Monarch Butterfly Migration Update: February 23, 2000

Today's Report Includes:

Field Notes from the Mexican Monarch Sanctuaries

Students Visit the Sierra Chincua With Dr. Calvert
"Students from Humboldt School in St. Paul, MN are here in Mexico this week, and accompanied me to the Sierra Chincua Sanctuary. We were treated to a display that was simply astounding. When we entered the woods and descended, we saw the first hint of the orange patina of basking monarchs. The surrounding forest was full of butterflies that seemed to float on invisible winds rising through the high canopy. After walking 10 minutes into the forest we came upon the area where the main clusters remain. Millions upon millions were basking in the sun.

How Many Millions of Monarchs?
"People always wonder how many butterflies they're actually seeing. In 1986, we did a 'mark & recapture' study at one of the sites in January. On the basis of that study we estimated 13,000,000 monarchs per hectare. Albeit crude, you can use that figure to estimate the total number of monarchs in a colony. The tree densities in these forests average 400 trees per hectare, so the number of monarchs per tree averages ____. (Of course the variation among trees is enormous-- from a handful to perhaps a hundred thousand--but with these numbers you can make an estimate.)

Challenge Question #6
"According to the numbers above, an average of how many monarchs are clustered on a single tree in a monarch colony?"

Challenge Question #7
"Using this average number of monarchs per tree, how many butterflies were clustered in the Sierra Chincua colony this January, when there were 350 trees filled with butterflies?"

"It's hard to imagine how many butterflies this truly is! To put the number into perspective, convert butterflies to people. Imagine giving one butterfly to every person in your state or province--and then in neighboring states/provinces--until all the butterflies are gone.

Challenge Question #8
"List the names of all the states and provinces in which people could receive a butterfly from this winter's Sierra Chincua population."

Shivering Butterflies Litter the Ground
"When you visit the colonies late in the day, the forest floor and stems of under-story plants are often covered with shivering butterflies. Shivering is most pronounced when temperatures are low. Shivering butterflies look like you might imagine from your own experiences in cold weather. The butterflies shiver their wings rapidly in an attempt to warm the muscles inside.

"You'll often hear people say the butterflies are trying to get warm enough to fly, but it may be that shivering helps them to crawl off the ground when they are too cold to fly. (Monarchs can crawl at temperatures as low as 5 degrees C. The monarch's flight threshold is about 13 degrees C, and in order to fly WELL, with lots of control, they need to attain thoracic temperatures pretty close to the temperatures that warm blooded mammals run - the upper 20s or even 30s. They can manage to get themselves airborne and glide--and occasionally flap with some control--at temperatures much lower than that, but they cannot fly well at temperatures in the teens.)

"Keep in mind that the temperature can sometimes drop to zero or even a few degrees below zero in the sanctuaries. Monarchs are paralyzed by temperatures this cold! When we were at the sanctuary this week it was during the peak warmth of the day. However, we did see some monarchs shivering that were down on the ground drinking water.

Why Do Monarchs Shiver?

Challenge Question #9
"Why do you think monarchs down on the ground drinking water were shivering?" And the question you must be wondering: "Why do monarchs work so hard to stay off of the ground?"

"I wondered how long a butterfly might be at risk on the ground--whether minutes, hours, half days, a full week. How long would it take one to reach necessary temperatures and move upward?
My impression from observations is that the sunflecks (small circumscribed areas where solar radiation is entering gaps in the canopy and striking the ground) move around the forest each day and probably cover virtually all locations. So if it is sunny the day after a butterfly is grounded, a sunfleck will find its position sometime during the next day, and warm the butterfly enough to move out of its predicament. If there is a period of cloudy weather it's a different story, and the risk period could be much longer. If buried by snow, monarchs might stay on the ground under the snow for more than a week. But they are not at much risk buried under the snow, except perhaps by being stepped on."

Dr. Bill Calvert
Reporting from Angangeuo, Michoacan, Mexico

Eligio Garcia's Monitoring Data From Sierra Chincua
"La mariposa ya se esta moviendo de lugar. El 16 febrero sólo quedan 85 árboles con mariposas. Los árboles ya no están tan llenos de mariposas, observe muchos apareamientos y hay algunas mariposas perchadas en los troncos de los árboles."

Once again, Eligio reports that the butterfly colony is continuing to move. This week, the main aggregation is at 3,140 m altitude and only occupies 85 trees.

How Much Land Do Butterflies Need?
The area occupied by a colony at any moment is very small. As you know, this winter only 350 trees were filled with butterflies in the Sierra Chincua at any one time. However, work by Eligio and other biologists shows how important it is to monitor the monarchs over time. From Eligio's thesis: "If we consider all the land the colony occupies--from the time the butterflies arrive in November until their departure in March--it has been estimated that a colony will utilize 60 hectares." Bill Calvert says above that there are 400 trees per hectare, so the 350 trees at Chincu are roughly 1 hectare. This means the colony uses 60 times more land during the season that it does at any one time during the season!
Life in Sanctuary Region
Making Tortillas With Maria Luisa

"Welcome to our home! It is small but humble. I invite you to stay awhile and learn how a typical Mexican family, living in the country, makes tortillas. We must start with an understanding of the seasons in our state of Michoacan. These seasons determine the way we can grow corn and eventually prepare tortillas."

Maria Luisa is a grandmother who lives in Los Remedios ejido. This ejido owns some of the most important land in the Sierra Chincua sanctuary. We hope these first-person accounts will portray the personal side of monarch conservation, as seen through the eyes of the children and families who live in the region. Watch for a new story each week, in both English and Spanish.

How to Respond to Today's Monarch Challenge Questions|