Monarch Butterfly Migration Update: September 3, 2004

Today’s Update Includes

• Highlights From the Migration Trail
• How Many Monarchs in Mexico This Winter? Dr. Taylor’s Prediction
• Challenge Question #2
• This Week’s Migration Focus: The Overnight Roost
• Try This! Too Cold to Fly Today?
• Dr. Brower: Pull Up a Chair and Watch
• Minnesota Roost: Five Monarchs Instead of Thousands
• Estimating the Number of Monarchs in a Roost: Let’s Practice
• Reminder: Symbolic Migration Deadline Oct 15
• Please Support Monarch Conservation in Mexico!

Click Map to Read Highlights Along the Migration Trail

One hundred and two people have now reported sightings of migrating monarchs. We have gathered many highlights for today’s map, so you can "click and read" the observers’ comments. Classroom suggestions for mapping the fall migration are provide here:

• Mapping the Fall Migration “Highlights”

In the north, everybody is commenting about the lackluster migration. There have been no reports of large aggregations or of large numbers in flight. Let’s see if numbers rise during the next week.

Interactive MapServer

This map shows the locations of all reports received.

In southern states, monarchs are not typically seen in the summer until late-August. As if on cue, during the last 10 days we received reports of first monarchs spontaneously from Arkansas, Oklahoma, North Carolina, South Carolina, Georgia, Virginia, Kansas and Texas.

It is not certain whether these late-summer sightings are early migrants coming down from the north, and/or members of the local population. (With each new generation there is an increase in monarch numbers. This could explain the butterflies’ sudden appearance.) Regardless of their origin, these butterflies are laying eggs. They will produce a fall generation, and add to the migration moving southward to Mexico.

Monarch Life Cycle
Egg	Larva	Chrysalis	Adult

Over-wintering Population in Mexico

According to Dr. Chip Taylor of Monarch Watch, "Late summer reproduction in the south, especially in Texas, could contribute more butterflies than normal to the migration due to well-distributed rainfall. Overall, the wintering population should be in the range of 4-6 hectares."

In December or January each year, the size of the monarch population at the over-wintering sites in Mexico is measured. Population estimates since 1996/1997 are shown on this graph.

Challenge Question #2
“If Dr. Taylor’s prediction is correct, how will this winter’s monarch population in Mexico compare those in past years? In your answer, list the years that the population was lower, higher and about the same as Dr. Taylor's prediction for this year.”

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

In the morning, the butterflies must warm up before they can fly. Monarchs cannot even crawl when their thoracic temperatures are below 5C (41F). To fly WELL, with lots of control, they need to attain thoracic temperatures in the upper 20’s or even 30’s C. Outdoor temperatures drop as the fall season progresses, so monarchs have a shorter and shorter window of time each day in which they can fly. This is one reason monarchs must leave the north before it gets too cold!

Try This! Too Cold to Fly Today?
Record and graph the daily temperatures in your hometown this fall. When does the temperature drop below 41F? As the season continues, for how many hours in each 24 hour day is the temperature below 41F? (Note that monarchs can bask in the sun and/or shiver to raise their temperatures above outdoor temperatures. Therefore, keep track of sunny and cloudy days to get a better sense of when monarchs could fly.)

Year after year, people often see monarchs at the same roosts along the migration trail. Where do these roosts tend to form? Do the monarchs find them by scent? Here Dr. Bill Calvert clears up some misconceptions:

• Do Monarchs Rest at the Same Roost Sites Every Fall?
• Does Scent Attract Monarchs to the Same Roosts?

Dr. Brower: Pull up a Chair and Watch!
Few people have studied monarch behavior at these roosts. We asked Dr. Lincoln Brower what he would do if he came upon one:

"I'd pull up a chair, grab a pair of binoculars and just sit and watch! I'd try to stay hour after hour, day after day--as long as the monarchs were there. People can contribute important observations by going with an open mind and documenting what they see. Science is often done with pre-conceived ideas, and so scientists can get hung up on their own theories, and forget to ask ‘What IS going on here?'" Field Studies You Can do at Roost Sites

Hear Dr. Lincoln Brower Audio File

Minnesota Roost: Five Monarchs Instead of Thousands
“I was expecting to have more butterflies to tag this year,” wrote Paul Viger from Campbell, MN. Monarchs roost in his yard every year, and Mr. Viger is an example of someone who’s contributing important observations through careful observation.

So far this year, only five butterflies have arrived! This is the forth year that he has made a daily count of roosting butterflies. At this time in 2001, over 6,000 butterflies had visited.

Monarchs at Fall Migration Roost Site in Minnesota 2001-2004	Monarch Reproduction in Upper Midwest 1996-2004
Mr. Paul Viger	Monarch Larva Monitoring Project

The graph on the left shows the number of butterflies as of September 1 each year from 2001-2004. Do Mr. Viger’s data correlate with the data Dr. Oberhauser collected this summer in the Upper Midwest? (See graph on the right.)

Why do daily monarch counts go up and down?

At any overnight roost, the number of visiting monarchs tends to rise and fall from one night to the next. Take a look at the data Mr. Viger collected during the Fall 2001 season. What relationship do you see between weather and migration? Why do you think monarchs behave this way?

• Analyzing Data from a Fall Migration Roost

How many monarchs do you see?

• Estimating the Number of Monarchs in a Roost

• About the Symbolic Migration

The Next Monarch Migration Update Will Be Posted on September 10, 2004.

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