, Calif. -- Monarchs occur throughout North America and migrate each winter to escape cold temperatures. The Rocky Mountains divide monarchs in to two populations: Eastern and Western. The Eastern population of monarchs flies up to 3,000 miles to Mexico for the winter. Monarchs that live west of the Rocky Mountains spend their winters on the coast of California, including areas on Vandenberg.

Monarchs that emerge from their chrysalises in late fall are unique; they do not mate or lay eggs, but almost immediately begin to migrate to warmer locations. As they migrate, monarchs nectar on flowers along the way, storing fat for their long winter journey. When temperatures fall below 55 degrees, monarchs cannot fly, therefore they must find roosting sites each night.

Monarchs are protected by law during their migration. As the California coast has developed, native habitat for migrating monarchs has decreased. Certain eucalyptus groves have been designated (protected by California Coastal Act) as sensitive natural resources which provide shelter and nectar sources for wintering monarchs. Although eucalyptus trees are not native, some groves provide the only available protection to the monarchs during critical wintering months.

Monarchs cluster on the branches, often towards the inside of the grove to stay warm. Monarchs move between clustering areas, also to regulate their temperature from becoming too high. If monarchs become too warm, their metabolisms will increase and they will deplete their stored-fat which needs to last the whole winter, to include their journey back home after winter.

Monarch butterflies can fly about 30 mph, when there is no a tail or head wind. They usually fly close to the ground, but have been found at altitudes of up to 12,000 feet. They can fly more than 400 miles without stopping, for more than 16 hours when crossing water. They can be taken hundreds of miles off course by wind, and still find their historical roosting sites, which are often specific trees that have been used for decades.

Scientist have many unanswered questions about the amazing monarch migration, including how they navigate over thousands of miles to the same tree, each year. This is still somewhat of a mystery to scientists. While biologists believe that monarchs generally use the sun's relative position in the sky for their navigation, there are also trace amounts of magnetic material in their head and thorax making their bodies into a living compass, which allow them to utilize the earth's magnetic field.

Because Vandenberg has many groves of eucalyptus trees available for the monarchs, there are generally smaller numbers of monarchs at each site, than other sites along the coastline. Over the holidays, take your family for a hike around eucalyptus stands and see if you can spot some of the wintering monarchs. Follow these tips to help find monarchs:

1. Leave Fido at home - monarchs are very sensitive to noise and movement, so even if dogs are leashed, they can still bark, which could cause monarchs to fly away.

2. As you approach the eucalyptus grove, use soft voices and watch for single monarchs flying 40-60 feet in the air, flying towards the grove. Follow these individuals where they enter the grove. It is easier for human eyes to see movement and their bright orange color will be easier to see against the blue sky, so finding the butterfly before it enters the grove often leads you to their roosting site.

3. Watch where you step! Cold temperatures can temporarily paralyze monarchs, and they fall from the tree branches onto the ground. When it warms up, they will be able to fly back up to the roost site, so be very careful not to step on any grounded monarchs while you are walking around the grove.

4. Bring a pair of binoculars, because often the monarchs are roosting about 40 feet above the ground, because of the temperature. When they roost, they fold their wings, so only the underside is exposed. This can make finding the roost a little bit tricky because their familiar orange coloring is covered. Once you are under the trees, allow your eyes to adjust to the shade and scan for the movement of a single butterfly flying to the roost site.
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