September 29, 2000

The Monarch Migration

Time to Fly  

monarch It's fall, and millions of monarch butterflies are migrating to warmer climates for the winter—heading either to the Californian coast or to certain mountains in Mexico. Why do the butterflies need to migrate?

Monarch butterflies know fall is here the same way that we do; they feel the chill in the air. While we adapt by putting on a sweater, the situation is much more serious for the monarchs. Temperatures below 55°F make it impossible for them to fly; temperatures below 40°F paralyze them. The monarchs originated in the tropics and can't live for long at temperatures below freezing.

At the same time that the air is cooling, the nectar supply that feeds the butterflies is dwindling. To survive, the insects begin migrating in late summer, flying with the wind to reach their winter homes.

Up to 100 million monarch butterflies migrate either to California or to Mexico each year. (This isn't the entire population. Some monarchs never make the migration.) There are more than 25 winter roosting sites along the Californian coast and about a dozen known sites in the Sierra Madre Oriental Mountains of Mexico. In both regions, butterflies depend upon trees for their survival. The insects cluster in pine and eucalyptus trees along the California coast and in ovamel trees in Mexico.

Wintering monarchs cluster together. The end result looks like massive clumps of feathery orange-and-black grapes. Each butterfly hangs with its wings over the butterfly beneath it, creating a shingle effect that buffers the bugs from the rain and creates warmth. The weight of the cluster also prevents the butterflies from being blown away.

Butterflies stay in their winter homes until about March, when they begin a quick retreat to their summer homes, at times traveling as fast as 30 mph.

Where Are the Butterflies Now?

  • The group Monarch Watch has estimated where the peak—or most—numbers of butterflies will be at various times during this year's fall migration. Use the information provided in the chart below to determine what region of the country is currently seeing the most butterflies.

  • Now look at Journey North's latest monarch butterfly migration map. Journey North is an organization that tracks migratory species with the help of students across the country. Determine whether the estimates of Monarch Watch are more or less on track.


Estimated Peak of Migration


Sept. 4


Sept. 11


Sept. 17

Sept. 24
Sept. 30


Oct. 6
Oct. 13
Oct. 20
Oct. 27
19.5 (Mexico)
Nov. 14
Why Butterflies Are in Danger  

Monarch butterflies are in danger of losing both their summer and winter habitats. Summer habitats are destroyed as more roads and new housing developments and business complexes encroach upon open space in North America (a phenomenon known as urban sprawl).

As land is developed, milkweed is killed. This is the only source of food for monarch butterfly larvae once they hatch from their eggs. Milkweed plants are also vulnerable to herbicides used by farmers, homeowners, landscapers, and gardeners. Herbicides are poison substances used to inhibit weed growth in gardens, lawns, and fields.

The butterflies don't have it easy in Mexico, either. The ovamel trees that they winter in also serve as a lumber source for local communities and big logging companies. Logging not only removes the trees, it opens up the forest canopy. The overhead holes expose the butterflies to the elements, increasing the chance that they will die.

There are only about a dozen known wintering sites in Mexico. Each site (approximately 7.5 acres) contains millions of butterflies. Damage to even one site can spell catastrophe for the monarch butterfly population. Recent findings report that 44% of the ovamel forest has been damaged or destroyed by logging.

Insect Attraction


Many people enjoy luring monarch butterflies to their gardens because the insects are so beautiful and entertaining to watch. There are many steps that you can take to ensure that butterflies will be drawn to your yard.

Butterflies like flowers with certain shapes, colors, and fragrances. They need a good surface on which to land because they can't hover for very long. Flowers such as daisies, azaleas, zinnias, black-eyed Susans, and sunflowers fit the bill. If a butterfly has its choice, it will go for the more colorful flowers, those with purple, orange or yellow blossoms.

Scent also plays a major role in attracting butterflies; the more fragrant the flower, the greater its appeal. Butterflies find their way to nectar—their food source—by following ultraviolet patterns on petals that are invisible to the human eye. It is best if your backyard contains flowers that bloom at different times so that the butterflies have access to nectar throughout the season.

Another surefire way to attract monarch butterflies is to have milkweed plants in your yard. These plants host the eggs and provide food for the caterpillars once they hatch. There are a few different types of milkweeds: swamp milkweed, showy milkweed, common milkweed, and butterfly weed.

Butterfly Bytes

  • Many monarchs begin appearing in Mexico around November 2, the Day of the Dead. Legend says they are the souls of ancestors, returning to participate in the festival.

  • How much do you know about monarch biology? Take this quiz.

  • Caterpillars grow several thousand times in size before pupating—the process during which they turn into butterflies. How tall and heavy would you be if your height and weight multiplied 3,000 times?

  • Butterflies rest with their wings closed. Moths rest with them open.

  • Butterfly antennas look like golf clubs.

  • Monarchs live up to 9 months.

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