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Monarch Butterfly Migration Update: October 11, 2001
Today's Report Includes:
- Highlights Along the Migration Trail
- How Many Monarchs per Minute? Challenge Question #11
- Does Scent Attract Monarchs to the Same Roosts?
- Journey North Map Server: A Look at the Future
Highlights From Along the Migration Trail
- Link to This Week's Migration Data (for classroom mapping).
Sightings from south of the U.S./Mexican border have begun to arrive. Meanwhile, monarchs
continue to move across Texas in good numbers, and the migration is still streaming down the East Coast. Late last
week, a strong cold front moved across the continent, touching all in its path. "On Monday we awoke to a heavy
frost and had to wear our winter coats to school," reported Mrs. Koonts' class in Bowie, MD. The same cold
front crossed the border into Mexico, carrying monarchs with it. From the state of Coahuila:
"Por fin vemos MONARCAS en Piedras Negras, Coahuila. Ayer domingo el dia muy fresco, tal
vez menos de 10 grados. Vimos unas cinco monarcas por minuto y en la carretera a Ciudad Acuna tambien. Todas inconfundibles
y con destino exacto al sur, " wrote Profesora Floria Herrera from Piedras Negras on October 7th."
Challenge Question #11:
"How many monarchs per minute did Profesora Herrera see? (And, if you're REALLY good at Spanish, send us a translation of her observations!)"
(To respond to this question, please follow the instructions
below.)
Here are comments from observers:
"Ya vi una el sabado 6 de este mes estaba en mi alberca y la moriposa estaba tomando agua, tambien un companero dijo que en Lamadrid, Coahuila habia muchas," wrote Ena Yaneth Cordoba Montoya from Monclova, Coahuila. (Reports from schools in northern Mexico come to us from the Mexican monarch migration tracking program, Correo Real, courtesy of its founder, Senora Rocio Trevino.)
The cold front had crossed Texas the previous day: "I was just outside (4:00 pm, October 5) and looked up and saw monarchs everywhere. There are 30 to 50 visible at any one time over the city right now. A strong front blew in today right at noon and the wind switched at that time. I didn't see anything flying at 1:40 (a hard rain was going) but the rain has ended for the moment and the monarchs are all scudding high and fast. For all of you south points south - here they come," wrote Forrest Mitchell from Stephenville, TX.
Ms. Caffey in Abilene, TX wrote, "Yesterday (10/06/01) at approximately 3:30 pm monarchs began to filter into our country area, and by dusk, approximately 3 to 4 thousand that had began to hang in one of our large mesquites and fruitless mulberries!"
"The front came through yesterday (10/5/01) about 7:30 pm and created a few scattered sprinkles. No monarchs. All day the 6th: No monarchs. Then at sunset they began tumbling down out of the skies from the north and clustering in the same trees they have for the last 22 years that I know about," reported Carol Cullar of Rio Bravo Nature Center near Eagle Pass, Texas.
Does Scent Attract Monarchs to the Same Roosts?
Earlier this fall, an observer watched roosts form night after night on the very same branch. "I am amazed that, of the hundreds of trees in our yard, the monarchs consistently manage to find the very same tree AND the same branch that the larger aggregations roosted on nights before. At first I thought it was just chance, but as the incoming butterflies select the same places night after night, I really wonder if the others leave a trail of some sort."
Dr. Calvert discusses these questions below. But before reading further, answer this question in your science journal, and/or discuss as a class:
Journaling Question:
"How could you test the hypothesis that scent attracts monarchs to roost on the same branch each night?"
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Now here's Dr. Calvert:
A person with hundreds of trees is in an excellent position to answer some very basic questions about roost formation.
To study this, a person should note the exact position of the roosts each year, along with daily observations of:
- Wind direction
- Humidity (dew point is the best measure of this),
- and, if in a drainage channel, their position with respect to the sides.
Has Anyone Done Research Into This?
One year along the migration route in Mexico's Sierra Madre Oriental, we encountered an immense flower field with
isolated huisache trees in it. Each evening monarchs that had been nectaring on the flowers would stop, and fly
towards the huisache trees. They would find each other and form transient, gregarious roosts. One evening we carefully
noted the branch they were on. The following day we cut that branch out of the tree and tied it in position in
another quadrant of the tree. That night we waited with anticipation. Well the wind had changed. The butterflies
did not go to our "special" limb, but rather went to the lee side of the tree, as is their custom.
So what can we conclude from this? Not much. The cut tree limb may have been a bit drier than the tree limbs attached
in the regular manner and therefore, rejected by the butterflies. But it seems that the dominant effect was the
wind and not some sort of pheromone deposited by butterflies on the branch the previous night.
Dr. Lincoln Brower and some of his students tried to find an attractant at a Mexican overwintering colony where
there were millions of butterflies. They found lots of chemicals associated with the fir (oyamel) trees, but nothing
that could be measured as a pheromone.
It seems likely that the butterflies are looking for these things:
- A tree where they can be off the ground and therefore safer.
- A place out of the wind where they will not be blown from their perches.
- Each other, so they can be safer in numbers.
- In dry habitats, they also seek trees in riparian areas where the humidity is high.
So How Do They Find Each Other?
On observing the formations of roosts in the evening, one is struck with several things:
- There is active patrolling amongst the canopies of trees. Many butterflies appear to be flying around looking for something.
- Some butterflies land on perches. When another butterfly approaches their position the perched butterflies open their wings, often rather abruptly.
- After this "disturbance" they may continue to open their wings for a few seconds more, usually much more slowly than initially.
Researchers have considered two possible meanings for this conspicuous behavior.
One is the obvious one: That the perched butterfly is signaling for other butterflies to come join it. But why
only signal when another butterfly is approaching? Again the answer may be the apparent one: Why bother to signal
when there is no one around?
Still there is another intriguing interpretation. The perched butterfly may be signaling to come join it, but with
conditions. The conditions would be to go to the periphery, a position more dangerous than the center. In any sort
of school or aggregation, it is always safer to be in the center. Your predator is likely to strike at the periphery.
So far no one has figured out a method of teasing out an answer to this question. Maybe you would like to try?
Journey North Map Server: A Look at the Future
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Technology is advancing quickly, and when more schools have fast connections our mapping system will include:
- Real-time input/output of data. (Presently, you can input your observations at any time, but the map is made manually by our staff once per week.)
- Interactive capabilities (so you can make your own maps of the species, regions and seasons of interest).
- Animations of historic migration data (so you can analyze dynamic patterns visually).
How to Respond to Today's Challenge Question:
1. Address an e-mail message to: jn-challenge-monarch@learner.org
2. In the Subject Line of your message write: Challenge Question #11.
3. In the body of the message, answer the question above.
The Next Monarch Migration Update Will Be Posted on October 18, 2001.
Copyright 2001 Journey North. All Rights Reserved.
Please send all questions, comments, and suggestions to our feedback form
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