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Monarch Butterfly Migration Update: February 21, 2002

Today's Report Includes:

First: A Quiet Walk Through the Monarch?s Forest
Take time, before reading on, to enjoy these images from the overwintering sites in Mexico.





Try This! What Makes You Wonder?
Imagine you were in the forest with Dr. Calvert. As questions come to mind, record what you?d like to ask, if you he were walking beside you. Send your questions to us, we?ll forward them to him in Mexico, and try to include the topics you?re wondering about in future updates. Write to:

Why Don?t Cold Butterflies Fall? Challenge Question #4

Here?s a burning question that comes to this writer?s mind: When you visit the monarch sanctuary on a cold day, the butterflies are motionless. They?re so cold can?t even crawl. So don?t you wonder...

Challenge Question #4
?Why don?t the butterflies 'lose their grip' and fall from the trees when they're paralyzed by the cold??

(To respond to this question, please follow the instructions below.)

Field Notes from by Dr. Bill Calvert
Wednesday, February 20, 2002

A Butterfly Blizzard
The monarchs performed on Tuesday, perhaps the first truly warm day after a long period of cool, rainy, windy weather. At Chincua, millions upon millions were in the air, filling the valley of Zapatero Canyon. Many millions more were basking in the surrounding treetops and wherever exposed to the sun. Surrounded by butterflies, one had the impression of a great, orange snowstorm. Witnessing this spectacular sight were members of the St. Hubert School in Chanhassen, MN, the Prairie Wood School of New London, MN, the Nettleton School and the Ordean Middle School of Duluth, MN. I didn?t realize you could see such a phenomenon this early in the season--it was truly, truly remarkable.

Challenge Question #5
?If the purpose is to conserve their lipid reserves while overwintering, why do you suppose the butterflies flew in blizzard proportions the first chance they had after a long period of dormancy??

(To respond to this question, please follow the instructions below.)

The reported heavy mortality caused by the storm was evident in both the Chincua and El Rosario Sanctuaries. Piles of dead butterflies were present, many layers deep, in both locations. Local kids searching through the piles turned up 10 tags in less than an hour?s effort. The storm of January 2002 has truly wrecked havoc with the overwintering migrants. But I want to convey the impression that there are lots and lots of butterflies. They?re here--and visible--by the millions.

Until next week,

But What If...Challenge Question #6

Fortunately the catastrophic storm occurred this year, when the monarch population was high. As this graph shows, even if we assume an 80% mortality rate, there would still be over half as many butterflies remaining after the storm as there were last winter, when the population was low.

The annual mid-winter population estimates were made by Eligio Garcia, of Mexico?s Instituto Nacional de Ecolocia. We estimated the post-storm population size by assuming 20% of the population survived. Much more information is needed to validate this estimate; it?s based on observations at only two colonies, for example, and the mortality rate might be much higher at the smaller, outlying colonies.

However, whatever the precise number, it leaves people concerned: What if this storm had hit last year, when the population was so low?

Challenge Question #6
?If 80% of LAST YEAR?S population had been killed by this single storm, how many hectares of monarchs would have remained?" (You may want to print a copy of the population graph, and add a "post-storm" bar so you can visualize your answer.)

(To respond to this question, please follow the instructions below.)

How Many Monarchs/Hectare?
A Surprise for Scientists After the Storm

Imagine the impossible prospect of counting the millions of butterflies in their winter sanctuaries! To estimate their numbers, Drs. Brower and Calvert did a ?mark, release and recapture? study at one of the sites back in January, 1986. On the basis of that study they estimated 13 million monarchs per hectare. Last May, Dr. Calvert reviewed the data and revised the estimate to 10 million per hectare.

Then came the storm of 2002! Butterflies cascaded to the ground by the millions, and along came Dr. Brower and Dave Kust to count them...

Photo: Dave Kust




In last week?s update, Dr. Brower shared preliminary mortality data from the samples he collected from the forest floor at Chincua. At that site, they found 2,241 dead monarchs per meter squared. This works out to be 22.41 million dead monarchs per hectare. He was astonished. This was more than twice the 10 million/hectare density they had originally estimated! And remember, this doesn?t even include the living butterflies that still remained in the trees.

The mortality estimates from the El Rosario colony indicate an even higher number of monarchs/hectare. This new information leaves scientists pondering the possibility of 3 to 4.5 times more monarchs per hectare than their original estimate. The sanctuaries may harbor many millions more butterflies than they imagined.

Estimates are Estimates! Challenge Question #7
When people hear numbers we tend to take them as fact. But let?s play with these two different density estimates and see how different the numbers of butterflies become. Just think: Nobody really knows how many butterflies there are! We can only estimate.

Are you ready? Before the storm this winter, monarchs covered a total area of 9.35 hectares in all colonies. (See graph above.) How many monarchs were there?

Challenge Question #7
?If you estimate 10 million monarchs per hectare, how many butterflies would be in those 9.35 hectares? What if you estimate 22 million monarchs per hectare??

(To respond to this question, please follow the instructions below.)

Discussion of Challenge Question #3
Why Count Males and Females?

"Why do you think the scientists were interested in knowing the number of male and female butterflies?"

Sorting male and female butterflies
Photo: Dave Kust

Mrs. Johnson's students at Union Park Elementary School in Orlando, Florida, responded, ?We think they are checking for numbers of male and female monarchs because they can find out if females or males are more able to survive the cold weather. Also, the numbers of males and females will determine how fast they can reproduce and increase the population to help make up the losses of millions of butterflies that were killed in the storm.?

These students are thinking along the same lines as scientists do! Whenever studying a population, knowing the number of males vs. females is important. What?s more, their size, condition, and lipid content might reveal differences between males and females that affected their survival.

Dr. Brower told us that the sex ratio is generally close to 50:50 in the eastern monarch population. (Remember, you can?t just assume this for all species!)

There are slight differences in the physical condition of males and females in a population. Dr. Brower explained, ?We have it documented, based on 5,211 monarchs collected throughout the year from the Eastern population, that:
  1. Females have higher lipid contents and generally weigh more than males.
  2. Males have greater lean mass.
  3. Males have longer wings.

(By the way, 2 and 3 are probably related to the fact that the males have to fly faster in order to court females successfully. Females probably need the extra lipid to go into egg production.)"

So did the scientists find the same number of dead females and males? Dr. Brower is still analyzing the data collected last month, but all samples appear to have had even sex ratios--except one! That sample, from El Rosario, had more males than females. The mortality rate at El Rosario was also higher. This exception will lead to interesting speculations. What do you think might have caused there to be more males in one of the samples from El Rosario?

Map of Over-wintering Monarchs

Please Report Your Sightings!
This map shows where monarchs have been reported this winter. Please send your observations NOW if monarchs are present in your area.

How to Respond to Today's Challenge Questions:

IMPORTANT: Answer only ONE question in each e-mail message.

1. Address an e-mail message to:
2. In the Subject Line of your message write: Challenge Question #4 (#5, #6, or #7).
3. In the body of EACH message, give your answer to ONE of the questions above.

The Next Monarch Migration Update Will Be Posted on February 28, 2002

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