POSTED 4 OCTOBER 2007
Attention, Northern Hemisphere: The migratory birds are heading south, bound for sunnier climes, where they can make a living in the winter. Monarch butterflies are riding the winds south toward refuges in California and especially Mexico. Out in the ocean, fish and marine mammals are tracing their own versions of the great cyclical pattern of movement we call migration.
Migration is a great mystery of biology. It's difficult to even know where many birds, fish, marine mammals and insects go. In some cases, it's not clear why they migrate in the first place.
And then there is the critical question of navigation: How do they find their way?
This edition of The Why Files will focus on winged migration. How do monarch butterflies migrate? How do birds? What do we know about the crafty techniques that these animals use to transit our planet?
Since the monarch butterfly migration burst into the public consciousness about a decade ago, it has come to exemplify the wonders, and the mysteries, of migration.
Each spring millions of monarchs fly north from their winter colonies in Mexico, and lay eggs on milkweeds in southern states. These butterflies die in a few weeks, but their offspring fly to breeding areas in the northern United States and Canada.
After two to four more generations, monarchs begin the great airborne trek back to small refuges in the mountains of central Mexico. There they gather in massive numbers on fir and pine trees at elevations of 10,000 to 12,000 feet. One hectare (2.5 acres) of trees can house feathery orange clumps containing 25 million to 70 million butterflies. Overall, the population of the Mexican refuges has reached an estimated 400 million!
Courtesy: © David Tenenbaum
The monarch migration raises questions:
- How could this half-gram insect fly up to 2,000 miles to its ancestral wintering grounds?
- How could these insects even find the refuges in the first place? (Remember, the insects that reach Mexico have never been there; they hatched in the North.)
- How can we preserve this marvelous butterfly's marvelous migration?
Life history of a regal butterfly
Like other moths and butterflies, monarchs pass through four life stages: egg, larva (caterpillar), pupa (chrysalis), and adult (butterfly). The eggs hatch in three to five days, and the new larvae promptly devours its spent eggshell.
As the caterpillar outgrows its skin, or cuticle, it forms a new skin, then sheds and eats the old one during the process called molting. After four molting cycles, the caterpillar forms a chrysalis, which hangs under a milkweed leaf. After a couple of weeks, the butterfly inside emerges from the cuticle and extends its wings, which become ready for flight in about an hour.
Monarch larvae live and feed exclusively on members of the milkweed family. Because these plants are toxic to most vertebrate animals, this feeding strategy makes monarchs poisonous, and most birds shun them.
Monarchs produce three to five generations in their summer territory, which is largely north of 40 degrees north latitude. Each summer generation lives from two to six weeks.
As fall approaches, the last generation of butterflies starts to fly toward Mexico, where it will spend the winter. With luck, and some smart survival tactics, these codgerly 'flies can live eight or nine months, and will die after laying eggs in Texas or nearby states. (Read more on the monarchical life cycle.) Although many other insects migrate, no other butterfly is known to overwinter in huge clusters at the same sites every year.
Why do monarchs migrate?
To stay alive and make a living. Monarchs can't survive temperatures much below freezing, so most monarchs east of the Rockies migrate to Mexico. (Western monarchs, however, overwinter along the California coast). Migrating monarchs make a good living by foraging on their mammoth North American feeding-and-breeding ground, then, when winter halts the growth of northern plants, retreating to warmer refuges in Mexico and California.
How do monarchs decide to head south?
To explore the migration, Monarch Watch, a non-profit organization that studies and conserves these butterflies, started enlisting thousands of volunteers to tag monarchs in 1992. Orley Taylor, who directs Monarch Watch and is a professor of biology at the University of Kansas, says the group has recovered about 12,000 of about 1 million tagged butterflies.
Photo taken near Monarch Butterfly Biosphere Reserve in Mexico, courtesy Monarch Watch
Taylor considers the tags, "A gold mine. Before the tagging program, I thought the migration timing was chaos, all driven by the weather. Weather does have an impact, but it's not the major driving factor."
Instead, the monarchs seem to be responding to changes in the sun's path through the sky. "The sun's arc changes every day, but it changes at a slightly different rate at different times of year," Taylor says. "At the beginning of the migration, the sun's arc is declining slowly in the sky day by day, and then it starts to move faster," and then slow again as winter approaches.
At all points, he adds, this rate of change "corresponds beautifully to the migration. It starts slowly, picks up speed, and then slows near the over-wintering sites." Knowing the sun's path, "I can predict when the migration is going to start, peak and end at any latitude."
How do monarchs find Mexico?
Answer unknown. Once the monarchs start moving, "They must set a course that is independent of celestial change," says Taylor. "A butterfly from Minnesota heads 180 degrees [due south], one from Wisconsin probably heads l90 degrees, while one from Atlanta must first head due west. In each portion of the country, a butterfly must pick up local information, whatever that is, and use it to pick a direction."
To fully understand migration, he adds, "That's where the real challenge is: How do they set those headings, and how do they set them in different parts of the country?"
How do monarchs find their mountaintop refuges?
Unknown. Remember, the monarchs that arrive in Mexico are gringos: they hatched in the north, have never seen Mexico, and they can't even ask directions because they don't know a word of Spanish. How do they find their way to the same ridgetops year after year?
Taylor says the answer probably emerges from "a combination of techniques -- a magnetic orientation overlaid on a response to elevation, river valleys, things like that." Although the butterflies seem to select the same mountains, and frequently even the same groups of trees to support their clusters, how they select these locations is not known.
What about the mechanics of migration?
As long-distance aviators, monarchs must know how to ride a good wind, and how to duck and cover in a bad wind, says Taylor. As we talked, he described the monarchs outside his office: "Today, the butterflies are low, out of the wind, they can't move into this wind. Tomorrow the wind will change to the northwest and they will use it to move to southeast. The wind will then change further and come from the north on the next day, aiding the butterflies as they progress southward. They ride these winds, and have all sorts of sensing capabilities to take advantage of thermals and winds speeds. This is like a military aircraft -- it's got all sorts of onboard computers."
And all these are housed in a brain smaller (much, much smaller) than a microchip, without so much as a GPS transceiver in sight!
In the monarch, evolution has perfected a flying machine that requires astonishingly little energy. The migrating monarch will "use as little powered flight as possible to get to Mexico," says Taylor. "They must live eight to nine months, and so they have to minimize the energy expenditure at every turn. On a sunny day with almost no wind, I expect them to ride a thermal [updraft] to 1,200 feet. If they can catch some wind for three to four hours, they can make tremendous headway."
The average monarch migrates about 25 miles a day, but they can make 100 miles if winds are favorable.
Preserving the monarch migration
Are the monarchs surviving? Typical of insects, their population surges and crashes. Taylor told us that heavy rains and freezing temperatures in 2002 and 2004 killed 80 percent and then 70 percent of the over-wintering monarchs, but the population returned close to normal in 2006 and 2007.
A much more severe threat is habitat loss, in the winter refuges and all along the migration routes through North America, Taylor says. "Since we are losing habitat, we are bound to see a decrease in numbers eventually."
Mexico has established the Monarch Butterfly Biosphere Reserve in the winter refuges, but a line on a map cannot ensure the survival of monarch's remarkable migration. Mexico's population is increasing, and farmers are moving up the slopes toward the refuges. When fields are burned before planting, billows of smoke disturb the butterflies.
Logging, while theoretically regulated, is another problem. In a 2007 interview, Taylor told us that deforestation had affected more than 5 percent of the Biosphere Reserve in 2005 alone. "We can't sustain a conservation effort if you take away 5 percent of the habitat in a single year. If Mexico cannot protect the forest and the monarchs, we could see the monarch population go down pretty seriously."
The answer is to meet both human and biological needs, he adds. "If we're going to solve this problem, we'll have to provide for the people, and the butterflies." But there is no simple solution. Although the butterflies attract tourists who bring money to the local economy, only a few local people benefit directly, and visitors numbering in the tens of thousands can harm the local ecosystem.
Although monarchs do not face extinction (they managed to colonize the Pacific and Europe in the mid 1800s), the survival of the massive migration is less certain, Taylor says. "They need these over-wintering sites; they have the best microclimate, and the butterflies can't move to other locations easily."
And once a migration pathway is eliminated, it's not clear whether it can be re-established.
Problems in the summer quarters, too
The monarchs spend about as much time in its summer region as in Mexico, and habitat loss is also a hazard up north. "Each day in the United States we lose 6,000 acres of habitat for wildlife to development," says Taylor. "Without milkweeds, monarch numbers will decline."
Courtesy: © David Tenenbaum
To confront habitat loss, Monarch Watch urges landowners and gardeners to protect and create habitats planting milkweeds for the caterpillars and nectar plants for the butterflies. "We have an impact on this butterfly throughout its annual cycle," says Taylor.
How birds migrate.
Megan Anderson, project assistant; Terry Devitt, editor; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive