A monarch butterfly feeds on thistle. (Photo by Mary Holland)

Prior to the mid-1970s the disappearance of monarch butterflies in the fall and their reappearance in the spring was a phenomenon not entirely understood. Migration was suspected, but the monarchs' destination remained unknown. In order to solve this mystery, a Canadian professor by the name of Fred Urquhuart came up with the idea of placing a tiny tag on the edge of one wing of a monarch, with information on it that allowed anyone finding a tagged, deceased butterfly to contact him. The recovery of many tagged monarchs eventually led to the discovery that the monarchs residing east of the Rocky Mountains overwinter in the oyamel fir forests on the tops of mountains in central Mexico—a location that requires these insects to make a journey of up to 2,000 miles.

The monarchs that emerge (and migrate) in the fall are unlike those that we see earlier in the summer in that they live six to nine months, as opposed to the six to eight weeks that butterflies emerging earlier live. It takes these migrants approximately two months to reach their destination, with most butterflies traveling between 50 and 100 miles per day. (The farthest ranging monarch butterfly recorded traveled 265 miles in one day.) It stretches the imagination to think of creatures that weigh less than one gram flying such a long distance, and it is even more astonishing when you consider that none of these butterflies has ever made the trip before. They travel only during the day and roost at night, often choosing the same trees to roost in year after year. Most of the monarchs that left Massachusetts in September are starting to arrive in Mexico right now.

In good years, as many as 500 million monarchs in eastern North America migrate to Mexico. Whether navigating by genetic programming, their antennal circadian clock (internal timing device), the magnetic pull of the earth, an internal compass set by the position of the sun, a combination of these factors, or some other means yet unknown to entomologists, all of the monarchs east of the Rocky Mountains arrive at and spend the winter in an area one millionth the size of their northern breeding grounds.

The butterflies need the specific microclimate that these hilltops provide. The temperature at these sites is cool, but not too cool (32 to 59 degrees Fahrenheit), which is crucial to the monarchs, for they can't afford to use up their energy reserves too quickly, as they would in a warmer environment. At the same time, they need to avoid freezing temperatures. The monarchs roost in the oyamel fir forests located approximately two miles above sea level, where the evergreen foliage provides shelter from the rain and serves as a buffer from the wind and snow as well as extreme temperatures. It is cold, but usually not quite freezing, and there is considerable fog and clouds at this elevation, which provide the necessary moisture for the butterflies.

Thousands of monarchs cluster on the branches of these firs, clinging to their needles, covering whole tree trunks and branches. Each butterfly hangs with its wings over the butterfly beneath it, creating a shingle effect that gives the insects some protection from the elements. The weight of the cluster also prevents the butterflies from being blown away. On sunny days, they often warm up enough to fly to nearby water, where they drink. As temperatures change, the monarchs may move slightly in order to change elevation, but for the most part they stay put.

The butterflies spend the better part of four months in this fashion. By March, when the temperature warms and the days begin to lengthen, the monarchs begin to breed and lay eggs as they start their journey north. These monarchs, which left their breeding grounds the previous fall, will not live to return to their homeland. Soon after breeding and laying eggs, they die, and the new generation of monarchs continues northward, breeding and laying eggs as they migrate. Each successive generation travels farther north; it is the great-grandchildren and great-great-grandchildren of the fall-migrating monarchs that return to our milkweed patches in the summer.

The survival rate of the overwintering monarchs depends on a number of factors, the most important being the weather conditions, which can potentially be lethal. During the winters of 2002 and 2004 heavy rain followed by a freeze killed as many as 80 percent of the monarchs in Mexico. Last winter was one of the worst on record, with the lowest recorded level of overwintering monarchs in the past 17 years. Between one-fourth and one-third as many butterflies came to Mexico last winter as in the preceding year.

Snow, sleet, and freezing rains decimated half the population that managed to arrive in Mexico last fall. Nonetheless, this summer was a productive one for monarchs in the Northeast. Fortunately, animals with a reproduction rate as high as monarchs can recover from short periods of high mortality. However, there are long-term conditions that threaten the health of the monarch population. Climate change (thought by many scientists to be responsible in part for recent torrential rains), illegal logging, pesticide use, and development on former farm lands north of Mexico in the U.S. are all thought to contribute to the decline in the monarch population. In 1986 the Mexican government created the Monarch Butterfly Special Biosphere Reserve, which protects a large portion of the five hilltops on which the monarchs remain during the winter.
 

You can read Mary's natural history blog at www.naturallycuriouswithmaryholland.wordpress.com.