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Monarch Migration Update: March 23, 1999

Today's Report Includes:

More Monarchs Sighted in Mexico & Texas
Rocio Trevino wrote from the Correo Real monarch migration program in Saltillo, Coahuilla, "You were right about the return of the monarchs, they were passing by my head and I had not noticed." (See sightings provided by Correo Real below.)

Further up the trail in Port Lavaca, TX, Harlen Aschen wrote on Sunday, "We FINALLY think we've seen a migrating monarch! It was very, very faded with 4 or 5 small holes in the wings that you could see the sky through. It flew off to the NNE into a gentle north breeze."

Here are today's data--NOW do you think the monarchs are migrating?

Final Field Notes from Mexico by Dr. Bill Calvert

This is my final report from Mexico as I'll be heading home next week. I will send my next report when I'm back in Austin.

A Quiet Departure This Year?
Nobody has witnessed masses of monarchs flying away this year. Dr. Lincoln Brower drove across the flyway on Sunday, March 21, and counted only 140 monarchs between Toluca to Zitacuaro. During the past week, we have traveled the area immediate north of the colonies of Herrada, Oxtotilpan, El Pelon and more recently Chincua, Rosario and San Andreas. The numbers seen traveling north have so far been disappointing, especially in the areas north of the southern colonies (Herrada and Pelon).

Nevertheless, Eligio Garcia reports that most monarchs have in fact departed. No butterflies are left in Herrada or Las Palomas, there are "precious few" in Picacho, and "very few" at El Rosario and Chincua. Last Friday at El Rosario, butterflies remained on only 25 of the 740 trees they covered during the peak of winter.

Challenge Question #17
"According to Eligio Garcia's estimate, what percent of the monarch population has departed from El Rosario?"

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

Physical Condition of the Butterflies as They Leave Mexico
What do the monarchs look like at this time of year, as they set off on their long trip northward? From an average mid-winter weight of 500 mg, the average weight is now down to about 400 mg. However, the departing monarchs APPEAR to be in remarkably good condition. Considering they have already flown as much as 2,000 miles and endured 5 months in Mexico, there is really not much sign of wear as they depart the colonies. However, for reasons I don't understand, when they appear in Texas a few weeks later, their wings are often so faded as to appear ghost-like. It seems that a lot of wear occurs on the trip north. I'm just speculating, but for some reason the heat and direct sun on the return journey seems to cost them a lot, perhaps because the sun is so strong at this latitude in March. (See below.)
Hot & Dusty Along the Migration Trail
I understand there are drought conditions in northern Mexico and Texas. Rocio Trevino noted the conditions in her region: "The weather is dry, windy and dusty." According to the Texas State Climatologist, John Griffiths, precipitation was way below normal for almost all of Texas during February. (See map of precipitation in Texas during February)

Challenge Question # 18
"What does drought mean to migrating monarchs? How do you think drought in northern Mexico and Texas might affect the butterflies?"

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

Discussion of Challenge Question #11
A few weeks ago, I asked if you could think of another cause of cascading that could be due to the presence of people? Here are the ideas sent by students:
  • They might feel the wind move or the dust coming up when the people walk by.
  • If people stamped their feet or ran, the butterflies might feel the ground moving.
  • They could possibly sense people by physically running into them.
  • Humans may carry a noticable scent if they wear perfume, smoke, eat spicy food or give off other body odors.

All of these are good guesses. But Joshua and Jessica at Citrus Elementary School in Vero Beach FL, thought of my answer: Carbon dioxide (CO2).

It's known that if you breathe on a cluster it will explode in your face. CO2 has a profound effect on many insects; mosquitoes are known to follow a CO2 gradient to find humans, for example. The implication is that the butterflies may associate carbon dioxide with predators, but this is poorly understood. People are beginning to worry about the effect of people breathing and causing cascading.

Subtropical Sun and Shadows
Challenge Question # 15
"Can you think of some reasons why the sun is so brilliant here in March?" There are a number of reasons, but the 3 of most importan are:
  1. The sun strikes the earth at much sharper angle here, and radiation doesn't have to travel through as much of the earth's atmosphere. For example, we are close to the Equinox--and the sun is nearly above the equator at zero degrees north. The butterfly colonies are located at 20 N so, at solar noon, the sun's rays strike the earth at an 70 degree angle. (To figure out the angle the sun's rays are striking your part of the earth for comparison, subtract your latitude from 90 degrees.)
  2. This is the 5th month of the dry season so the air contains very little moisture. (Moisture also attenuates radiation.)
  3. The butterfly colonies are at a very high elevation, where the air is "thin" and water vapor and dust particles are relatively rare. (Such particles and molecules scatter light.)

Until next week,
Bill Calvert

Weather Report from Escuela Pedro Ascencio
Profesor Gilberto Salazar Vargas reports that it rained on Saturday for the FIRST time in over 30 days, when just 2 cm rain was received. Also, on Sunday the town of Angangueo celebrated the first day of spring, and the final day of the monarch festival. The entrance to Sierra Chincua was officially closed.

Plot last week's temperatures in the sanctuary region. Is the monarch's over-wintering region in Mexico as warm as you expected it to be?

Week of March 15, 1999
Temperatures in Monarch Over-wintering Region


Temperatura Maxima (C)

Minima (C)


Helada o Rocio





























2 cm







How to Respond to Today's Monarch Challenge Questions

IMPORTANT: Please 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 # 17 (or Challenge Question # 18)
3. In the body of EACH message, answer ONE of the questions above.

The Next Monarch Migration Update Will Be Posted on March 30, 1999.

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