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Monarch Butterfly Migration Update: March 7, 2002

Today's Report Includes:

Field Notes From Mexico by Dr. Bill Calvert
March 6, 2002

In the past week, the butterflies at Sierra Chincua have moved down the canyon another 200, maybe even 300, meters from where they were before. A few dense clusters remain, but most are late-season clusters. They?re spread out and they?re small.

This dispersion from dense packing later in the season suggests the reason for the tight winter aggregations: The close-packing serves to warm the butterflies during the cold part of the season. However, as of yet, there is no hard evidence that packing supplies heat for the butterflies.

Whatever the reason, as the ambient temperature heats up, the butterflies leave the colonies and do not return to the dense clusters. Instead they move downslope and reform in smaller clusters, higher in the canopy and more widely dispersed in the forest.

It?s still very chilly here, and we have witnessed no massive movements yet. There are only a few butterflies down watering in Angangeo, not the usual business of butterflies flying through the town en mass. I don?t think they?ve started back yet. It may be because it?s just cooler this year. I think it?s affecting the butterflies, and I would predict a later return this year. We?re having the usual clear days, which usually stimulates them, but they were behaving like mid-February not early-March colonies.

Mating is beginning, we?re seeing more and more mating pairs every time. We saw about 10 in 2 hour period at Chincua, for example.

Today we?re going to a new colony on Cerro Pelon. Evidently the same one that Prince Charles is going to visit this Friday. I?ll tell you about that next week.

Until then,

Pictures and Thoughts about Camouflage in the Colonies
By Elizabeth Howard

When visiting the monarch colonies I was struck by the camouflage apparent in the photos below. I was expecting to see bright orange butterflies! But on a cool day, the resting butterflies looked almost grey. The colors and patterns of their closed wings match the oyamel forest elegantly. They blend in with the oyamel bark and lichens so well that they become almost invisible.

Look at the tree trunk covered with monarchs. Notice the single butterfly with open wings at the base of the truck. (Click on images to enlarge.) Its vivid orange wings make all of the others, with closed wings, seem ghostlike in comparison.

Next, look closely at the butterflies against the oyamel bark. Notice here how closely the colors and patterns of the veined wings match the colors and patterns of the bark. Also, light seems to hit the tips of the raised lichens, so that the white spots on the margins of the monarch wings also help to conceal the butterflies.

From the distance, look how well the monarchs match the overall grey appearance of the oyamel trunks. Their bodies also blend in against the rough texture the lichens create on the bark. This photo shows why it's easy to walk right past the colonies, they're so well concealed.

Maybe the patterns of the underwings conceal monarchs equally during all seasons, but I did enjoy noting how elegantly they matched the oyamel forest. It made me wonder if it's natural selection during the wintering season that has produced these colors and patterns.

A Careful Look at Monarch Wings

As you know, monarchs have four wings--two forewings and two hindwings. Each wing, of course, has a topside and an underside. That makes 8 surfaces!

If you have a real monarch, dead or alive, take a close at it look now. Look closely at the undersides of the two hindwings. Also pay careful attention to the tips of the undersides of the forewings. Describe what you see.

Now go back and look at the way the monarchs fold their wings when resting at the overwintering sites.

Challenge Question #10
?How does the resting position of a monarch help to camouflage the butterfly in the oyamel forest? In your answer, include the names of the wing surfaces that are important for camouflage.?

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

Try This! Classroom Camouflage Safari
Students create camouflaged butterflies that blend in with the classroom habitat. Next, butterfly predators test their skill at finding the concealed butterflies.

Estimates are Estimates!
Discussion of Challenge Question #7

After the January storm, scientists realized there may be many more monarchs in a colony than they had imagined. Instead of the 10 million/hectare estimate they?d always used, they now estimate there may be 22 million/hectare--or more!

Challenge Question #7 asked you to estimate the number of monarchs this winter in the 9.35 hectares the colonies cover. We asked, ?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?? This table shows there?s a difference of 144 million monarchs if you use the new density estimate!

How Many Butterflies This Winter?

Estimate Used

Number of Butterflies

10 million/hectare

95 million

22 million/hectare

209 million


114 million!

What If? Discussion of Challenge Question #6

We asked, ?If 80% of LAST YEAR?S population had been killed by this single storm, how many hectares of monarchs would have remained?"

Here?s the monarch math needed to answer: Last year the area covered with monarchs measured 2.8 hectares. If 80% of last year?s population had died, 20% would have remained. Multiplying 20% X 2.8 hectares = .56 hectares.

Lipid Loss: Discussion of Challenge Questions #8 and #9
Miss Bailey's Third Graders at Citrus Elementary School in Vero Beach, Florida, answered both questions!

"As a class, our students came to the consensus that the average monarch burns about 80 mg of lipids during the overwintering season, from November through March. If a monarch tag weighs 10 mg, then the average monarch loses eight times that weight by the end of the overwintering season."

Milkweed Emerging on the Migration Trail?
Record cold spread across the monarch?s breeding range in eastern North America last week. Even on the Texas Gulf Coast temperatures dropped into the low 20s, freezing back many plants.

Departure from normal

Minimum Temperatures

According to Harlen Aschen in Victoria, Texas, it hasn?t been this cold in early March since 1989. "How will the local milkweeds, that we would expect to be emerging about now, tolerate temperature into the low to mid twenties?" he wondered.

We hope you will help us monitor the spring emergence of the monarch's food plant across North America. This will be a fascinating year to note the timing of the migration!

Please report the FIRST MILKWEED Leaves to Emerge This Spring!

Note: We've changed our protocol this year. Because it?s so hard to catch the exact date milkweed shoots emerge from the ground, we'll be watching for the first leaves to open. Please watch carefully for milkweed to emerge, then note the date the first leaves unfold from the stem.

Winter Monarch Sightings: Pre-Migration
This map shows where people have reported monarchs this winter. We need your help before the monarchs arrive from Mexico.

** Please Report Your Winter Monarch Sightings NOW! ***

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 # 10.
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 March 14, 2002

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