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Monarch Migration Update: March 4, 2005

Today's Report Includes:

Field Notes from Mexico from Dr. Bill Calvert
"Just wanted to let you know we're heading to Mexico tomorrow to meet Bill Calvert and others to visit the monarchs," wrote an excited teacher from Minnesota on Wednesday. We will have news directly from the overwintering sites Mexico in the coming two weeks, as the monarchs prepare to head north.

From What do Monarchs Need Shelter? Discussion of CQ #4
The monarch's winter forest is one of the most elegant examples of shelter in nature. Millions of monarchs fly across the continent to find safe harbor there. From what do monarchs need shelter? Students who answered last week’s Challenge Question #4 said the forest provides shelter from snow, ice, hail, wind, rain, and other effects of storms; from too much heat or too much cold; and from the predators that might eat them. You can read in detail how these students from Michigan, Rhode Island, New Hampshire responded:

The Forest as Shelter: Microclimate is the Key
"The forest acts like a blanket retaining warmth during the cold nights and keeping out the heat during the day," explains Dr. Calvert. The forest reduces variations in temperature and humidity, to the great benefit of the butterflies, he says.

This is because forest produces what is called a "microclimate." A microclimate is the climate of a small, local area where the climate differs from the general climate due to the unique amounts of sunlight, wind, moisture, and other elements this local area receives.
Can you read what Dr. Calvert wrote on the roof of this VW bus? It was parked near the monarch sanctuary on a winter night.

Not Too Hot, Not Too Cold: Preserving a Delicate Balance
The graph below shows the average minimum temperature at the overwintering sites for each month of the year. On average, overnight low temperatures hover just above freezing while the monarchs are in Mexico. These temperatures are similar to those in your refrigerator! The forest shelters monarchs by preserving a very delicate balance: If temperatures were too warm, the butterflies would burn their stored fat too quickly; if temperatures were too cold, they would freeze to death.
  • Remember: This graph shows the average temperatures. This means the daily low temperatures can fluctuate above and below the temperatures shown.

With regard to temperature, "The really critical time for the monarchs is January and early February when it's the coldest time of the season," says Dr. Brower. "We know temperatures below -8 degrees C will kill about 50% of the monarchs. If they're WET, it will kill close to 80-90% of the monarchs."

The Forest as Blanket, Umbrella, and Hot Water Bottle
Dr. Brower often says the forest acts as a blanket, an umbrella, and a hot water bottle. If the forest is cut or thinned, he says, monarchs will not have the shelter they need. Describe what Dr. Brower means, using each of his analogies.

How is the monarch's forest like a blanket, an umbrella, and a hot water bottle?
Photos courtesy of Dr. Lincoln Brower, Sweet Briar College

Analyzing Real Data: Before and After a Storm
"After a snowstorm, I'd expect that most butterflies would survive, even many of those stuck in the snow," says Dr. Calvert. "Here is a sample set of data so you can see how things changed after a February snowstorm." Based on the data, is a butterfly safer in a tree or on the ground?

How Many Monarchs? Before or After the Long Winter?
Population estimates, shown on the graph below, are made before mid-January each year. They show us how many butterflies reached Mexico in the fall and were still alive by mid-winter. However, the size of the spring migration also depends on how many monarchs survive the rest of the winter, when storms are likely to occur. (Data are not available every year, even though storms may have occurred.)

Notice the impact storms can have on population size. Why have scientists been particularly worried about a storm this year?

Dew and Other Dangers: The Effect of Cold, Wet Weather
Challenge Question #4 stated that monarchs need shelter from dew, and we asked you to consider why. Students said wet wings would make flight more difficult, and Dr. Brower explained above that wet butterflies freeze to death more easily. Here Dr. Calvert describes another danger of dew:

"When dew freezes, the water turns into ice crystals. The ice crystals can puncture the butterfly's body through the exoskeleton."

For more information, see: The Effect of Cold, Wet Weather on Monarchs

How Are These Monarchs Avoiding Predators? CQ #5
Each of the three pictures below shows monarchs avoiding predators. Describe what you see very carefully. Then speculate how the forest helps to protect monarchs from predators.

Challenge Question #5
"How do you think the forest helps monarchs avoid predators? Inspect the photos below for clues, then send us your answer."

Dr. Lincoln Brower, Sweet Briar College 

Ask the Expert Opens Today (1 pm Friday, March 4)
Once again this year, monarch biologist Dr. Karen Oberhauser has volunteered to respond to students' questions. We are thrilled to offer this opportunity to you! We will accept questions for the next two weeks:

How to Respond to Today's Monarch Challenge Question

1. Address an e-mail message to:

2. In the Subject Line of your message write: Challenge Question #5

3. In the body of your message, answer the question above.

The Next Monarch Migration Update Will Be Posted on March 11, 2005

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